Pretreatment technologies for lignocellulosic biomass: Research progress, mechanisms, and prospects

Fan, J., Lu, Y., An, N., Zhu, W., Li, M., Gao, M., Wang, X., Wu, C., and Wang, Y. (2025). "Pretreatment technologies for lignocellulosic biomass: Research progress, mechanisms, and prospects," BioResources 20(2), Page numbers to be added.

Abstract

Lignocellulose, which consists of cellulose, hemicellulose, and lignin, has very stable properties. Among them, cellulose makes up 30% to 50% of the content, and hemicellulose makes up 20% to 43%. Cellulose and hemicellulose can be converted into fermentable sugar through saccharification, and then into bioresources through fermentation. Pretreatment methods such as high temperature and high pressure, acid and alkali cooking, enzymatic digestion can effectively decompose the lignocellulose structure, remove lignin, increase the porosity of lignocellulose, specific surface area, etc., increase the efficiency of saccharification, and improve the utilization of lignocellulose. Pretreatment is a key stage in the production process of bioresources. However, the pretreatment process produces by-products known as inhibitors such as acetic acid, furfural, and phenols. These inhibitors tend to inhibit the activity of biological enzymes, impede the saccharification of cellulose and hemicellulose, disrupt the integrity of the cell membrane of the fermenting bacteria, lead to mutation of the fermenting bacteria, and result in a decrease in the yield of the bioresource. This paper reviews recent advances in pretreatment methods, analyzes the reasons for the emergence of inhibitors, and summarizes methods to reduce the effects of inhibitors.

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Pretreatment Technologies for Lignocellulosic Biomass: Research Progress, Mechanisms, and Prospects

Jiamei Fan ,^a Yuan Lu,^b Ning An,^b Wenbin Zhu,^c,dMingxi Li ,^aMing Gao ,^dXiaona Wang,^d Chuanfu Wu,^dand Ying Wang ,^a,b,c*

DOI: 10.15376/biores.20.2.Fan

Keywords: Fermentation; Lignocellulose; Structure; Pretreatment; Inhibitors

Contact information: a: Department of Biological Science, College of Life Sciences, Sichuan Normal University, Chengdu 610101, Sichuan, China; b: Chengdu Environmental Investment Group Co., LTD, Chengdu 610042, Sichuan, China; c: National-Regional Joint Engineering Research Center for Soil Pollution Control and Remediation in South China, Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and Management, Institute of Eco-environmental and Soil Sciences, Guangdong Academy of Sciences, Guangzhou 510650, Guangdong, China; d: School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China;

* Corresponding author: wangyingcqu@gmail.com

Graphical Abstract

INTRODUCTION

With the increasing consumption of fossil energy and the worsening energy crisis, the production of bioresources through biotechnology to replace fossil energy have the potential to alleviate the energy crisis and reduce pollution emissions (Wang et al. 2024). Among the many raw materials used in the production of bioresources, lignocellulose is favored by researchers because of its large global stock, cheap price, easy availability, and renewability (Oliveira et al. 2018).

Lignocellulose is mainly composed of cellulose, hemicellulose, and lignin, of which cellulose and hemicellulose are the main components utilized in biotechnology for the making such products as ethanol. There is great interest in the saccharification of cellulose and hemicellulose into fermentable sugars such as glucose and xylose, and then microorganisms ferment the fermentable sugars into bioresources through metabolic activities (Wang et al. 2015). However, the utilization of hemicellulose and cellulose is hindered by the complex polymer compound lignin, which consists mainly of guaiacyl (G), p-hydroxyphenyl (H), and butyryl (S) units (Ekielski and Mishra 2020). Scholars have found through research that a variety of methods such as crushing and crushing, high temperature and pressure cooking, acid and alkali soaking, and microbial degradation are effective in separating and removing lignin, thus exposing cellulose and hemicellulose, a step known as pretreatment (Ojo and de Smidt 2023). Therefore, the production of bioresources from lignocellulose requires pretreatment, saccharification, and fermentation steps. Currently, pretreatment technologies are divided into four main categories, physical pretreatment, chemical pretreatment, biological pretreatment and combined pretreatment. With the improvement of science and technology, the diversity of radiation pretreatment in physical pretreatment has increased, ultrasound and gamma irradiation are applied to the pretreatment of lignocellulose (Chen et al. 2022, Kucharska et al. 2018), and in the category of chemical pretreatment, the application of deep crystalline solvents and ionic solutions to the pretreatment test of lignocellulose has also increased (Liu et al. 2012; 2021).

After the pretreatment step, the structure of lignocellulose becomes fluffy and porous, the specific surface area is greatly increased, and the previously smooth planes become rough. These changes provide more attachment points for enzymes or cells in the saccharification step, which promotes the yield of fermentable sugars (Wang et al. 2024). Pretreatment can increase the utilization of lignocellulose, but some by-products are produced during pretreatment, such as acetyl shedding of hemicellulose to form acetic acid (AA) in high-temperature pretreatment, formation of aldehydes from monosaccharides in high-temperature acidic environment of inorganic acid pretreatment, and formation of phenolic compounds from lignin in high-temperature alkaline or acidic environments. These substances alter the pH environment, reduce enzyme activity, and disrupt cellular integrity in ways that impede the hydrolysis and fermentation of lignocellulosic biomass and reduce renewable energy production, and are referred to herein as inhibitors. (Klinke et al. 2004; Yee et al. 2018).

This paper summarizes and classifies the existing pretreatment methods from the structure of lignocellulose and discusses the principle of inhibitor generation and the mechanism of inhibitors affecting renewable energy production. Finally, taking the production fermentation of lactic acid as an example, it summarizes the feasible methods to reduce the impact of inhibitors on renewable energy production.

LIGNOCELLULOSE COMPOSITION AND STRUCTURE

Lignocellulosic biomass is the most widespread source of renewable organic compounds on earth. The global production of lignocellulose, including agricultural waste, garden waste, and part of municipal organic solid waste, is over 220 billion tons per year (Zagrodnik et al. 2021). Using lignocellulose as a raw material to produce bioresources can reduce production costs and improve inedible biomass utilization. Lignocellulose is mainly composed of cellulose, hemicellulose, and lignin (Fig. 1).

Fig. 1. Basic structure and composition of lignocellulose and the unique characteristics of the three basic structures

Cellulose

Cellulose is a renewable carbohydrate composed of 10000 to 15000 glucose monomers. The cellulose content in lignocellulose in general lies between 30.0% and 50.0%. For wheat straw it has been reported to be 30.0%, for corncob 45.0%, and for corn straw 37.5%. Two glucose molecules are linked by a β-1,4-glycosidic bond in which one glucose molecule is rotated 180° relative to the other; this arrangement produces the smallest structural unit of cellulose, called cellobiose (Sun et al. 2002). Subsequently, the glucose chains are linked via hydrogen bonds to form a layer, then the layers are linked to each other by hydrogen bonding into a three-dimensional structure (Fig. 1). The ordered crystalline and disordered amorphous regions resulted from the layers being held together by hydrogen bonds and van der Waals forces. Cellulose consists of crystalline and amorphous regions, making it insoluble in water and conventional organic solvents but soluble under highly alkaline or high-temperature conditions (Monte et al. 2017).

Hemicellulose

Hemicellulose is the second component of lignocellulose after cellulose. The hemicellulose content is usually between 20.0% and 43.0%. It consists of a variety of secondary structures, including O-acetyl-4-O-methylglucuronoxylan, arabinoxylan, xyloglucan, and arabinogalactan (Fig. 1). Unlike cellulose, hemicellulose chains have side groups. They also lack crystalline domains. The molecular chains of hemicellulose are shorter than those of cellulose. Certain hemicellulose chains are readily soluble in water or in a variety of solvents, including acids, alkalis, and organic solvents. Hemicellulose can be divided into two categories: water soluble and alkali soluble (Zhou et al. 2021). Notably, the furanose and pyranose sugar units composed of hemicellulose undergo dehydration upon dissolution, resulting in the formation of furfural and 5-hydroxymethyfurfural (HMF); they also shed the acetyl groups of the branched chains, forming AA, which is a common inhibitor during fermentation and affects the hydrolysis efficiency of enzymes and microbial activity (Feng et al. 2022).

Lignin

Lignin is the primary structural support in plant cells and is mainly composed of G, H, and S units, in which the G, H, and S are formed by the dehydrogenation of coniferyl alcohol, coumaric alcohol, and sinapyl alcohol (Fig. 1). G, H, and S are linked by various chemical bonds such as β-β, β-5, β-O-4, β-1, C-C, and 5-5 to create lignin units with a molecular weight exceeding 10,000 (Rajesh et al. 2019). Lignin acts as the “glue” to connected to carbohydrates via phenyl glycoside bonds, benzyl ether bonds, and gamma esters bonds, making lignocellulose a tightly integrated whole that increases the firmness of plant cells, reduces the adsorption of cellulose and hemicellulose on enzymes, and reduces the utilization rate of lignocellulose.

PRETREATMENT TECHNOLOGY

The purpose of pretreatment of lignocellulose generally is to make the cellulose component accessible to the action of cellulase enzymes. In some cases, the goal of pretreatment is to separate cellulose and hemicellulose and remove lignin, thus increasing the accessible surface area and pore structure of hemicellulose and cellulose enzymes, effectively promoting enzymatic hydrolysis, and obtaining more fermentable sugars. Compared with lignocellulose without pretreatment, the recovery rate of cellulose and hemicellulose from lignocellulose after pretreatment can be improved, and the pretreatment process can also effectively remove poorly used lignin (Fig. 2). Pretreatment methods include physical (thermal pretreatment, mechanical pretreatment, irradiation pretreatment), chemical (acid pretreatment, alkali pretreatment, inorganic salt pretreatment, organic solvents pretreatment, deep eutectic solvents (DES) pretreatment, ionic liquids (ILs) pretreatment), biological (fungal pretreatment, bacterial pretreatment, and enzymatic pretreatment), and comprehensive treatment methods (combination of multiple pretreatment methods for better pretreatment results) (Fig. 3).

