Chemical pretreatments of wood chips prior to alkaline pulping – A review of pretreatment alternatives, chemical aspects of the resulting liquors, and pulping outcomes

Abstract

The chemical industry is being forced to evaluate new strategies for more effective utilization of renewable feedstocks to diminish the use of fossil resources. In this literature review, the integration of both acidic and alkaline pretreatment phases of hardwood and softwood chips with chemical pulping is discussed. Depending on the pretreatment conditions, high-volume sulfur-free fractions with varying chemical compositions can be produced. In case of acidic pretreatments, the major products include carbohydrates (mono-, oligo-, and polysaccharides), whereas under alkaline (i.e., aqueous NaOH) pretreatment conditions, the sulfur-free fractions of aliphatic carboxylic acids, lignin, and extractives are primarily obtained. All these fractions are potentially interesting groups of compounds and can be used in a number of applications. Finally, the effects of pretreatments on pulping are also considered. Although it is believed that there are important advantages to be gained by integrating this type of renewable raw material-based production, in particular, with kraft pulping, sulfur-free pulping methods such as soda-AQ and oxygen/alkali delignification processes are also briefly discussed.

Full Article

Chemical Pretreatments of Wood Chips Prior to Alkaline Pulping – A Review of Pretreatment Alternatives, Chemical Aspects of the Resulting Liquors, and Pulping Outcomes

Joni T. Lehto* and Raimo J. Alén

The chemical industry is being forced to evaluate new strategies for more effective utilization of renewable feedstocks to diminish the use of fossil resources. In this literature review, the integration of both acidic and alkaline pretreatment phases of hardwood and softwood chips with chemical pulping is discussed. Depending on the pretreatment conditions, high-volume sulfur-free fractions with varying chemical compositions can be produced. In case of acidic pretreatments, the major products include carbohydrates (mono-, oligo-, and polysaccharides), whereas under alkaline (i.e., aqueous NaOH) pretreatment conditions, the sulfur-free fractions of aliphatic carboxylic acids, lignin, and extractives are primarily obtained. All these fractions are potentially interesting groups of compounds and can be used in a number of applications. Finally, the effects of pretreatments on pulping are also considered. Although it is believed that there are important advantages to be gained by integrating this type of renewable raw material-based production, in particular, with kraft pulping, sulfur-free pulping methods such as soda-AQ and oxygen/alkali delignification processes are also briefly discussed.

Keywords: Autohydrolysis; Biorefining; Biochemicals; By-products; Carbohydrates; Delignification; Extractives; Hydrolysis; Lignin; Organic acids; Pretreatment; Pulping

Contact information: Laboratory of Applied Chemistry, Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 University of Jyväskylä, Finland; *Corresponding author: joni.t.lehto@jyu.fi

INTRODUCTION

Sustainable economic growth requires safe, environmentally friendly, and sustainable resources for industrial production (Hardy 2004; Kamm and Kamm 2004; Sánchez and Cardona 2008; Amidon et al. 2011). Due to the depleting resources of fossil fuels, increased concern of global warming, and increased demand for energy and materials, fossil carbon sources, such as petroleum and natural gas, must be replaced by renewable raw materials. Chemicals, biofuels, and biocomposites manufactured from renewable resources, such as forest and agricultural biomass, have been considered to be the most promising alternatives for replacing traditional raw materials (Goldstein 1981; Sinsky 1983; Stevens 2004; Carvalheiro et al. 2008; Cherubini 2010; Dautzenberg et al. 2011; Alén 2011; Liu et al. 2012a,b; Viikari et al. 2012). While the needed energy production can be based on various alternative production systems (wind, solar, hydropower, biomass, nuclear fission, and fusion), the production of materials, such as chemicals, polymers, and fuels, mainly depends on biomass. For these reasons, different biorefinery processes integrated with modern pulp and paper mills have gained great interest, especially during the last few decades (Kamm et al. 2006; Ragauskas et al. 2006; Huang et al. 2010; Walton et al. 2010a; Alén 2011; Marinova et al. 2014). This approach includes the modification of a conventional pulp mill to incorporate elements of an integrated forest biorefinery (IFBR) mill (van Heiningen 2006; Hämäläinen et al. 2011; Baijpai 2012; Resalati et al. 2012; Moshkelani et al. 2013). It has been estimated (Connor 2007) that in North America there are approximately 200 chemical pulp mills and high-yield pulp mills that could also economically host biorefineries. There are also another 100 large, nonintegrated paper mills with heat and energy sinks large enough to support a biorefinery. Internationally, it has been estimated that there are over 450 pulp and paper mills and another 400-500 nonintegrated paper mills that are sound potential sites for biorefineries.

As such, conventional chemical pulp mills can be considered as typical examples of chemical/thermochemical biorefineries utilizing different technological innovations to fractionate and convert woody biomass into a wide range of products, such as cellulose, extractives-derived by-products, lignin-based materials, hemicelluloses, and many other minor products (Herrick and Hegert 1977; Alén 1990; Kamm et al. 2006; Huang et al. 2010; Chirat et al. 2012; Sanglard et al. 2013; Benali et al. 2014; Leskelä et al. 2014). However, it can be concluded that modern pulp mills could serve as a promising platform for even more efficient use of wood and other lignocellulosic materials. A modern IFBR approach could include the opportunity to produce not only the main product (pulp fiber), but also value-added green chemicals, such as fuel grade ethanol, as well as cellulose and hemicelluloses together with their derivatives; this could include low-volume/high-value lignin/extractives-based fine chemicals, thus resulting in an enhanced utilization of feedstocks with the simultaneous increase in profitability and decrease in greenhouse gas emissions (Mendes et al. 2009; Mansoornejad et al. 2013; Martin-Sampedro et al. 2014). In addition to the more efficient use of pulp woods, the analogous processing of agricultural and forest residues could be considered as well. This approach could result in the improved use of energy and raw materials with the simultaneous increase in profitability and expansion of the product portfolio of mills.

One of the most promising biorefinery techniques is based on the various chemical pretreatment processes by which wood chips can be treated and partially solubilized under varying conditions (pH, temperature, and treatment time) prior to delignification (Carvalheiro et al. 2008; Marinova et al. 2009; von Weymarn 2011; Chirat et al. 2012; Galbe and Zacchi 2012; Cordeiro et al. 2013; Behera et al. 2014; Gomes et al. 2014). Different pretreatments of lignocellulosics produce carbohydrates- and lignin-containing hydrolysates, which can be further converted into desired chemicals, biomaterials, or biofuels (Kamm and Kamm 2004; Sánchez and Cardona 2008; Alvira et al. 2010; Zhu and Pan 2010; Chiaramonti et al. 2012; Mood et al. 2013; Silva-Fernandes et al. 2015). In addition, using various pretreatments, it is possible to increase the reactivity of feedstock material, leading to enhanced pulping performance and recovery of potential by-streams (lignin and hydroxy acids) (Hendriks and Zeeman 2009; Kumar et al. 2009; Zhu et al. 2010). However, in each case, a general prerequisite for finding a realizable design concept is that the presence of all the feedstock constituents is taken into account when planning target-oriented economic processes for the manufacture of useful products. Hence, the main principle is to maximize the value of lignocellulosic biomass and, conversely, to minimize the production of waste.

Biomass pretreatment technology plays an important role in the modern forest industry and in other biorefinery concepts based on lignocellulosic feedstocks (Mosier et al. 2005; Carvalheiro et al. 2008; Yoon et al. 2008; Yoon and van Heiningen 2008). Chemical pulp mills use large quantities of both softwood and hardwood feedstocks to produce cellulosic pulp. However, the chemical pulping process is rather unselective, and a significant part of raw wood material is dissolved during delignification into the cooking liquor (“black liquor”), which is burned in the recovery furnace for the recovery of cooking chemicals and the production of energy. This dissolved organic fraction contains, in addition to carbohydrate-derived degradation products (aliphatic carboxylic acids), the degradation products of other significant wood polymeric component lignin, together with minor amounts of extractives. The low heating value of carbohydrates and their degradation products, compared to that of lignin, encourages the partial removal of these constituents by different pretreatment stages prior to the main pulping stage (Tunc and van Heiningen 2008a,b; Al-Dajani and Tschirner 2010; Pu et al. 2011; Vila et al. 2011; Chirat et al. 2012; Liu et al. 2012c; Martin-Sampedro et al. 2014). Lignocellulosic material (LCM) feedstocks are transformed into three major output streams (cellulose, hemicelluloses, and lignin) together with their degradation products by chemical, physical, and biological pretreatments (Duff and Murray 1996). These materials can be processed further into various value-added end products, such as bioethanol for energy purposes, polymeric composites, or different biochemicals (Gírio et al. 2010; Alvira et al. 2010; Agbor et al. 2011; Liu et al. 2011b; Rodríguez-López et al. 2012).

Several pretreatment processes and technologies have been suggested for the fractionation and recovery of valuable components from LCMs (Sánchez and Cardona 2008; Tunc and van Heiningen 2008a,b; Kumar et al. 2009; Zhu et al. 2010; Song et al. 2011; Si et al. 2015). Biomass pretreatments change the structure of LCMs and remove structural constituents from the feedstocks, making the pretreated materials more accessible for further fractionation and conversion techniques, such as alkaline pulping (Yang and Wyman 2004; Hendriks and Zeeman 2009; Gírio et al. 2010). The alteration of wood structure can include the removal of lignin and hemicelluloses, reduction of the crystallinity of cellulose, and increasing the porosity of the LCMs. In addition to the pretreated wood material, carbohydrate-rich hydrolysates are formed (Mok and Antal 1992; Garrote et al. 2001; Song et al. 2008; Li et al. 2010).

Pretreatment processes must meet several requirements. The process must be efficient enough to improve the formation of free carbohydrates (mono-, oligo-, and polysaccharides), which can be further hydrolyzed to fermentable sugars or used as polymers, but at the same time, the production of high quality pulp must be preserved. The degradation and loss of carbohydrates and formation of inhibitory substances, which can disturb subsequent hydrolysis and fermentation processes, must be avoided. Finally, the process must be cost effective. In this review, the following three basic chemical pretreatment processes are discussed (the total amount of material removed is typically in range 15 to 25% of the initial feedstock):

– Hot-water extraction,

– acidic extraction, and

– alkaline extraction.

Other possible pretreatments (left out from this review) include, for example, physical (milling, grinding, etc.), enzymatical/biological, and some chemical/physicochemical (ammonia fiber explosion (AFEX), carbon dioxide explosion, organosolv, ionic liquids, etc.) pretreatments.

HOT-WATER EXTRACTION

Water is a unique solvent for any industry. It has a relatively high boiling point for its mass, a high dielectric constant, a high polarity, and a highly hydrogen-bonded structure (Ramos et al. 2002; Smith 2002; Krogell 2015). However, the properties of water are dramatically altered as the temperature and pressure are raised and the hydrogen-bonded lattice is disrupted due to the increasing thermal motion of the water molecules (Yu et al. 2008). Between 100 °C and 374 °C, heated water can have a permittivity very similar to that of typical organic solvents, making the organic non-polar compounds more soluble in water (Wiboonsirikul and Adachi 2008). The increased diffusitivity characteristics, the reduced viscosity, polarity, relative permittivity, and surface tension together with enhanced solvent properties towards organic solutes allow more efficient mass-transfer reactions from solid samples (such as wood) compared to the extractions conducted under ambient conditions. Low surface tension and low viscosity promote better penetration of water into the matrix particles, thus enhancing the extraction. Low surface tension between water and the matrix permit better contact of solutes with water, and as the surface tension of water is decreased, the solvent cavity is easily formed, allowing the solute to dissolve more quickly in the solvent. In addition to these physical properties, the enhanced solvent properties of water can be related to the increased vapor pressures and accelerated thermal desorption of the compounds. As a solvent, water is cheap, non-flammable, non-toxic, abundant, easily disposable, and environmentally friendly; thus, it has a great potential to replace commonly used organic solvents.

Hot-water extraction (HWE) conducted at 50 to 100 °C has been used for a long time to extract organic components at the atmospheric pressure from solid sample matrices (Willför and Holmbom 2004; Kronholm et al. 2007). However, the extracted compounds have usually been relatively polar and readily soluble in water at these low temperatures. Pressurized hot-water extraction (PHWE, often known also as sub-critical water extraction, superheated water extraction, extraction with water at elevated temperatures and pressures, and near critical water extraction) has become a popular green extraction method for several compounds present in many different biological matrices (Vegas et al. 2004; Amidon and Liu 2009; Kim et al. 2009; Teo et al. 2010; Singh et al. 2014). PHWE is typically performed at the temperatures above 100 °C and below 374 °C (the critical temperature of water). The enhancement on the extraction efficiency of PHWE can be attributed to improvement in the solubility and mass transfer effect, and to increased disruption of surface equilibria (Teo et al. 2010; Kilpeläinen et al. 2013; Penttilä et al. 2013; Kilpeläinen 2015). The PHWE has the ability to recover and dissolve both polar and non-polar components from biological matrices, which makes this method an interesting alternative for fractionation of LCMs. Such compounds as phenolic, polycyclic aromatic compounds, oils, and proteins have all been extracted from environmental samples by PHWE. However, within the context of IFBR processes, the main emphasis of the PHWE has often been focused on the production of various hemicelluloses-derived carbohydrate fractions (Kilpeläinen et al. 2012).

Several factors, such as temperature, pressure, extraction time, and the analyte characteristics affect the efficiency of PHWE (Wiboonsirikul and Adachi 2008; Teo et al. 2010; Kleen et al. 2011; Hanim et al. 2012; Borrega et al. 2013a,b; Krogell et al. 2015; Krogell 2015). Other parameters include the flow rate, the type of reaction vessel, and the use of varying modifiers and additives. Temperature is the main parameter affecting the physicochemical properties of water and the efficiency of PHWE (Kronholm et al. 2007). When the temperature is increased, the solute-matrix interactions caused by van der Waals’ forces, hydrogen bonding, and dipole attractions of the solute molecules and active sites of the sample matrix are disrupted. The thermal energy overcomes the cohesive (solute-solute) and adhesive (solute-matrix) interactions by decreasing the activation energy required for desorption (Richter et al. 1996). Pressure has only minor effects on the extraction recoveries during PHWE, as raising the pressure has only minor effects on the relative permittivity and solvent strengths of liquid water. During PHWE, a certain minimum pressure is required to maintain the water in a liquid state at the treatment temperatures (100 to 374 °C). In addition, since water has to be efficiently pushed through the sample, some samples may require higher extraction pressures. Extraction time is strongly dependent on the treatment temperature, on the desired extraction yield, and on the nature of the sample matrix components (Kronholm et al. 2007). By increasing the treatment temperatures (i.e., more energy is provided), the treatment times can be decreased and vice versa.

PHWE can be carried out from several minutes to several hours, depending on used temperature and type of the used sample (Kronholm et al. 2007). Sample characteristics, such as the chemical composition of physical nature of the matrix, porosity, surface to volume ratio, size, and mass have a great influence on PHWE efficiency (Kronholm et al. 2007; Wiboonsirikul and Adachi 2008; Kleen et al. 2011; Krogell 2015). Extraction efficiency, rate, and recovery of various analytes can be greatly increased by increasing the porosity and the surface to volume ratio of the sample. In addition, PHWE efficiency varies according to whether the analytes are deposited in, adsorbed on, or chemically bonded to the sample matrix.

PHWE performed with LCMs is often called autohydrolysis (Garrote et al. 1999a,b; Carvalheiro et al. 2008; Al-Dajani et al. 2009; Testova et al. 2009,2011). Autohydrolysis technology covers a wide range of treatments, including water- and steam-based processes (Garrote et al. 1999a,b; Li et al. 2005). On an industrial scale, autohydrolysis is applied to the production of dissolving pulps based on the pre-hydrolysis kraft process (PHK) (Leschinsky et al. 2009a,b; Schild et al. 2010). During the first part of the autohydrolysis, catalytical hydronium ions come solely from the water autoionization reactions (Zumdahl and Zumdahl 2007). Hydronium ions act as a catalyst, especially during the early hydrolysis reactions of LCMs, leading to selective cleavage of the glycosidic linkages and in particular the acetyl groups of the hemicelluloses (Garrote et al. 2001; Chen et al. 2010; Liu 2010).

In the second stage of the autohydrolysis, hydronium ions coming from acetic and uronic acids start acting as catalysts in autohydrolysis (Garrote et al. 1999a,b; Carvalheiro et al. 2008). The contribution of hydronium ions originating from organic acids is higher than that from water autoionization catalyzing the hydrolysis of the linkages between hemicelluloses and lignin (Chua and Wayman 1979; Wayman and Chua 1979; Bujanovic et al. 2012), as well as the hydrolysis of the glycosidic bonds of the carbohydrate chains, resulting in chain degradation and reduced molar masses (Lai 2001; Kamerling and Gerwig 2007; Leschinsky et al. 2008a,b; Li and Gellerstedt 2008; Borrega et al. 2011a,b).

Depending on the severity of treatment conditions (temperature, treatment time, and pressure), autohydrolysis can result in the formation of mono-, oligo-, and polysaccharides (up to 60 – 80% of the total dissolved organics) (Gullón et al. 2012; Lehto et al. 2014a,b) and dissolved lignin, together with a variety of their degradation products (Sears et al. 1971; Nabarlatz et al. 2004; Paredes et al. 2008; El Hage et al. 2010). The benefits of the autohydrolysis process over the acid pretreatments are that corrosion problems are limited, no waste sludges are generated, economic and operational costs are low, and cellulose is not significantly degraded under the applied conditions (Garrote et al. 1999a,b; Lei et al. 2010; Xiao et al. 2011). The disadvantage of autohydrolysis process is that it is not specific, and various side processes take place during the treatment, leading to hydrolysates with very complex chemical compositions that need further processing (Garrote et al. 2003; Vázquez et al. 2005; Leschinsky et al. 2009a,b; Gütsch and Sixta 2011).

Determining a relationship between the materials removed from LCMs by autohydrolysis and the process variables, such as temperature and time, is essential in order to predict optimal operation conditions for the production of the desired lignocellulosic products and allow a comparison of operations carried out at different temperatures and times (Tunc and van Heiningen 2009). One such operational parameter is the so-called pre-hydrolysis factor (P-factor), which combines the effect of treatment time with the applied temperature in one parameter, similar to the H-factor for kraft cooking (Brasch and Free 1965; Sixta et al. 2006; Pedersen and Mayer 2010; Saukkonen et al. 2012a). This single variable can then be used as a tool for estimating and comparing autohydrolysis processes with different temperatures and time. When the activation energy for the cleavage of glycosidic bonds of the carbohydrate materials in wood is coined with the P-factor concept (Lin 1979), Equation 1 can be employed for the P-factor concept (Sixta et al. 2006; Gütsch et al. 2012),

 (1)

where t is reaction time in hours, T is temperature in Kelvins, and k is a rate constant. The activation energy used in Eq. 1 is 125.6 kJ/mol. Typical autohydrolysis pretreatment times range from minutes to several hours, depending on the wanted outcomes (i.e., molecular properties of the dissolved hemicelluloses and lignin, formation of unwanted degradation products, and pulping properties of the pretreated wood)

Besides the conventional autohydrolysis, microwave-assisted HWE has been used for extraction of hemicelluloses with high yield from spruce chips (Lundqvist et al. 2002,2003). This treatment strongly shortens the required extraction time, since in this case heating the water to the desired temperature is extremely fast. However, the microwave-assisted extraction is difficult to up-scale and can be considered rather expensive.

In addition, surface active agents or surfactants as additives have been used for facilitating the diffusion of hemicelluloses out from the wood matrix and preventing unspecific adsorption of cellulase enzymes to lignin as well as for improving the solubility and removal of lignin from the wood matrix (Kristensen et al. 2007; Wei et al. 2011). The use of surfactants in autohydrolysis is mainly aiming at the enzymatic degradation of cellulose and hemicelluloses to manufacture of ethanol and other value-added products.

ACIDIC PRETREATMENTS

Acidic pretreatments of LCMs are usually performed with some common organic (Tunc et al. 2014) or mineral acids, such as sulfuric acid (H₂SO₄), hydrochloric acid (HCl), nitric acid (HNO₃), or trifluoroacetic acid (F₃CCO₂H, TFA), of which H₂SO₄ is the most commonly used acid (Carvalheiro et al. 2008; Marzialetti et al. 2008). Acidic pretreatment is often combined with other hydrolysis techniques, such as enzymatic hydrolysis, which would proceed too slowly without the pretreatment (Uçar 1990). The main aim of these hydrolysis combinations is usually the production of hydrolysate containing fermentable sugars, which can be further processed into various biochemicals, such as bioethanol (Cara et al. 2008). Acid hydrolysis has been utilized as a pretreatment method for numerous lignocellulosic feedstocks, such as agricultural wastes (Binod et al. 2010; Romero et al. 2010; Talebnia et al. 2010; Rocha et al. 2014; Singh et al. 2014), but also for woody biomass (Söderström et al. 2003; Jensen et al. 2010; Wei et al. 2012). The pretreatment time and temperature are strongly dependent on the concentration of the used acid and on the wanted outcomes. Increasing the acid concentration can be used to shorten the needed pretreatment time and to lower the treatment time, and vice versa. However, typical treatment times in acidic pretreatments range from minutes (even seconds) to few hours.

However, the removal of constituents from LCMs by acidic hydrolysis techniques involves complex physical changes and chemical reactions that are related to the complexity of the feedstock material itself (Harris et al. 1984; Niemelä and Sjöström 1988b). For this reason, the hydrolysates are chemically rather complex. They contain a mixture of various carbohydrates (Garrote et al. 1999a,b; Carvalheiro et al. 2008; Al-Dajani et al. 2009; Teo et al. 2010) and a minor amount of other organics, including aliphatic carboxylic acids (“volatile acids”, i.e., acetic and formic acids) (Tunc and van Heiningen 2011), “acidic” carbohydrate degradation products (furans, i.e., 2-furaldehyde or furfural (FF) and 5-(hydroxymethyl)furfural (HMF)), as well as heterogeneous fractions of lignin- and extractives-derived materials (Larsson et al. 1999). All these non-carbohydrate materials may be regarded as harmful substances, especially when considering the biochemical utilization of carbohydrates in hydrolysates, and usually some kind of purification or detoxification of acidic hydrolysates is required (Delgenes et al. 1996; Olsson and Hahn-Hägerdal 1996; Palmqvist and Hahn-Hägerdal 2000a,b; Nilvebrandt et al. 2001). Several purification possibilities, such as solvent extraction (Cruz et al. 1999; Garrote et al. 2003; Vázquez et al. 2005; Moure et al. 2006; Parajó et al. 2008), resin treatment (Nilvebrandt et al. 2001; Conde et al. 2008; Schwartz and Lawoko 2010), and membrane assisted filtration (such as ultrafiltration) (Mänttäri et al. 1997; Bhattacharya et al. 2005; Pizzichini et al. 2005; Vegas et al. 2006; Sjöman et al. 2008) have been proposed for production of detoxified substrates for subsequent fermentation processes.

Among the acidic pretreatments, processes conducted with diluted mineral acids have been the most favored applications for industrial purposes, mainly due to the minor corrosion problems caused by the used diluted acid combined with reasonably high carbohydrate yields dissolved from LCMs (Carvalheiro et al. 2008; Yang and Wyman 2009). Compared to the concentrated acid hydrolysis, which is very expensive method due to the significant challenges and costs in both operational and equipment design, dilute acid hydrolysis generates lower amounts of degradation products and significantly fewer corrosion problems in hydrolysis tanks and pipes. Although concentrated acid hydrolysis processes have the advantage of operating at low or medium temperatures, leading to the reduction of operating costs (Gírio et al. 2010), the type of used equipment needs to be resistant to an extremely corrosive environment. Special attention must be paid to the recovery of the used acid and to the neutralization of the solution. These requirements are the key challenges that must be overcome to keep the concentrated acid pretreatments economically viable.

Dilute acid hydrolysis has been used primarily for the removal of hemicelluloses from LCMs, leaving the cellulose fraction more amenable for further treatments (Gírio et al. 2010; Lenihan et al. 2010). Dilute acid hydrolysis can result in the recovery yields up to 95% of the theoretical amounts expected from hemicellulose-derived fragments, depending on the applied pretreatment conditions (i.e.,temperature, treatment time, and acid concentration). The solid residue from the dilute acid hydrolysis contains mainly cellulose and lignin, which can be subjected to further processing, such as chemical pulping or enzymatic hydrolysis (Parajó et al. 1993,1994; Fredrick et al. 2008).

