Towards sustainable production of bio-based lactic acid via a bio-based technical route: Recent developments and the use of palm kernel cakes in the bioconversion

Abstract

The continued reliance on non-renewable fossil resources has led to serious environmental issues. In light of these concerns, the transition from non-renewable sources to more sustainable ones have been explored, as exemplified by the production of bio-based lactic acid via lignocellulosic biomass bio-refinery process. Malaysia, the second-largest producer of palm oil in the world, generates abundant, cheap, and underutilized oil palm biomass in the form of palm kernel cakes. Comprised of 50% fermentable hexose sugars, palm kernel cakes have emerged as an interesting feedstock substitute in the production of bio-based fine chemicals, e.g., lactic acid. This paper focuses on current work based on selected literature published in the 21st century on the exploitation of palm kernel cakes as a novel feedstock in bio-refinery processes after addressing the current global demand and potential commodity applications of bio-based lactic acid. It then discusses current research on potential lactic acid-producing microorganisms, with particular attention to bacteria, and different pretreatment methods for carbohydrate recovery from palm kernel cakes. It also highlights the potential of oil palm biomass, especially palm kernel cakes, as a promising commodity that contributes to sugar platforms in value-added products, e.g., biofuel, bioenergy, ethanol, acids, and fine chemicals.

Full Article

Towards Sustainable Production of Bio-based Lactic Acid via a Bio-based Technical Route: Recent Developments and the Use of Palm Kernel Cakes in the Bioconversion

Nuraishah Abd Rahim,^a Abdullah Amru Indera Luthfi,^a,b,* Peer Mohamed Abdul,^a,b Jamaliah Md Jahim,^a,b and Nurul Adela Bukhari ^a,c

The continued reliance on non-renewable fossil resources has led to serious environmental issues. In light of these concerns, the transition from non-renewable sources to more sustainable ones have been explored, as exemplified by the production of bio-based lactic acid via lignocellulosic biomass bio-refinery process. Malaysia, the second-largest producer of palm oil in the world, generates abundant, cheap, and underutilized oil palm biomass in the form of palm kernel cakes. Comprised of 50% fermentable hexose sugars, palm kernel cakes have emerged as an interesting feedstock substitute in the production of bio-based fine chemicals, e.g., lactic acid. This paper focuses on current work based on selected literature published in the 21^st century on the exploitation of palm kernel cakes as a novel feedstock in bio-refinery processes after addressing the current global demand and potential commodity applications of bio-based lactic acid. It then discusses current research on potential lactic acid-producing microorganisms, with particular attention to bacteria, and different pretreatment methods for carbohydrate recovery from palm kernel cakes. It also highlights the potential of oil palm biomass, especially palm kernel cakes, as a promising commodity that contributes to sugar platforms in value-added products, e.g., biofuel, bioenergy, ethanol, acids, and fine chemicals.

DOI: 10.15376/biores.17.2.Rahim

Keywords: Biomass conversion; Hydrolysis; Lactic acid; Palm kernel cake; Pretreatment

Contact information: a: Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi, Selangor 43600 Malaysia; b: Research Centre for Sustainable Process Technology (CESPRO), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi, Selangor 43600 Malaysia; c: Energy and Environment Unit, Engineering & Processing Research Division, Malaysian Palm Oil Board (MPOB), 6, Persiaran Institusi, Bandar Baru Bangi, Kajang, Selangor 43000 Malaysia;

* Corresponding author: amru@ukm.edu.my

INTRODUCTION

The environmental ramifications of the continued dependence on non-renewable crude oil and the escalating demand for fuel and commodity chemicals have steadily contributed to the rise of greenhouse gas emissions, the current climate crisis, and diminishing fossil-based resources. Attempts to substitute the unstable supply of petroleum-based resources with sustainable chemical platform counterparts are underway (Luthfi et al. 2016). One such alternative sustainable chemical platform is lactic acid.