Fig. 2. Cellulose and hemicellulose recovery and lignin removal of lignocellulose in reported researches by various pretreatment methods.. (a) cellulose and hemicellulose recovery rate of lignocellulose before and after pretreatment (LHW, liquid hot water (Gunes et al. 2022); RT, rolling thread (Deng and Li 2021); DES, deep eutectic solvents (Huang et al. 2020); AS+X (ammonium sulfite+xylanase), ammonium sulﬁte + xylanase (Yu et al. 2020); Alkail (Wang et al. 2017); SA, sulfamic acid (Song et al. 2022);SA (sulfamic acid)/NaCl, sulfamic acid/sodium chloride solutions (Song et al. 2022)); (b) lignin removal rate of lignocellulose before and after pretreatment (LHW, liquid hot water (Gunes et al. 2022); HBAW, high-boiling point alcohol/water (Liu et al. 2017); DES, deep eutectic solvents (Huang et al. 2020); Alkail (Wang et al. 2017); Na₂SO₃, sodium sulfite (Chen et al. 2019); CO₂+AE, CO₂+Alkail explosion (Triwahyuni et al. 2023); Na₃PO₄.12H₂O + ZnCl₂ (Hassan et al. 2020)). The data were cited from the related values in published papers.

Physical Pretreatment

Thermal pretreatment includes liquid hot water (LHW), high-pressure steam, and steam explosions (Hendriks and Zeeman 2009). Heat pretreatment separates hemicellulose as a result of the acetyl groups shed from the molecular chains of hemicellulose, which acidifies the environment and promotes hemicellulose dissolution. With the dissolution of hemicellulose, the lignocellulose structure becomes fluffy and disordered, and the cellulose is exposed to the solution (Fan et al. 2013). The steam explosion equipment consists of a steam generator and a pressurizer, which first creates a high-pressure saturated steam environment and then releases steam. The sudden decrease in pressure results in an explosion that fractures the lignocellulose into uneven fragments. High temperatures and pressures open the aromatic rings of lignin, leading to the separation of lignin from lignocellulose. Additionally, steam explosions disrupt the relationship between crystalline cellulose and disordered regions. Gunes et al. (2022) studied the subsequent effect of pretreatment of Miscanthus × giganteus biomass with LHW, enzymatic hydrolysis was improved, and the concentration of fermentative sugar was increased. The primary objective of the thermal pretreatment is to eliminate hemicellulose and enhance its solubility, which is advantageous for subsequent enzymatic hydrolysis. Nevertheless, the removal of lignin is not efficient, and the thermal pretreatment process results in the production of inhibitors, such as furan and AA (Ko et al. 2015).

Mechanical pretreatment includes ball milling, rolling treat, shearing, and ultrasonic, etc. (Ouajai and Shanks 2006; Hendriks and Zeeman 2009). The lignocellulose can be cut into small pieces via the force of ball milling, shearing, and thread rolling, then increasing the specific surface area of the lignocellulose of enzymatic hydrolysis (Cao et al. 2023). After ball milling, the crystallinity index of pretreated hemp fibres decreased. After thread rolling, the cell wall of pretreated corn stalks was torn. Then the torn fibres could be converted into a fluffy structure, which was conducive to the contact of the enzyme with the cellulose (Ouajai and Shanks 2006). Ultrasonic pretreatment is a more appealing technology that disintegrates long chain organic compounds thanks to the vibration and the high-pressure environment generated inside the ultrasonic bath (Kucharska et al. 2018).

Irradiation pretreatment includes electron-beam irradiation (EBI), microwave irradiation, and gamma irradiation. Irradiation pretreatment transfers energy to the irradiated lignocellulose in the form of electron lines or radioisotopes generated by an electron accelerator. Ionizing radiation promotes ionization and excitation inside lignocellulose, releases orbital electrons, forms free radicals, and realizes the fission of the internal structure of lignocellulose (Guo et al. 2020). EBI pretreatment causes lignocellulose to split and layer rapidly, and the structure becomes coarse and porous (Fei et al. 2019). Microwave radiation pretreatment can be used to heat lignocellulose evenly in a short time, thereby promoting its hydrolysis and avoiding a large loss of cellulose (Ahorsu et al. 2019). Gamma irradiation pretreatment can effectively induce lignocellulosic cell disruption and lysis, thereby increasing the concentration of fermentation sugars in the solution (Chen et al. 2022). Irradiation pretreatment requires attention to the dose of irradiation used, as excessive irradiation can lead to the production of acetic and formic acids, and gamma pretreatment requires attention to the safety of its operation.

Physical pretreatment is environmentally friendly and easy to operate. Since it does not require a large amount of chemical reagents, it reduces chemical pollution. However, mechanical pretreatment, as part of physical pretreatment, has limitations. The equipment costs are high, and energy consumption is substantial. Some methods such as crushing and cutting only change the macroscopic structure of lignocellulose. It’s difficult to break down lignocellulose at the molecular level. Therefore, it often needs to be combined with chemical and biological pretreatment methods. Thermal pretreatment and irradiation pretreatment can generate formic acid, acetic acid, and furan – based organic compounds during processing. These substances can inhibit subsequent enzymatic hydrolysis and fermentation reactions, which influence product yield. The generation of inhibitors is related to the composition of raw materials. Different biomasses vary in the content and structure of cellulose, hemicellulose, and lignin, and thus the types of inhibitors produced also differ. It is necessary to rationally select processing temperatures, time, irradiation doses, etc., according to the composition of raw materials to minimize the generation of inhibitors.

Chemical Pretreatment

Inorganic solvent pretreatment includes acid, alkali, and inorganic salt pretreatments (Ojo and de Smidt 2023). Acids are widely used in pretreatment, including sulfuric, hydrochloric, aminosulfonic, and nitric acids. Because the hemicellulose molecular chain contains many hydroxyl groups, the acidic environment is enriched with hydrogen ions, and the hydrated hydrogen ions protonate the hemicellulose molecular chain glycosidic bond of oxygen atoms, thereby breaking the glycosidic bond in the molecular chain. Acid pretreatment can dissolve hemicellulose in the liquid and catalyze the substitution reaction of the lignin aromatic ring and the dehydration-condensation reaction of the sugar, removing the lignin, thereby exposing the cellulose, promoting enzymatic digestion, and increasing lactic acid production (Kucharska et al. 2018). As one of the classical pretreatment methods, acids are widely used in pretreatment. When sugarcane leaves were pretreated with acid, hemicellulose was completely removed, and the glucose yield was greater than 80% (Martins et al. 2022). With formic acid pretreatment, approximately 90.1% of cellulose and 87.1% of lignin were removed from corncobs (Qiao et al. 2021). Furthermore, the temperature must be controlled when acid is used as pretreatment, and cellulose can be dissolved when the temperature is higher than 160°C, decreasing the yield of fermentative sugar. It is also important to highlight that acids facilitate the condensation reaction of polysaccharides on the aromatic ring of lignin, resulting in the generation of pseudo-lignin, which subsequently inhibits enzymatic hydrolysis reaction (Martins et al. 2022).

Alkaline pretreatment includes the use of sodium hydroxide, calcium hydroxide, and ammonia (Guo et al. 2011; Ojo and de Smidt 2023). Alkaline pretreatment can break the ester bonds among lignin, cellulose, and hemicellulose and separate them, thus increasing the availability of fermentative sugars. Triwahyuni et al. (2023) used carbon dioxide and caustic soda for the pretreatment of empty oil palm fruit bunches; the rate of delignification reached an impressive 80.5%, and the glucose yield was 99.3%. The lignin in the straw consisted of G and S units with a minute amount of H. The G unit in lignin reacts with ammonia or hydroxide, which separates the lignin and facilitates the hydrolysis of hemicellulose and cellulose. Consequently, alkaline pretreatment is commonly used to preprocess lignocellulose from agricultural straw (Guo et al. 2011). According to Wang et al. (2017), the removal of lignin reached 67.5% and the hydrolysis efficiency increased by 2.12 times after the alkali pretreatment of Sophora flavescens with sodium hydroxide, and the efficiency of alkaline pretreatment was 3% higher than that without pretreatment. An unfavorable aspect of alkali pretreatment is that the pretreated biomass needs to be washed and sewage is produced, which increases production costs.

Fig. 3. Classification of lignocellulosic pretreatment technologies

Inorganic salt pretreatment involves the use of sulfate, sulfite, ferric chloride, and ferrous chloride (Wei et al. 2019). Metal ions primarily affect the separation of lignin from hemicellulose. The lower the lignin content in the biomass, the more noticeable the impact, and the higher the valence metal ions, the higher the removal of hemicellulose (Idrees et al. 2013). In the pretreatment of sulfate and sulfite, sulfate reacts with lignin to form sulfonated lignin, increasing the hydrophilicity of lignin and promoting the exposure of cellulose. When water hyacinth and bagasse were treated with Na₂S, lignin was almost completely removed (Sewsynker-Sukai et al. 2018). After pretreatment with Na₃PO₄.12H₂O-ZnCl₂, the silica on the surface of the oil palm empty fruit bunch (OPEFB) was removed, the internal lignin content was greatly reduced, and the crystal strength of the material was also reduced (Hassan et al. 2020). However, the residual metal ions after pretreatment are a major drawback, and metal ions can affect the protein activity of microorganisms, inhibiting the activity of the strain and reduces the yield (Huo et al. 2018).