In addition to the concentrated and dilute acid hydrolysis, pretreatments conducted with extremely low acid (ELA) additions can be used to hydrolyze LCMs under very low acid concentrations (< 0.1%) and at high temperature (as high as 220 °C) (Ojumu et al. 2003; Ojumu and Ogunkunle 2005). The advantages of the ELA hydrolysis process (resembling pretreatments conducted with pressurized hot water (PHW)) include limited corrosion effects, leading to the need for cheaper standard grade stainless steel equipment, instead of using expensive high nickel alloy. For this reason, significant cost advantages can be achieved in the equipment and operational design (Kim et al. 2001; Gurgel et al. 2012). In addition, a process using ELA qualifies as a ‘green technology’ because it has only minimal environmental effects. Recent advances in the process design have brought ELA processes into a position where it can compete on par with the enzymatic hydrolysis processes in the overall process economics.

The acid-catalyzed hydrolysis of glycosidic linkages in polysaccharides and the cleavage of α- and β-aryl ether bonds in lignin are the primary degradation reactions that occur when LCMs are treated in acidic media (Sjöström 1993; Brunow et al. 1999; Vuorinen and Alén 1999; Ishii and Shimizu 2001; Sakakibara and Sano 2001; Sixta et al. 2006; Lange 2007; Wang et al. 2012). The hydrolytic reactions are often accompanied by further chemical reactions, such as dehydration, degradation, and condensation reactions. Acid hydrolysis of the glycosidic linkage involves an initial protonation of the glycosidic oxygen, followed by decomposition of the resulting conjugate acid to the cyclic carbonium ion (Vuorinen and Alén 1999; Ishii and Shimizu 2001; Lange 2007). After the rapid addition of water, free carbohydrate is liberated. The alternative mechanism involves a protonation of the ring oxygen atom to form a conjugated acid, followed by a ring opening to give an acyclic carbonium ion (Sixta et al. 2006). After this stage, the addition of water to the protonated hemiacetal leads to the hydrolysis and to a liberation of a free monosaccharide. Hydrolysis process is affected by the physical structures and conformation of the carbohydrate components and the nature and concentration of the used acid. Due to the amorphous nature, different ring structures, and carbon atom configurations, hemicelluloses degrade faster under acidic conditions than cellulose.

In addition to the hydrolytic reactions, the acidic treatment of carbohydrate-containing materials produces different degradation products originating from pentoses and hexoses (Li et al. 2005; Zeng et al. 2010; Liu et al. 2013b). From pentoses (such as xylose), the main degradation product formed under acidic conditions is FF (Qi and Xiuyang 2007; Agirrezabal-Telleria et al. 2013,2014; Liu et al. 2014; López et al. 2014). From hexoses, which are more stable than pentoses under acidic conditions, the primary degradation product is HMF. However, HMF can be further degraded to formic and levulinic acids (Marzialetti et al. 2008; Zeng et al. 2010).

Acidic degradation of lignin proceeds mainly via the cleavage of α- and β-aryl ether linkages (Sakakibara and Sano 2001). The acidic cleavage reactions of the bonds linking the lignin precursors together into a web-like structure entails several reaction routes and mechanisms, and they yield a wide variety of different intermediates and end products. The reaction routes and formed products are dependent on the nature of the lignin and on the presence and the abundance of different bonds linking the lignin monomers together.

ALKALINE PRETREATMENTS

Alkaline extraction of pulp wood chips prior to pulping can be considered as one promising and well integrated biorefinery process combined with an existing industrial alkaline pulping process (Yoon et al. 2011; Luo et al. 2012; Stoklosa and Hodge 2012; von Schenck et al. 2013; Singh et al. 2014). Alkaline pretreatment can shorten the cooking times and lower the need for alkali charge of the subsequent pulp cooking process. Also, alkaline pulping liquors, such as white and green liquors, can be used for pretreatments as such (Ban et al. 2003a,b, 2004; Ban and Lucia 2003, 2005; Ban et al. 2009; Walton et al. 2010a; Wu et al. 2010; Yoon and van Heiningen 2010; Gonzalez et al. 2011; Jun et al. 2012; Meng et al. 2014; Kim et al. 2015; Wang et al. 2015). Pretreatments conducted with white and green liquors utilize proven technologies for the sophisticated recovery of all the used chemicals at a very high efficiency, and they can be easily integrated into existing pulp mill operations (Jin et al. 2010). For this reason, no additional operational equipment is needed for the production of pretreatment chemicals.

Alkaline pretreatment is chemically similar to kraft pulping, in which acetyl groups, uronic acids, hemicelluloses, and lignin are partly removed. For this reason, it increases the porosity, reactivity, and accessibility of the remaining feedstock towards, for example, chemical pulping, and enzymatic hydrolysis (Minor and Springer 1993; Hu et al. 2008; McIntosh and Vancov 2010; Ju et al. 2011; Chen et al. 2013; Kallioinen et al. 2013). The mechanism of the alkaline hydrolysis has been suggested to be the saponification of the intermolecular ester bonds crosslinking hemicelluloses and lignin (Sun and Cheng 2002). Particularly, alkyl aryl linkages of lignin are readily cleaved under alkaline conditions, facilitating enhanced dissolution of lignin (Park and Kim 2012).

When compared to hot-water and acidic pretreatment methods, mild alkaline pretreatments have advantages, as they generally maintain the yield and quality of the produced pulp and cause minor sugar degradation (Al-Dajani and Tschirner 2008; Chen et al. 2012; Jun et al. 2012; Sim et al. 2012; Huang and Ragauskas 2013a,b). The difference between acidic and alkaline hydrolysis lies in the nature of the extracted carbohydrates (De Lopez et al. 1996). Whereas the acidic hydrolysis conditions often produce significant amounts of monosaccharides from LCMs, the extraction under alkaline conditions produces oligosaccharides and polysaccharides (Jiang et al. 2014; Lehto and Alén 2013, 2015a). Furthermore, carbohydrates are susceptible to further degradation under alkaline conditions, leading to significant formation (up to 60-70% of the dissolved organic material) (Lehto and Alén 2013, 2015a) of various degradation products, such as monocarboxylic and dicarboxylic hydroxy acids, together with volatile acids, such as acetic and formic acids (Niemelä 1990a,b; Sjöström 1991).

Alkaline pretreatment processes are often conducted at low temperatures (below 100 °C) and pressures, and they can be carried out even at ambient pressure (Hu et al. 2008). On the other hand, the treatment times are usually long, ranging from hours to even days. The most employed chemicals in alkaline pretreatments of LCMs include NaOH and Ca(OH)₂, together with KOH, Na₂CO₃, and NH₃(Mosier et al. 2005; Carvalheiro et al. 2008; Alvira et al. 2010). Addition of an oxidizing agent (oxygen or H₂O₂) to an alkaline pretreatment process can be used to improve the performance of the extraction by favoring the removal of lignin.

Pretreatments performed with NaOH have received the most interest among alkaline pretreatment processes due to their widespread utilization in the course of alkaline pulping (Mosier et al. 2005). NaOH causes fiber swelling, increases the internal surface area of cellulose, and decreases the degree of polymerization and crystallinity of wood polysaccharides. These phenomena together promote the cleavage of structural linkages between lignin and carbohydrates and simultaneous disruption of lignin structure (Sun and Cheng 2002; Alvira et al. 2010; El Mansouri et al. 2011; Bujanovic et al. 2012; Mao et al. 2012; Xiao et al. 2012; Lehto et al. 2015). However, the process is quite complicated, involving several reactive and nonreactive phenomena, such as dissolution of non-degraded polysaccharides, peeling reactions, hydrolysis of glycosidic bonds and acetyl groups, and decomposition of dissolved polysaccharides into various degradation products (i.e., hydroxy acids) (Mirahmadi et al. 2010). The increase in digestibility has been shown to be more enhanced in hardwoods when compared to softwoods. This has been explained by the compositional differences between these two feedstocks, as hardwoods contain a higher proportion of total carbohydrates to lignin compared that for softwoods (Helmerius et al. 2010).

In addition to the different carbohydrate to lignin ratios, the types of hemicelluloses also have their effects on alkaline treatments (Helmerius et al. 2010). Softwood glucomannans are degraded by the alkaline peeling reactions rapidly under alkaline conditions, while the deacetylated, solubilized oligomeric xylan is more stable due to the 4-O-methylglucuronic acid side chains. As xylan is the dominant hemicellulosic component in many commercially utilized hardwoods (such as silver birch, Betula pendula), alkaline pretreatment has been found to represent a more suitable extraction method for hardwoods than for softwoods. In addition to the chemical nature of the hardwood hemicelluloses, more open vascular structure of hardwoods renders them more amenable to chemical pretreatments.

CHEMICAL COMPOSITION AND UTILIZATION OF THE PRETREATMENT LIQUORS

Chemical Composition of Pretreatment Liquors

Pretreatment hydrolysates obtained from LCMs under different treatment conditions are complex mixtures containing mainly soluble carbohydrates and other wood-derived components, representing a promising source of renewable raw materials for the production of different biochemicals and biofuels (Kumar et al. 2008; Menon and Rao 2012; Lehto and Alén 2013, 2015; Lehto et al. 2014a,b). The main purpose of the pretreatment process has often been the recovery of the main hemicelluloses, such as xylan or glucomannan, which can be further processed to desired product applications, for example, viafermentation or fractionation processes (Garrote et al. 2001; Parajó et al. 2008; Song et al. 2008). However, in addition to carbohydrates (and depending on the applied pretreatment conditions), especially acidic pretreatment liquors contain a wide variety of “acidic” sugar degradation products (FF and HMF together with “volatile” carboxylic acids such as acetic and formic acids), extractives, and lignin-derived phenolic compounds (Luo et al. 2002; Persson et al.2002; Moure et al. 2006; Barakat et al. 2012; Lehto and Alén 2012, 2013, 2015a; Lehto et al. 2014a,b). On the other hand, alkaline pretreatments produce significant amounts of various carbohydrates-derived “non-volatile” carboxylic acids, such as hydroxy mono- and dicarboxylic acids (Lehto and Alén 2013, 2015a). Many of these carbohydrates-derived degradation products and non-saccharide compounds are potentially inhibitory to the microorganisms used especially in the fermentation processes, and for this reason further fractionation of the produced hydrolysates is usually necessary (Converti and Del Borghi 1998; Taherzadeh et al. 1999; Palmqvist and Hahn-Hägerdahl 2000a,b; Walton et al. 2010b; Lehto and Alén 2012). However, these non-saccharide components can also serve as a potential raw material source for the production of many low-volume/high-value components, such as pharmaceuticals.

Fractionation of Pretreatment Effluents

Several fractionation and purification methods have been proposed for the fractionation and production of purified component fractions from pretreatment hydrolysates (Palmqvist and Hahn-Hägerdahl 2000a,b; Moure et al. 2006; Peng et al. 2012). Such methods as solvent extraction, precipitation with chemicals, chromatographic purification, treatments with adsorption resins (with or without charged groups), and filtration with various membranes have all been used for the fractionation of these complex aqueous mixtures.

Solvent extraction (usually conducted with some common organic solvent, such as ethyl acetate or diethyl ether) is a useful method for removing especially non-saccharide components from lignocellulosic hydrolysates, yielding both a selectively refined aqueous phase containing carbohydrates and a solvent-soluble fraction mainly composed of phenolics and extractive-derived compounds (Cruz et al. 1999; Palmqvist and Hahn-Hägerdahl 2000a,b; Garrote et al. 2003; Vázquez et al. 2005; Moure et al. 2005, 2006; Lehto and Alén 2012).

Solvent precipitations performed as single or multistage operations (usually conducted with ethanol, acetone, and 2-propanol) have been successfully employed for the fractionation of carbohydrate components (mainly in their polymeric forms) from lignocellulosic hydrolysates (Kalapathy and Proctor 2001; Vegas et al. 2004; Moure et al. 2006; Peng et al. 2009). In particular, sequential treatments of biomass with hot water and dilute alkali combined with graded ethanol precipitation with increasing alcohol concentrations have been shown to be very effective methods for the fractionation of hemicelluloses from biomass and biomass-derived hydrolysates.

At an analytical level, chromatographic fractionation, such as high-performance anion-exchange chromatography (HPAEC) and size-exclusion chromatography (SEC), have been employed for the production of low-volume, very high-purity hemicellulose fractions from lignocellulosic hydrolysates (Moure et al. 2006). In addition, chromatographic techniques have been employed for refining samples prior to structural characterization of the hemicelluloses by a ¹³C nuclear magnetic resonance (NMR) spectroscopy, matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and nanospray mass spectrometry (Kabel et al. 2002a,b).

Several types of resins and activated charcoals have been used for the purification of plant extracts and lignocellulosic hydrolysates (Miyafuji et al. 2003; Canilha et al. 2004; Moure et al. 2006; Villareal et al. 2006; Ou et al. 2007; Conde et al. 2008; Wickramasinghe and Grzenia 2008; Schwartz and Lawoko 2010; Lehto and Alén 2012). Acrylic, polystyrene-based, and formaldehyde-phenol resins have been used to recover anthocyanins, flavonoids, and hydroxycinnamates from different types of plant extracts. Moreover, adsorbent resins have been used to recover and concentrate phenolics from aqueous and synthetic solutions, and ion-exchange (anionic and cationic) resins have been utilized for the purification of lignocellulosic hydrolysates (Nilvebrandt et al. 2001; Jeong et al. 2014; Trinh et al. 2014). Under defined conditions, the adsorption of phenolic compounds can be favored with respect to other compounds (such as carbohydrates) present in the solution. Polymeric resins are highly porous, and a variety of compounds can be adsorbed and desorbed, depending on the polarity of the solvent. Using polar solvents, such as water, resins can absorb organic and non-polar species, whereas in the presence of non-polar solvents, some resins (especially those of acrylic and phenolic nature) exhibit hydrophilic or slightly polar behavior and can adsorb slightly polar species. In the case of ion-exchange resins, anion-exchange resins have been found to be especially effective in removing different types of compounds (phenolics, furan aldehydes, and aliphatic acids) from lignocellulosic hydrolysates (Conde et al. 2008).

Membrane-assisted processing of lignocellulosic hydrolysates has gained importance in fractionation of hemicelluloses in particular from aqueous extracts (Moure et al. 2006; Vegas et al. 2006; Nabarlatz et al. 2007; Sjöman et al. 2006, 2007, 008; Al Manarash et al. 2012a,b; Sainio et al. 2013; Ajao et al. 2014; Kallioinen et al. 2014). In the pulp and paper industry, membrane technology has already been implemented for recycling chemicals, for cleaning process effluents, and for removing impurities from water circulation systems (Mänttäri et al. 1997, 2002; Koivula et al. 2011, 2012, 2013; Krawczyk et al. 2013). Several different membrane filtration processes, such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), have been used in the pulp and paper industry, mainly for concentration and fractionation of spent liquors, to remove color and to treat bleaching effluents (Bhattacharya et al. 2005; Pizzichini et al. 2005). UF, for example, has been suggested for the recovery of hemicelluloses from the process waters of pulp mills producing thermomechanical pulp (TMP) (Persson and Jönsson 2010). Membrane separation processes can be tailored to meet the wanted degree of purification by choosing the appropriate membranes with convenient membrane weight cut offs (MWCOs) and the right operational conditions, such as applied pressures. The choice of membrane and operating conditions has a considerable impact on the purification efficiency, on the process problems (such as membrane fouling), and the economics of the fractionation process (Mänttäri et al. 2000; Persson and Jönsson 2010; Koivula et al. 2011; Puro et al. 2011).

Fermentation of LCM Hydrolysates

LCM-derived hydrolysates can be fermented with different microorganisms, the aim being at the production of ethanol or other platform chemicals, such as small-size organic acids (Ragauskas et al. 2006; Koutinas et al. 2008; Sánchez and Cardona 2008; Hörhammer et al. 2011; Boucher et al. 2014). For fermentation purposes, biomass is converted into sugar solutions, and these substrates are then fermented by case-specific microorganisms to overproduce metabolic products. After purification, the metabolic products could be used as platform molecules for the production of various bulk chemicals through biological/chemical processing routes. Several platform chemicals are already industrially produced via fermentation, such as ethanol, citric acid, glutamic acid, lactic acid, and 1,3-propanediol (Viikari and Alén 2011).

However, achieving an economically feasible fermentation process of LCM hydrolysates faces several obstacles affecting the overall efficiency of the bioconversion process. The first problem is that carbohydrate solutions produced with present day hydrolysis processes are generally very dilute. The second problem is that a large pentosan fraction originating from hemicelluloses cannot be fermented by traditional brewing yeasts. Hexoses can be fermented quite easily to ethanol, whereas work on efficient fermentation of pentoses into ethanol is still in progress (Chirat et al. 2009). One of the most profound weaknesses of early efforts to develop effective wood-to-chemicals bioconversion processes has been the lack of organisms capable of fermenting pentose sugars (Delgenes et al. 1996; Olsson and Hahn-Hägerdal 1996). As wood hydrolysates are often rich in hemicelluloses-derived pentoses (such as xylose), considerable effort has been subjected towards the isolation and production of microorganisms that can produce fermentation products with high yield from both hexoses and pentoses and that can withstand both the produced fermentation products and the inhibitory compounds present in the hydrolysates (Larsson et al. 2001; Katahira et al. 2006; Hahn-Hägerdahl et al. 2007; Tian et al. 2011; Shupe and Liu 2012). Effective pentose fermenting microorganisms (both naturally occurring and recombinant species) have been found among bacteria, yeasts, and fungi. Of the naturally occurring yeasts, Candida shehatae, Pachysolen tannophilus, and Pichia stipitis have been found to be the most promising alternatives for pentose fermentation. Of the genetically engineered yeasts, the recombinant strains of Saccharomyces cerevisiae have gained considerable interest (van Maris et al. 2006). In addition to genetically engineered yeasts, several bacteria (such as Escherichia coli) have been genetically modified and harnessed for ethanol production.

Production of fuel alcohols, such as ethanol or butanol from LCMs, has gained growing global attention during the past few decades (Brandberg et al. 2004; Tang et al. 2006; Margeot et al. 2009; Chin et al. 2010; Limayem and Ricke 2012; Sun and Liu 2012; Lee et al. 2013). Depletion of fossil resources and growing demands for energy and chemicals have forced industries to search for alternative material sources. Compared with the finite supply of fossil fuels, ethanol produced from abundant and renewable LCMs can play a vital role in achieving sustainable development by reducing greenhouse gas emissions (Xu and Liu 2009). Production of fuel ethanol from LCMs includes the degradation of the lignocellulosic structure to a mixture of fermentable sugars, followed by the fermentation and distillation of the fermentation broth to obtain 95% ethanol (Olsson and Hahn-Hägerdal 1996). The ethanol yield of the process is an important parameter with regards to the process economy, since the costs of raw material and process operation are typically associated with the material passing through the process and not the amount of product manufactured. For producing of fuel ethanol, yeast fermentation is considered to be a mature technology, where future scientific improvements will result only in a minor cost benefit (Duff and Murray 1996). Hexoses are readily fermented by yeasts (such as Saccharomyces cerevisiae, also known as Brewer’s yeast) (Olsson and Hahn-Hägerdal 1996). In addition to conventional yeast fermentation, certain bacteria are capable of fermenting hexoses to ethanol (Chirat et al. 2009).

There are certain advantages when using bacteria instead of yeasts for alcohol fermentation (Chirat et al. 2009). During bacterial fermentation, less biomass is generated than in yeast fermentation. For this reason, higher ethanol yields are achieved through the use of anaerobic ethanologenic bacteria, such as Zymomonas mobilis or Zymomonas anaerobia. In addition to improved yield, the ethanol fermentation by bacteria offers a number of other advantages over the traditional yeast fermentation. Sugar uptake rates and ethanol productivity are generally higher in bacterial fermentation, bacteria have higher ethanol tolerance, and they are less sensitive to low pH and inhibitory compounds found in lignocellulosic hydrolysates. In addition to ethanol, the fermentative production of several other alcohols from biomass has been suggested (Felipe et al. 1996; Parajó et al 1997a,b; Kamm et al. 2006; Viikari and Alén 2011). Such alcohols as butanol, xylitol, n-propanol, 1,2-propanediol, 1,3-propanediol, glycerol, 2,3-butanediol, and 1,2,4-butanetriol have all been produced by fermentative pathways from lignocellulosic biomass.

Several small-size organic acids produced by fermentative methods are considered to have high “building block potential”, either as monomers for the production of novel polyesters and polyamides or as a starting material for a variety of commodity chemicals currently produced petrochemically (Kamm et al. 2006). Several carboxylic acids are already produced by fermentative pathways and utilized by industry. Such common organic acids include, for example, lactic, citric, and tartaric acids (Viikari and Alén 2011). Of these, especially lactic acid has gained a great interest, as it can be converted into various chemicals, including acrylic acid, propylene glycol, acetaldehyde, and 2,3-pentanedione (Melzoch et al. 1997; Woiciechowski et al. 1999; Guo et al. 2010).

Lactic acid has a wide range of applications in food, pharmaceutical, and cosmetic industries; in recent years demands for lactic acid have increased due to its application in the manufacturing of biodegradable polymers (poly(lactic acid), PLA) and green solvents (Melzoch et al. 1997; Hofvendahl and Hahn-Hägerdahl 2000; Zhang et al. 2007; Guo et al. 2010). Lactic acid has been commercially produced from fermenting glucose, starch, liquefied starch or sucrose, and it has been estimated that 60 to 80% of the production costs are related to the high cost of substrate raw materials. Thus, using low-cost raw material, such as lignocellulosic biomass, is required for building an economical and sustainable lactic acid industry (Parajó et al. 1996).

Considerable efforts have been directed to the search for organisms capable of fermenting lignocellulosic hydrolysates into lactic acid (Guo et al. 2010; Abdel-Rahman et al. 2011). Many industrial microorganisms producing lactic acid lack the ability to ferment xylose, which is an important component present in lignocellulosic hydrolysates. Therefore, efforts have been made to find wild strains of microorganisms or to genetically engineer such common microorganisms as Escherichia coli for the utilization of hydrolysates rich in pentoses. In addition to the complex carbohydrate composition, inhibitory compounds present in lignocellulosic hydrolysates affect the efficiency of the lactic acid fermentation process, as is also the case in alcohol fermentation. Hence, the discoveries of the strains of microorganisms with abilities to not only ferment various sugars, but also to resist fermentation inhibitors, are of great interest to researchers and industry.

In addition to the three most commonly produced acids (lactic, citric, and tartaric acids), the production and exploitation of several other small-size organic acids derived from lignocellulosics has been investigated (Kim et al. 2004; Kamm et al. 2006; Liu et al. 2013a). Such acids as propionic, pyruvic, 3-hydroxypropanoic (3-HPA), butyric, 3-hydroxybutanoic, aspartic, glutamic, succinic, fumaric, malic, and itaconic acids have been highlighted as being potential chemicals produced from LCM feedstocks. Of these, especially 3-HPA, four-carbon diacids (malic, fumaric, and succinic acids), itaconic, aspartic, and glutamic acids have the potential to become economically viable products if low-cost fermentation routes can be developed and scaled up to the industrial level.

Polysaccharide-based Biodegradable Films and Hydrogels

In recent years, various polymeric applications manufactured from LCMs have gained growing attention (Edlund et al. 2011; Stevanic et al. 2011; Wang et al. 2012; Oinonen et al. 2013). In this approach, hemicelluloses represent many promising properties for the manufacture of value-added polymeric composites and blends, which could replace materials currently manufactured from fossil resources or even metals, such as aluminum (Alekhina et al. 2014). Natural biopolymers are considered to have many advantageous properties when compared to their synthetic counterparts, as they are abundant, low-cost, biocompatible, nontoxic, biodegradable, and environmentally safe (Peng et al. 2011b; Šimkovic et al. 2011). Of the several possible hemicelluloses-derived polymeric applications, the manufacture of various polymeric films and hydrogels has garnered the most interest.

Due to their ability to form dense macromolecular networks, hemicelluloses (especially xylans and glucomannans) reveal many promising features for the manufacturing of edible and packaging films, and coatings used in various packaging applications (Bachegul et al. 2012; Mikkonen and Tenkanen 2012; Egüés et al. 2013; Ruiz et al. 2013; Alekhina et al. 2014). These bio-based polymers can be used instead of aluminum foil or synthetic polymers, resulting in the decreased carbon footprint of the packaging industry (Oinonen et al. 2013; Alekhina et al. 2014). Hemicelluloses-derived polymers are hydrophilic, and as such, they demonstrate notable barrier characteristics against oils and fats (Stevanic et al. 2011). However, water vapor permeability and oxygen permeability also are crucial properties, especially when considering many food packaging applications (Mikkonen et al. 2010). Consequently, modification of the barrier properties of hemicelluloses-derived biopolymers against water and moisture must be significantly improved before they can be considered suitable for various food packaging solutions (Saxena et al. 2011; Escalante et al. 2012). Such modifications may include several possibilities, such as etherification, esterification, oxidation, and polymerization pathways (Ren et al. 2012).