The organic solvents used for lignocellulose pretreatment include glycerol, ethylenediamine (EDA), maleic acid, n-propylamine, isobutanol, acetone, and tetrahydrofuran (Tang et al. 2017; Karnaouri et al. 2021; Chen et al. 2019; Risanto et al. 2022). Organic solvent pretreatment can be performed under mild conditions, and the solvent recovery efficiency after the treatment is very high. Two-step organosolv pretreatment allowed 86% glycerol and 92% ethanol recovery with 81.5% lignin removal (Song et al. 2022). Different organic solvents have varying effects on lignocellulose. Lignocellulose can disintegrate because of the strong affinity of EDA for the hydrogen-oxygen bonds between hemicellulose and cellulose. With pretreatment with EDA, the lignin and hemicellulose were removed, the surface features of the rice straw became rougher, and the pore volume and external surface of lignocellulose also increased, achieving a higher lactic acid concentration of 92.5 g/L (Chen et al. 2019). When sugarcane leaf was pretreated with maleic acid, the amorphous cellulose of the sugarcane matrix was affected, the surface of the matrix became rough, and the voids increased and were clearly visible (Risanto et al. 2022).

Organic solvent pretreatment has a certain selection effect on the lignocellulose components. After methanol pretreatment, pine and beech almost completely dissolved the hemicellulose. The main function of organic peroxide acids is to remove lignin, which can be tested at room temperature to prevent the formation of furfural and HMF at high temperatures. Glycerol solvent pretreatment can effectively extract sulfur-free lignin and promote lignin recycling. Nevertheless, organic solvent pretreatment has various disadvantages: methanol is toxic, has a low boiling point, and can easily form toxic vapors, and organic peroxide acids are expensive and increase production costs (Zhao et al. 2009).

Deep eutectic solvents are homogeneous mixtures of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA). The melting points of the mixture are lower than those of single compounds (Liu et al. 2021). DESs are simple to prepare, highly stable, easily degradable, and exhibit good biocompatibility. The HBDs of a DES generally include glycerol, ethylene glycol, 4-hydroxybenzoic acid, acetic acid, and other organic acids. The HBA are generally betaine (Ba), acetamide (Am), and acetylcholine (ChCl). DESs have a highly selective extraction ability for lignin because they can form hydrogen bonds with lignin via the proton supply and electron acceptance ability of DES, break ether bonds and hydrogen bonds in lignin and deconstruct lignocellulose. In a study by Xu et al. (2020), corncob pretreatment by DES was analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), which revealed that the surface of the corncob became rough and loose, with obvious layered fractures, and the lignin and hemicellulose that covered the cellulose were destroyed. The higher crystallinity index values shown by XRD also indicate that the pretreatment removed lignin and hemicellulose. In a report by Liu et al. (2019), the wheat straw was pretreated via the DES, which is composed of benzyltriethyl ammonium chloride (TEBAC)/LA, and the removal rate of lignin reached 79.73 ± 0.93. Numerous studies have demonstrated that DES can be effectively used to pretreat lignocellulose. Despite the presence of DES in the pretreated liquid, cellulase and xylanase maintained their stability.

Ionic liquids are organic salts composed of organic cations, organic anions, or inorganic anions with a melting point near ambient temperature (Liu et al. 2012). ILs can be divided into four categories according to the type of organic cation used: quaternary ammonium, N-alkylpyridine, N-alkyl isoquinoline, and 1-alkyl-3-methylimidazole. Cellulose can be dissolved in certain ILs and recovered using water or ethanol. Wheat straw was heated with 1-ethyl-3-methylimidazolium acetate [EMIM]. Lignin was removed in large quantities, and the internal crystal structure of cellulose was significantly destroyed. After enzymatic hydrolysis, it was fermented by Lactobacillus breve, and the lactic acid yield reached 0.49 g/g (Grewal and Khare 2018). Dadi et al. (2006) treated lignocellulose with 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), which increased enzymatic hydrolysis efficiency by a factor of 50. ILs can be recycled after lignocellulosic pretreatment without consuming significant amounts of energy. This is a green, environmentally friendly, and promising pretreatment method. The disadvantages of IL pretreatment are that the presence of IL inhibits microbial activity, is toxic to cells, and disrupts cell membranes.

After chemical pretreatment of lignocellulose, three major groups of inhibitors are produced: aldehydes, which are produced by dehydration of sugars under high-temperature acidic pretreatment conditions; weak acids, which are mainly derived from hydrolysis of acetyl groups in hemicellulose; and phenols, which are mainly produced during the degradation of lignin. In addition, residual inorganic ions are part of the inhibitors during pretreatment with inorganic salt solutions. These inhibitors interfere with the protein structure of the enzyme and affect the saccharification process. In order to reduce the production of inhibitors, one can choose the appropriate treatment according to the compositional characteristics of the lignocellulosic material. For example, when the content of hemicellulose in lignocellulose is high (e.g. from 20.0% to 43.0%.), one can choose organic solvents to treat hemicellulose relatively gently to reduce the production of acetic acid. When the cellulose content in lignocellulose is very high (e.g. between 30.0% and 50.0%), it is necessary to pay attention to the pretreatment temperature, the concentration of the acid solution and the length of pretreatment, to reduce the production of aldehyde inhibitors.

Biological Pretreatment

Biological pretreatment includes fungal, bacterial, and enzymatic pretreatment (Fig. 4). Fungal pretreatment includes white rot, brown rot, and soft rot fungi (Tian et al. 2018). Fungi secrete a series of extracellular enzymes to corrode and digest lignocellulose, transforming it from macromolecular to small molecular substances. Phanerochaete chrysosporium secretes lignin peroxidase (LiP), manganese peroxidase (MnP), and laccases. Laccase is a polyphenol oxidase containing four copper atoms that can react with lignin (Wu et al. 2022). The presence of laccase and MnP enables white-rot fungi to degrade lignin selectively. When white-rot fungi are used in the pretreatment of lignocellulose, hemicellulose and cellulose are retained as much as possible, and lignin is removed to the greatest extent. With the use of lignin and hemicellulose, cellulose is also catalyzed and utilized by fungi. Corn stover was pretreated with the white rot strain Blood Red Radish NRRL-FP-103506-Sp at 28 °C and 74.0% humidity for 30 days, and it was found that the loss of lignin reached 51.0 ± 1.2%, and the loss of hemicellulose and cellulose were 50.7 ± 2.1% and 25.4 ± 0.3%, respectively (Saha et al. 2016). Therefore, fungal pretreatment should focus on the processing time (Bao et al. 2022).

Fig. 4. Three ways of lignocellulose biological pretreatment. (a) fungal pretreatment; (b) bacterial pretreatment; (c) enzyme pretreatment.

Many types of bacteria are used for biological pretreatment. The most common way to break down the lignocellulose of bacteria is the β-ketoadipic acid pathway. Sodré et al. (2021) identified 10 species of bacteria that can break down the lignin via a β-ketoadipic acid pathway. Bacillus lignophilus L1 breaks down lignin via the gentianic acid pathway, benzoic acid pathway, and β-adipic acid ketone pathway. At 50 °C, lignin was the only carbon source, and B. lignophilus L1 degraded 38.9% of lignin within 7 days. The living environments and directional screening of bacteria are more extensive than those of fungi. Flavobacterium beibuense, Algoriphagus ratkowskyi, Pseudomonas putida, and Halomonas meridiana decomposed lignocellulose in high-salt environments. Arthrobacter sp. C2 can decompose lignin at a low temperature of 15 °C and has the activity of decomposing lignin in the range of pH 3 to 10 (Wu et al. 2022).

Biological enzymes play an important role in biological pretreatment. Laccase can degrade lignin through cleavage of the lignin side chain and a demethylation reaction and can use H₂O₂ as an oxidant to convert Mn²⁺ into a smaller chelating agent, Mn³⁺, and then it can penetrate the dense structure. The β-ether enzyme also can degrade lignin without the cofactors (Tian et al. 2018). The purpose of lignocellulose pretreatment to promote the lactic acid yield can be achieved by adding enzymes, thereby eliminating the consumption of nutrients during strain growth (Fig. 4). As an indispensable enzyme in lignocellulosic bio-pretreatment, laccase has been genetically engineered to achieve rapid mass production in Escherichia coli with good heat resistance and thermal stability. In addition, the use of composite enzymes for lignocellulose treatment is superior to the use of single enzymes because of the synergistic effect between the composite enzymes (Wu et al. 2022).

Biological pretreatment can be carried out at an ambient temperature ranging from 25 to 50 °C. This not only significantly saves energy and reduces costs but also ensures the safety of the treatment process. Fungi have a stronger ability to decompose lignin compared to bacteria, and the pretreatment cost of fungi is lower than that of enzyme treatment (Saini and Sharma 2021). However, fungi require time to grow, which will extend the pretreatment cycle. Bacterial pretreatment has low energy consumption, does not require chemical reagents, and has mild conditions. Nevertheless, many bacteria are difficult to cultivate successfully in the laboratory, and their performance cannot be stably inherited over successive generations, resulting in unstable pretreatment effects. Biological enzymes can efficiently hydrolyze biomass raw materials and improve the treatment efficiency. However, biological enzymes cannot regenerate themselves and need to be supplemented at an additional cost, which increases the overall treatment cost.

Combined Pretreatment Method

To improve the effectiveness of pretreatment in increasing bioresources yield, researchers have proposed comprehensive pretreatment; the combination of any two or even three of the aforementioned pretreatment methods can be primarily classified as a combination of physical and biological pretreatment. Expansion combined with Irpex lacteus fungal treatment to degrade wheat straw reduced the crystallinity of cellulose and destroyed lignin chemical bonds, which reduced the structural resistance of subsequent I. lacteus treatments and increased the enzyme activity of I. lacteus. Compared with I. lacteus, the pretreatment cycle was greatly reduced during co-processing (Cao et al. 2023). Pre-fermentation combined with acid pretreatment easily converted the water-soluble carbohydrates (WSCs) contained in bagasse into inhibitors during the pretreatment process. Pre-fermentation converted 98% of WSC into lactic acid, increasing total lactic acid production by 180% (Qiu et al. 2022). For microwave-assisted alkali pretreatment, a mixed solution of vinasse and NaOH was placed in a microwave, and after the pretreatment, lignin was removed and certain amounts of hemicellulose and cellulose were increased (Cao et al. 2019). Organic amine and organosolv synergistically pretreated corn stover using n-propylamine as a catalyst and aqueous ethanol as a solvent resulted in a delignification of 81.7%, and the total sugar yield was increased to 83.2% (Tang et al. 2017). The defining characteristic of joint preprocessing is that the preceding processing can offer certain advantages to subsequent processing; consequently, it is not possible to categorize and summarize this process in a straightforward manner. The combined pretreatment methods that have been practiced are listed in Table 1.

The pretreatment process usually causes lignocellulose to produce three types of inhibitors: acetates, furfural, and phenols. Hemicellulose contains a large number of acetyl groups in its structure, which are shed from the molecular chain of hemicellulose to form acetic acid under high temperature or acidic treatment conditions. Cellulose and hemicellulose are partially decomposed into monosaccharides such as hexose and pentose under pretreatment conditions, and under high temperatures and acidic conditions, pentose removes three molecules of water and undergoes molecular rearrangement to form furfural, and hexose removes three molecules of water and then undergoes molecular rearrangement under acidic conditions to form HMF. The main purpose of pretreatment technology is to remove lignin, and a variety of high temperatures, acids and alkalis, and strong oxidizing conditions will promote the connection of the structure of lignin bond breakage. Various high temperatures, acidic and alkaline conditions, and strong oxidizing conditions will cause the linkages in the structure of lignin to break, such as the main linkage of lignin, β-O-4, the common linkage β-O-4, and C-C, etc., and their breakage will lead to the production of phenolic inhibitors. Demethylation of the methoxy functional group in lignin or in alkaline environments generates phenolic inhibitors (Chandel et al. 2011).

EFFECT OF PRETREATMENT ON HYDROLYSIS AND FERMENTATION

Enzymatic hydrolysis of Cellulose and Hemicellulose

After pretreatment, the cellulose and hemicellulose in lignocellulose are initially broken down into fragmented structures of short sugar chains. The saccharification step breaks down polysaccharides into fermentable sugars such as glucose, xylose, arabinose, etc., and then microorganisms convert the sugars into bioresources through metabolic activity (Zhou et al. 2021). The cellulase system consists of endoglucanase, exoglucanase, and β – glucosidase. Endoglucanase randomly cleaves the β – 1,4 – glycosidic bonds within cellulose, breaking the long-chain cellulose. Exoglucanase cleaves cellobiose units from the non-reducing end of the cellulose chain, ultimately producing a large amount of cellobiose. Subsequently, β-glucosidase hydrolyzes cellobiose into glucose. The hemicellulase system includes xylanase, mannanase, arabinofuranosidase, etc. Different enzymes act on different glycosidic bonds of hemicellulose. Xylanase acts on the β-1,4-glycosidic bonds of the xylan main chain, degrading xylan into oligosaccharides and xylose. Mannanase hydrolyzes the β-1,4-glycosidic bonds in the mannan main chain. Arabinofuranosidase hydrolyzes the arabinofuranosidic bonds on the side chains of hemicellulose, causing the side chains to detach from the main chain, which facilitates the enzymatic hydrolysis of the main chain (Wang et al. 2015; Li et al. 2024). Other enzymes, such as galactosidase and glucuronidase, act on the corresponding glycosidic bonds (Ojo and de Smidt 2023). Pretreatment opens the structure of lignocellulose, separates lignin, cellulose and hemicellulose, and increases the contact sites for enzymes, promoting saccharification efficiency. However, the various inhibitors produced in the pretreatment can also have an effect on the saccharification step (Yang et al. 2024).

Saccharification strategies can be categorized into two types based on the source of enzymes: off-site and on-site. Separate hydrolytic fermentation (SHF) and simultaneous saccharification and fermentation (SSF), as well as separate hydrolytic co-fermentation (SHCF) and simultaneous saccharification co-fermentation (SSCF), which are derived from the combination of these two processes and the downstream fermentation process, are off-site saccharification strategies, where individually produced enzymes are added for saccharification to produce fermentable sugars. The two production processes of consolidated bioprocessing (CBP) and consolidated bio-saccharification (CBS), on the other hand, belong to the on-site method saccharification process, i.e., the enzyme production and saccharification process are concentrated in one system, and this saccharification strategy reduces the cost of enzyme production and isolation. Regardless of the saccharification strategy, the enzymes in the saccharification system are affected by inhibitors (Wang et al. 2024).

The inhibition of the enzyme in the saccharification step is simply divided into two types. One is the effect of inhibitors produced in the pretreatment step on the enzyme. Phenolic compounds will change the spatial structure of the enzyme, so that the center of the enzyme activity of the conformation of the enzyme activity is changed to reduce the activity of the enzyme. The reaction of the enzyme proteins weakens the stability of the enzyme. Phenolic inhibitors also adsorb on the surface of the substrate, hindering the contact between the enzyme and the substrate. Acid inhibitors affect the pH of the glycation system, inhibit secretion of enzyme synthesis, and inactivate the enzyme by altering its three-dimensional structure. Furfural inhibitors react with the functional groups of amino acids, thereby altering the chemical structure and properties of the enzyme and reducing its affinity for the substrate. On the other hand, as saccharification proceeds, cellobiose, glucose, and xylose also inhibit the saccharification process. There is mainly glucose inhibition of β-glucosidase, xylan inhibition of cellulase, and cellobiose inhibition of cellobiose hydrolase. High concentrations of xylose also inhibit the binding of cellobiose to the enzyme’s active site. When the saccharification step is in the same environment as the fermentation step, cellulase activity is also inhibited as the concentration of fermentation products increases (Wang et al. 2024).

The positive effect of pretreatment on fermentation

Good reaction conditions for enzymatic hydrolysis can be achieved by pretreatment. Scanning electron microscope (SEM) analysis in many pretreatment studies has shown that pretreatment would destroy the dense and ordered structure of the lignocellulose surface and expand the internal space of the material. After DES treatment, the biomass surface of aloe vera leaves became rough and disordered, and the epidermal structure of aloe vera leaves were destroyed (Rajeswari and Jacob 2022); after high boiling alcohol/water (HBAW) pretreatment, bamboo chips were disrupted and fragmented, after LHW pretreatment, large substrate pore volume was developed (Liu et al. 2017). In the rolling pretreatment, the fiber of corn stalk was separated and the top of the pile was cracked, the internal structure of the material was fluffy, and the specific surface area and pore size were significantly improved (Deng and Li 2021). After ball milling, the specific surface area of hemp doubled, and the pore volume expanded by 5.6 times (Ouajai and Shanks 2006). Fourier transform infrared (FTIR) analysis of many pretreatment studies has shown that various chemical bonds in lignocellulose, especially in hemicellulose and lignin, are broken under the action of temperature and chemical reagents. In one study, under the action of chemical reagents (or aqueous solution), lignocellulose was dissolved and generated sulfonated lignin, cellulose was exposed thereby promoting enzymatic hydrolysis, which showed that the peak intensity at 1734 cm⁻¹ in the FTIR spectra of wheat straw pretreated by EBI was significantly reduced (Guo et al. 2020). Compared with the FTIR spectra of natural OPEFB, there were no bands at 1735 and 1740 cm⁻¹ in the FTIR spectra of maleic acid pretreated OPEFB, indicating that pretreatment can effectively break down lignocellulose (Risanto et al. 2022).

Here, the fermentation production of lactic acid is used as an example to analyze the influence of the pretreatment step. Pretreatment increased lactic acid production (Table 2) by increasing the concentration of fermented sugars. The fermentable sugar concentration of bagasse was increased to 80.0 g/L after alkali pretreatment, and the fermentative sugar yield of olefinic acid pretreated bagasse increased from 0.13 to 0.65 g/g (Katepogu et al. 2022). Owing to the chemical properties of lignocellulose itself, especially those of hemicellulose, which is easily soluble at high temperatures, most of the hemicellulose is lost during pretreatment (Chang et al. 2012). Qiao et al. (2021) removed approximately 90.10% of xylan via formic acid cooking pretreatment. Similarly, alkaline pretreatment can remove a significant amount of lignin (43.0%) and hemicellulose from eucalyptus sawdust (Camesasca et al. 2021; Qiao et al. 2021). In contrast to previous knowledge, hemicellulose can also be converted to lactic acid by microorganisms when hydrolyzed to xylose, arabinose, and other pentoses. Therefore, appropriate pretreatment methods can be selected according to the characteristics of the fermentation strains. Lactobacillus delbrueckii sp. bulgaricus can make good use of cellulose to produce lactic acid, but the limitation is that the strain cannot use pentose; therefore, when selecting the pretreatment method, one needs only to consider the retention rate of cellulose (Karnaouri et al. 2020; Zhang et al. 2022). Pentose and hexose can be converted to lactic acid by some strains, such as Lactobacillus pentosus CECT4023T and Enterococcus mundtii QU 25, and the pretreatment method with a higher hemicellulose retention rate is more suitable for use; for example, pretreatment with Na₃PO₄.12H₂O resulted in a hemicellulose removal of 9.8% (Wang et al. 2014; Hassan et al. 2020).