The second class of polymeric application manufactured from hemicelluloses includes the production of hydrogels (Gabrielii et al. 2000; Peng et al. 2011a; Pohjanlehto et al. 2011). Due to the high hydrophilicity, the abundance of hydroxyl groups together with biodegradability, nontoxicity, and responsiveness to environmental stimuli (pH, ionic strength, solvent composition, temperature, as well as electric and magnetic fields) hemicelluloses-derived hydrogels have attracted attention, especially in the manufacture of biomedical product applications, such as controlled drug delivery systems, biological scaffolds for tissue engineering, biosensors, immobilized carriers for the encapsulation of living cells, and barrier materials for regulating biological adhesions (Söderqvist-Lindblad et al. 2005; Edlund and Albertsson 2008; Sun et al. 2013; Maleki et al. 2014). In addition to medical applications, such alternatives as superadsorbents in hygiene products, water treatment, and drying agents have been manufactured from LCM-derived hemicelluloses.

Xylooligosaccharides

Sugar oligomers (especially xylooligosaccharides or XOs) have been found to contain several interesting health effects (such as prebiotic activity), which makes them interesting alternatives for acting as ingredients for functional foods, and replacing commercial antioxidants manufactured via chemical synthesis (Kabel et al.2002a,c; Carvalheiro et al. 2004; Garrote et al. 2004b; Parajó et al.2004; Vázquez et al. 2006; Aachary and Prapulla 2010). XOs are used as food ingredients due to their technological properties and health effects (Vázquez et al. 2000). The sweetness of dimeric xylobiose is equivalent to 30% of that of sucrose, and the sweetness of other XOs is moderate and possesses no off-taste. XOs are stable over a wide range of pH (2.5 to 8.0) and temperatures (up to 100 °C), which makes XOs promising alternatives, for example, for inulin. This permits their utilization in many dietary applications, such as low-pH juices and carbonated drinks. For food ingredient purposes, XOs have an acceptable odor, are non-cariogenic and are contain less energy (i.e., are of low-calorie) which means, that they can be incorporated into anti-obesity diets. XOs save insulin secretion from the pancreas and stimulate intestinal mineral absorption. From a nutritional point of view, XOs behave as non-digestible oligosaccharides, which are not degraded in the stomach (Kabel et al. 2002c; Vegas et al. 2006). XOs can be mixed with other prebiotics to achieve a synergistic effect, or they can be a part of symbiotic preparations together with probiotic microorganisms.

The health effects of XOs are mainly related to their effects on the gastrointestinal flora (Vázquez et al. 2000; Kabel et al. 2002a,c; Parajó et al. 2004; Moure et al. 2006; Vázquez et al. 2006; Aachary and Prapulla 2010). Tests carried out with humans have demonstrated that beneficial bacteria can utilize XOs as their carbon sources. As ingested xylobiose is not excreted in feces or urine and as XOs cannot be hydrolyzed either by saliva, pancreatin, or gastric juice, the bacterial utilization of the XOs is the only possibility. XOs promote the growth of certain beneficial bacteria (especially Bifidobacteriaand Lactobacilli) in the gastrointestinal tract. Reported advantageous effects of Bifidobacteria on human health include such effects as suppressing activity of entero putrefactive bacteria, preventing the formation of detrimental products, such as toxic amines, repression of pathogenic bacteria due to the production of short-chain organic acids, facilitating decreased pH in the gastrointestinal tract, as well as the digestion and absorption of nutrients.

In addition to the enhanced growth of beneficial bacteria, suppression of the growth of some detrimental bacteria (Clostridium difficile), which lack the ability of using XOs as a carbon source, has been reported (Vázquez et al. 2000; Moure et al. 2006; Aachary and Prapulla 2010). The beneficial effects of XOs to gastrointestinal flora are accompanied with the observation that XOs can help to reduce the concentrations of so-called secondary bile acids, which negatively affect the colon and present dose-dependent toxic potential related to mutagenic and tumor-promoting properties that these acids may have. These properties together result in the ability to prevent gastrointestinal infections, to reduce the duration of diarrhea episodes, to maintain the fecal water content within normal levels, and to enhance cecal epithelial cell proliferation. As XOs have been found to contain several positive effects (both cellular and physiological), they fulfill the requirements for prebiotic compounds. XOs are commercially utilized as food ingredients especially in Japan, where about 60 companies use XOs as raw materials.

Utilization of XOs as food additives is by far the most important application discovered for XOs (Vázquez et al. 2000; Moure et al. 2006; Aachary and Prapulla 2010). XOs can be used in the formulation of so-called functional foods in which XOs are used in combination with, for example, soya milk, nutritive preparations, special preparations for health food for elderly people and children, or as active components of symbiotic preparations. The term “functional food” refers to a food product fortified with added ingredients that can positively affect the consumer’s health. As the knowledge and awareness of the links between health, nutrition, and diet is rapidly growing, the markets for functional foods and for their ingredients are quickly expanding. The manufacture of probiotics is growing, but the long-term exploitation of XOs as health promoters is dependent on several factors. The health effects must be scientifically proven with clinical tests, the consumers must be informed and assured of the beneficial effects, markets have to be created, and product quality must fulfill consumer expectations.

In addition to the health effects related to the gastrointestinal tract, several other biological effects have been proposed for the XOs (Vázquez et al. 2000; Moure et al. 2006; Aachary and Prapulla 2010). Such effects include antioxidant activities, blood and skin related effects, antiallergy, antimicrobial, anti-infection, and anti-inflammatory properties, cytotoxic activity, immunomodulatory action, as well as cosmetic and a variety of other properties. In addition to the biological effects to human health, XOs have been used in phytopharmaceutical and feed applications.

Non-Carbohydrate Components

In addition to the carbohydrates, lignocellulosic hydrolysates contain a variety of non-carbohydrate products originating from lignin, extractives, and initial carbohydrates (Fenske et al. 1998; Nilvebrandt et al. 2001; Klinke et al. 2002,2004; Vázquez et al. 2005, Montané et al. 2006; Conde et al. 2008). As many of these compounds are potentially inhibitory during fermentation processes and because the purity requirements of XOs used in functional foods are high, these components must be fractionated from the hydrolysates. Fractionation can be performed by various common laboratory techniques, such as liquid-liquid extraction performed with suitable organic solvents (i.e., ethyl acetate, diethyl ether, various alcohols, and alkenes) or with various adsorption resins (Converti et al. 2000; Klinke et al. 2004; Zautsen et al. 2009; Pienkos and Zhang 2009; Soto et al. 2011). However, the non-carbohydrate phenolic component fraction has been shown to contain certain beneficial properties, including antioxidant, antimicrobial, and biological activities, which makes them potentially suitable for food or cosmetic applications (Barclay et al. 1997; Lu et al. 1998; Cruz et al. 2001; Garrote et al. 2004a, 2007; González et al. 2004; Dong et al. 2011; Soto et al. 2011). The utilization of the non-carbohydrate fraction could be of scientific and economic interest, fostering an integrated multiproduct process, in which not only carbohydrate fraction, but also non-carbohydrate fraction is commercially utilized.

Phenolic compounds of natural origin containing antioxidant activity exhibit great potential in replacing synthetic compounds (Ogata et al. 1997; Cruz et al. 2001; Garrote et al. 2004a; González et al. 2004; Ugartondo et al. 2008). Raw LCM is cheap, renewable, and abundant, and the concerns regarding the safety of the synthetically manufactured products and the consumer preferences for natural products make these compounds interesting both commercially and environmentally. Interest in manufacturing natural antioxidants from various sources has grown, and LCMs are a promising source of these components (González et al. 2004; Garrote et al. 2004a). Many non-carbohydrate components present in lignocellulosic hydrolysates originate from lignin, and thus they are of a phenolic nature. On the other hand, the antioxidant activity of phenolics formed during hydrolysis of LCMs is well known (Cruz et al. 2001; Garrote et al.2004a). The antioxidant activity of the compound is dependent on the chemical structure of the compound (Cruz et al. 2005; Egüés et al. 2012). Lignin monomers and dimers have been shown to be effective antioxidants. Simple benzoic and cinnamic acid-related phenolic acids are absorbed via organisms’ metabolic pathways and have a role in the antioxidant defense. Esters of phenolic acids are more active than phenolic acids, whereas oligomers and condensed tannins are more active than monomeric phenols.

The mode of action of phenolic components in biological systems seems to be related with the effects to membrane permeability, decreased electron transport through the membranes, and the possible interference of the metabolic synthesis of macromolecules and nucleic acids (Garrote et al. 2004a). The inhibitory effect of the phenolic component is dependent on the size of the molecule but also on the functionality of the compound (Garrote et al. 2004a; García et al. 2010; Ponomarenko et al. 2014). A linear relationship between the number of carbon atoms in an alkyl side chain and antimicrobial activity has been observed. In addition, the presence of different functional groups present in phenolic components has been shown to affect the toxicity of the compound. The inhibitory potential of the functional groups attached to the benzene ring is estimated to increase in the order: COOH > p-OH > CHO > CH=CH. A variety of pharmacological and biochemical activities (anticarcinogenic, antiatherogenic, and anti-inflammatory) has been reported for phenolic compounds. Phenolic compounds produced by colonic microflora from flavonoids can have protective activities in the colon, whereas caffeic and ferulic acids are protective against nitrite ions. Ferulic acid esters are active agents in cosmetics, as they contain antioxidant and ultraviolet absorption properties.

ALKALINE PULPING OF PRETREATED WOOD

Industrial pulping refers to processes by which wood or other LCMs are converted into a fibrous end product (i.e., pulp) by chemical, mechanical, or by combination of these methods (Alén 2000; Sixta et al. 2006). Of these methods, chemical pulping accounts for 70% of the total pulp production, with the sulfate process (i.e., kraft process) being the most utilized chemical pulping method worldwide. During chemical pulping, the web-like structure of lignin is degraded, and lignin is dissolved through chemical reactions at elevated temperatures (Prinsen et al. 2013). The liberated fibers can then be separated from the cooking liquor, washed, and bleached. However, delignification is not a highly selective process. Simultaneously with the lignin removal, significant parts of the hemicelluloses and some cellulose together with extractives are also degraded. The total yield of cellulosic fiber ranges from 45% to 55% depending on the wood source and the applied pulping process. In addition to pulp, only turpentine and tall oil from kraft pulping and lignosulfonates from sulfite pulping have so far achieved considerable value as by-products, although the recovery of other degraded organics (i.e., degradation products of carbohydrates and lignin) could be an interesting alternative to using them as fuel in recovery boilers.

Pretreatment processes have a profound effect on the chemical composition of wood and on the subsequent delignification behavior of the pretreated feedstocks (Borrega et al. 2011a,b; Duarte et al. 2012; Coelho dos Santos Muguet et al. 2013; Huang and Ragauskas 2013a,b; Vila et al. 2013; Lehto and Alén 2015a,b,c). In general, autohydrolysis and mild acidic pretreatments have been proposed to enhance the delignification rates of both softwoods and hardwoods when compared to the untreated feedstocks, resulting in savings in the costs of pulping and bleaching chemicals, reduced cooking times, and lower energy demand (Yoon and van Heiningen 2008; Lu et al. 2012; Chirat et al. 2013; Hamaguchi et al. 2013; Runge and Zhang 2013). It has been revealed in previous studies (Reguant et al. 1997; Jahan et al. 2009; Lu et al. 2012; Lehto and Alén 2015b,c) that the kappa number of the produced pulp can be lowered significantly by applying a pretreatment stage prior to pulping. The enhanced delignification of the pretreated wood has been explained by the improved penetration of cooking chemicals caused by the increased pore volume and permeability of the fiber cell wall together with the hydrolytic cleavage of lignin structure, removal of hemicelluloses and their degradation products, and cleavage of LCC bonds. In addition, it has been shown in previous studies (Lehto and Alén 2015b,c; Lehto et al. 2015) that applied pretreatments have a profound effect on the formation of various degradation products originated from lignin and carbohydrates. The content of various aliphatic hydroxy acids was significantly higher in those black liquors originated from pretreated (hot-water extracted) feedstocks. This was assumed to be caused by partial removal of acetyl groups already during the pretreatments, leading to decreased content of acetic acid (i.e., decreased need for its neutralization during cooking) in the corresponding black liquors. Furthermore, higher-molar-mass lignin was found to be dissolving during cooking from pretreated feedstocks, when compared to the reference cooks without pretreatments (Lehto et al. 2015). As described earlier in this chapter, this was thought to happen due to the more open structure of pretreated wood, thus enhancing the diffusion of higher-molar-mass lignin from the wood matrix.

Besides positive aspects, negative effects from autohydrolysis and mild acidic pretreatments on pulping and pulp quality also exist (Yoon and van Heiningen 2008; Duarte et al. 2011; Saukkonen et al. 2012b; Hamaguchi et al. 2013; Vena et al. 2013a,b; Liu et al. 2012d; 2015). The most profound negative effects include the reduced yield of pulp compared to the reference kraft cooks produced without pre-extraction, reduced refining response, and decreased strength (tensile and burst strengths) properties of the produced pre-hydrolysis kraft (PHK) pulps. Alkaline pretreatments in general have only very minor negative effects on modified pulping and pulp quality (Huang and Ragauskas 2013a,b; Johakimu and Andrew 2013; Vena et al. 2013a,b)

Applying a pretreatment stage prior to delignification has been extensively used in the manufacture of dissolving pulp. In general, dissolving pulp refers to a cellulose product having very high cellulose content (95% or higher) suitable for manufacturing of various cellulose derivatives, such as cellulose esters and ethers, viscose, and cellophane (Biermann 1996; Sixta 2006). It is known that 85 to 88% of the total dissolving pulp is manufactured from wood by PHK and acid sulfite processes combined with subsequent purification stages, such as hot and cold caustic extraction (HCE and CCE), respectively (Sixta and Schild 2009; Radiotis et al. 2011; Sixta et al. 2011; Borrega et al. 2013a; Wang et al. 2014). The PHK process is a variation of the typical kraft pulping process, with the addition of a pre-hydrolysis stage to extract hemicellulose fraction from wood chips prior to cooking (Liu et al. 2011b; Saeed et al. 2012; Mateos-Espejel et al. 2013). The main objective of the drastic manufacturing process is to remove the non-cellulosic carbohydrates (i.e., hemicelluloses) as completely as possible and to produce pulp, containing a low amount (3 to 4%) of hemicelluloses compared to typical paper-grade kraft pulp (10%). Even though the subsequent purification processes (HCE and CCE) cause considerable increases in production costs due to high yield loss and high chemical charges, they are still needed because even small amounts of non-cellulosic polysaccharides may influence the processability and properties of the final product. However, in addition to the production of high-grade dissolving pulp, the PHK process allows the recovery of various organic components (such as acetic acid and carbohydrates) from the pre-hydrolysis liquors (Saeed et al. 2012; Ahsan et al.2014).

Kraft Pulping

The most important chemical pulping process used today is the kraft process (Gellerstedt 2001; Dimmel and Gellerstedt 2010). Its advantages include excellent paper strength and permanence, low energy requirements, sophisticated chemical recycle (i.e., low chemical costs), and insensitivity to wood species. The disadvantages are low pulp yields (40 to 55% of wood dry solids (DS)), high capital investment costs, brown unbleached pulp color, high cost of bleaching chemicals, significant amounts of organic components in the bleaching effluents, and strong odors. In conventional kraft cooking, lignin is dissolved with strong alkaline aqueous cooking liquor (i.e., “white liquor”) containing mainly sodium hydroxide (NaOH) and sodium sulfide (Na₂S) as active cooking chemicals (Alén 2000; Sixta et al. 2006). The reactions occurring during kraft cooking are complex and not fully understood (Alén 2000; Sixta et al. 2006; Lapierre 2010). The main active chemical agents in the kraft process are hydroxide (HO^–) and hydrosulfide (HS^–) anions. It is generally known that the HS^– plays an important role in kraft pulping by accelerating delignification through reacting with lignin, whereas carbohydrate reactions are primarily affected by alkalinity (i.e., HO^–). Furthermore, white liquor contains small amounts of Na₂SO₄, Na₂CO₃, Na₂S₂O₃, NaCl, and CaCO₃, together with other salts and non-process elements. The concentrations of active cooking chemicals are conventionally expressed as active alkali (AA; NaOH + Na₂S) and effective alkali (EA: NaOH + ½ Na₂S). Both batch and continuous cooking digesters are used in kraft cooking, typical cooking times being 1 to 3 h and temperatures from 150 ºC to 180 ºC. The overall effect of cooking time and temperature is usually presented by a single numerical value, the so-called H-factor (Vroom 1957). During kraft cooking, roughly half of the wood material is dissolved. The delignification proceeds in three distinct phases (Dimmel and Gellerstedt 2010). The first phase of delignification is characterized as an “extraction phase”, during which the selectivity of the delignification is rather low, resulting in the removal from 15% to 25% of the initial lignin, but simultaneously leading to a loss of as much as 40% of the hemicelluloses. As the process continues, delignification is accelerated due to the increasing temperature. During this second phase (i.e., “bulk delignification phase”), the rate of the delignification is controlled by chemical reactions and follows as a first order reaction. The rate of delignification remains high during the bulk delignification stage until about 90% of the initial lignin has been dissolved. The final delignification phase (i.e., “residual delignification”) proceeds slowly and the loss of carbohydrates is significantly increased. The selectivity of delignification for kraft pulping is rather low, leading to the degradation of wood polysaccharides already at comparatively low temperatures as the chips come into contact with the cooking liquor (Alén 2000; Sixta et al. 2006; Pakkanen et al. 2013). As the cooking process proceeds, the white liquor gradually becomes enriched with an extremely complex mixture of degraded lignin, carbohydrate degradation products, and inorganic anions and cations, and is finally converted to black liquor (Alén et al. 1984, 1985; Niemelä et al. 1985; Niemelä and Sjöström 1986a,b, 1988a; Niemelä 1988a,b,c, 1989, 1990a,b; Alén 2000b; Sixta et al. 2006; Hellstén et al. 2013).

After cooking, the spent cooking liquor (i.e., black liquor) is separated from the pulp, concentrated to 65–80% dry solids content, and burned in the recovery boiler for the recovery of cooking chemicals and the production of energy (Adams 1997; Vakkilainen 2008). After combustion, the inorganic fraction (i.e., smelt) containing Na₂CO₃ and Na₂S together with a minor amount of Na₂SO₄ is dissolved in water to form “green liquor” (Hupa 1997; Vakkilainen 2008). Green liquor reacts with lime (CaO) in the causticizing stage during which Na₂CO₃ is converted to NaOH, which can be used for white liquor. The efficiency of the chemical conversion and recovery is about 90%; thus, white liquor still contains small amounts of Na₂CO₃ and other sodium salts.

Soda-Anthraquinone Pulping

Kraft process is the dominant pulping method in the world. However, the sulfur compounds formed during kraft process can cause odor problems already at extremely low concentrations (Martínez et al. 1997; Bordado and Gomes 2002, 2003). Alternative pulping methods mainly comprise sulfur-free alkaline processes, usually in the presence of some catalysts, such as soda cooking combined with the AQ catalyst (Biermann 1996; Feng et al. 2002a,b; Ban et al. 2009; Prinsen et al. 2013; Ikeda and Magara 2015). Without the catalyst, sulfur-free alkaline cooking has some disadvantages. When NaOH is the only cooking chemical, the pulping process is slower and yields less and weaker pulp due to the alkaline degradation of carbohydrates, as strongly alkaline cooking conditions decomposes carbohydrates by peeling reactions and alkaline hydrolysis. Soda-AQ pulping is mainly used for delignification of non-wood materials, such as straw, reed canary grass, various agricultural residues, and annual plants (Feng and Alén 2001; Finell and Nilsson 2004; Hedjazi et al. 2009). Of the woody materials, hardwoods are generally more suitable for soda-AQ cooking than softwoods. Hardwoods can be delignified with soda-AQ cooking to pulps that present similarly in terms of yield, strength, and bleachability than corresponding kraft pulps. However, due to the possibly carcinogenic effects, European Food Safety Authority (EFSA) (EFSA 2012) and Confederation of European Paper Industries (CEPI) (Anteroinen 2013) have recommended that the utilization of anthraquinone should be decreased.

As a catalyst, AQ can be used in kraft, soda, and alkaline sulfite processes to increase delignification and decrease carbohydrate degradation (Prinsen et al. 2013). The general beneficial phenomena caused by AQ include increased delignification rates, improved selectivity, reduced alkali charges, and improved pulp properties (Fengel and Wegener 1989). The chemical mechanism of alkaline AQ pulping is generally well known (Dimmel 1996; Dimmel and Gellerstedt 2010). AQ acts as a redox catalyst, and during the pulping process, its function is twofold. AQ reacts with aldehyde end groups (reducing end groups) present in carbohydrate chains and oxidizes them to alkali-stable aldonic acids with simultaneous formation of anthrahydroquinone (AHQ) (Chai et al. 2007). The catalytic cycle is completed by lignin, which reacts with AHQ and oxidizes it back to AQ (Lundquist et al. 1981; Venica et al. 2008a,b). The reaction between lignin and AHQ induces the cleavage of β-aryl ether linkages via a quinone-methide intermediate, leading to adducts prone to fragmentation under alkaline conditions. Simultaneously with the cleavage of β-aryl ether linkages, AHQ is regenerated to AQ. In addition to this reductive reaction pathway, alternative mechanisms for lignin degeneration caused by AQ have been proposed. It has been suggested that γ-CH₂OH end-groups present in lignin side chains are oxidized by AQ to γ-CHO end-groups, leading to side chains susceptible for alkali catalyzed fragmentation, including reverse aldol and β-elimination type reactions. Furthermore, some degradation of C-C bonds (C-α and C-β) can take place.

Oxygen-Alkali Delignification

Oxygen-alkali delignification can be defined as a process that utilizes oxygen in the presence of alkali to remove residual lignin from unbleached pulp before bleaching stage (McDonough 1996; Alén 2000; Susilo and Bennington 2007; Gellerstedt 2010). However, oxygen-alkali delignification is not a selective process; together with the removal of residual lignin, significant degradation of pulp polysaccharides takes place, leading to a formation of a wide range of water-soluble degradation products, such as hydroxy acids, volatile acids, carbohydrates, and methanol (Sjöström 1980; Theander 1980; Alén and Sjöström 1991; Gellerstedt 2001; Laitinen et al. 2002; Salmela et al. 2008). Because of the non-selectivity, typically 35 to 55% of the residual lignin can be removed before the selectivity of the process decreases and degradation of carbohydrates start to decrease the properties of the produced pulp (Yang et al. 2003; Fu et al. 2005; Shin et al. 2006).

The reaction mechanism of oxygen-alkali delignification is extremely complex and not fully understood (Alén 2000). Under alkaline conditions, oxygen is an efficient oxidizing agent for organic compounds, resulting in the formation of various reactive oxygen intermediates, such as hydroperoxyl and hydroxyl radicals, hydrogen peroxide, and hydroperoxide anions. The complicated oxidation process of oxygen delignification involves several radical chain reactions, which are combined with various organic compounds mainly originating from lignin, carbohydrates, and some extractives (Starnes Jr. 1980; McDonough 1996; Alén 2000; Akim et al. 2001; Kalliola et al. 2011). From lignin, oxidative degradation leads to the formation of various aliphatic carboxylic acids via different intermediates, such as catechol, quinone, and muconic acids (Kuitunen et al. 2011; Rovio et al. 2011). As in the kraft process, the main degradation products originating from carbohydrates during oxygen-alkali process include various carboxylic acids.

As some of the reactive oxygen-containing intermediates are non-selective oxidizing agents and can cause significant carbohydrate losses, their formation must be controlled. Especially, hydroxyl radicals are believed to be responsible for a major part of the damages caused to carbohydrates, particularly at higher degrees of delignification. Their formation is catalyzed by various transition metals, such as Fe, Cu, and Mn, which are present in trace amounts in unbleached pulp. Due to this detrimental effect, some additives (such as magnesium sulfate) are usually added to pulp to prevent the function of these transition metals (Starnes Jr. 1980; Alén 2000).

SUMMARY COMMENTS

For several reasons, a more versatile utilization of renewable resources such as wood (“biomass”) for the production of green energy (e.g., electricity and liquid fuels biodiesel and ethanol) as well as organic chemicals with a higher added value will be essential already in the near future. Although the major constituent of biomass is cellulose, it also contains substantial amounts of other polysaccharides as well as non-carbohydrate constituents (mainly lignin and extractives). The prerequisite for a realizable biorefinery concept is that the presence of all these constituents is taken into consideration when planning target-oriented economic processes for the manufacture of useful products.

Many different driving forces are shaping developments in the forest industry, and these have consequences for its continuity. It is obvious that in pulp production the present manufacture of by-products remains rather limited, presenting chemists with new challenges in developing more effective and flexible processes. In general, it can be concluded that chemical pulping offers many attractive biorefinery possibilities in the form of new by-products originated from various side-streams. One of the most promising approaches is based on the various pretreatments of chips for producing soluble fractions under varying conditions prior to delignification.