Negative effects of inhibitors on fermentation

As shown in Table 2, pretreatment can enhance enzymatic hydrolysis and increase yield, but it may also introduce certain inhibitors. Pretreatment generally causes lignocellulose to produce three types of inhibitors: furans (HMF and 2-furfural), organic acids (AA, formic acid, and levulinic acid), and phenols (vanillin, syringaldehyde, 4-hydroxybenzaldehyde, pinealdehyde, ferulic acid and cinnamic acid) (Klinke et al. 2004; Yee et al. 2018). Inhibitors can enter the microbial cells, change the permeability of the cell membrane, destroy the integrity of the cell membrane, inhibit the growth and metabolism of microorganisms, reacting with nucleic acids and causing DNA damage, react with proteins, change the spatial structure of enzymes, interfere with enzyme activity, and change the original metabolic pathway. Acetic acid will also change the pH of the fermentation broth, bringing acid-base stress to microorganisms from the outside, increasing the energy burden of microorganisms, acetic acid can also enter the cell, reducing intracellular pH, affecting the survival of cells. Phenolic inhibitors have a certain degree of fat solubility, will destroy the cell’s phospholipid bilayer, interfering with the normal physiological function of the cell. In addition, the condensation reaction between furfural and phenolic substances will form pseudo-lignin, which will be attached to the substrate and hinder the metabolic utilization of sugar by microorganisms (Chandel et al. 2013, Kucharska et al. 2018). Some solvents used in the pretreatment process, if not completely removed, will also become a class of substances that inhibit the metabolic activities of microorganisms, such as ILS, and ILS remaining in the pretreatment solution will lead to microbial poisoning (Wahlström and Suurnäkki 2015). During EBI pretreatment, cellulose reacts with the amino compounds produced by EBI to form copolymers, thereby inhibiting the catalytic activity of the enzyme (Fei et al. 2019). After pretreatment with EDA, the residual EDA did not affect the activity of the hydrolysis enzyme, but it inhibited the production and metabolism of lactic acid bacteria (Chen et al. 2019). In summary, inhibitors can affect the conversion and utilization of sugar by microorganisms and reduce product yield (Fig. 5).

Fig. 5. Effect of inhibitors on Lactic acid production. (a) inhibitors interfere with the biofilm, causing its integrity to be lost; (b) Inhibitors interfere with the cell membrane and hinder the absorption and utilization of sugar by bacteria; (c) Inhibitor-induced gene mutation; (d) Pseudolignin deposition, hinder the enzymatic hydrolysis of sugar; (e) inhibitors lead to a decrease in enzyme activity; (f) The hydrolysis of weak acid will affect the pH value of fermentation broth.

Strategies to Overcome the Adverse Effects of Inhibitors

The presence and influence of inhibitors can be reduced in four aspects.

I: Appropriate pretreatment method selection to reduce the production of inhibitors. Qiu et al. (2023) reported that pre-fermentation and pretreatment can decrease the generation of inhibitors. The WSCs in bagasse are converted to furan inhibitors during acid pretreatment, and the pre-fermentation step converts 98.00% of the soluble carbohydrates into lactic acid, which reduces HMF formation.

II: A detoxification step is performed after pretreatment to reduce the inhibitors. The produced inhibitors can be removed or reduced in concentration by washing and cleaning the pretreated substrate with organic solvents. Chen et al. (2019) reported that the concentration of EDA can be decreased from 5.61 wt.% to 0.70 wt.% via washing in dilute EDA pretreated rice straw.

Table 1. Studies of Lignocellulosic Pretreatment in Recent Years

Table 2. The Inhibitor Content in the Liquid after Pretreatment of Lignocellulosic Biomass, the Sugar Content in the Hydrolysate and the Lactic Acid Yield

Atmospheric glycerol solvent (AGO) can remove lignin components in lignocellulose well, but a large amount of residual AGO will adhere to the pretreatment substrate, which seriously affects the efficiency of enzymatic hydrolysis. Washing AGO with ethanol solution, xylose concentration increased from 8.50% to 62.0% (Song et al. 2022). Suitable strains can be selected for biological detoxification of the culture medium during fermentation. In the experiment of dry acid pretreatment of wheat straw, fungus was used to successfully decompose the inhibitor with a total concentration

of 29.30 mg/g dry matter to a concentration of 2.20 mg/g, and the lactic acid yield was as high as 130.00 g/L after detoxification (Sodréet al. 2021; He et al. 2023).

III: An advanced fermentation production process was selected to reduce the concentration of inhibitors. Before fermentation, nanofiltration (NF) and reverse osmosis membranes were used to detoxify the inhibitors and concentrate the fermentable sugars. Under a certain pressure range, the NF membrane had good conductivity for inhibitors; formic acid was 90.30%, AA was 88.30%, furfural was 98.10%, HMF was 95.50%, and vanillin was 88.50%. Therefore, the NF membrane had excellent detoxification performance. Pan et al. using the batch feeding strategy for detoxification. A high inhibitor concentration of 13.50 g/L was achieved during the cork sequential pretreatment of sulfuric acid and steam (Pan et al. 2019). The use of and fed-batch fermentation process batch via Pediococcus acidilactici TY112 can convert inhibitors, with the lactic acid yield reaching 0.86 g/g (Campos et al. 2022).

IV: A reasonable selection of fermentation strains can reduce the effects of inhibitors to a certain extent. Pediococcus pentosaceus HLV1 can produce lactate under the stress of inhibitors (Katepogu et al. 2022), Ouyang et al. (2020) tested Bacillus coagulans CC17A, which can digest multiple inhibitors in the hydrolysate and 35.5 g lactic acid was produced from 80 g wheat straw, and the combination of Pseudomonas putida KT2440 and B. coagulans NL01 could directly produce lactic acid in highly toxic hydrolysates. P. acidilactici TY112 is a genetically engineered bacterium that cannot metabolize the fermentable sugars in the culture medium, but it can convert 100.0% of the organic acids and furan inhibitors produced by pretreatment, and the removal rate of most monoaromatic compounds is 90.0% (Aulitto et al. 2019). After the engineered bacteria are converted to remove most of the inhibitors, the yield of LA synthesized by B. coagulans NL01 using the hydrolysate was 0.80 g/g (Zou et al. 2021). The microbial community DUT47 was found to be robust to inhibitor compounds and could produce lactic acid 0.50 g/g in a sulfuric acid-pretreated sugarcane solution without detoxification treatment (Sun et al. 2021).

CONCLUSIONS AND PERSPECTIVES

Lignocellulose represents a significant raw material for bioresources production. The composition of lignocellulose includes cellulose, hemicellulose, and lignin. These polymers are held together by a multitude of chemical bonds, resulting in a highly stable and complex structure. Pretreatment represents an effective method for disrupting the overall structure of lignocellulose, facilitating the separation of lignin, the exposure of cellulose and hemicellulose, and enhancing the utilization of lignocellulose.

The primary objective of physical pretreatment is to destroy the physical properties of lignocellulose and disrupt its crystal structure. This process is relatively straightforward and can be easily operation. Chemical pretreatment can select the appropriate treatment solution according to the specific type of lignocellulose components present, this method can effectively remove lignin while retaining hemicellulose and cellulose content. Biological pretreatment can be conducted under mild conditions and is an energy-efficient and environmentally friendly process. Nevertheless, it should be noted that the application of pretreatment techniques may result in the formation of a range of inhibitory compounds, including furfural, acetic acid, and phenolics. Inhibitors can be classified into three primary categories: 1) those that result from the further deoxygenation of monosaccharide substances during the pretreatment process, leading to the formation of furfural inhibitors. 2) acetic acid, which is formed by the deacetylation of sugar-rich acetyl groups, is the primary source of acid inhibitors. 3) various phenolic inhibitors are produced by lignin groups during the pretreatment process. Inhibitors affect both the saccharification step and the fermentation step. Inhibitors bind to the active site of the enzyme, preventing the enzyme from binding to polysaccharides, inhibitors react with groups of amino acids, altering the structure of the enzyme and affecting enzyme stability, inhibitors react with the enzyme, altering the enzyme’s three-dimensional structure and rendering it catalytically incapable, and in the on-site saccharification strategy. Inhibitors also affect the activity of the cell, and the acidic environment created by acidic inhibitors exhausts the cell to resist the acidic environment, decreasing enzyme production. These effects are specific to the effect of inhibitors on the saccharification step. The specific effects of inhibitors on the fermentation step are reflected in the effects of various substances on cell integrity and enzyme activity. Inhibitors can enter the microbial cell, change the permeability of the cell membrane, damage the integrity of the cell membrane, damage the cell DNA, inhibit the growth and metabolism of microorganisms, interfere with the activity of enzymes, and change the metabolic pathway. Acetic acid also alters the pH of the fermentation broth, bringing acid-base stress to the microorganisms and increasing the energy burden of the microorganisms.

By analyzing the lignocellulosic pretreatment test and LA fermentation test in recent years, several feasible solutions to reduce the impact of inhibitors on LA production were summarized: improving pretreatment methods, detoxifying pretreatment solutions, optimizing fermentation production methods, and cultivating resistant strains. Taking this as a reference, it provides some new ideas for realizing more efficient bioresources production, alleviating energy crisis and creating environmentally friendly and green industrial products in the future.

ACKNOWLEDGMENTS

This work was supported by the National Key R&D Program of China (Grant NO. 2022YFE0118800), the National Natural Science Foundation of China (Grant NO. 22276012), the Ecological Environment Science & Technology Project of Anhui Province (Grant NO. 2023hb0015), the GDAS’ Project of Science and Technology Development (Grant NO. 2019GDASYL-0102005), and the Guangdong Foundation for Program of Science and Technology Research (Grant NO. 2023B1212060044).

REFERENCES CITED

Ahorsu, R., Cintorrino, G., Medina, F., and Constantí, M. (2019). “Microwave processes: A viable technology for obtaining xylose from walnut shell to produce lactic acid by Bacillus coagulans,” J. Cleaner Prod. 231, 1171-1181. DOI: 10.1016/j.jclepro.2019.05.289

Aulitto, M., Fusco, S., Nickel, D. B., Bartolucci, S., Contursi, P., and Franzén, C. J. (2019). “Seed culture pre-adaptation of Bacillus coagulans MA-13 improves lactic acid production in simultaneous saccharification and fermentation,” Biotechnol. Biofuels 12, 2-11. DOI: 10.1186/s13068-019-1382-2

Bao, X., Guo, G., Huo, W. J., Li, Q. J., Xu, Q., and Chen, L. (2022). “Ensiling pretreatment fortified with laccase and microbial inoculants enhances biomass preservation and bioethanol production of alfalfa stems,” Sci. Total Environ. 857, article 159442. DOI: 10.1016/j.scitotenv.2022.159442

Camesasca, L., de Mattos, J. A., Vila, E., Cebreiros, F., and Lareo, C. (2021). “Lactic acid production by Carnobacterium sp. isolated from a maritime Antarctic Lake using eucalyptus enzymatic hydrolysate,” Biotechnol Rep 31, article e00643. DOI: 10.1016/j.btre.2021.e00643

Campos, J., Tejada, L. G., Bao, J., and Lidén, G. (2022). “Fed-batch strategies for biodetoxification in production of optically pure lactic acid from softwood hydrolysate using Pediococcus acidilactici,” Process Biochem 125, 162-170. DOI: 10.1016/j.procbio.2022.12.027

Cao, X., Zuo, S., Cai, R., Yang, F., Jiang, X., and Xu, C. (2023a). “Expansion combined with Irpex lacteus fungal treatment for enhancing buckwheat straw degradation,” Biochem. Eng. J. 197, article 108994. DOI: 10.1016/j.bej.2023.108994

Cao, Y., Liu, H., Shan, J., Sun, B., Chen, Y., Ji, L., Ji, X., Wang, J., Zhu, C., and Ying, H. (2023b). “Ammonia-mechanical pretreatment of wheat straw for the production of lactic acid and high-quality lignin,” Fermentation 9, article 177. DOI: 10.3390/fermentation9020177

Cao, Y., Wang, J., Wang, Q., Liu, J., Liu, T., Knapp, C. W., and Wang, Y. (2019). “Effect of β-glycosidase supplementation on vinasse saccharification and L-lactic acid fermentation,” BioResources 14(1), 1379-1389. DOI: 10.15376/BIORES.14.1.1379-1389

Chandel, A. K., Silva, S. S., and Singh, O. V. (2011). “Detoxification of lignocellulosic hydrolysates for improved bioconversion of bioethanol,” in: Biofuel Production – Recent Developments and Prospects, M. A. S. Bernardes (ed.), InTech 10, pp. 225-246. DOI: 10.5772/16454

Chandel, A. K., Silva, S. S., and Singh, O. V. (2013). “Detoxification of lignocellulose hydrolysates: Biochemical and metabolic engineering toward white biotechnology,” Bioenergy Res 6, 388-401. DOI: 10.1007/s12155-012-9241-z

Chang, J., Cheng, W., Yin, Q., Zuo, R., Song, A., Zheng, Q., Wang, P., Wang, X., and Liu, J. (2012). “Effect of steam explosion and microbial fermentation on cellulose and lignin degradation of corn stover,” Bioresource Technology 104, 587-592. DOI: 10.1016/j.biortech.2011.10.070

Chawla, S. K. and Goyal, D. (2022). “Optimization of pretreatment of wheat straw using response surface methodology for production of lactic acid using Bacillus sonorenesis strain DGS15,” Bioenergy Res 16, 967-978. DOI: 10.1007/s12155-022-10439-9

Chen, H., Jiang, L., Cheng, Y., Lu, J., Lv, Y., Yan, J., and Wang, H. (2019). “Improving enzymatic hydrolysis efficiency of corncob residue through sodium sulfite pretreatment,” Appl. Microbiol. Biotechnol 103, 7795-7804. DOI: 10.1007/s00253-019-10050-7

Chen, H., Huo, W., Wang, B., Wang, Y., Wen, H., Cai, D., Zhang, C., Wu, Y., and Qin, P. (2019a). “L-lactic acid production by simultaneous saccharification and fermentation of dilute ethylediamine pre-treated rice straw,” Ind. Crops Prod. 141, article 111749. DOI: 10.1016/j.indcrop.2019.111749

Chen, Y., Yin, Y., and Wang, J. (2022). “Biohydrogen production using macroalgal biomass of Laminaria japonica pretreated by gamma irradiation as substrate,” Fuel. DOI: 10.1016/j.fuel.2021.122179

Chuetor, S., Ruiz, T., Barakat, A., Laosiripojana, N., Champreda, V., and Sriariyanun, M. (2021). “Evaluation of rice straw biopowder from alkaline-mechanical pretreatment by hydro-textural approach,” Bioresource Technol. 323, article 124619. DOI: 10.1016/j.biortech.2020.124619

Cubas-Cano, E., Gonzalez-Fernandez, C., Ballesteros, I., and Tomas-Pejo, E. (2020). “Efficient utilization of hydrolysates from steam-exploded gardening residues for lactic acid production by optimization of enzyme addition and pH control,” Waste Manage 107, 235-243. DOI: 10.1016/j.wasman.2020.04.003

Dadi, A. P., Varanasi, S., and Schall, C. A. (2006). “Enhancement of cellulose saccharification kinetics using an ionic liquid pretreatment step,” Biotechnol. Bioeng. 95, 904-910. DOI: 10.1002/bit.21047

Deng, L., and Li, J. (2021). “Thread rolling: An efficient mechanical pretreatment for corn stover saccharification,” Energies 14, article 542. DOI: 10.3390/en14030542

Ekielski, A., and Mishra, P. K. (2020a). “Lignin for bioeconomy: The present and future role of technical lignin,” Int. J. Mol. Sci. 22, article 63. DOI: 10.3390/ijms22010063

Fan, J., De bruyn, M., Budarin, V. L., Gronnow, M. J., Shuttleworth, P. S., Breeden, S., Macquarrie, D. J., and Clark, J. H. (2013). “Direct microwave-assisted hydrothermal depolymerization of cellulose,” J. Am. Chem. Soc. 135, 11728-11731. DOI: 10.1021/ja4056273

Fei, X., Jia, W., Wang, J., Chen, T., and Ling, Y. (2019). “Study on enzymatic hydrolysis efficiency and physicochemical properties of cellulose and lignocellulose after pretreatment with electron beam irradiation,” Int. J. Biol. Macromol. 145, 733-739. DOI: 10.1016/j.ijbiomac.2019.12.232

Feng, C., Zhu, J., Hou, Y., Qin, C., Chen, W., Nong, Y., Liao, Z., Liang, C., Bian, H., and Yao, S. (2022). “Effect of temperature on simultaneous separation and extraction of hemicellulose using p-toluenesulfonic acid treatment at atmospheric pressure,” Bioresource Technol. 348, article 126793. DOI: 10.1016/j.biortech.2022.126793

Grewal, J., and Khare, S. K. (2018). “One-pot bioprocess for lactic acid production from lignocellulosic agro-wastes by using ionic liquid stable Lactobacillus brevis,” Bioresource Technol. 251, 268-273. DOI: 10.1016/j.biortech.2017.12.056

Gunes, K., Sargin, S., and Celiktas, M. S. (2022). “Investigation of lactic acid production by pressurized liquid hot water from cultivated Miscanthus × giganteus,” Prep. Biochem. Biotechnol. 53, 22-30. DOI: 10.1080/10826068.2022.2035745

Guo, P., Mochidzuki, K., Cheng, W., Zhou, M., Gao, H., Zheng, D., Wang, X., and Cui, Z. (2011). “Effects of different pretreatment strategies on corn stalk acidogenic fermentation using a microbial consortium,” Bioresource Technol. 102, 7526-7531. DOI: 10.1016/j.biortech.2011.04.083

Guo, X., Li, H., Yan, H., Dai, Y., Luo, X., Yang, X., and Kong, L. (2020). “Production of organic carboxylic acids by hydrothermal conversion of electron beam irradiation pretreated wheat straw,” Biomass Convers. Biorefin. 10, 997-1006. DOI: 10.1007/s13399-019-00471-9

Hassan, N., Kharil Anwar, N. A., and Idris, A. (2020). “Strategy to enhance the sugar production using recyclable inorganic salt for pre-treatment of oil palm empty fruit bunch (OPEFB),” BioResources 15(3), 4912-4931. DOI: 10.15376/biores.15.3.4912-4931

He, N., Chen, M., Qiu, Z., Fang, C., Lidén, G., Liu, X., Zhang, B., and Bao, J. (2023). “Simultaneous and rate-coordinated conversion of lignocellulose derived glucose, xylose, arabinose, mannose, and galactose into D-lactic acid production facilitates D-lactide synthesis,” Bioresource Technol. 377, article 128950. DOI: 10.1016/j.biortech.2023.128950

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

Huang, Z., Feng, G., Lin, K., Pu, F., Tan, Y., Tu, W., Han, Y., Hou, X., Zhang, H., and Zhang, Y. (2020). “Significant boost in xylose yield and enhanced economic value with one-pot process using deep eutectic solvent for the pretreatment and saccharification of rice straw,” Ind. Crops Prod. 152, article 112515. DOI: 10.1016/j.indcrop.2020.112515