In this review, the integrated process alternatives including acidic and alkaline pretreatments of wood chips mainly for producing carbohydrates- and lignin-derived fractions have been briefly discussed. It is obvious that even a partial recovery of chip organics makes already a high volume production possible. One further attraction of this strategy is that such a biorefinery production can be readily integrated with the existing delignification process whose main product, fiber, is already a well-established product. The pulp is suitable for versatile utilization in paper or paperboard, and depending on the cooking conditions can be used as dissolving pulp for the production of cellulose derivatives and regenerated cellulose. However, the main prerequisite for pretreatments is that they can be made by causing only slight modifications to the fiber production process. The reason for this is that the pulp industry is capital-intensive and the equipment, once installed must have a long lifetime. Therefore, sweeping changes of any significance in mill process are even in most cases not possible.

The main points illuminated in this review are as follows:

1. Autohydrolysis of wood chips with water offers a promising, economic, and environmentally-friendly tool for recovering carbohydrate-rich materials prior to pulping, thus creating a possibility for more efficient use of wood raw material enabling the production of carbohydrate-based value-added chemicals and other materials. However, in this case, the modification of the pulp mill environment must be optimized to meet the requirements set for the overall process performance.
2. Aqueous pretreatments with an acid addition have also been realized for producing carbohydrate-rich hydrolysates from biomass. The addition of inorganic or organic acid into the pretreatment has a profound effect on the overall process, both in terms of equipment needed, and products formed. In addition, the recycling of the acid is also normally needed.
3. Aqueous alkaline pretreatment can be considered as one of the most attractive alternatives for producing carbohydrates- and lignin-rich hydrolysates within the existing pulp mill environment. This is due to the fact that modern pulping processes are mainly based on alkaline delignification, thus offering a logical integration of the alkaline pretreatment stage with cooking. In addition, opposite to autohydrolysis and acidic pretreatments, alkaline pretreatments have been found to cause generally only minor negative effects on pulp quality.
4. Depending on the process parameters, pretreatment liquors contain a variety of potential raw materials for producing a wide range of biomass-based platform chemicals or utilized, for example, in various polymeric applications. Carbohydrates can be converted into chemicals such as ethanol or carboxylic acids via fermentation. Lignin (especially sulfur-free fractions) is suitable for producing many chemicals and chemical mixtures that are currently derived from petroleum-based sources. It is most likely to be used as a bio-based additive to polymers, but also as low-cost carbon fiber, activated carbon, resins (“phenol mixtures”) in the plastic industry, and as many other straightforward products without significant modification, including binders, surface or dispersing agents, emulsifiers, and sequestrants. Aliphatic carboxylic acids from alkaline pretreatments form an interesting group of compounds and can be used as single components, or as more or less purified mixtures, in a number of applications. Of this group, formic and acetic acids as well as “lower-molar-mass” hydroxy acids glycolic and lactic acids are already today commercially important chemicals. In addition, “higher-molar-mass” hydroxy acids 3,4-dideoxypentonic, 3-deoxypentonic, xyloisosaccharinic, and glucosiosaccharinic acids (present in their lactone forms) can, for example, be converted into many valuable derivatives by reduction (i.e., production of polyalcohols), oxidation (i.e., production of polycarboxylic acids) or utilization in various polymers, detergents, and emulsifiers (i.e., production of esters). Together with these, mixtures containing high-molar-mass lactones of 3,4-dideoxy-pentonic, 3-deoxy-pentonic, xyloisosaccharinic, and glucoisosaccharinic acids, could be advantageously utilized in the preparation of various detergents and emulsifiers (i.e., surfactants).
5. Alkaline pulping of pretreated wood can be performed for producing not only various grades of chemical pulps, but also a wide spectrum of by-products recovered from the black liquor. With this respect, it would be a benefit if sulfur-free delignification is applied. However, in order to maintain the quality of pulp, combining pretreatments with pulping operations must be very finely tuned. This includes carefully determined pretreatment conditions (i.e., temperature, time, and applied additives) , which remove sufficient amounts of organics from wood to make their utilization and refining economically viable, but which simultaneously preserve the quality of fiber. In addition, the changes caused by the pretreatments must be taken into consideration during the subsequent pulping operations, in order to avoid “overcooking” of the material, which could lead to severely decreased strength properties of the produced pulp and lowered yield.
6. Finally, it should be pointed out that the established pulping processes effectively separate, in much the same way as the new biomass processing technology (“reactive fractionation”), most lignin and a significant proportion of hemicelluloses (as such or in the form of aliphatic carboxylic acids) from cellulose, creating a relatively pure fiber and thus, laying a natural foundation to integrated biorefining of wood resources. For this reason, chemical pulping offers many attractive possibilities in the form of new by-products originated from various side-streams.

ACKNOWLEDGMENT

Financial support from the Fortum Foundation is gratefully acknowledged.

REFERENCES CITED

Aachary, A. A. and Prapulla, S. G. (2010). “Xylooligosaccharides (XOS) as an emerging prebiotic: Microbial synthesis, utilization, structural characterization, bioactive properties, and applications,” Comprehens. Rev. Food Sci. Food Saf. 10(1), 2-16. DOI: 10.1111/j.1541-4337.2010.00135.x

Abdel-Rahman, M. A., Tashiro, Y., and Sonomoto, K. (2011). “Lactic acid production from lignocellulose-derived sugars using lactic acid bacteria: Overview and limits,” J. Biotechnol. 156(4), 286-301. DOI: 10.1016/j.jbiotec.2011.06.017

Adams, T. N. (1997). “General characteristics of kraft black liquor recovery boilers,” in: Kraft Recovery Boilers, Adams, T. N. (ed.), TAPPI Press, Atlanta, GA, USA, pp. 3-38.

Agbor, V. B., Cicek, N., Sparling, R., Berlin, A., and Levin, D. B. (2011). “Biomass pretreatment: Fundamentals towards application,” Biotechnol. Adv. 29(6), 675-685. DOI: 10.1016/j.biotechadv.2011.05.005

Agirrezabal-Telleria, I., Gandarias, I., and Arias, P. I. (2013). “Production of furfural from pentose-rich biomass: Analysis of process parameters during simultaneous furfural stripping,” Bioresour. Technol. 143(5), 258-264. DOI: 10.1016/j.biortech.2013.05.082

Agirrezabal-Telleria, I., Gandarias, I., and Arias, P. I. (2014). “Heterogeneous acid-catalysts for the production of furan-derived compounds (furfural and hydroxymethylfurfural) from renewable carbohydrates: A review,” Catal. Tod. 234, 42-58. DOI: 10.1016/j.cattod.2013.11.027

Ahsan, L., Jahan, M. S., and Ni, Y. (2014). “Recovering/concentrating of hemicellulosic sugars and acetic acid by nanofiltration and reverse osmosis from prehydrolysis liquor of kraft based hardwood dissolving pulp process,” Bioresour. Technol.155, 111-115. DOI: 10.1016/j.biortech.2013.12.096

Ajao, O., Rahni, M., Marinova, M., Chadjaa, H., and Savadogo, O. (2014). “Hemicelluloses prehydrolysate concentration by nanomembrane filtration: Feasibility and effect of operating conditions,” Conf. Proc., NWBC 2014, Nordic Wood and Biorefinery Conference, Stockholm, Sweden, 25.-27.3.2014, pp. 180-185.

Akim, L. G., Colodette, J. L., and Argyropoulos, D. S. (2001). “Factors limiting oxygen delignification of kraft pulp,” Can. J. Chem.79, 201-210. DOI: 10.1139/v01-007

Al-Dajani, W. W., and Tschirner, U. W. (2008). “Pre-extraction of hemicelluloses and subsequent kraft pulping Part I: Alkaline extraction,” TAPPI J. 7(6), 3-8.

Al-Dajani, W. W., and Tschirner, U. W. (2010). “Pre-extraction of hemicelluloses and subsequent ASA and ASAM pulping: Comparison of autohydrolysis and alkaline extraction,” Holzforschung 64(4), 411-416. DOI: 10.1515/hf.2010.064

Al-Dajani, W. W., Tschirner, U. W., and Jensen, T. (2009). “Pre-extraction of hemicelluloses and subsequent kraft pulping Part II: Acid and autohydrolysis,” TAPPI J. 8(9), 30-37.

Alekhina, M., Mikkonen, K. S., Alén, R., Tenkanen, M., and Sixta, H. (2014). ”Carboxymethylation of alkali extracted xylan for preparation of bio-based packaging films,” Carbohydr. Polym. 100, 89-96. DOI: 10.1016/j.carbpol.2013.03.048

Alén, R. (1990). “Conversion of cellulose-containing materials into useful products,” in: Cellulose Sources and Exploitation – Industrial Utilization, Biotechnology, and Physico-Chemical Properties, Kennedy, J. F., Phillips, G. O., and Williams, P. A. (eds.), Ellis Horwood, Chichester, England, pp. 453-464.

Alén, R. (2000). “Basic chemistry of wood delignification,” in: Forest Products Chemistry, Stenius P. (ed.), Series: Papermaking Science and Technology, Book 3, Fapet Oy, Helsinki, Finland, pp. 58-104.

Alén, R. (2011). “Principles of biorefining,” in: Biorefining of Forest Resources, Alén, R. (ed.), Paper Engineers´ Association, Helsinki, Finland, pp. 18-114.

Alén, R., Lahtela, M., Niemelä, K., and Sjöström, E. (1985). “Formation of hydroxy carboxylic acids from softwood polysaccharides during alkaline pulping,” Holzforschung 39(4), 235-238. DOI: 10.1515/hfsg.1985.39.4.235

Alén, R., Niemelä, K., and Sjöström, E. (1984). “Gas-liquid chromatographic separation of hydroxy monocarboxylic acids and dicarboxylic acids on a fused-silica capillary column,” J. Chromatogr.301, 273-276. DOI: 10.1016/S0021-9673(01)89197-4

Alén, R., and Sjöström, E. (1991). “Formation of low-molecular-mass compounds during the oxygen delignification of pine kraft pulp,” Holforschung 45(1), 83-86. DOI: 10.1515/hfsg.1991.45.s1.83

Al Manarash, M., Kallioinen, M., Ilvesniemi, H., and Mänttäri, M. (2012a). ”Recovery of galactoglucomannan from wood hydrolysate using regenerated cellulose ultrafiltration membranes,” Bioresour. Technol. 114, 375-381. DOI: 10.1016/j.biortech.2012.02.014

Al Manarash, M., Kallioinen, M., and Mänttäri, M. (2012b). “Concentration and purification of galactoglucomannans from wood pressurized hot water extraction liquors by high shear rate ultrafiltration,” Proc. Eng. 44, 1163-1165. DOI: 10.1016/j.proeng.2012.08.711

Alvira, P., Tomás-Pejó, E., Ballesteros, M., and Negro, M. J. (2010). “Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review,” Bioresour. Technol. 101(13), 4851-4861. DOI: 10.1016/j.biortech.2009.11.093

Amidon, T. E., Bujanovic, B., Liu, S., and Howard, J. R. (2011). “Commercializing biorefinery technology: A case for the multi-product pathway to a viable biorefinery,” Forests 8(4), 929-947. DOI: 10.3390/f2040929

Amidon, T. E., and Liu, S. (2009). “Water-based woody biorefinery,” Biotechnol. Adv. 27(5), 542-550. DOI: 10.1016/j.biotechadv.2009.04.012

Anteroinen, S. (2013). “Papers in order – Suspected carcinogen anthraquinone not used in Metsä Group pulp mills,” ECHO Metsä Fibre Customer Magazine 1, 26-27.

Bachegul, E., Toraman, H. E., Ozkan, N., and Bakir, U. (2012). “Evaluation of alkaline pretreatment temperature on a multi-product basis for the co-production of glucose and hemicellulose based films from lignocellulosic biomass,” Bioresour. Technol. 103(1), 440-445. DOI: 10.1016/j.biortech.2011.09.138

Baijpai, P. (2012). “Integrated forest biorefinery,” in: Biotechnology for Pulp and Paper Processing, Bajpai, P. (ed.), Springer, New York, NY, USA, pp. 375-402. DOI: 10.1007/978-1-4614-1409-4

Ban, W., Liu, Q., Guo, J., Mao, H., and Lucia, L. A. (2009). “A study of anthraquinone- fortified green liquor pretreatments of loblolly pine chips,” Holzforschung 63(3), 272-277. DOI: 10.1515/HF.2009.055

Ban, W., and Lucia, L. A. (2003). “Kraft green liquor pretreatment of softwood chips. 1. Chemical sorption profiles,” Ind. Eng. Chem. Res.42(3), 646-652. DOI: 10.1021/ie020491r

Ban, W., and Lucia, L. A. (2005). “Kinetic profiling of green liquor-modified kraft pulping,” Ind. Eng. Chem. Res. 44(9), 2948-2954. DOI: 10.1021/ie048762g

Ban, W., Singh, J., and Lucia, L. A. (2003a). “Kraft green liquor pretreatment of softwood chips. Part III: Lignin chemical modifications,” Holzforschung 57(3), 275-281. DOI: 10.1515/HF.2003.041

Ban, W., Song, J., and Lucia, L. A. (2004). “Insight into the chemical behavior of softwood carbohydrates during high-sulfidity green liquor pretreatment,” Ind. Eng. Chem. Res. 43(6), 1366-1372. DOI: 10.1021/ie034030x

Ban, W., Wang, S., and Lucia, L. A. (2003b). “The relationship of pretreatment pulping parameters with respect to selectivity: Optimization of green liquor pretreatment conditions for improved kraft pulping,” Paperi ja Puu 85(7), 1-7.

Barakat, A., Monlau, F., Steyer, J.-P., and Carrere, H. (2012). “Effect of lignin-derived and furan compounds found in lignocellulosic hydrolysates on biomethane production,” Bioresour. Technol. 104, 90-99. DOI: 10.1016/j.biortech.2011.10.060

Barclay, L. R. C., Xi, F., and Norris, J. Q. (1997). “Antioxidant properties of phenolic model compounds,” J. Wood Chem. Technol.17(1-2), 73-90. DOI: 10.1080/02773819708003119

Behera, S., Arora, R., Nandhagopal, N., and Kumar, S. (2014). “Importance of chemical pretreatment for bioconversion of lignocellulosic biomass,” Renew. Sust. En. Prod. 36, 91-106. DOI: 10.1016/j.rser.2014.04.047

Benali, M., Périn-Levasseur, Z., Savulescu, L., Kouisni, L., Jemaa, N., Kudra, T., and Paleologou, M. (2014). “Implementation of lignin-based biorefinery into a Canadian softwood kraft pulp mill: Optimal resources integration and economic viability assessment,” Biom. Bioen. 67, 473-482. DOI: 10.1016/j.biombioe.2013.08.022

Bhattacharya, P. K., Jayan, R., and Bhattacharjee, C. (2005). “A combined biological and membrane-based treatment of prehydrolysis liquor from pulp mill,” Separ. Purif. Technol. 45(2), 119-130. DOI: 10.1016/j.seppur.2005.03.003

Biermann, C. J. (1996). Handbook of Pulping and Papermaking, Academic Press, San Diego, CA, USA.

Binod, P., Sindhu, R., Singhania, R. R., Vikram, S., Devi, L., Nagalakshmi, S., Kurien, N., Sukumaran, R. K., and Pandey, A. (2010). “Bioethanol production from rice straw: and overview,” Bioresour. Techn. 101(13), 4767-4774. DOI: 10.1016/j.biortech.2009.10.079

Bordado, J. C. M. and Gomes, J. F. P. (2002). “Atmospheric emissions of kraft pulp mills,” Chem. Eng. Proc. 41(8), 667-671. DOI: 10.1016/S0255-2701(01)00184-2

Bordado, J. C. M. and Gomes, J. F. P. (2003). “Emission and odour control in kraft pulp mills,” J. Clean. Prod. 11(7), 797-801. DOI: 10.1016/S0959-6526(02)00101-4

Borrega, M., Nieminen, K., and Sixta, H. (2011a). “Degradation kinetics of the main carbohydrates in birch wood during hot water extraction in a batch reactor at elevated temperatures,” Bioresour. Technol. 102(22), 10724-10732. DOI: 10.1016/j.biortech.2011.09.027

Borrega, M., Nieminen, K., and Sixta, H. (2011b). “Effects of hot water extraction in a batch reactor on the delignification of birch wood,” BioResources 6(2), 1890-1903.

Borrega, M., Niemelä, K., and Sixta, H. (2013a). “Effect of hydrothermal intensity on the formation of degradation products from birchwood,” Holzforschung 67(8), 871-879. DOI: 10.1515/hf-2013-0019

Borrega, M., Tolonen, L. K., Bardot, F., Testova, L., and Sixta, H. (2013b). “Potential of hot water extraction of birch wood to produce high-purity dissolving pulp after alkaline pulping,” Bioresour. Technol. 135, 665-671. DOI: 10.1016/j.biortech.2012.11.107

Boucher, J., Chirat, C., and Lachenal, D. (2014). “Extraction of hemicelluloses from wood in a pulp biorefinery, and subsequent fermentation into ethanol,” En. Conv. Manag. 88, 1120-1126. DOI: 10.1016/j.enconman.2014.05.104

Brandberg, T., Franzén, C. J., and Gustafsson, L. (2004). “The fermentation performance of nine strains of Saccharomyces cerevisiae in batch and fed-batch cultures in dilute-acid wood hydrolysate,” J. Biosci. Bioeng. 98(2), 122-125. DOI: 10.1016/S1389-1723(04)70252-2

Brasch, D. J. and Free, K. W. (1965). “Prehydrolysis-kraft pulping of Pinus radiata grown in New Zealand,” TAPPI J. 48(4), 245-248.

Brunow, G., Lundquist, K., and Gellerstedt, G. (1999). “Lignin,” in:Analytical Methods in Wood Chemistry, Pulping and Papermaking, Sjöström, E., and Alén, R. (eds.), Springer, Heidelberg, Germany, pp. 77-120.

Bujanovic, B. M., Goundalkar, M. J., and Amidon, T. E. (2012). “Increasing the value of a biorefinery based on hot-water extraction: Lignin products,” TAPPI J. 11(1), 19-26.

Canilha, L., de Almeida e Silva, J. B., and Solenzal, A. I. N. (2004). “Eucalyptus hydrolysate detoxification with activated charcoal adsorption of ion-exchange resins for xylitol production,” Proc. Biochem. 39(12), 1909-1912. DOI: 10.1016/j.procbio.2003.09.009

Cara, C., Ruiz, E., Oliva, J. M., Sáez, F., and Castro, E. (2008). “Conversion of olive tree biomass into fermentable sugars by dilute acid pretreatment and enzymatic saccharification,” Bioresour. Technol. 99(6), 1869-1876. DOI: 10.1016/j.biortech.2007.03.037

Carvalheiro, F., Duarte, L. C., and Gírio, F. M. (2008). “Hemicellulose biorefineries: A review on biomass pretreatments,” J. Sci. Ind. Res. 67, 849-864.

Carvalheiro, F., Esteves, M. P., Parajó, J. C., Pereira, H., and Gírio, F. M. (2004). “Production of oligosaccharides by autohydrolysis of brewery’s spent grain,” Bioresour. Technol. 91(1), 93-100. DOI: 10.1016/S0960-8524(03)00148-2

Chai, X.-S., Samp, J., Hou, Q. X., Yoon, S.-H., and Zhu, J. Y. (2007). “Possible mechanism for anthraquinone species diffusion in alkaline pulping,” Ind. Eng. Chem. Res. 46(15), 5245-5249. DOI: 10.1021/ie0615741

Chen, B.-Y., Chen, S.-W., and Wang, H. T. (2012). “Use of different alkaline pretreatments and enzyme models to improve low-cost cellulosic biomass conversion,” Biomass Bioen. 39, 182-191. DOI: 10.1016/j.biombioe.2012.01.012

Chen, X., Lawoko, M., and van Heiningen, A. (2010). “Kinetics and mechanism of autohydrolysis of hardwoods,” Bioresour. Technol.101(20), 7812-7819. DOI: 10.1016/j.biortech.2010.05.006

Chen, Y., Stevens, M. A., Zhu, Y., Holmes, J., and Xu, H. (2013). “Understanding of alkaline pretreatment parameters for corn stover enzymatic saccharification,” Biotechnol. Biof. 6(2013). (http://www.biotechnologyforbiofuels.com/content/6/1/8).

Cherubini, F. (2010). “The biorefinery concept: Using biomass instead of oil for producing energy and chemicals,” Environm. Conv. Manag. 51(7), 1412-1421. DOI: 10.1016/j.enconman.2010.01.015

Chiaramonti, D., Prussi, M., Ferrero, S., Oriani, L., Ottonello, P., Torre, P., and Cherchi, F. (2012). “Review of pretreatment processes for lignocellulosic ethanol production, and development of an innovative method,” Biom. Bioen. 46, 25-35. DOI: 10.1016/j.biombioe.2012.04.020

Chin, K. L., H’ng, P. S., Wong, L. J., Tey, B. T., and Paridah, M. T. (2010). “Optimization study of ethanolic fermentation from oil palm trunk, rubberwood and mixed hardwood hydrolysates using Saccharomyces cerevisiae,” Bioresour. Technol. 101(9), 3287-3291. DOI: 10.1016/j.biortech.2009.12.036

Chirat, C., Boiron, L., and Lachenal, D. (2013). “Bleaching ability of pre-hydrolyzed pulps in the context of a biorefinery mill,” TAPPI J. 12(11), 49-53.

Chirat, C., Lachenal, D., and Sanglard, M. (2012). “Extraction of xylans from hardwood chips prior to kraft cooking,” Proc. Biochem. 47(3), 381-385. DOI: 10.1016/j.procbio.2011.12.024

Chirat, C., Pipon, G., Viardin, M. T., Lachenal, D., Lloyd, J. A., and Suckling, I. (2009). “Hemicelluloses extraction from eucalyptus and softwood wood chips: Pulp and ethanol production,” Proc. 15th ISWFPC, International Symposium on Wood, Fiber and Pulping Chemistry, Oslo, Norway, 15.-18.5.2009.

Chua, M. G. S., and Wayman, M. (1979). “Characterization of autohydrolysis aspen (P. tremuloides) lignins. Part 1. Composition and molecular weight distribution of extracted autohydrolysis lignin,” Can. J. Chem. 57(10), 1141-1149. DOI: 10.1139/v79-187

Coelho dos Santos Muguet, M., Ruuttunen, K., Jääskeläinen, A.-S., Colodette, J. L., and Vuorinen, T. (2013). “Defibration mechanisms of autohydrolyzed Eucalyptus wood chips,” Cellulose 20(5), 2647-2654. DOI: 10.1007/s10570-013-0023-3

Conde, E., Moure, A., Domínguez, H., and Parajó, J. C. (2008). “Fractionation of antioxidants from autohydrolysis of barley husks,” J. Agric. Food Chem. 56(22), 10651-10659. DOI: 10.1021/jf801710a

Connor, E. (2007). “The integrated forest biorefinery: the pathway to our bio-future,” Proc. International chemical recovery conference: efficiency and energy management, Quebec City, QC, Canada, 29.- 1.6.2007.

Converti, A., and Del Borghi, M. (1998). “Inhibition of the fermentation of oak hemicellulose acid-hydrolysate by minor sugars,” J. Biotechnol. 64(2-3), 211-218. DOI: 10.1016/s0168-1656(98)00109-6

Converti, A., Domínguez, J. M., Perego, P., Silvério da Silva, S., and Zilli, M. (2000). “Wood hydrolysis and hydrolysate detoxification for subsequent xylitol production,” Chem. Eng. Technol. 23(11), 1013-1020. DOI: 10.1002/1521-4125(200011)23:11<1013::AID-CEAT1013>3.0.CO;2-C

Cordeiro, N., Ashori, A., Hamzeh, Y., and Faria, M. (2013). “Effects of hot water pre-extraction on surface properties of bagasse soda pulp,” Mat. Sci. Eng. C. 33(2), 613-617. DOI: 10.1016/j.msec.2012.10.005

Cruz, J. M., Domínguez, J. M., Domínguez, H., and Parajó, J. C. (1999). “Solvent extraction of hemicellulosic wood hydrolysates: A procedure useful for obtaining both detoxified fermentation media and polyphenols with antioxidant activity,” Food Chem. 67(2), 147-153. DOI: 10.1016/s0308-8146(99)00106-5

Cruz, J. M., Domínguez, J. M., Domínguez, H., and Parajó, J. C. (2001). “Antioxidant and antimicrobial effects of extracts from hydrolysates of lignocellulosic materials,” J. Agric. Food Chem.49(5), 2459-2464. DOI: 10.1021/jf001237h

Cruz, J. M., Domínguez, H., and Parajó, J. C. (2005). “Anti-oxidant activity of isolates from acid hydrolysates of Eucalyptus globulus wood,” Food Chem. 90(4), 503-511. DOI: 10.1016/j.foodchem.2004.05.018

Dautzenberg, G., Gerhardt, M., and Kamm, B. (2011). “Bio based fuels and fuel additives from lignocellulose feedstock via the production of levulinic acid and furfural,” Holzforschung 65(4), 439-451. DOI: 10.1515/hf.2011.081

Delgenes, J. P., Moletta, R., and Navarro, J. M. (1996). “Effects of lignocellulose degradation products on ethanol fermentations of glucose and xylose by Saccharomyces cerevisiae, Zymomonas mobilis, Pichia stipitis and Candida shehatae,” Enz. Microb. Technol.19(3), 220-225. DOI: 10.1016/0141-0229(95)00237-5

De Lopez, S., Tissot, M., and Delmas, M. (1996). “Integrated cereal straw valorization by an alkaline pre-extraction of hemicellulose prior to soda-anthraquinone pulping. Case study of barley straw,” Biom. Bioener. 10(4), 201-211. DOI: 10.1016/0961-9534(95)00031-3

Dimmel, D. (1996). Pulping with Anthraquinone: Fundamental Chemistry, IPST Technical Paper Series Number 626, Institute of Paper Science and Technology, Atlanta, GA, USA.