Huo, D., Dan-ni, X., Yang, Q., Liu, Q., Hou, Q., and Tao, Z. (2018). “Improving the efficiency of biomass pretreatment and enzymatic saccharification process by metal chlorides,” BIOB 13, 1909-1916. DOI: 10.15376/biores.13.1.1909-1916

Idrees, M., Adnan, A., Qureshi, F. A. (2013). “Optimization of sulfide/sulfite pretreatment of lignocellulosic biomass for lactic acid production,” Biomed. Res. Int. 2013, article 934171. DOI: 10.1155/2013/934171

Karnaouri, A., Asimakopoulou, G. E., Kalogiannis, K.G., Lappas, A. A., and Topakas, E. (2020). “Efficient d-lactic acid production by Lactobacillus delbrueckii subsp. bulgaricus through conversion of organosolv pretreated lignocellulosic biomass,” Biomass Bioenergy 140, article 105672. DOI: 10.1016/j.biombioe.2020.105672

Karnaouri, A., Asimakopoulou, G., Kalogiannis, K. G., Lappas, A. A., and Topakas, E. (2021). “Efficient production of nutraceuticals and lactic acid from lignocellulosic biomass by combining organosolv fractionation with enzymatic/fermentative routes,” Bioresource Technol. 341, article 125846. DOI: 10.1016/j.biortech.2021.125846

Katepogu, H., Wee, Y. J., Chinni, S. V., Gopinath, S. C. B., Syed, A., Bahkali, A. H., Elgorban, A. M., and Lebaka, V. R. (2022). “Lactic acid production from sugarcane field residue as renewable and economical bioresource by newly isolated Pediococcus pentosaceus HLV1,” Biomass Convers. Biorefin. 13, 14927-14937. DOI: 10.1007/s13399-022-03267-6

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

Ko, J. K., Kim, Y., Ximenes, E., and Ladisch, M. R. (2015). “Effect of liquid hot water pretreatment severity on properties of hardwood lignin and enzymatic hydrolysis of cellulose,” Biotechnol. Bioeng. 112, 252-262. DOI: 10.1002/bit.25349

Kucharska, K., Rybarczyk, P., Hołowacz, I., Łukajtis, R., Glinka, M., and Kamiński, M. (2018). “Pretreatment of lignocellulosic materials as substrates for fermentation processes,” Molecules 23, article 2937. DOI: 10.3390/molecules23112937

Li, M., Zhu, W. B., Fan, J., Gao, M., Wang, X., Wu, C., Wang, Y., and Lu, Y. (2024). “Carbon catabolite repression during the simultaneous utilization of lignocellulose-derived sugars in lactic acid production: Influencing factors and mitigation strategies,” Environ. Res. 2024, article 120484. DOI: 10.1016/j.envres.2024.120484

Liu, C., Wang, F., Stiles, A.R., and Guo, C. (2012). “Ionic liquids for biofuel production: Opportunities and challenges,” Appl. Energy 92, 406-414. DOI: 10.1016/J.APENERGY.2011.11.031

Liu, J., Li, R., Shuai, L., You, J., Zhao, Y., Chen, L., Li, M., Chen, L., Huang, L., and Luo, X. (2017a). “Comparison of liquid hot water (LHW) and high boiling alcohol/water (HBAW) pretreatments for improving enzymatic saccharification of cellulose in bamboo,” Ind Crops Prod 107, 139-148. DOI: 10.1016/j.indcrop.2017.05.035

Liu, Y., Zheng, J., Xiao, J., He, X., Zhang, K., Yuan, S., Peng, Z., Chen, Z., and Lin, X. (2019). “Enhanced enzymatic hydrolysis and lignin extraction of wheat straw by triethylbenzyl ammonium chloride/lactic acid-based deep eutectic solvent pretreatment,” ACS Omega 4, 19829-19839. DOI: 10.1021/acsomega.9b02709

Liu, Y., Zheng, X., Tao, S., Hu, L., Zhang, X., and Lin, X. (2021). “Process optimization for deep eutectic solvent pretreatment and enzymatic hydrolysis of sugar cane bagasse for cellulosic ethanol fermentation,” Renew. Energ. 177, 259-267. DOI: 10.1016/j.renene.2021.05.131

Martins, J. R., Schmatz, A. A., Salazar-Bryan, A. M., and Brienzo, M. (2022). “Effect of dilute acid pretreatment on the sugarcane leaf for fermentable sugars production,” Sugar Tech. 24, 1540-1550. DOI: 10.1007/s12355-021-01106-y

Mishra, V., and Jana, A. K. (2019). “Sweet sorghum bagasse pretreatment by Coriolus versicolor in mesh tray bioreactor for selective delignification and improved saccharification,” Waste Biomass Valori. 10, 1-14. DOI: 10.1007/s12649-018-0276-z

Monte, L. S., Escócio, V. A., de Sousa, A. M. F., Furtado, C. R. G., Leite, M. C. A. M., Visconte, L. L. Y., and Pacheco, E. B. A. V. (2017). “Study of time reaction on alkaline pretreatment applied to rice husk on biomass component extraction,” Biomass Convers Biorefin 8, 189-197. DOI: 10.1007/s13399-017-0271-9

Nalawade, K., Baral, P., Patil, S. P., Pundir, A., Kurmi, A. K., Konde, K., Patil, S., and Agrawal, D. (2020). “Evaluation of alternative strategies for generating fermentable sugars from high-solids alkali pretreated sugarcane bagasse and successive valorization to L (+) lactic acid,” Renewable Energy 157, 708-717. DOI: 10.1016/j.renene.2020.05.089

Niu, D., Zuo, S., Ren, J., Li, C., Zheng, M., Jiang, D., and Xu, C. (2020). “Effect of wheat straw types on biological delignification and in vitro rumen degradability of wheat straws during treatment with Irpex lacteus,” Anim. Feed Sci. Tech. 267, article 114558. DOI: 10.1016/j.anifeedsci.2020.114558

Ojo, A. O., and de Smidt, O. (2023). “Lactic Acid: A comprehensive review of production to purification,” Processes 11, article 688. DOI: 10.3390/pr11030688

Oliveira, R. A., Komesu, A., Rossell, C. E. V., and Filho, R. M. (2018). “Challenges and opportunities in lactic acid bioprocess design – From economic to production aspects,” Sustainable Chem. Pharm. 15, article 100206. DOI: 10.1016/j.scp.2019.100206

Ouajai, S., and Shanks, R. (2006). “Solvent and enzyme induced recrystallization of mechanically degraded hemp cellulose,” Cellulose 13, 31-44. DOI: 10.1007/s10570-005-9020-5

Ouyang, S., Zou, L., Qiao, H., Shi, J., Zheng, Z., and Ouyang, J. (2020). “One-pot process for lactic acid production from wheat straw by an adapted Bacillus coagulans and identification of genes related to hydrolysate-tolerance,” Bioresource Technol. 315, article 123855. DOI: 10.1016/j.biortech.2020.123855

Pan, L., He, M., Wu, B., Wang, Y., Hu, G., and Ma, K. (2019). “Simultaneous concentration and detoxification of lignocellulosic hydrolysates by novel membrane filtration system for bioethanol production,” J. Cleaner Prod. 227, 1185-1194. DOI: 10.1016/j.jclepro.2019.04.239

Pérez-Rodríguez, N., García-Bernet, D., and Domínguez, J. M. (2016). “Effects of enzymatic hydrolysis and ultrasounds pretreatments on corn cob and vine trimming shoots for biogas production,” Bioresource Technol. 221, 130-138. DOI: 10.1016/j.biortech.2016.09.013

Pontes, R. V., Romaní, A., Michelin, M., Domingues, L., Teixeira, J. A., and Nunes, J. O. (2021). “L-lactic acid production from multi-supply autohydrolyzed economically unexploited lignocellulosic biomass,” Ind. Crops Prod. 170, article 113775. DOI: 10.1016/j.indcrop.2021.113775

Qiao, H., Ouyang, S., Shi, J., Zheng, Z., and Ouyang, J. (2021). “Mild and efficient two-step pretreatment of lignocellulose using formic acid solvent followed by alkaline salt,” Cellulose 28, 1283-1293. DOI: 10.1007/s10570-020-03622-8

Qiu, Z., Han, X., Fu, A., Jiang, Y., Zhang, W., Jin, C., Li, D., Xia, J., He, J., Deng, Y., Xu, N., Liu, X., He, A., Gu, H., and Xu, J. (2022). “Enhanced cellulosic D-lactic acid production from sugarcane bagasse by pre-fermentation of water-soluble carbohydrates before acid pretreatment,” Bioresource Technol. 368, article 128324. DOI: 10.1016/j.biortech.2022.128324

Rajesh, Banu. J., Kavitha, S., Yukesh, Kannah, R., Poornima, Devi, T., Gunasekaran, M., Kim, S. H., and Kumar, G. (2019). “A review on biopolymer production via lignin valorization,” Bioresource Technol. 290, article 121790. DOI: 10.1016/j.biortech.2019.121790

Rajeswari, G., and Jacob, S. (2022). “Co-fermentation of lactic acid and acetone-butanol-ethanol (ABE) from the deep eutectic solvent-pretreated Aloe vera leaf rind through sequential valorization of holocellulose,” Biomass Convers Biorefin. DOI: 10.1007/s13399-022-03688-3

Risanto, L., Adi, D. T. N., Fajriutami, T., Teramura, H., Fatriasari, W., Hermiati, E., Kahar, P., Kondo, A., and Ogino, C. (2022). “Pretreatment with dilute maleic acid enhances the enzymatic digestibility of sugarcane bagasse and oil palm empty fruit bunch fiber,” Bioresource Technol. 369, article 128382. DOI: 10.1016/j.biortech.2022.128382