Dimmel, D., and Gellerstedt, G. (2010). “Chemistry of alkaline pulping,” in: Lignin and Lignans – Advances in Chemistry, Heitner, C., Dimmel, D., and Schmidt, J. (eds.), CRC Press, Boca Raton, FL, USA, pp. 350-391.

Dong, X., Dong, M., Lu, Y., Turley, A., Jin, T., and Wu, C. (2011). “Antimicrobial and antioxidant activities of lignin from residue of corn stover to ethanol production,” Ind. Crops Prod. 34(3), 1629-1634. DOI: 10.1016/j.indcrop.2011.06.002

Duarte, G. V., Gamelas, J. A. F., Ramarao, B. V., Amidon, T. E., and Ferreira, P. T. (2012). “Properties of extracted Eucalyptus globuluskraft pulps,” TAPPI J. 11(4), 47-55.

Duarte, G. V., Ramarao, B. V., Amidon, T. E., and Ferreira, P. T. (2011). “Effect of hot water extraction on hardwood kraft pulp fibers (Acer saccharum, sugar maple),” Ind. Eng. Chem. Res. 50(17), 9949-9959. DOI: 10.1021/ie200639u

Duff, S. J. B. and Murray, W. D. (1996). “Bioconversion of forest products industry waste cellulosics to fuel ethanol: A review,” Bioresour. Technol. 55(1), 1-33. DOI: 10.1016/0960-8524(95)00122-0

Edlund, U., and Albertsson, A.-C. (2008). “A microspheric system: Hemicellulose-based hydrogels,” J. Bioact. Comp. Polym. 23(2), 171-186. DOI: 10.1177/0883911507088400

Edlund, U., Voepel, J., and Albertsson, A.-C. (2011). “Renewable materials from hemicellulose containing biomass,” Conf. Proc., NWBC 2011, Nordic Wood and Biorefinery Conference, Stockholm, Sweden, 22.-24.3.2011, pp. 126-131.

Egüés, I., Eceiza, A., and Labidi, J. (2013). “Effect of different hemicelluloses characteristics on film forming properties,” Ind. Crops Prod. 47, 331-338. DOI: 10.1016/j.indcrop.2013.03.031

Egüés, I., Sanchez, C., Mondragon, I., and Labidi, J. (2012). “Antioxidant activity of phenolic compounds obtained by autohydrolysis of corn residues,” Ind. Crops Prod. 36(1), 164-171. DOI: 10.1016/j.indcrop.2011.08.017

El Hage, R., Chrusciel, L., Desharnais, L., and Brosse, N. (2010). “Effect of autohydrolysis of Miscantus x giganteus on lignin structure and organosolv delignification,” Bioresour. Technol. 101(23), 9321-9329. DOI: 10.1016/j.biortech.2010.06.143

El Mansouri, N.-E., Yuan, Q., and Huang, F. (2011). “Characterization of alkaline lignins for use in phenol-formaldehyde and epoxy resins,” BioResources 6(3), 2647-2662.

Escalante, A., Conçalves, A., Bodin, A., Stepan, A., Sandström, C., Toriz, G., and Gatenholm, P. (2012). “Flexible oxygen barrier films from spruce xylan,” Carbohydr. Polym. 87(4), 2381-2387. DOI: 10.1016/j.carbpol.2011.11.003

European Food Safety Authority (EFSA) (2012). “Reasoned opinion on the review of the existing maximum residue levels (MRLs) for anthraquinone according to Article 12 of Regulation (EC) No 396/2005,” EFSA J. 10, 2761.

Felipe, M. G. A., Alves, L. A., Silva, S. S., Roberto, I. C., Mancilha, I. M., and Almeida e Silva, J. B. (1996). “Fermentation of eucalyptus hemicellulosic hydrolysate to xylitol by Candida guilliermondii,” Bioresour. Technol. 56(2-3), 281-283. DOI: 10.1016/0960-8524(96)00031-4

Feng, Z., and Alén, R. (2001). “Soda-AQ pulping of reed canary grass,” Ind. Crops. Prod. 14(1), 31-39. DOI: 10.1016/S0926-6690(00)00086-8

Feng, Z., Alén, R., and Niemelä, K. (2002a). “Formation of aliphatic carboxylic acids during soda-AQ pulping of kenaf bark,” Holzforschung 56(4), 388-394. DOI: 10.1515/HF.2002.061

Feng, Z., Alén, R., and Pakkanen, H. (2002b). “Characterization of black liquors from soda-AQ pulping of reed canary grass (Phalaris arundinacea L.),” Holzforschung 56(3), 298-303. DOI: 10.1515/HF.2002.048

Fengel, D., and Wegener, G. (1989). Wood – Chemistry, Ultrastructure, Reactions, Walter de Gruyter, Berlin, Germany.

Fenske, J. J., Griffin, D. A., and Penner, M. H. (1998). “Comparison of aromatic monomers in lignocellulosic biomass prehydrolysates,” J. Ind. Microbiol. Biotechnol. 20(6), 364-368. DOI: 10.1038/sj.jim.2900543

Finell, M., and Nilsson, C. (2004). “Kraft and soda-AQ pulping of dry fractionated reed canary grass,” Ind. Crops. Prod. 19(2), 155-165. DOI: 10.1016/j.indcrop.2003.09.002

Fredrick, W. J., Jr., Lien, S. J., Courchene, C. E., DeMartini, N. A., Ragauskas, A. J., and Iisa, K. (2008). “Co-production of ethanol and cellulose fiber from southern pine: A technical and economic assessment,” Biom. Bioen. 32(12), 1293-1302. DOI: 10.1016/j.biombioe.2008.03.010

Fu, S., Singh, J. M., Wang, S., and Lucia, L. (2005). “Investigation of the chemistry of oxygen delignification of low kappa softwood kraft pulp using an organic/inorganic chemical selectivity system,” J. Wood Chem. Technol. 25(3), 95-108. DOI: 10.1080/02773810500191526

Gabrielii, I., Gatenholm, P., Glasser, W. G., Jain, R. K., and Kenne, L. (2000). “Separation, characterization and hydrogel-formation of hemicellulose from aspen wood,” Carbohydr. Polym. 43(4), 367-374. DOI: 10.1016/S0144-8617(00)00181-8

Galbe, M., and Zacchi, G. (2012). “Pretreatment: The key to efficient utilization of lignocellulosic materials,” Biom. Bioen. 46, 70-78. DOI: 10.1016/j.biombioe.2012.03.026

García, A., Toledano, A., Andrés, M. Á., and Labidi, J. (2010). “Study of the antioxidant capacity of Miscanthus sinensis lignins,” Proc. Biochem. 45(6), 935-940. DOI: 10.1016/j.procbio.2010.02.015

Garrote, G., Cruz, J. M., Domínguez, H., and Parajó, J. C. (2003). “Valorisation of waste fractions from autohydrolysis of selected lignocellulosic materials,” J. Chem. Technol. Biotechnol. 78(4), 392-398. DOI: 10.1002/jctb.760

Garrote, G., Cruz, J. M., Moure, M., Domínguez, H., and Parajó, J. C. (2004a). “Antioxidant activity of byproducts from the hydrolytic processing of selected lignocellulosic materials,” Trends Food Sci. Technol. 15(3-4), 191-200. DOI: 10.1016/j.tifs.2003.09.016

Garrote, G., Domínguez, H., and Parajó, J. C. (1999a). “Hydrothermal processing of lignocellulosic materials,” Holz als Roh- und Werkst. 57(3), 191-203. DOI: 10.1007/s001070050039

Garrote, G., Domínguez, H., and Parajó, J. C. (1999b). “Mild autohydrolysis: An environmentally friendly technology for xylooligosaccharide production from wood,” J. Chem. Technol. Biotechnol. 74(11), 1101-1109. DOI: 10.1002/(SICI)1097-4660(199911)74:11<1101::AID-JCTB146>3.0.CO;2-M

Garrote, G., Domínguez, H., and Parajó, J. C. (2001). “Generation of xylose solutions from Eucalyptus globulus wood by autohydrolysis-posthydrolysis processes: Posthydrolysis kinetics,” Bioresour. Technol. 79(2), 155-164. DOI: 10.1016/s0960-8524(01)00044-x

Garrote, G., Domínguez, H., and Parajó, J. C. (2004b). “Production of substituted oligosaccharides by hydrolytic processing of barley husks,” Ind. Eng. Chem. Res. 43(7), 1608-1614. DOI: 10.1021/ie0342762

Garrote, G., Falqué, E., Domínguez, H., and Parajó, J. C. (2007). “Autohydrolysis of agricultural residues: Study of reaction byproducts,” Bioresour. Technol. 98(10), 1951-1957. DOI: 10.1016/j.biortech.2006.07.049

Gellerstedt, G. (2001). “Pulping chemistry,” in: Wood and Cellulosic Chemistry, Hon, D., and Shiraishi, N. (eds.), 2^nd edition, Marcel Dekker, Inc., New York, NY, USA, pp. 859-905.

Gellerstedt, G. (2010). “Chemistry of pulp bleaching,” in: Lignin and lignans – Advances in Chemistry, Heitner, C., Dimmel, D., and Schmidt, J. (eds.), CRC Press, Boca Raton, FL, USA, pp. 393-429.

Gírio, F. M., Fonseca, C., Carvalheiro, F., Duarte, L. C., Marques, S., and Bogel-Łukasik, R. (2010). “Hemicelluloses for fuel ethanol: A review,” Bioresour. Technol. 101(13), 4775-4800. DOI: 10.1016/j.biortech.2010.01.088

Goldstein, I. S. (1981). Organic Chemicals from Biomass, CRC Press, Boca Raton, FL, USA.

Gomes, F. J. B., Santos, F. A., Colodette, J. L., Demuner, I. F., and Batalha, L. A. R. (2014). “Literature review on biorefinery processes integrated to the pulp industry,” Nat. Resourc. 5(9), 419-432. DOI: 10.4236/nr.2014.59039

González, J., Cruz, J. M., Domínguez, H., and Parajó, J. C. (2004). “Production of antioxidants from Eucalyptus globulus wood by solvent extraction of hemicellulose hydrolysates,” Food Chem. 84(2), 243-251. DOI: 10.1016/S0308-8146(03)00208-5

Gonzalez, R., Treasure, T., Phillips, R., Jameel, H., and Saloni, D. (2011). “Economic of cellulosic ethanol production: Green liquor pretreatment for softwood and hardwood, greenfield and repurpose scenarios,” BioResources 6(3), 2551-2567.

Gütsch, J. S., Nousiainen, T., and Sixta, H. (2012). “Comparative evaluation of autohydrolysis and acid-catalyzed hydrolysis of Eucalyptus globulus wood,” Bioresour. Technol. 109, 77-85. DOI: 10.1016/j.biortech.2012.01.018

Gütsch, J. S. and Sixta, H. (2011). “Purification of Eucalyptus globulus water prehydrolyzates using the HiTAC process (high-temperature adsorption on activated charcoal),” Holzforschung 65(4), 511-518. DOI: 10.1515/hf.2011.065

Gullón, P., Romaní, A., Vila, C., Garrote, G., and Parajó, J. C. (2012). “Potential of hydrothermal treatments in lignocellulose biorefineries,” Biofuels Bioprod. Bioref. 6, 219-232. DOI: 10.1002/bbb.339

Guo, W., Jia, W., Li, Y., and Chen, S. (2010). “Performances of Lactobacillus brevis for producing lactic acid from hydrolysate of lignocellulosics,” Appl. Biochem. Biotechnol. 161(1-8), 124-136. DOI: 10.1007/s12010-009-8857-8

Gurgel, L. V. A., Marabezi, K., Ramos, L. A., and da Silva Curvelo, A. A. (2012). “Characterization of depolymerized residues from extremely low acid hydrolysis (ELA) of sugarcane bagasse cellulose: Effects of degree of polymerization, crystallinity and crystallite size on thermal decomposition,” Ind. Crops Prod. 36(1), 560-571. DOI: 10.1016/j.indcrop.2011.11.009

Hämäläinen, S., Näyhä, A., and Pesonen, H.-L. (2011). “Forest biorefineries – A business opportunity for the Finnish forest cluster,” J. Clean. Prod. 19(16), 1884-1891. DOI: 10.1016/j.jclepro.2011.01.011

Hahn-Hägerdahl, B., Karhumaa, K., Fonseca, C., Spencer-Martins, I., and Gorwa-Grauslund, M. F. (2007). “Towards industrial pentose-fermenting yeast strains,” Appl. Microbiol. Biotechnol. 74(5), 937-953. DOI: 10.1007/s00253-006-0827-2

Hamaguchi, M., Kautto, J., and Vakkilainen, E. (2013). “Effects of hemicellulose extraction on the kraft pulp mill operation and energy use: Review and case study with lignin removal,” Chem. Eng. Res. Des. 91(7), 1284-1291. DOI: 10.1016/j.cherd.2013.02.006

Hanim, S. S., Norsyabilah, R., Nor Suhaila, M. H., Noraishah, A., and Siti Kartina, A. K. (2012). “Effects of temperature, time and pressure on the hemicelluloses yield extracted using subcritical water extraction,” Proc. Eng. 42, 562-565. DOI: 10.1016/j.proeng.2012.07.448

Hardy, J. (2004). “Green chemistry and sustainability,” in: Renewable Bioresources – Scope and Modification for Non-food Applications, Stevens, C. V. and Verhé, R. (eds.), John Wiley & Sons Ltd, Chichester, West Sussex PO19 8SQ, England, pp. 1-29.

Harris, J. F., Scott, R. W., Springer, E. L., and Wenger, T. H. (1984). “Factors influencing dilute sulfuric acid prehydrolysis of southern red oak wood,” in: Progress in Biomass Conversion, Tillman, D. A. and Jahn, E. C. (eds.), Vol. 5, Academic Press, Orlando, FL, USA, pp. 102-141.

Hedjazi, S., Kordsachia, O., Patt, R., Latibari, A. J., and Tschirner, U. (2009). “Alkaline sulfite-anthraquinone (AS/AQ) pulping of wheat straw and totally chlorine free (TCF) bleaching of pulps,” Ind. Crops. Prod. 29(1), 27-36. DOI: 10.1016/j.indcrop.2008.03.013

Hellstén, S., Lahti, J., Heinonen, J., Kallioinen, M., Mänttäri, M., and Sainio, T. (2013). “Purification process for recovering hydroxy acids from soda black liquor,” Chem. Eng. Res. Des. 91(12), 2765-2774. DOI: 10.1016/j.cherd.2013.06.001

Helmerius, J., von Walter, J. V., Rova, U., Berglund, K. A., and Hodge, D. B. (2010). “Impact of hemicellulose pre-extraction for bioconversion on birch kraft pulp properties,” Bioresour. Technol.101(15), 5996-6005. DOI: 10.1016/j.biortech.2010.03.029

Hendriks, A. T. W. M., and Zeeman, G. (2009). “Pretreatments to enhance the digestibility of lignocellulosic biomass,” Bioresour. Technol. 100(1), 10-18. DOI: 10.1016/j.biortech.2008.05.027

Herrick, F. W., and Hegert, H. L. (1977). “Utilization of chemicals from wood: Retrospect and prospect,” in: The Structure, Biosynthesis, and Degradation of Wood, Recent Advances in Phytochemistry,Loewus, F. A. and Runecles, V. C. (eds.), Vol. 11, Plenium Press, New York, NY, USA, pp. 443-515.

Hörhammer, H., Walton, S., and van Heiningen, A. (2011). “A larch based biorefinery: Pre-extraction and extract fermentation to lactic acid,” Holzforschung 65(4), 491-496. DOI: 10.1515/hf.2011.085

Hofvendahl, K., and Hahn-Hägerdahl, B. (2000). “Factors affecting the fermentative lactic acid production from renewable resources,” Enz. Microb. Technol. 26(2-4), 87-107. DOI: 10.1016/S0141-0229(99)00155-6

Hu, G., Heitmann, J. A., and Rojas, O. J. (2008). “Feedstock pretreatment strategies for producing ethanol from wood, bark, and forest residues,” BioResources 3(1), 270-294.

Huang, F., and Ragauskas, A. (2013a). “Integration of hemicellulose pre-extraction in the bleach-grade pulp production process,” TAPPI J. 12(10), 55-61. DOI: 10.1021/ie302242h

Huang, F., and Ragauskas, A. (2013b). “Extraction of hemicellulose from loblolly pine woodchips and subsequent kraft pulping,” Ind. Eng. Chem. Res. 52(4), 1743-1749.

Huang, H.-J., Ramaswamy, S., Al-Dajani, W. W., and Tschirner, U. (2010). “Process modeling and analysis of pulp mill-based integrated biorefinery with hemicellulose pre-extraction for ethanol production: A comparative study,” Bioresour. Technol. 101(2), 624-631. DOI: 10.1016/j.biortech.2009.07.092

Hupa, M. (1997). “Recovery Boiler Chemistry,” in: Kraft Recovery Boilers, Adams, T. N. (ed.), TAPPI Press, Danvers, MA, USA, pp. 39-60.

Ikeda, T., and Magara, K. (2015). “Chemical properties of softwood soda-anthraquinone lignin,” J. Wood Chem. Technol. 35(3), 167-177. DOI: 10.1080/02773813.2014.905605

Ishii, T., and Shimizu, K. (2001). “Chemistry of cell wall polysaccharides,” in: Wood and Cellulosic Chemistry, Hon, D., and Shiraishi, N. (eds.), 2^nd edition, Marcel Dekker, Inc., New York, NY, USA, pp. 175-212.

Jahan, M. S., Saeed, A., Yonghao, N., and He, Z. (2009). “Pre-extraction and its impact on the alkaline pulping of bagasse,” Biob. Mat. Bioen. 3(4), 380-385. DOI: 10.1166/jbmb.2009.1053

Jensen, J. R., Morinelly, J. E., Gossen, K. R., Brodeur-Campbell, M. J., and Shonnard, D. R. (2010). “Effects of dilute acid pretreatment conditions on enzymatic hydrolysis monomer and oligomer sugar yields for aspen, balsam, and switchgrass,” Bioresour. Technol.101(7), 2317-2325. DOI: 10.1016/j.biortech.2009.11.038

Jeong, S.-Y., Trinh, L.T.P., Lee, H.-J., and Lee, J.-W. (2014). “Improvement of the fermentability of oxalic acid hydrolysates by detoxification using electrodialysis and adsorption,” Bioresour. Technol. 152, 444-449. DOI: 10.1016/j.biortech.2013.11.029

Jiang, H., Chen, Q., Ge, J., and Zhang, Y. (2014). “Efficient extraction and characterization of polymeric hemicelluloses from hybrid poplar,” Carbohydr. Polym. 101, 1005-1012. DOI: 10.1016/j.carbpol.2013.10.030

Jin, Y., Jameel, H., Chang, H., and Phillips, R. (2010). “Green liquor pretreatment of mixed hardwood for ethanol production in a repurposed kraft pulp mill,” J. Wood Chem. Technol. 30(1), 86-104. DOI: 10.1080/02773810903578360

Johakimu, J., and Andrew, J. (2013). “Hemicellulose extraction from south African Eucalyptus grandis using green liquor and its impact on kraft pulping efficiency and paper making properties,” BioResources 8(3), 3490-3504. DOI: 10.15376/biores.8.3.3490-3504

Ju, Y.-H., Huynh, L.-H., Kasim, N. S., Guo, T.-J., and Wang, J. H. (2011). “Analysis of soluble and insoluble fractions of alkali and subcritical water treated sugarcane bagasse,” Carbohydr. Polym.83(2), 591-599. DOI: 10.1016/j.carbpol.2010.08.022

Jun, A., Tschirner, U. W., and Tauer, Z. (2012). “Hemicellulose extraction from aspen chips prior to kraft pulping utilizing kraft white liquor,” Biom. Bioen. 37, 229-236. DOI: 10.1016/j.biombioe.2011.12.008

Kabel, M. A., Carvalheiro, F., Garrote, G., Avgerinos, E., Koukios, E., Parajó, J. C., Gírio, F. M., Schols, H. A., and Voragen, A. G. J. (2002a). “Hydrothermally treated xylan rich by-products yield different classes of xylo-oligosaccharides,” Carbohydr. Polym. 50(1), 47-56. DOI: 10.1016/S0144-8617(02)00045-0

Kabel, M. A., Kortenoeven, L., Schols, H. A., and Voragen, A. G. J. (2002b). “In vitro fermentability of differently substituted xylo-oligosaccharides,” J. Agric. Food. Chem. 50(21), 6205-6210. DOI: 10.1021/jf020220r

Kabel, M. A., Schols, H. A., and Voragen, A. G. J. (2002c). “Complex xylo-oligosaccharides identified from hydrothermally treated Eucalyptus wood and brewery´s spent grain,” Carbohydr. Polym. 50(2), 191-200. DOI: 10.1016/S0144-8617(02)00022-X

Kalapathy, U., and Proctor, A. (2001). “Effect of acid extraction and alcohol precipitation conditions on the yield and purity of soy hull pectin,” Food Chem. 73(4), 393-396. DOI: 10.1016/S0308-8146(00)00307-1

Kallioinen, A., Hakola, M., Riekkola, T., Repo, T., Leskelä, M., von Weymarn, N., and Siika-aho, M. (2013). “A novel alkaline oxidation pretreatment for spruce, birch and sugar cane bagasse,” Bioresour. Technol. 140, 414-420. DOI: 10.1016/j.biortech.2013.04.098

Kallioinen, M., Nevalainen, T., and Mänttäri, M. (2014). “Recovery of highly concentrated hemicellulose fractions from spruce and birch extracts,” Conf. Proc., NWBC 2014, Nordic Wood and Biorefinery Conference, Stockholm, Sweden, 25.-27.3.2014, pp. 186-191.

Kalliola, A., Kuitunen, S., Liitiä, T., Rovio, S., Ohra-aho, T., Vuorinen, T., and Tamminen, T. (2011). “Lignin oxidation mechanisms under oxygen delignification conditions. Part 1. Results from direct analyses,” Holzforschung 65(4), 567-574. DOI: 10.1515/hf.2011.101

Kamerling, J. P., and Gerwig, G. J. (2007). “Strategies for the structural analysis of carbohydrates,” in: Comprehensive Glycoscience, Analysis of Glucans, Boons, G.-J., Lee, Y.C., Suzuki, A., Taniguchi, N., and Voragen, A. G. J. (eds.), Elsevier Ltd., Oxford, UK, pp. 1-69.

Kamm, B., and Kamm, M. (2004). “Principles of biorefineries,” Appl. Microbiol. Biotechnol. 64(2), 137-145. DOI: 10.1007/s00253-003-1537-7

Kamm, B., Kamm, M., Gruber, P. R., and Kromus, S. (2006). “Biorefinery systems – An overview,” in: Biorefineries – Industrial Processes and Products, Status Quo and Future Directions, Kamm, B., Gruber, P. R., and Kamm, M. (eds.), Vol. 1, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, pp. 3-40.

Katahira, S., Mizuike, A., Fukuda, H., and Kondo, A. (2006). “Ethanol fermentation from lignocellulosic hydrolysate by a recombinant xylose- and cellooligosaccharide-assimilating yeast strain,” Appl. Microbiol. Biotechnol. 72(6), 1136-1143.

Kilpeläinen, P. (2015). Pressurized Hot Water Flow-through Extraction of Birch Wood, Doctoral Thesis, Laboratory of Wood and Paper Chemistry, Åbo Akademi University, Åbo, Finland.