Saha, B. C., Qureshi, N., Kennedy, G. J., and Cotta, M. A. (2016). “Biological pretreatment of corn stover with white-rot fungus for improved enzymatic hydrolysis,” Int. Biodeterior. 109, 29-35. DOI: 10.1016/j.ibiod.2015.12.020

Saini, S., and Sharma, K. K. (2021). “Fungal lignocellulolytic enzymes and lignocellulose: A critical review on their contribution to multiproduct biorefinery and global biofuel research,” Int. J. Biol. Macromol. 193, 2304-2319. DOI: 10.1016/j.ijbiomac.2021.11.063

Senila, L., Cadar, O., Kovacs E, Gal, E., Dan, M., Stupar, Z., Simedru, D., Senila, M., and Roman, C. (2023). “L-Poly (lactic acid) production by microwave irradiation of lactic acid obtained from lignocellulosic wastes,” Int. J. Mol. Sci. 24, article 9817. DOI: 10.3390/ijms24129817

Sewsynker-Sukai, Y., Suinyuy, T. N., and Kana, E. G. (2018). “Development of a sequential alkalic salt and dilute acid pretreatment for enhanced sugar recovery from corn cobs,” Energy Convers Manage 160, 22-30. DOI: 10.1016/j.enconman.2018.01.024

Sodré, V., Vilela, N., Tramontina, R., and Squina, F. M. (2021). “Microorganisms as bioabatement agents in biomass to bioproducts applications,” Biomass Bioenerg 151, article 106161. DOI: 10.1016/j.biombioe.2021.106161

Song, G., Sun, C., Hu, Y., Wang, C., Xia, C., Tu, M., Zhang, E., Show, P. L., and Sun, F. F. (2022). “Construction of anhydrous two-step organosolv pretreatment of lignocellulosic biomass for efficient lignin membrane-extraction and solvent recovery,” J. Phys. Energy 5, article 014015. DOI: 10.1088/2515-7655/acacc7

Song, W., Li, J., Xiao, Y., Chen, H., Sun, Y., Zhang, S., Li, Y., Chen, G., and Wang, G. (2020). “Building an operational framework for pretreatment corn stover via sulfamic acid/NaCl and application,” Biomass Convers. Biorefin. 12, 5627-5634. DOI: 10.1007/s13399-020-00975-9

Sun, Y., and Cheng, J. (2002). “Hydrolysis of lignocellulosic materials for ethanol production: a review,” Bioresource Technol. 83, 1-11. DOI: 10.1016/S0960-8524(01)00212-7

Sun, Y., Li, X., Wu, L., Yi, L., Fan, L., Zhilong, X., and Tong, Y. (2021). “The advanced performance of microbial consortium for simultaneous utilization of glucose and xylose to produce lactic acid directly from dilute sulfuric acid pretreated corn stover,” Biotechnol. Biofuels 14, article 233. DOI: 10.1186/s13068-021-02085-8

Tang, C., Shan, J., Chen, Y., Zhong, L., Shen, T., Zhu, C., and Ying, H. (2017). “Organic amine catalytic organosolv pretreatment of corn stover for enzymatic saccharification and high-quality lignin,” Bioresource Technol. 232, 222-228. DOI: 10.1016/j.biortech.2017.02.041

Tian, S., Zhao, R., and Chen, Z. (2018). “Review of the pretreatment and bioconversion of lignocellulosic biomass from wheat straw materials,” Renew. Sust. Energ. Rev. 91, 483-489. DOI: 10.1016/j.rser.2018.03.113

Triwahyuni, E., Miftah, A. K., Muryanto, M., Maryan, R., and Sudiyani, Y. (2023). “Conversion of oil palm empty fruit bunch into bioethanol through pretreatment with CO₂ as impregnating agent in alkali explosion,” Biomass Convers. Bioref. DOI: 10.1007/s13399-023-04102-2

Wahlström, R., and Suurnäkki, A. (2015). “Enzymatic hydrolysis of lignocellulosic polysaccharides in the presence of ionic liquids,” Green Chem. 17, 694-714. DOI: 10.1039/C4GC01649A

Wang, J., Gao, M., Liu, J., Wang, Q., Wang, C., Yin, Z., and Wu, C. (2017). “Lactic acid production from Sophora flavescens residues pretreated with sodium hydroxide: Reutilization of the pretreated liquor during fermentation,” Renew. Sust. Energ. Rev. 92, 284-306. DOI: 10.1016/j.rser.2018.04.033

Wang, Y., Abdel-Rahman, M. A., Tashiro, Y., Xiao, Y., Zendo, T., Sakai, K., and Sonomoto, K. (2014). “l-(+)-Lactic acid production by co-fermentation of cellobiose and xylose without carbon catabolite repression using Enterococcus mundtii QU 25,” RSC Adv. 4(42), 22013-22021. DOI: 10.1039/c4ra02764g

Wang, Y., Tashiro, Y., and Sonomoto, K. (2015). “Fermentative production of lactic acid from renewable materials: Recent achievements, prospects, and limits,” J. Biosci. Bioeng. 119, 10-18. DOI: 10.1016/j.jbiosc.2014.06.003

Wang, Y., Zhang, Y., Cui, Q., Feng, Y., and Xuan, J. (2024). “Composition of lignocellulose hydrolysate in different biorefinery strategies: Nutrients and inhibitors,” Molecules 29, article 2275. DOI: 10.3390/molecules29102275

Wei, W., Zhang, H., and Jin, Y. (2019). “Comparison of microwave-assisted zinc chloride hydrate and alkali pretreatments for enhancing eucalyptus enzymatic saccharification,” Energ. Convers. Manage. 186, 42-50. DOI: 10.1016/j.enconman.2019.02.054

Wu, Z., Peng, K., Zhang, Y., Wang, M., Yong, C., Chen, L., Qu, P., Huang, H., Sun, E., and Pan, M. (2022). “Lignocellulose dissociation with biological pretreatment towards the biochemical platform: A review,” Mater. Today Bio. 16, article 100445. DOI: 10.1016/j.mtbio.2022.100445

Xu, G., Li, H., Xing, W., Gong, L., Dong, J., and Ni, Y. (2020a). “Facilely reducing recalcitrance of lignocellulosic biomass by a newly developed ethylamine-based deep eutectic solvent for biobutanol fermentation,” Biotechnol. Biofuels 13, article 166. DOI: 10.1186/s13068-020-01806-9

Yang, B., Tang, Z., Koffi, P. A., He, Y., and Ma, C. (2024). “Significantly enhancing enzymatic saccharification of poplar waste through the effective removal of lignin and hemicellulose by 1-butanol-Na3PO4-water treatment,” Ind. Crop. Prod. 222, article 119890. DOI: 10.1016/j.indcrop.2024.119890

Yee, K. L., Jansen, L. E., Lajoie, C. A., Penner, M. H., Morse, L., and Kelly, C. J. (2018). “Furfural and 5-hydroxymethyl-furfural degradation using recombinant manganese peroxidase,” Enzyme Microb. Technol. 108, 59-65. DOI: 10.1016/j.enzmictec.2017.08.009

Yu, G., Liu, S., Feng, X., Zhang, Y., Liu, C., Liu, Y. J., Li, B., Cui, Q., and Peng, H. (2020). “Impact of ammonium sulfite-based sequential pretreatment combinations on two distinct saccharifications of wheat straw,” RSC Adv. 10, 17129-17142. DOI: 10.1039/d0ra01759k

Zagrodnik, R., Duber, A., and Seifert, K. (2021). “Hydrogen production during direct cellulose fermentation by mixed bacterial culture: The relationship between the key process parameters using response surface methodology,” J. Cleaner Prod. 314, article 127971. DOI: 10.1016/j.jclepro.2021.127971

Zhang, B., Li, H., Chen, L., Fu, T., Tang, B., Hao, Y., Li, J., Li, Z., Zhang, B., Chen, Q., Chen, Q., Nie, C., You, Z-Y., Guan, C-Y., and Peng, Y. (2022a). “Recent advances in the bioconversion of waste straw biomass with steam explosion technique: A comprehensive review,” Processes 10, article 1959. DOI: 10.3390/pr10101959

Zhao, X., Cheng, K., and Liu, D. (2009). “Organosolv pretreatment of lignocellulosic biomass for enzymatic hydrolysis,” Appl. Microbiol. Biotechnol. 82, 815-827. DOI: 10.1007/s00253-009-1883-1

Zhou, Z., Liu, D., and Zhao, X. (2021). “Conversion of lignocellulose to biofuels and chemicals via sugar platform: An updated review on chemistry and mechanisms of acid hydrolysis of lignocellulose,” Renewable Sustainable Energy Rev. 146, article 111169. DOI: 10.1016/j.rser.2021.111169

Zhu, W. B., Sun, H., Zhang, Y., Wang, N., Li, Y., Liu, S., Gao, M., Wang, Y., and Wang, Q. (2023). “Improving lactic acid yield of hemicellulose from garden garbage through pretreatment of a high solid loading coupled with semi-hydrolysis using low enzyme loading,” Bioresource Technol. 384, article 129330. DOI: 10.1016/j.biortech.2023.129330

Zou, L., Ouyang, S., Hu, Y., Zheng, Z., and Ouyang, J. (2021). “Efficient lactic acid production from dilute acid-pretreated lignocellulosic biomass by a synthetic consortium of engineered Pseudomonas putida and Bacillus coagulans,” Biotechnol. Biofuels 14, article 227. DOI: 10.1186/s13068-021-02078-7

Article submitted: November 14, 2024; Peer review completed: December 20, 2024; Revised version received: February 16, 2025; Further revision received and accepted: March 13, 2025; Published: March 24, 2025.

DOI: 10.15376/biores.20.2.Fan