Kilpeläinen, P., Kitunen, V., Pranovich, A., Ilvesniemi, H., and Willför, S. (2013). ”Pressurized hot water flow-through extraction of birch sawdust with acetate pH buffer,” BioResources 8(4), 5202-5218. DOI: 10.15376/biores.8.4.5202-5218

Kilpeläinen, P., Leppänen, K., Spetz, P., Kitunen, V., Ilvesniemi, H., Pranovich, A., and Willför, S. (2012). “Pressurised hot water extraction of acetylated xylan from birch sawdust,” Nord. Pulp Pap. Res. J. 27(4), 680-688. DOI: 10.3183/NPPRJ-2012-27-04-p680-688

Kim, Y., Hendrickson, R., Mosier, N. S., and Ladisch, M. R. (2009). “Liquid hot water pretreatment of cellulosic biomass,” in: Biofuels – Methods and Protocols, Mielenz, J. R. (ed.), Humana Press Springer, New York, NY, USA, pp. 93-102.

Kim, S. J., Kwon, H. S., Kim, G. H., and Um, B. H. (2015). “Green liquor extraction of hemicellulosic fractions and subsequent organic acid recovery from the extracts using liquid-liquid extraction,” Ind. Crops Prod. 67, 395-402. DOI: 10.1016/j.indcrop.2015.01.040

Kim, J. S., Lee, Y. Y., and Torget, R. W. (2001). “Cellulose hydrolysis under extremely low sulfuric acid and high-temperature conditions,” Appl. Biochem. Biotechn. 91-93(1-9), 331-340. DOI: 10.1385/ABAB:91-93:1-9:331

Kim, D. Y., Yim, S. C., Lee, P. C., Lee, W. G., Lee, S. Y., and Chang, H. N. (2004). “Batch and continuous fermentation of succinic acid from wood hydrolysate by Mannheimia succiniciproducens MBEL55E,” Enz. Microb. Technol. 35(6-7), 648-653. DOI: 10.1016/j.enzmictec.2004.08.018

Kleen, M. A., Liitiä, T. M., and Tehomaa, M. M. (2011). “The effect of the physical form and size of raw materials in pressurized hot water extraction of birch,” Proc. 16^th ISWFPC, 16th International Symposium on Wood, Fibre and Pulping Chemistry, Tianjin, China, 8.6.-10.6.2011.

Klinke, H. B., Ahring, B. K., Schmidt, A. S., and Thomsen, A. B. (2002). “Characterization of degradation products from alkaline wet oxidation of wheat straw,” Bioresour. Technol. 82(1), 15-26. DOI: 10.1016/S0960-8524(01)00152-3

Klinke, H. B., Thomsen, A. B., and Ahring, B. K. (2004). “Inhibition of ethanol-producing yeast and bacteria by degradation products produced during pre-treatment of biomass,” Appl. Microbiol. Biotechnol. 66(1), 10-26. DOI: 10.1007/s00253-004-1642-2

Koivula, E., Kallioinen, M., Preis, S., Testova, L., Sixta, H., and Mänttäri, M. (2011). “Evaluation of various pretreatment methods to manage fouling in ultrafiltration of wood hydrolysates,” Separ. Purif. Technol. 83, 50-56. DOI: 10.1016/j.seppur.2011.09.006

Koivula, E., Kallioinen, M., Sainio, T., Antón, E., Luque, S., and Mänttäri, M. (2013). “Enhanced membrane filtration of wood hydrolysates for hemicelluloses recovery by pretreatment with polymeric adsorbents,” Bioresour. Technol. 143, 275-281. DOI: 10.1016/j.biortech.2013.05.129

Koivula, E., Kallioinen, M., Sainio, T., Luque, S., and Mänttäri, M. (2012). “Adsorption to improve filtration performance in treatment of wood-based hydrolysates,” Proc. Eng. 44, 1384-1386. DOI: 10.1016/j.proeng.2012.08.796

Koutinas, A. A., Du, C., Wang, R. H., and Webb, C. (2008). “Production of chemicals from biomass,” in: Introduction to Chemicals from Biomass, Clark, J., and Deswarte, F. (eds.), John Wiley & Sons Ltd., Chichester, United Kingdom, 78-100.

Krawczyk, H., Oinonen, P., and Jönsson, A.-S. (2013). “Combined membrane filtration and enzymatic treatment for recovery of high molecular mass hemicelluloses from chemithermomechanical pulp process water,” Chem. Eng. J. 225, 292-299. DOI: 10.1016/j.cej.2013.03.089

Kristensen, J. B., Börjesson, J., Bruun, M. H., Tjerneld, F., and Jørgensen, H. (2007). “Use of surface active additives in enzymatic hydrolysis of wheat straw lignocellulose,” Enzyme Microb. Technol. 40(4), 888-895. DOI: 10.1016/j.enzmictec.2006.07.014

Krogell, J. (2015). Intensification of Hemicellulose Hot-water Extraction from Spruce Wood by Parameter Tuning, Doctoral Thesis, Laboratory of Wood and Paper Chemistry, Åbo Akademi University, Åbo, Finland.

Krogell, J., Eränen, K., Pranovich, A., and Willför, S. (2015). “In-line high-temperature pH control during hot-water extraction of wood,” Ind. Crops Prod. 67, 114-120. DOI: 10.1016/j.indcrop.2015.01.026

Kronholm, J., Hartonen, K., and Riekkola, M.-L. (2007). “Analytical extractions with water at elevated temperatures and pressures,” Tr. Anal. Chem. 26(5), 396-412. DOI: 10.1016/j.trac.2007.03.004

Kuitunen, S., Kalliola, A., Tarvo, V., Tamminen, T., Rovio, S., Liitiä, T., Ohra-aho, T., Lehtimaa, T., Vuorinen, T., and Alopaeus, V. (2011). “Lignin oxidation mechanisms under oxygen delignification conditions. Part 3. Reaction pathways and modeling,” Holzforschung 65(4), 587-599. DOI: 10.1515/hf.2011.100

Kumar, P., Barret, D. M., Delwiche, M. J., and Stroeve, P. (2009). “Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production,” Ind. Eng. Chem. Res. 48(8), 3713-3729. DOI: 10.1021/ie801542g

Kumar, R., Singh, S., and Singh, O. V. (2008). “Bioconversion of lignocellulosic biomass: Biochemical and molecular perspectives,” J. Ind. Microbiol. Biotechnol. 35(5), 377-391. DOI: 10.1007/s10295-008-0327-8

Lai, Y.-Z. (2001). “Chemical degradation,” in: Wood and Cellulosic Chemistry, Hon, D. N.-S., and Shiraishi, N. (eds.), Marcell Dekker, Inc., New York, NY, USA, 78-100.

Laitinen, E., Tornberg, J., and Alén, R. (2002). “A laser-induced fluorescence (LIF) method for monitoring oxygen-alkali delignification of softwood kraft pulp,” Anal. Lett. 35(15), 2539-2547. DOI: 10.1081/AL-120016543

Lange, J.-P. (2007). “Lignocellulose Conversion: An Introduction to Chemistry, Process and Economics,” in: Catalysis for Renewables – From Feedstock to Energy Production, Centi, G., and van Santen, R.A. (eds.), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, pp. 21-52.

Lapierre, C. (2010). “Determining lignin structure by chemical degradations,” in: Lignin and Lignans, Advances in Chemistry, Heitner, C., Dimmel, D. R., and Schmidt, J. A. (eds.), Boca Raton, FL, USA, pp. 11-42.

Larsson, S., Cassland, P., and Jönsson, L. F. (2001). “Development of a Saccharomyces cerevisiae strain with enhanced resistance to phenolic fermentation inhibitors in lignocellulose hydrolysates by heterologous expression of laccase,” Appl. Environm. Microbiol. 67(3), 1163-1170. DOI: 10.1128/AEM.67.3.1163-1170.2001

Larsson, S., Palmqvist, E., Hahn-Hägerdal, B., Tengborg, C., Stenberg, K., Zacchi, G., and Nilvebrandt, N.-O. (1999). “The generation of fermentation inhibitors during dilute acid hydrolysis of softwood,” Enz. Microb. Technol. 24(3-4), 151-159. DOI: 10.1016/S0141-0229(98)00101-X

Lee, H.-J., Lim, W.-S., and Lee, J.-W. (2013). “Improvement of ethanol fermentation from lignocellulosic hydrolysates by the removal of inhibitors,” J. Ind. Eng. Chem. 19(6), 2010-2015. DOI: 10.1016/j.jiec.2013.03.014

Lehto, J., and Alén, R. (2012). “Purification of hardwood-derived autohydrolysates,” BioResources 7(2), 1813-1823. DOI: 10.15376/biores.7.2.1813-1823

Lehto, J., and Alén, R. (2013). “Alkaline pre-treatment of hardwood chips prior to delignification,” J. Wood Chem. Technol. 33(2), 77-91. DOI: 10.1080/02773813.2012.748077

Lehto, J., and Alén, R. (2015a). “Alkaline pre-treatment of softwood chips prior to delignification,” J. Wood Chem. Technol. 35(2), 146-155. DOI: 10.1080/02773813.2014.902964

Lehto, J., and Alén, R. (2015b). “Organic materials in black liquors of soda-AQ pulping of hot-water-extracted birch (Betula pendula) sawdust,” Holzforschung 69(3), 257-264. DOI: 10.1515/hf-2014-0094

Lehto, J., and Alén, R. (2015c). “Organic material dissolved during oxygen-alkali pulping of hot-water-extracted birch sawdust,” TAPPI Journal 14(4), 237-244.

Lehto, J., Alén, R., and Malkavaara, P. (2014a). “Multivariate correlation between analytical data for various organics dissolved during autohydrolysis of silver birch (Betula pendula) chips and treatment conditions,” BioResources 9(3), 4958-4970. DOI: 10.15376/biores.9.3.4958-4970

Lehto, J., Alén, R., and Malkavaara, P. (2014b). “Multivariate correlation between analysis data on dissolved organic material from Scots pine (Pinus sylvestris) chips and their autohydrolysis pre-treatment conditions,” BioResources 9(1), 93-104.

Lehto, J., Pakkanen, H., and Alén, R. (2015). “Molecular mass distribution of sulfur-free lignin from alkaline pulping preceded by hot-water-extraction,” Appita J. 68(2), 149-158.

Lei, Y., Liu, S., Li, J., and Sun, R. (2010). “Effect of hot-water extraction on alkaline pulping of bagasse,” Biotechnol. Adv. 28(5), 609-612. DOI: 10.1016/j.biotechadv.2010.05.009

Lenihan, P., Orozco, A., O’Neill, E., Ahmad, M. N. M., Rooney, D. W., and Walker, G. M. (2010). “Dilute acid hydrolysis of lignocellulosic biomass,” Chem. Eng. J. 156(2), 395-403. DOI: 10.1016/j.cej.2009.10.061

Leschinsky, M., Sixta, H., and Patt, R. (2009a). “Detailed mass balances of the autohydrolysis of Eucalyptus globulus at 170 ºC,” BioResources 4(2), 687-703. DOI: 10.1515/hf.2008.117

Leschinsky, M., Zuckerstätter, G., Weber, H. K., Patt, R., and Sixta, H. (2008a). “Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 1: Comparison of different lignin fractions formed during water prehydrolysis,” Holzforschung 62(6), 645-652. DOI: 0.1515/hf.2008.133

Leschinsky, M., Zuckerstätter, G., Weber, H. K., Patt, R., and Sixta, H. (2008b). “Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 2: Influence of autohydrolysis intensity,” Holzforschung 62(6), 653-658.

Leschinsky, M., Weber, H. K., Patt, R., and Sixta, H. (2009b). “Formation of insoluble components during autohydrolysis of Eucalyptus globulus,” Lenzing. Ber. 87, 16-25.

Leskelä, M., Kokko, A., and Suurnäkki, A. (2014). “FuBio programs: Building a bridge from pulp mills to new businesses and new markets using cellulose and lignin,” Conf. Proc., NWBC 2014, Nordic Wood and Biorefinery Conference, Stockholm, Sweden, 25.-27.3.2014, pp. 57-61.

Li, J., and Gellerstedt, G. (2008). “Improved lignin properties and reactivity by modifications in the autohydrolysis process of aspen wood,” Ind. Crops Prod. 27(2), 175-181. DOI: 10.1016/j.indcrop.2007.07.022

Li, J., Henriksson, G., and Gellerstedt, G. (2005). “Carbohydrate reactions during high-temperature steam treatment of aspen wood,” Appl. Biochem. Biotechnol. 125(3), 175-188. DOI: 10.1385/ABAB:125:3:175

Li, H., Saeed, A., Jahan, M. S., Ni, Y., and van Heiningen, A. (2010). “Hemicellulose removal from hardwood chips in the pre-hydrolysis step of the kraft-based dissolving pulp production process,” J. Wood. Chem. Technol. 30(1), 48-60. DOI: 10.1080/02773810903419227

Limayem, A., and Ricke, S. C. (2012). “Lignocellulosic biomass for bioethanol production: Current perspectives, potential issues and future prospects,” Prog. En. Comb. Sci. 38(4), 449-467. DOI: 10.1016/j.pecs.2012.03.002

Lin, C. K. (1979). Prehydrolysis-alkaline Pulping of Sweetgum Wood, Ph.D. Thesis, NC State University, Raleigh, NC, USA.

Liu, S. (2010). “Woody biomass: Niche position as a source of sustainable renewable chemicals and energy and kinetics of hot-water extraction/hydrolysis,” Biotechnol. Adv. 28(5), 563-582. DOI: 10.1016/j.biotechadv.2010.05.006

Liu, S., Abrahamson, L. P., and Scott, G. M. (2012a). “Biorefinery: Ensuring biomass as a sustainable renewable source of chemicals, materials and energy,” Biom. Bioen. 39, 1-4. DOI: 10.1016/j.biombioe.2010.12.042

Liu, S., Bischoff, K. M., Leathers, T. D:, Qureshi, N., Rich, J. O., and Hughes, S. R. (2013a). “Butyric acid from anaerobic fermentation of lignocellulosic biomass hydrolysates by Clostridium tyrobutyricumstrain RPT-4213,” Bioresour. Technol. 143, 322-329. DOI: 10.1016/j.biortech.2013.06.015

Liu, Z., Fatehi, P., Jahan, M. S., and Ni, Y. (2011a). “Separation of lignocellulosic materials by combined processes of pre-hydrolysis and ethanol extraction,” Bioresour. Technol. 102(2), 1264-1269. DOI: 10.1016/j.biortech.2010.08.049

Liu, W., Hou, Q., Mao, C., Yuan, Z., and Li, K. (2012d). “Effect of hemicellulose pre-extraction on the properties and bleachability of aspen (Populus tremuloides) chemithermomechanical pulp,” Ind. Eng. Chem. Res. 51(34), 11122-11127. DOI: 10.1021/ie300265s

Liu, H., Hu, H., Jahan, M. S., and Ni, Y. (2013b). “Furfural formation from the pre-hydrolysis liquor of a hardwood kraft-based dissolving pulp production process,” Bioresour. Technol. 131, 315-320. DOI: 10.1016/j.biortech.2012.12.158

Liu, H., Hu, H., Baktash, M. M., Jahan, M. S., Ahsan, L., and Ni, Y. (2014). “Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process,” Biom. Bioen. 66, 320-327. DOI: 10.1016/j.biombioe.2014.02.003

Liu, J., Li, M., Luo, X., Chen, L., and Huang, L. (2015). “Effect of hot-water extraction (HWE) severity on bleached pulp based biorefinery performance of eucalyptus during the HWE-Kraft-ECF bleaching process,” Bioresour. Technol. 181, 183-190. DOI: 10.1016/j.biortech.2015.01.055

Liu, S., Lu, H., Hu, R., Shupe, A., Lin, L., and Liang, B. (2012b). “A sustainable woody biomass biorefinery,” Biotechnol. Adv. 30(4), 785-810. DOI: 10.1016/j.biotechadv.2012.01.013

Liu, Z., Ni, Y., Fatehi, P., and Saeed, A. (2011b). “Isolation and cationization of hemicelluloses from pre-hydrolysis liquor of kraft-based dissolving pulp production process,” Biom. Bioen. 35(5), 1789-1796. DOI: 10.1016/j.biombioe.2011.01.008

Liu, W., Yuan, Z., Mao, C., Hou, Q., and Li, K. (2012c). “Extracting hemicelluloses prior to aspen chemi-thermomechanical pulping: Effects of pre-extraction on pulp properties,” Carbohydr. Polym. 87(1), 322-327. DOI: 10.1016/j.carbpol.2011.07.050

López, F., García, M. T., Feria, M. J., García, J. C., de Diego, C. M., Zamudio, M. A. M., and Díaz, M. J. (2014). “Optimization of furfural production by acid hydrolysis of Eucalyptus globulus in two stages,” Chem. Eng. J. 240, 195-201. DOI: 10.1016/j.cej.2013.11.073

Lu, F.-J., Chu, L.-H., and Gau, R.-J. (1998). “Free radical-scavenging properties of lignin,” Nutrit. Canc. 30(1), 31-38. DOI: 10.1080/01635589809514637

Lu, H., Hu, R., Ward, A., Amidon, T. E., Liang, B., and Liu, S. (2012). “Hot-water extraction and its effects on soda pulping of aspen woodchips,” Biom. Bioen. 39, 5-13. DOI: 10.1016/j.biombioe.2011.01.054

Lundquist, K., Simonson, R., and Tingsvik, K. (1981). “Investigation of lignins from soda and soda/anthraquinone cooking of spruce wood,” Paperi Puu 63(11)709-712.

Lundqvist, J., Jacobs, A., Palm, M., Zacchi, G., Dahlman, O., and Stålbrand, H. (2003). “Characterization of galactoglucomannan extracted from spruce (Picea abies) by heat-fractionation at different conditions,” Carbohydr. Polym. 51(2), 203-211. DOI: 10.1016/S0144-8617(02)00111-X

Lundqvist, J., Teleman, A., Junel, L., Zacchi, G., Dahlman, O., Tjerneld, F., and Stålbrand, H. (2002). “Isolation and characterization of galactoglucomannan from spruce (Picea abies),” Carbohydr. Polym. 48(1), 29-39. DOI: 10.1016/S0144-8617(01)00210-7

Luo, C., Brink, D. L., and Blanch, H. W. (2002). “Identification of potential fermentation inhibitors in conversion of hybrid poplar hydrolysate to ethanol,” Biom. Bioen. 22(2), 125-138. DOI: 10.1016/S0961-9534(01)00061-7

Luo, J., Genco, J. M., and Zou, H. (2012). “Extraction of hardwood biomass using dilute alkali,” TAPPI J. 11(6), 19-27.

Mänttäri, M., Nuortila-Jokinen, J., and Nyström, M. (1997). “Evaluation of nanofiltration membranes for filtration of paper mill total effluents,” Filtr. Separ. 34(3) 275-280. DOI: 10.1016/S0015-1882(97)84794-5

Mänttäri, M., Pihlajamäki, A., and Nyström, M. (2002). “Comparison of nanofiltration and tight ultrafiltration membranes in the filtration of paper mill process water,” Desalin. 149(1-3) 131-136. DOI: 10.1016/S0011-9164(02)00744-0

Mänttäri, M., Puro, L., Nuortila-Jokinen, J., and Nyström, M. (2000). “Fouling effects of polysaccharides and humic acid in nanofiltration,” J. Membr. Sci. 165(1), 1-17. DOI: 10.1016/S0376-7388(99)00215-X

Maleki, L., Edlund, U., and Albertsson, A.-C. (2014). “Unrefined wood hydrolysates are viable reactants for the reproducible synthesis of highly swellable hydrogels,” Carbohydr. Polym. 108, 281-290. DOI: 10.1016/j.carbpol.2014.02.060

Mansoornejad, B., Pistikopoulos, E. N., and Stuart, P. (2013). “Metrics for evaluating the forest biorefinery supply chain performance,” Comp. Chem. Eng. 57, 125-139. DOI: 10.1016/j.compchemeng.2013.03.031

Mao, J. Z., Zhang, L. M., and Xu, F. (2012). “Fractional and structural characterization of alkaline lignins from Carex meyerianaKunth,” Cellul. Chem. Technol. 46(3-4), 193-205.

Margeot, A., Hahn-Hägerdal, B., Edlund, M., Slade, R., and Monot, F. (2009). “New improvements for lignocellulosic ethanol,” Curr. Op. Biotechnol. 20(3), 372-380. DOI: 10.1016/j.copbio.2009.05.009

Marinova, M., Mateos-Espejel, E., Jemaa, N., and Paris, J. (2009). “Addressing the increased energy demand of a kraft mill biorefinery: The hemicellulose extraction case,” Chem. Eng. Res. Des. 87(9), 1269-1275. DOI: 10.1016/j.cherd.2009.04.017

Marinova, M., Perrier, M., and Paris, J. (2014). “A sustainable strategy for the forest biorefinery,” Conf. Proc., NWBC 2014, Nordic Wood and Biorefinery Conference, Stockholm, Sweden, 25.-27.3.2014, pp. 2-8.

Martin-Sampedro, R., Eugenio, M. E., Moreno, J. A., Revilla, E., and Villar, J. C. (2014). “Integration of a kraft pulping mill into a forest biorefinery: Pre-extraction of hemicellulose by steam explosion versus steam treatment,” Bioresour. Technol. 153, 236-244. DOI: 10.1016/j.biortech.2013.11.088

Martínez, J. M., Reguant, J., Salvadó, J., and Farriol, X. (1997). “Soda-anthraquinone pulping of a softwood mixture: Applying a pseudo-kinetic severity parameter,” Bioresour. Technol. 60(2), 161-167. DOI: 10.1016/s0960-8524(97)00010-2

Mateos-Espejel, E., Radiotis, T., and Jemaa, N. (2013). “Implications of converting a kraft pulp mill to a dissolving pulp operation with a hemicellulose extraction stage,” TAPPI J. 12(2), 29-38.

Marzialetti, T., Olarte, M. B. V., Sievers, C., Hoskins, T. J. C., Agrawal, P. K., and Jones, C. W. (2008). “Dilute acid hydrolysis of loblolly pine: A comprehensive approach,” Ind. Eng. Chem. Res.47(19), 7131-7140. DOI: 10.1021/ie800455f

McDonough, T. J. (1996). “Oxygen delignification,” in: Pulp Bleaching – Principles and Practice, Dence, C. W., and Reeve, D. W. (eds.), TAPPI Press, Atlanta, GA, USA, pp. 213-239.

McIntosh, S., and Vancov, T. (2010). “Enhanced enzyme saccharification of Sorghum bicolor straw using dilute alkali pretreatment,” Bioresour. Technol. 101(17), 6718-6727. DOI: 10.1016/j.biortech.2010.03.116

Melzoch, K., Votruba, J., Hábová, V., and Rychtera, M. (1997). “Lactic acid production in a cell retention continuous culture using lignocellulosic hydrolysate as a substrate,” J. Biotechn. 56(1), 25-31. DOI: 10.1016/S0168-1656(97)00074-6

Mendes, C. V. T., Carvalho, M. G. V. S., Baptista, C. M. S. G., Rocha, J. M. S., Soares, B. I. G., and Sousa, G. D. A. (2009). “Valorisation of hardwood hemicelluloses in the kraft pulping process by using an integrated biorefinery concept,” Food Bioprod. Proc. 87(3), 197-207. DOI: 10.1016/j.fbp.2009.06.004

Meng, X., Geng, W., Ren, H., Jin, Y., Chang, H.-M., and Jameel, H. (2014). “Enhancement of enzymatic saccharification of poplar by green liquor pretreatment,” BioResources 9(2), 3236-3247. DOI: 10.15376/biores.9.2.3236-3247

Menon, V., and Rao, M. (2012). “Trends in bioconversion of lignocellulose: Biofuels, platform chemicals & biorefinery concept,” Proc. En. Comb. Sci. 38(4), 522-550. DOI: 10.1016/j.pecs.2012.02.002

Mikkonen, K. S., Heikkilä, M. I., Helén, H., Hyvönen, L., and Tenkanen, M. (2010). “Spruce galactoglucomannan films show promising barrier properties,” Carbohydr. Polym. 79(4), 1107-1112. DOI: 10.1016/j.carbpol.2009.10.049

Mikkonen, K. S., and Tenkanen, M. (2012). “Sustainable food-packaging materials based on future biorefinery products: Xylans and mannans,” Trends Food Sci. Technol. 28(2), 90-102. DOI: 10.1016/j.tifs.2012.06.012

Minor, J., and Springer, E. L. (1993). “Improved penetration of pulping reagents into wood,” Paperi ja Puu 75(4), 241-246.

Mirahmadi, K., Kabir, M. M., Jeihanipour, A., Karimi, K., and Taherzadeh, M. J. (2010). “Alkaline pretreatment of spruce and birch to improve bioethanol and biogas production,” BioResources 5(2), 928-938. DOI: 10.15376/biores.5.2.928-938

Miyafuji, H., Danner, H., Neureiter, M., Thomasser, C., Bvochora, J., Szolar, O., and Braun, R. (2003). “Detoxification of wood hydrolysates with wood charcoal for increasing the fermentability of hydrolysates,” Enz. Microb. Technol. 32(3-4), 396-400. DOI: 10.1016/S0141-0229(02)00308-3

Mood, S. H., Golfeshan, A. H., Tabatabaei, M., Jouzani, G. S., Najafi, G. H., Gholami, M., and Ardjmand, M. (2013). “Lignocellulosic biomass to bioethanol, a comprehensive review with focus on pretreatment,” Ren. Sust. En. Prod. 27, 77-93. DOI: 10.1016/j.rser.2013.06.033

Mok, W.S.-L., and Antal, M. J. (1992). “Uncatalyzed solvolysis of whole biomass hemicellulose by hot compressed liquid water,” Ind. Eng. Chem. Res. 31(4), 1157-1161. DOI: 10.1021/ie00004a026

Montané, D., Nabarlatz, D., Martorell, A., Torné-Fernández, V., and Fierro, V. (2006). “Removal of lignin and associated impurities from xylo-oligosaccharides by activated carbon adsorption,” Ind. Eng. Chem. Res. 45(7), 2294-2302. DOI: 10.1021/ie051051d

Moshkelani, M., Marinova, M., Perrier, M., and Paris, J. (2013). “The forest biorefinery and its implementation in the pulp and paper industry: Energy overview,” Appl. Therm. Eng. 50(2), 1427-1436. DOI: 10.1016/j.applthermaleng.2011.12.038

Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y. Y., Holtzapple, M., and Ladisch, M. (2005). “Features of promising technologies for pretreatment of lignocellulosic biomass,” Bioresour. Technol. 96(6), 673-686. DOI: 10.1016/j.biortech.2004.06.025

Moure, A., Domínguez, H., and Parajó, J. C. (2005). “Antioxidant activity of liquors from aqueous treatments of Pinus radiata wood,” Wood Sci. Technol. 39(2), 129-139. DOI: 10.1007/s00226-004-0279-8

Moure, A., Gullón, P., Domínguez, H., and Parajó, J. C. (2006). “Advances in the manufacture, purification and applications of xylo-oligosaccharides as food additives and nutraceuticals,” Proc. Biochem. 41(9), 1913-1923. DOI: 10.1016/j.procbio.2006.05.011

Nabarlatz, D., Farriol, X., and Montané, D. (2004). “Kinetic modeling of the autohydrolysis of lignocellulosic biomass for the production of hemicellulose-derived oligosaccharides,” Ind. Eng. Chem. Res. 43(15), 4124-4131. DOI: 10.1021/ie034238i

Nabarlatz, D., Torras, C., Garcia-Valls, R., and Montané, D. (2007). “Purification of xylo-oligosaccharides from almond shells by ultrafiltration,” Separ. Purif. Technol. 53(3), 235-243. DOI: 10.1016/j.seppur.2006.07.006

Niemelä, K. (1988a). “GLC-MS studies on pine kraft black liquors, Part I. Identification of monomeric compounds,” Holzforschung 42(3), 169-173. DOI: 10.1515/hfsg.1988.42.3.169

Niemelä, K. (1988b). “GLC-MS studies on pine kraft black liquors, Part II. Identification of hydroxy acids with a stilbene structure,” Holzforschung 42(3), 175-176. DOI: 10.1515/hfsg.1988.42.3.169

Niemelä, K. (1988c). “The formation of 2-hydroxy-2-cyclopenten-1-ones from polysaccharides during kraft pulping of pine wood,” Carbohydr. Res. 184, 131-137. DOI: 10.1016/0008-6215(88)80011-9

Niemelä, K. (1989). “GLC-MS studies on pine kraft black liquors, Part V. Identification of catechol compounds,” Holzforschung 43(2), 99-103. DOI: 10.1515/hfsg.1989.43.2.99

Niemelä, K. (1990a). “Conversion of xylan, starch, and chitin into carboxylic acids by treatment with alkali,” Carbohydr. Res. 204, 37-49. DOI: 10.1016/0008-6215(90)84019-Q

Niemelä, K. (1990b). “The formation of hydroxy monocarboxylic acids and dicarboxylic acids by alkaline thermochemical degradation of cellulose,” J. Chem. Techn. Biotechnol. 48(1), 17-28. DOI: 10.1002/jctb.280480103

Niemelä, K., Alén, R., and Sjöström, E. (1985). “The formation of carboxylic acids during kraft and kraft-anthraquinone pulping of birch wood,” Holzforschung 39(3), 167-172. DOI: 10.1515/hfsg.1985.39.3.167

Niemelä, K. and Sjöström, E. (1986a). “Alkaline degradation of mannan,” Holzforschung 40(1), 9-14. DOI: 10.1515/hfsg.1986.40.1.9

Niemelä, K., and Sjöström, E. (1986b). “Simultaneous identification of aromatic and aliphatic low molecular weight compounds from alkaline pulping liquor by capillary gas-liquid chromatography-mass spectrometry,” Holzforschung 40(6), 361-368. DOI: 10.1515/hfsg.1986.40.6.361

Niemelä, K., and Sjöström, E. (1988a). “The conversion of cellulose into carboxylic acids by a drastic alkali treatment,” Biomass 15(4), 223-231. DOI: 10.1016/0144-4565(88)90058-3

Niemelä, K., and Sjöström, E. (1988b). “Identification of the products of hydrolysis of carboxymethylcellulose,” Carbohydr. Res. 180(1), 43-52. DOI: 10.1016/0008-6215(88)80062-4

Nilvebrandt, N.-O., Reimann, A., Larsson, S., and Jönsson, L. F. (2001). “Detoxification of lignocelluloses hydrolysates with ion-exchange resins,” Appl. Biochem. Biotechn. 91-93(1-9), 35-49. DOI: 10.1385/ABAB:91-93:1-9:35

Ogata, M., Hoshi, M., Shimotohno, K., Urano, S., and Endo, T. (1997). “Antioxidant activity of magnolol, honokiol, and related phenolic compounds,” J. Amer. Oil Chem. Soc. 74(5), 557-562. DOI: 10.1007/s11746-997-0180-3

Oinonen, P., Areskogh, D., and Henriksson, G. (2013). “Enzyme catalyzed cross-linking of spruce galactoglucomannan improves its applicability in barrier films,” Carbohydr. Polym. 95(2), 690-696. DOI: 10.1016/j.carbpol.2013.03.016

Ojumu, T. V., AttahDaniel, B. E., Betiku, E., and Solomon, B. O. (2003). “Auto-hydrolysis of lignocellulosics under extremely low sulphuric acid and high temperature conditions in batch reactor,” Biotechnol. Bioprocess. Eng. 8(5), 291-293. DOI: 10.1007/BF02949219

Ojumu, T. V., and Ogunkunle, O. A. (2005). “Production of glucose from lignocellulosic under extremely low acid and high temperature in batch process, auto-hydrolysis approach,” J. Applied. Sci. 5(1), 15-17. DOI: 10.3923/jas.2005.15.17

Olsson, L., and Hahn-Hägerdal, B. (1996). “Fermentation of lignocellulosic hydrolysates for ethanol production,” Enz. Microb. Technol. 18(5), 312-331. DOI: 10.1016/0141-0229(95)00157-3

Ou, S., Luo, X., Xue, F., Huang, C., Zhang, N., and Liu, Z. (2007). “Separation and purification of ferulic acid in alkaline-hydrolysate from sugarcane bagasse by activated charcoal adsorption/anion macroporous resin exchange chromatography,” J. Food Eng. 78(4), 1298-1304. DOI: 10.1016/j.jfoodeng.2005.12.037

Pakkanen, H., and Alén, R. (2013). “Alkali consumption of aliphatic carboxylic acids during alkaline pulping of wood and nonwood feedstocks,” Holzforschung 67(6), 643-650.

Pakkanen, H., Paloheimo, T., and Alén, R. (2013). “Characterization of dissolved material during the initial phase of softwood kraft pulping,” TAPPI J. 11(1), 35-41.

Palmqvist, E., and Hahn-Hägerdal, B. (2000a). “Fermentation of lignocellulosic hydrolysates. I: Inhibition and detoxification,” Bioresour. Techn. 74(1), 17-24. DOI: 10.1016/S0960-8524(99)00160-1

Palmqvist, E., and Hahn-Hägerdal, B. (200b). “Fermentation of lignocellulosic hydrolysates. II: Inhibitors and mechanisms of inhibition,” Bioresour. Techn. 74(1), 25-33.

Parajó, J. C., Alonso, J. L., and Santos, V. (1996). “Lactic acid from wood,” Proc. Biochem. 31(3), 271-280. DOI: 10.1016/0032-9592(95)00059-3

Parajó, J. C., Dominguez, H., and Domínguez, J. M. (1997a). “Improved xylitol production with Debaromyces hansenii Y-7426 from raw or detoxified wood hydrolysates,” Enz. Microb. Technol. 21(1), 18-24. DOI: 10.1016/S0141-0229(96)00210-4

Parajó, J. C., Domínguez, H., and Domínguez, J. M. (1997b). “Xylitol production from Eucalyptus wood hydrolysates extracted with organic solvents,” Proc. Biochem. 32(7), 599-604. DOI: 10.1016/S0032-9592(97)00016-2

Parajó, J. C., Domínguez, H., Moure, A., Díaz-Reinoso, B., Conde, E., Soto, M. L., Conde, M. J., and González-López, N. (2008). “Recovery of phenolic antioxidants released during hydrolytic treatments of agricultural and forest residues,” Electr. J. Environ. Agric. Food Chem. 8(7), 3243-3249.

Parajó, J. C., Garrote, G., Cruz, J. M., and Dominguez, H. (2004). “Production of xylooligosaccharides by autohydrolysis of lignocellulosic materials,” Trends Food Sci. Technol. 15(3-4), 115-120. DOI: 10.1016/j.tifs.2003.09.009

Parajó, J. C., Vázquez, D., Alonso, J. L., Santos, V., and Domínguez, H. (1993). “Prehydrolysis of Eucalyptus wood with dilute sulphuric acid: Operation at atmospheric pressure,” Holz Roh Werkst. 51(5), 357-363. DOI: 10.1007/BF02663809

Parajó, J. C., Vázquez, D., Alonso, J. L., Santos, V., and Domínguez, H. (1994). “Prehydrolysis of Eucalyptus wood with dilute sulphuric acid: Operation in autoclave,” Holz Roh Werkst. 52(2), 102-108. DOI: 10.1007/BF02615474

Paredes, J. J., Jara, R., Shaler, S. M., and van Heiningen, A. (2008). “Influence of hot water extraction on the physical and mechanical behavior of OSB,” For. Prod. J. 58(12), 56-62.

Park, Y. C., and Kim, J. S. (2012). “Comparison of various alkaline pretreatment methods of lignocellulosic biomass,” Energy 47(1), 31-35. DOI: 10.1016/j.energy.2012.08.010

Pedersen, M., and Meyer, A. S. (2010). “Lignocellulose pretreatment severity – Relating pH to biomatrix opening,” New Biotechnol. 27(6), 739-750. DOI: 10.1016/j.nbt.2010.05.003

Peng, F., Peng, P., Xu, F., and Sun, R.-C. (2012). “Fractional purification and bioconversion of hemicelluloses,” Biotechnol. Adv.30(4), 879-903. DOI: 10.1016/j.biotechadv.2012.01.018

Peng, F., Ren, J.-L., Xu, F., Bian, J., Peng, P., and Sun, R.-C. (2009). “Comparative study of hemicelluloses obtained by graded ethanol precipitation from sugarcane bagasse,” J. Agric. Food Chem. 57(14), 6305-6317. DOI: 10.1021/jf900986b

Peng, X.-W., Ren, J.-L., Zhong, L.-X., Peng, F., and Sun, R.-C. (2011a). “Xylan-rich hemicelluloses-graft-acrylic acid ionic hydrogels with rapid responses to pH, salt, and organic solvents,” J. Agric. Food Chem. 59(15), 8208-8215. DOI: 10.1021/jf201589y

Peng, X.-W., Ren, J.-L., Zhong, L.-X., and Sun, R.-C. (2011b). “Nanocomposite films based on xylan-rich hemicelluloses and cellulose nanofibers with enhanced mechanical properties,” Biomacromol. 12(9), 3321-3329. DOI: 10.1021/bm2008795

Penttilä, P. A., Kilpeläinen, P., Tolonen, L., Suuronen, J.-P., Sixta, H., Willför, S., and Serimaa, R. (2013). “Effects of pressurized hot water extraction on the nanoscale structure of birch sawdust,” Cellulose20(5), 2335-2347. DOI: 10.1007/s10570-013-0001-9

Persson, P., Andersson, J., Gorton, L., Larsson, S., Nilvebrandt, N.-O., and Jönsson, L. J. (2002). “Effect of different forms of alkali treatment on specific fermentation inhibitors and on the fermentability of lignocellulose hydrolysates for production of fuel ethanol,” J. Agric. Food Chem. 50(19), 5318-5325. DOI: 10.1021/jf025565o

Persson, T., and Jönsson, A.-S. (2010). “Isolation of hemicelluloses by ultrafiltration of thermomechanical pulp mill process water – Influence of operating conditions,” Chem. Eng. Res. Des. 88(12), 1548-1554. DOI: 10.1016/j.cherd.2010.04.002

Pienkos, P. T., and Zhang, M. (2009). “Role of pretreatment and conditioning processes on toxicity of lignocellulosic biomass hydrolysates,” Cellulose 16(4), 743-762. DOI: 10.1007/s10570-009-9309-x

Pizzichini, M., Russo, C., and Di Meo, C. (2005). “Purification of pulp and paper wastewater, with membrane technology, for water reuse in a closed loop,” Desalination 178(1-3), 351-359. DOI: 10.1016/j.desal.2004.11.045

Pohjanlehto, H., Setälä, H., Kammiovirta, K., and Harlin, A. (2011). “The use of N,N’-diallylaldardiamines as cross-linkers in xylan derivatives-based hydrogels,” Carbohydr. Res. 346(17), 2736-2745.

Ponomarenko, J., Dizhbite, T., Lauberts, M., Viksna, A., Dobele, G., Bikovens, O., and Telysheva, G. (2014). “Characterization of softwood and hardwood LignoBoost kraft lignins with emphasis on their antioxidant activity,” BioResources 9(2), 2051-2068. DOI: 10.15376/biores.9.2.2051-2068

Prinsen, P., Rencoret, J., Gutierrez, A., Liitiä, T., Tamminen, T., Colodette, J. L., Alvaro Berbis, M., Jimenez-Barbero, J., Martinez, A. T., and del Río, J. C. (2013). “Modification of the lignin structure during alkaline delignification of Eucalyptus wood by kraft, soda-AQ, and soda-O₂ cooking,” Ind. Eng. Chem. Res. 52(45), 15702-15712. DOI: 10.1021/ie401364d

Pu, Y., Treasure, T., Gonzalez, R., Venditti, R., and Jameel, H. (2011). “Autohydrolysis pretreatment of mixed hardwoods to extract value prior to combustion,” BioResources 6(4), 4856-4870.

Puro, L., Kallioinen, M., Mänttäri, M., and Nyström, M. (2011). “Evaluation of behavior and fouling potential of wood extractives in ultrafiltration of pulp and paper mill process water,” J. Membr. Sci.368(1-2), 150-158. DOI: 10.1016/j.memsci.2010.11.032

Qi, J., and Xiuyang, L. Ü. (2007). “Kinetics of non-catalyzed decomposition of D-xylose in high temperature liquid water,” Chin. J. Chem. Eng. 15(5), 666-669. DOI: 10.1016/S1004-9541(07)60143-8

Radiotis, T., Zhang, X., Paice, M., and Byrne, V. (2011). “Optimizing production of xylose and xylooligomers from wood chips,” Conf. Proc., NWBC 2011, Nordic Wood and Biorefinery Conference, Stockholm, Sweden, 22.-24.3.2014, pp. 92-99.

Ragauskas, A. J., Nagy, M., and Kim, D. H. (2006). “From wood to fuels – Integrating biofuels and pulp production,” Ind. Biotechnol. 2(1), 55-65. DOI: 10.1089/ind.2006.2.55

Ramos, L., Kristenson, E. M., and Brinkman, U. A. T. (2002). “Current use of pressurized liquid extraction and subcritical water extraction in environmental analysis,” J. Chromatogr. A. 975(1), 3-29. DOI: 10.1016/S0021-9673(02)01336-5

Reguant, J., Martínez, J. M., Montané, D., Salvadó, J., and Farriol, X. (1997). “Cellulose from softwood via prehydrolysis and soda/anthraquinone pulping,” J. Wood. Chem. Technol. 17(1-2), 91-110. DOI: 10.1080/02773819708003120

Ren J., Peng, X., Zhong, L., Peng, F., and Sun, R. (2012). “Novel hydrophobic hemicelluloses: Synthesis and characteristic,” Carbohydr. Polym. 89(1), 152-157. DOI: 10.1016/j.carbpol.2012.02.064

Resalati, H., Kermanian, H., Fadavi, F., and Feizmand, M. (2012). “Effect of hot-water and mild alkaline extraction on soda-AQ pulping of wheat straw,” BioResources 1(1), 71-80.

Richter, B. E., Jones, B. A., Ezzell, J. L., and Porter, N. L. (1996). “Accelerated solvent extraction: A technique for sample preparation,” Anal. Chem. 68(6), 1033-1039. DOI: 10.1021/ac9508199

Rocha, M.-V. P., Rodrigues, T. H. S., de Albuquerque, T. L., Gonçalves, L. R. B., and de Macedo, G. R. (2014). “Evaluation of dilute acid pretreatment on cashew apple bagasse for ethanol and xylitol production,” Chem. Eng. J. 243, 234-243. DOI: 10.1016/j.cej.2013.12.099

Rodríguez-López, J., Romaní, A., González-Muñoz, M. J., Garrote, G., and Parajó, J. C. (2012). “Extracting value-added products before pulping: Hemicellulosic ethanol from Eucalyptus globulus wood,” Holzforschung 66(5), 591-599. DOI: 10.1515/hf-2011-0204

Romero, I., Ruiz, E., Castro, E., and Moya, M. (2010). “Acid hydrolysis of olive tree biomass,” Chem. Eng. Res. Des. 88(5-6), 633-640. DOI: 10.1016/j.cherd.2009.10.007

Rovio, S., Kuitunen, S., Ohra-aho, T., Alakurtti, S., Kalliola, A., and Tamminen, T. (2011). “Lignin oxidation mechanisms under oxygen delignification conditions. Part 2. Advanced methods for the detailed characterization of lignin oxidation mechanisms,” Holzforschung65(4), 575-585. DOI: 10.1515/hf.2011.088

Ruiz, H. A., Cerqueira, M. A., Silva, H. D., Rodríguez-Jasso, R. M., Vicente, A. A., and Teixeira, J. A. (2013). “Biorefinery valorization of autohydrolysis wheat straw hemicellulose to be applied in a polymer-blend film,” Carbohydr. Polym. 92(2), 2154-2162. DOI: 10.1016/j.carbpol.2012.11.054

Runge, T., and Zhang, C. (2013). “Hemicellulose extraction and its effect on pulping and bleaching,” TAPPI J. 12(10), 45-52.

Saeed, A., Jahan, M. S., Li, H., Liu, Z., Ni, Y., and van Heiningen, A. (2012). “Mass balances of components dissolved in the pre-hydrolysis liquor of kraft-based dissolving pulp production process from Canadian hardwoods,” Biom. Bioen. 39, 14-19. DOI: 10.1016/j.biombioe.2010.08.039

Sainio, T., Kallioinen, M., Nakari, O., and Mänttäri, M. (2013). “Production and recovery of monosaccharides from lignocellulose hot water extracts in a pulp mill biorefinery,” Bioresour. Technol. 135, 730-737. DOI: 10.1016/j.biortech.2012.08.126

Sakakibara, A., and Sano, Y. (2001). “Chemistry of lignin,” in: Wood and Cellulosic Chemistry, Hon, D. and Shiraishi, N. (eds.), 2^ndedition, Marcel Dekker, Inc., New York, NY, USA, pp. 109-173.

Salmela, M., Alén, R., and Vu, M. T. H. (2008). “Description of kraft cooking and oxygen-alkali delignification of bamboo by pulp and dissolving material analysis,” Ind. Crops Prod. 28(1), 47-55. DOI: 10.1016/j.indcrop.2008.01.003

Sánchez, Ó. J., and Cardona, C. A. (2008). “Trends in biotechnological production of fuel ethanol from different feedstocks,” Bioresour. Technol. 99(13), 5270-5295. DOI: 10.1016/j.biortech.2007.11.013

Sanglard, M., Chirat, C., Jarman, B., and Lachenal, D. (2013). “Biorefinery in a pulp mill: simultaneous production of cellulosic fibers from Eucalyptus globulus by soda-anthraquinone cooking and surface-active agents,” Holzforschung 67(5), 481-488. DOI: 10.1515/hf-2012-0178

Saukkonen, E., Kautto, J., Rauvanto, I., and Backfolck, K. (2012a). “Characteristics of prehydrolysis-kraft pulp fibers from Scots pine,” Holzforschung 66(7), 801-808. DOI: 10.1515/hf-2011-0158

Saukkonen, E., Lyytikäinen, K., and Backfolk, K. (2012b). “Alkaline xylan extraction of bleached kraft pulp – Effect of extraction time on pulp chemical composition and physical properties,” TAPPI J. 11(4), 37-43.

Saxena, A., Elder, T. J., and Ragauskas, A. J. (2011). “Moisture barrier properties of xylan composite films,” Carbohydr. Polym.84(4), 1371-1377. DOI: 10.1016/j.carbpol.2011.01.039

Schild, G., Sixta, H., and Testova, L. (2010). “Multifunctional alkaline pulping, delignification and hemicellulose extraction,” Cellul. Chem. Technol. 44(1-3), 35-45.

Schwartz, T. J., and Lawoko, M. (2010). “Removal of acid-soluble lignin from biomass extracts using Amberlite XAD-4 resin,” BioResources 5(4), 2337-2347.

Sears, K. D., Beélik, A., Casebier, R. L., Engen, R. J., Hamilton, J. K., and Hergert, H. L. (1971). “Southern pine prehydrolyzates: Characterization of polysaccharides and lignin fragments,” J. Polym. Sci.: Part C 36(1), 425-443.

Shin, S.-J., Schroeder, L. R., and Lai, Y.-Z. (2006). “Understanding factors contributing to low oxygen delignification of hardwood kraft pulps,” J. Wood. Chem. Technol. 26(1), 5-20. DOI: 10.1080/02773810600582087

Shupe, A. M., and Liu, S. (2012). “Ethanol fermentation from hydrolysed hot-water wood extracts by pentose fermenting yeasts,” Biom. Bioenergy 39, 31-38. DOI: 10.1016/j.biombioe.2011.08.010

Si, S., Chen, Y., Fan, C., Hu, H., Li, Y., Huang, J., Liao, H., Hao, B., Li, Q., Peng, L., and Tu, Y. (2015). “Lignin extraction distinctively enhances biomass enzymatic saccharification in hemicellulose-rich Miscanthus species under various alkali and acid pretreatments,” Bioresour. Technol. 183, 248-254. DOI: 10.1016/j.biortech.2015.02.031

Silva-Fernandes, T., Duarte, L. C., Carvalheiro, F., Loureiro-Dias, M. C., Fonseca, C., and Gírio, F. (2015). “Hydrothermal pretreatment of several lignocellulosic mixtures containing wheat straw and two hardwood residues available in Southern Europe,” Bioresour. Technol. 183, 213-220. DOI: 10.1016/j.biortech.2015.01.059

Sim, K., Youn, H. J., Cho, H., Shin, H., and Lee, H. L. (2012). “Improvements in pulp properties by alkali pre-extraction and subsequent kraft pulping with controlling H-factor and alkali charge,” BioResources 7(4), 5864-5878. DOI: 10.15376/biores.7.4.5864-5878

Šimkovic, I., Gedeon, O., Uhliariková, I., Mendichi, R., and Kirschnerová, S. (2011). “Xylan sulphate films,” Carbohydr. Polym. 86(1), 214-218. DOI: 10.1016/j.carbpol.2011.04.034

Singh, R., Shukla, A., Tiwari, S., and Srivastava, M. (2014). “A review on delignification of lignocellulosic biomass for enhancement of ethanol production potential,” Renew. Sustain. Energy Rev. 32, 713-728. DOI: 10.1016/j.rser.2014.01.051

Sinsky, A. J. (1983). “Organic chemicals from biomass: An overview,” in: Organic Chemicals from Biomass, Wise, D. L. (ed.), Benjamin/Cummins Publishing Company, London, UK, pp. 1-67.

Sixta, H., Borrega, M., Testova, L., Costabel, L., Alekhina, M., and Guetsch, J. (2011). “Progress and challenges in the separation and purification of xylan from hardwood,” Conf. Proc., NWBC 2011, Nordic Wood and Biorefinery Conference, Stockholm, Sweden, 22.-24.3.2011, pp. 76-84.

Sixta, H., Potthast, A., and Krotschek, A. W. (2006). “Chemical pulping processes,” in: Handbook of Pulp, Sixta, H. (ed.), Wiley-VCH Verlag GmbH, Weinheim, Germany, pp. 325-366. DOI: 10.1002/9783527619887

Sixta, H., and Schild, G. (2009). “A new generation kraft process,” Lenzing. Ber. 87, 26-37.

Sjöman, E. Mänttäri, M., Nyström, M., Koivikko, H., and Heikkilä, H. (2006). ”Nanofiltration of monosaccharide containing solution to recover xylose,” Desalin. 199(1), 348-349. DOI: 10.1016/j.desal.2006.03.082

Sjöman, E., Mänttäri, M., Nyström, M., Koivikko, H., and Heikkilä, H. (2007). “Separation of xylose from glucose by nanofiltration from concentrated monosaccharide solutions,” J. Membr. Sci. 292(1-2), 106-115. DOI: 10.1016/j.memsci.2007.01.019

Sjöman, E., Mänttäri, M., Nyström, M., Koivikko, H., and Heikkilä, H. (2008). “Xylose recovery from different hemicelluloses hydrolyzate feeds,” J. Membr. Sci. 310(1-2), 268-277. DOI: 10.1016/j.memsci.2007.11.001

Sjöström, E. (1980). “Behavior of pulp polysaccharides during oxygen-alkali delignification,” in: Chemistry of Delignification with Oxygen, Ozone, and Peroxides, Gratzl, J. S., Nakano, J., and Singh, R. P. (eds.), UNI Publishers CO., LTD., Tokyo, Japan, pp. 61-77.

Sjöström, E. (1991). “Carbohydrate degradation products from alkaline treatment of biomass,” Biom. Bioenergy 1(1), 61-64. DOI: 10.1016/0961-9534(91)90053-F

Sjöström, E. (1993). Wood Chemistry – Fundamentals and Applications, 2^nd edition, Academic Press, San Diego, CA, USA.

Smith, R. M. (2002). “Extractions with superheated water,” J. Chromatogr. A. 975(1), 31-46. DOI: 10.1016/S0021-9673(02)01225-6

Söderström, J., Pilcher, L., Galbe, M., and Zacchi, G. (2003). “Two-step steam pretreatment of softwood by dilute H₂SO₄ impregnation for ethanol production,” Biom. Bioen. 24(6), 475-486. DOI: 10.1016/S0961-9534(02)00148-4

Söderqvist-Lindblad, M., Albertsson, A.-C., Ranucci, E., Laus, M., and Giani, E. (2005). “Biodegradable polymers from renewable sources: rheological characterization of hemicellulose-based hydrogels,” Biomacromol. 6(2), 684-690. DOI: 10.1021/bm049515z

Song, T., Pranovich, A., and Holmbom, B. (2011). “Characterisation of Norway spruce hemicelluloses extracted by pressurised hot-water extraction (ASE) in the presence of sodium bicarbonate,” Holzforschung 65(1), 35-42. DOI: 10.1515/hf.2011.015

Song, T., Pranovich, A., Sumerskiy, I., and Holmbom, B. (2008). “Extraction of galactoglucomannan from spruce wood with pressurised hot water,” Holzforschung 62(6), 659-666. DOI: 10.1515/HF.2008.131

Soto, M., Moure, A., Domínguez, H., and Parajó, J.C. (2011). “Recovery, concentration and purification of phenolic compounds by adsorption: A review,” J. Food Eng. 105(1), 1-27. DOI: 10.1016/j.jfoodeng.2011.02.010

Starnes Jr., W.H. (1980). “Mechanisms of autoxidation in neutral or alkaline media,” in: Chemistry of Delignification with Oxygen, Ozone, and Peroxides, Gratzl, J. S., Nakano, J., and Singh, R. P. (eds.), UNI Publishers CO., LTD., Tokyo, Japan, pp. 3-25.

Stevanic, J. S., Joly, C., Mikkonen, K. S., Pirkkalainen, K., Serimaa, R., Rémond, C., Toriz, G., Gatenholm, P., Tenkanen, M., and Salmén, L. (2011). “Bacterial nanocellulose-reinforced arabinoxylan films,” J. Appl. Polym. Sci. 122(2), 1030-1039. DOI: 10.1002/app.34217

Stevens, C.V. (2004). “Industrial products from carbohydrates, wood and fibres,” in: Renewable Bioresources – Scope and Modification for Non-food Applications, Stevens, C. V., and Verhé, R. (eds.), John Wiley & Sons Ltd, Chichester, West Sussex PO19 8SQ, England, pp. 160-188.

Stoklosa, R. J., and Hodge, D. B. (2012). “Extraction, recovery, and characterization of hardwood and grass hemicelluloses for integration into biorefining processes,” Ind. Eng. Chem. Res. 51(34), 11045-11053. DOI: 10.1021/ie301260w

Sun, Y., and Cheng, J. (2002). “Hydrolysis of lignocellulosic materials for ethanol production: A review,” Bioresour. Techn. 83(1), 1-12. DOI: 10.1016/s0960-8524(01)00212-7

Sun, Z., and Liu, S. (2012). “Production of n-butanol from concentrated sugar maple hemicellulosic hydrolysate by Clostridia acetobutylicum ATCC824,” Biomater. Bioenergy 39, 39-47. DOI: 10.1016/j.biombioe.2010.07.026

Sun, X.-F., Wang, H.-H., Jing, Z.-X., and Mohanathas, R. (2013). “Hemicellulose-based pH-sensitive and biodegradable hydrogel for controlled drug delivery,” Carbohydr. Polym. 92(2), 1357-1366. DOI: 10.1016/j.carbpol.2012.10.032

Susilo, J., and Bennington, C. P. J. 2007). “Modelling kappa number and pulp viscosity in industrial oxygen delignification systems,” Chem. Eng. Res. Des. 85(6), 872-881. DOI: 10.1205/cherd06167

Taherzadeh, M. J., Niklasson, C., and Lidén, G. (1999). “Conversion of dilute-acid hydrolysates of spruce and birch to ethanol by fed-batch fermentation,” Bioresour. Techn. 69(1), 59-66. DOI: 10.1016/S0960-8524(98)00169-2

Talebnia, F., Karakashev, D., and Angelidaki, I. (2010). “Production of bioethanol from wheat straw: An overview on pretreatment, hydrolysis and fermentation,” Bioresour. Techn. 101(13), 4744-4753. DOI: 10.1016/j.biortech.2009.11.080

Tang, Y., An, M., Liu, K., Nagai, S., Shigematsu, T., Morimura, S., and Kida, K. (2006). “Ethanol production from acid hydrolysate of wood biomass using the flocculating yeast Saccharomyces cerevisiaestrain KF-7,” Proc. Biochem. 41(4), 909-914. DOI: 10.1016/j.procbio.2005.09.008

Teo, C. C., Tan, S. N., Yong, J. W. H, Hew, C. S., and Ong, E. S (2010). “Pressurized hot water extraction (PHWE),” J. Chromatogr. A. 1217(16), 2484-2494. DOI: 10.1016/j.chroma.2009.12.050

Testova, L., Chong, S.-L., Tenkanen, M., and Sixta, H. (2011). “Autohydrolysis of birch wood,” Holzforschung 65(4), 535-542. DOI: 10.1515/hf.2011.073

Testova, L., Vilonen, K., Pynnönen, H., Tenkanen, M., and Sixta, H. (2009). “Isolation of hemicelluloses from birch wood: Distribution of wood components and preliminary trials in dehydration of hemicelluloses,” Lenzing. Ber. 87, 58-65.

Theander, O. (1980). “Carbohydrate reactions in oxygen-alkali delignification processes,” in: Chemistry of Delignification with Oxygen, Ozone, and Peroxides, Gratzl, J. S., Nakano, J., and Singh, R. P. (eds.), UNI Publishers CO., LTD., Tokyo, Japan, pp. 43-59.

Tian, S., Zhu, J., and Yang, X. (2011). “Evaluation of an adapted inhibitor-tolerant yeast strain for ethanol production from combined hydrolysate of softwood,” Appl. En. 88(5), 1792-1796. DOI: 10.1016/j.apenergy.2010.11.037

Trinh, L. T. P., Kundu, C., Lee, J.-W., and Lee, H.-J. (2014). “An integrated detoxification process with electrodialysis and adsorption from the hemicellulose hydrolysates of yellow poplar,” Bioresour. Technol. 161, 280-287. DOI: 10.1016/j.biortech.2014.03.042

Tunc, M. S., Chedda, J., van der Heide, E., Morris, J., and van Heiningen, A. (2014). “Pretreatment of hardwood chips viaautohydrolysis supported by acetic and formic acid,” Holzforschung68(4), 401-409. DOI: 10.1515/hf-2013-0102

Tunc, M. S., and van Heiningen, A. R. P. (2008a). “Hydrothermal dissolution of mixed southern hardwoods,” Holzforschung 62(5), 539-545. DOI: 10.1515/HF.2008.100

Tunc, M. S. and van Heiningen, A. R. P. (2008b). “Hemicellulose extraction of mixed southern hardwood with water at 150 °C: Effect of time,” Ind. Eng. Chem. Res. 47(18), 7031-7037. DOI: 10.1021/ie8007105

Tunc, M. S., and van Heiningen, A. R. P. (2009). “Autohydrolysis of mixed southern hardwoods: Effect of P-factor,” Nord. Pulp. Pap. Res. J. 24(1), 46-51. DOI: 10.3183/NPPRJ-2009-24-01-p046-051

Tunc, M. S., and van Heiningen, A. R. P. (2011). “Characterization and molecular weight distribution of carbohydrates isolated from the autohydrolysis extract of mixed southern hardwoods,” Carbohydr. Polym. 83(1), 8-13. DOI: 10.1016/j.carbpol.2010.07.015

Uçar, G. (1990). “Pretreatment of poplar by acid and alkali for enzymatic hydrolysis,” Wood Sci. Technol. 24(2), 171-180. DOI: 10.1007/BF00229052

Ugartondo, V., Mitjans, M., and Vinardell, M. P. (2008). “Comparative antioxidant and cytotoxic effects of lignins from different sources,” Bioresour. Technol. 99(14), 6683-6687. DOI: 10.1016/j.biortech.2007.11.038

Vakkilainen, E. (2008). “Chemical recovery,” in: Chemical Pulping, Part 2, Recovery of Chemicals and Energy Tikka, P. (ed.), Paperi ja Puu Oy, Helsinki, Finland, pp. 10-35.

van Heiningen, A. (2006). “Converting a kraft pulp mill into an integrated forest biorefinery,” Pulp Paper Can. 107(6), 38-43.

van Maris, A. J. A., Abbot, D. A., Bellissimi, E., van den Brink, J., Kuyper, M., Luttik, M. A. H., Wisselink, H. W., Scheffers, W. A., van Dijken, J. P., and Pronk, J. T. (2006). “Alcoholic fermentation of carbon sources in biomass hydrolysates by Saccharomyces cerevisiae: Current status,” Anton. Leeuwenh. 90(4), 391-418. DOI: 10.1007/s10482-006-9085-7

Vázquez, M. J., Alonso, J. L., Domínguez, H., and Parajó, J. C. (2000). “Xylooligosaccharides: Manufacture and applications,” Trends Food Sci. Technol. 11(11), 387-393. DOI: 10.1016/S0924-2244(01)00031-0

Vázquez, M. J., Alonso, J. L., Domínguez, H., and Parajó, J. C. (2006). “Enhancing the potential of oligosaccharides from corncob autohydrolysis as prebiotic food ingredients,” Ind. Crops Prod. 24(2), 152-159. DOI: 10.1016/j.indcrop.2006.03.012

Vázquez, M. J., Garrote, G., Alonso, J. L., Domínguez, H., and Parajó, J. C. (2005). “Refining of autohydrolysis liquors for manufacturing xylooligosaccharides: evaluation of operational strategies,” Bioresour. Technol. 96(8), 889-896. DOI: 10.1016/j.biortech.2004.08.013

Vegas, R., Alonso, J. L., Domínguez, H., and Parajó, J. C. (2004). “Processing of rice husk autohydrolysis liquors for obtaining food ingredients,” J. Agric. Food Chem. 52(24), 7311-7317. DOI: 10.1021/jf049142t

Vegas, R., Luque, S., Alvarez, J. R., Alonso, J. L., Domínguez, H., and Parajó, J. C. (2006). “Membrane-assisted processing of xylooligosaccharide-containing liquors,” J. Agric. Food Chem. 54(15), 5430-5436. DOI: 10.1021/jf060525w

Vena, P. F., Brienzo, M., García-Aparicio, M. P., Görgens, J. F., and Rypstra, T. (2013a). “Hemicelluloses extraction from giant bamboo (Bambusa balcooa Roxburgh) prior to kraft or soda-AQ pulping and its effect on pulp physical properties,” Holzforschung 67(8), 863-870. DOI: 10.1515/hf-2012-0197

Vena, P. F., Garcia-Aparicio, M. P., Brienzo, M., Görgens, J. F., and Rypstra, T. (2013b). “Effect of alkaline hemicellulose extraction on kraft pulp fibers from Eucalyptus grandis,” J. Wood Chem. Technol.33(3), 157-173. DOI: 10.1080/02773813.2013.773040

Venica, A. D., Chen, C.-L., and Gratzl, J. S. (2008a). “Soda-AQ delignification of poplar wood. Part 1: Reaction mechanism and pulp properties,” Holzforschung 62(6), 627-636. DOI: 10.1515/hf.2008.118

Venica, A. D., Chen, C.-L., and Gratzl, J. S. (2008b). “Soda-AQ delignification of poplar wood. Part 2: Further degradation of initially dissolved lignins,” Holzforschung 62(6), 637-644. DOI: 10.1515/hf.2008.119

Viikari, L., and Alén, R. (2011). ”Biochemical and chemical conversion of forest biomass,” in: Biorefining of Forest Resources, Alén, R. (ed.), Paper Engineers´ Association, Helsinki, Finland, pp. 225-261.

Viikari, L., Vehmaanperä, J., and Koivula, A. (2012). ”Lignocellulosic ethanol: From science to industry,” Biom. Bioen. 46, 13-24. DOI: 10.1016/j.biombioe.2012.05.008

Vila, C., Fransisco, J. L., Santos, V., and Parajó, J. C. (2013). “Effects of hydrothermal processing on the cellulosic fraction of Eucalyptus globulus wood,” Holzforschung 67(1), 33-40. DOI: 10.1515/hf-2012-0046

Vila, C., Romero, J., Fransisco, J. L., Garrote, G., and Parajó, J. C. (2011). “Extracting value from Eucalyptus wood before kraft pulping: Effects of hemicelluloses solubilization on pulp properties,” Bioresour. Technol. 102(8), 5251-5254. DOI: 10.1016/j.biortech.2011.02.002

Villareal, M. L. M., Prata, A. M. R., Felipe, M. G. A., and Almeida e Silva, J. B. (2006). “Detoxification procedures of eucalyptus hemicellulose hydrolysate for xylitol production by Candida guilliermondii,” Enz. Microb. Technol. 40(1), 17-24. DOI: 10.1016/j.enzmictec.2005.10.032

von Schenck, A., Berlin, N., and Uusitalo, J. (2013). “Ethanol from Nordic wood raw material by simplified alkaline soda cooking pre-treatment,” Appl. Energy 102, 229-240. DOI: 10.1016/j.apenergy.2012.10.003

von Weymarn, N. (2011). “The sugar platform in the wood biorefinery,” Conf. Proc., NWBC 2011, Nordic Wood and Biorefinery Conference, Stockholm, Sweden, 22.-24.3.2011, p. 138.

Vroom, K. (1957). “The “H” factor: A means of expressing cooking times and temperatures as a single variable,” Pulp Pap. Can. 58(3), 228-231.

Vuorinen, T., and Alén, R. (1999). “Carbohydrates,” in: Analytical Methods in Wood Chemistry, Pulping and Papermaking, Sjöström, E., and Alén, R. (eds.), Springer, Heidelberg, Germany, pp. 37-75.

Walton, S. L., Hutto, D., Genco, J. M., van Walsum, G. P., and van Heiningen, A. R. P. (2010a). “Pre-extraction of hemicelluloses from hardwood chips using an alkaline wood pulping solution followed by kraft pulping of the extracted wood chips,” Ind. Eng. Chem. Res.49(24), 12638-12645. DOI: 10.1021/ie100848p

Walton, S., van Heiningen, A., and van Walsum, P. (2010b). “Inhibition effects on fermentation of hardwood extracted hemicelluloses by acetic acid and sodium,” Bioresour. Technol.101, 1935-1940. DOI: 10.1016/j.biortech.2009.10.043

Wang, W., Meng, X., Min, D., Song, J., and Jin, Y. (2015). “Effects of green liquor pretreatment on the chemical composition and enzymatic hydrolysis of several lignocellulosic biomasses,” BioResources 10(1), 709-720.

Wang, H., Pang, B., Wu, K., Kong, F., Li, B., and Mu, X. (2014). “Two stages of treatments for upgrading bleached softwood paper grade pulp to dissolving pulp for viscose production,” Biochem. Eng. J. 82, 183-187. DOI: 10.1016/j.bej.2013.11.019

Wang, K., Yang, H., Yao, X., Xu, F., and Sun, R.-C. (2012). “Structural transformation of hemicelluloses and lignin from triploid poplar during acid-pretreatment based biorefinery process,” Bioresour. Technol. 116, 99-106. DOI: 10.1016/j.biortech.2012.04.028

Wayman, M., and Chua, M. G. S. (1979). “Characterization of autohydrolysis aspen (P. tremuloides) lignins. Part 4. Residual autohydrolysis lignin,” Can. J. Chem. 57(19), 2612-2616. DOI: 10.1139/v79-422

Wei, L., Shrestha, A., Tu, M., and Adhikari, S. (2011). “Effects of surfactant on biochemical and hydrothermal conversion of softwood hemicellulose to ethanol and furan derivatives,” Process Biochem. 46(9), 1785-1792. DOI: 10.1016/j.procbio.2011.06.001

Wei, W., Wu, S., and Liu, L. (2012). “Enzymatic saccharification of dilute acid pretreated eucalyptus chips for fermentable sugar production,” Bioresour. Technol. 110, 302-307. DOI: 10.1016/j.biortech.2012.01.003

Wiboonsirikul, J., and Adachi, S. (2008). “Extraction of functional substances from agricultural products or by-products by subcritical water treatment,” Food Sci. Technol. Res. 14(4), 319-328. DOI: 10.3136/fstr.14.319

Wickramasinghe, S. R., and Grzenia, D. L. (2008). “Adsorptive membranes and resins for acetic acid removal from biomass hydrolysates,” Desalin. 234(1-3), 144-151. DOI: 10.1016/j.desal.2007.09.080

Willför, S., and Holmbom, B. (2004). “Isolation and characterisation of water soluble polysaccharides from Norway spruce and Scots pine,” Wood Sci. Technol. 38(3), 173-179. DOI: 10.1007/s00226-003-0200-x

Woiciechowski, A. L., Soccol, C. R., Ramos, L. P., and Pandey, A. (1999). “Experimental design to enhance the production of L-(+)-lactic acid from steam-exploded wood hydrolysate using Rhizopus oryzae in a mixed-acid fermentation,” Proc. Biochem. 34(9), 949-955. DOI: 10.1016/S0032-9592(99)00012-6

Wu, S.-F., Wang, H.-M., Jameel, H., and Phillips, R. (2010). “Novel green liquor pretreatment of loblolly pine chips to facilitate enzymatic hydrolysis into fermentable sugars for ethanol production,” J. Wood Chem. Technol. 30(3), 205-218. DOI: 10.1080/02773811003746717

Xiao, L.-P., Shi, Z.-J., Xu, F., and Sun, R.-C. (2012). “Characterization of lignins isolated with alkaline ethanol from the hydrothermal pretreated Tamarix ramosissima,” Bioenergy Res. 6(2), 519-532. DOI: 10.1007/s12155-012-9266-3

Xiao, L.-P., Sun, Z.-J., Shi, Z.-J., Xu, F., and Sun, R.-C. (2011). “Impact of hot compressed water pretreatment on the structural changes of woody biomass for bioethanol production,” BioResources6(2), 1576-1598.

Xu, J., and Liu, S. (2009). “Optimization of ethanol production from hot-water extracts of sugar maple chips,” Renew. Energy 34(11), 2353-2356. DOI: 10.1016/j.renene.2009.03.025

Yang, R., Lucia, L., Ragauskas, A. J., and Jameel, H. (2003). “Oxygen delignification chemistry and its impact on pulp fibres,” J. Wood Chem. Technol. 23(1), 13-29. DOI: 10.1081/WCT-120018613

Yang, B., and Wyman, C. E. (2009). “Dilute acid and autohydrolysis pretreatment,” in: Biofuels – Methods and Protocols, Mielenz, J. R. (ed.), Humana Press Springer, New York, NY, USA, pp. 103-114.

Yang, B., and Wyman, C. E. (2004). “Effect of xylan and lignin removal by batch and flowthrough pretreatment on the enzymatic digestibility of corn stover cellulose,” Biotechn. Bioen. 86(1), 88-95. DOI: 10.1002/bit.20043

Yoon, S.-H., and van Heiningen, A. (2008). “Kraft pulping and papermaking properties of hot-water pre-extracted loblolly pine in an integrated forest products biorefinery,” TAPPI J. 7(7), 22-27.

Yoon, S.-H., and van Heiningen, A. (2010). “Green liquor extraction of hemicelluloses from southern pine in an Integrated Forest Biorefinery,” J. Ind. Eng. Chem. 16(1), 74-80. DOI: 10.1016/j.jiec.2010.01.018

Yoon, S.-H., Macewan, K., and van Heiningen, A. (2008). “Hot-water pre-extraction from Loblolly pine (Pinus taeda) in an integrated forest products biorefinery,” TAPPI J. 7(6), 27-32.

Yoon, S.-H., Tunc, M. S., and van Heiningen, A. (2011). “Near-neutral pre-extraction of hemicelluloses and subsequent kraft pulping of southern mixed hardwoods,” TAPPI J. 10(1), 7-15.

Yu, Y., Lou, X., and Wu, H. (2008). “Some recent advances in hydrolysis of biomass in hot-compressed water and its comparisons with other hydrolysis methods,” Energy & Fuels 22(1), 46-60. DOI: 10.1021/ef700292p

Zautsen, R. R. M., Maugeri-Filho, F., Vaz-Rossell, C. E., Straathof, A. J. J., van der Wielen, L. A. M., and de Bont, J. A. M. (2009). “Liquid-liquid extraction of fermentation inhibiting compounds in lignocellulose hydrolysate,” Biotechnol. Bioeng. 102(5), 1354-1360. DOI: 10.1002/bit.22189

Zeng, W., Cheng, D., Zhang, H., Chen, F., and Zhan, X. (2010). “Dehydration of glucose to levulinic acid over MFI-type zeolite in subcritical water at moderate conditions,” Reac. Kinet. Mech. Cat.100(2), 377-384. DOI: 10.1007/s11144-010-0187-x

Zhang, Z. Y., Jin, B., and Kelly, J. M. (2007). “Production of lactic acid from renewable materials by Rhizopus fungi,” Biochem. Eng. J.35(3), 251-263. DOI: 10.1016/j.bej.2007.01.028

Zhu, J. Y., and Pan, X. J. (2010). “Woody biomass pretreatment for cellulosic ethanol production: Technology and energy consumption evaluation,” Bioresour. Technol. 101(13), 4992-5002. DOI: 10.1016/j.biortech.2009.11.007

Zhu, J. Y., Pan, X., and Zalesny, R. S., Jr. (2010). “Pretreatment of woody biomass for biofuel production: Energy efficiency, technologies, and recalcitrance,” Appl. Microbiol. Biotechnol. 87(3), 847-857. DOI: 10.1007/s00253-010-2654-8

Zumdahl, S. S., and Zumdahl, S. A. (2007). Chemistry, 7^th Edn., Houghton Mifflin Company, Boston, MI, USA.

Article submitted: May 26, 2015; Peer review completed: July 10, 2015; Revised version received: August 11, 2015; Accepted: August 14, 2015; Published: September 9, 2015.

DOI: 10.15376/biores.10.4.Lehto