Lactic acid, or 2-hydroxy-propanoic acid (C₃H₆O₃), is a colorless and odorless monocarboxylic acid ranked by the National Laboratories of the U.S. Department of Energy as a second-tier building block chemical (Werpy and Petersen 2004). Discovered in the late 1700s, it is a crucial organic acid molecule required for the anaerobic metabolism of living organisms (Costa et al. 2020). Lactic acid can be found in the form of isomers L and D-lactic acid (Oliveira et al. 2018a). Considered “Generally Recognized as Safe” (GRAS) by the Food and Drug Administration (FDA) (Food and Drug Administration 2015), lactic acid was also listed as one of the most useful building blocks for multiple products (Din et al. 2021). It has recently gained substantial attention in various fields of application, e.g., cosmetics, pharmaceuticals, food, chemicals, intermediate building blocks, and the textile, paper, and detergent industries (Abdel-Rahman et al. 2021).

Lactic acid can be obtained synthetically from acetaldehyde via the lactonitrile pathway. It involves the reaction between acetaldehyde and hydrogen cyanide (HCN), which is catalyzed by a basic catalyst under high temperature and pressure conditions. Sulphuric acid is then further applied to recover and purify the product for the resultant lactic acid (Komesu et al. 2017). The energy consumption required for the high temperature and pressure makes up 30% to 40% of the total operational costs of conventional lactic acid production (Komesu et al. 2017). Apart from its need for expensive raw materials and inefficient energy usage, this process of producing lactic acid also produces a racemic mixture of D- and L- lactic acid. D-lactic acid has been found to be unsafe for humans and thus incurs a high downstream cost, ultimately increasing the total production cost of lactic acid (Mazzoli 2020).

Alternatively, bio-based lactic acid can be derived from carbohydrate feedstocks found in abundant and inexpensive agricultural residues (Manandhar and Shah 2020). Malaysia as the world’s second-largest producer of palm oil generates abundant, cheap, and sugar-rich oil palm biomass in the form of palm kernel cakes (PKC) (Boonsawang et al. 2017). Constituting about 50% fermentable hexose sugars present in the form of galactomannan and glucan, PKC has emerged as a sound and sustainable source for lactic acid production. Lignocellulose feedstock from oil palm biomass contains polysaccharides that can be hydrolyzed via chemicals and/or a cocktail of cellulolytic enzymes under mild operating conditions to liberate fermentable sugars in the form of hexose and pentose sugars. Many microorganisms, including lactic acid bacteria, fungi, and yeast, can utilize these sugars and convert them into a large amount of high purity L-lactic acid through fermentation (Idrees et al. 2013; Lin et al. 2020). It is estimated that 90% of the current global lactic acid supply is derived from bio-based routes and its future demand is expected to increase at an annual rate of more than 12% from 2020 to 2025 (Hassan et al. 2019; Marketsandmarkets.com 2020).

The stereospecificity of pure optical L-lactic acid obtained from bio-based processes is determined by the L- or D-lactate dehydrogenases (LDH) gene type and lactate racemases that could interchange between both of the genes. Hence, studies demonstrated that optically pure lactic acid can be obtained via the deletion, disruption, and genome shuffling of the LDH gene encoded for the enzyme catalyzing the production of unnecessary isomers. Accordingly, it was found that a high productivity of L-lactic acid can be attained in L. lactis strain with UV-mutagenesis due to the enhancement of glucose alongside a reduction in NADH oxidase (Nox) activity of the cell (Hatti-Kaul et al. 2018). It is estimated that 90% of the bio-based lactic acid production worldwide incorporates the lactic acid bacteria for fermentation due to its high environmental adaptability, high titers and productivity, and high tolerance to a wide range of temperatures and pHs (Sauer et al. 2008; Oliveira et al. 2018a).

CURRENT GLOBAL DEMAND

The global lactic acid supply is nearly 270,000 t per year, while the demand for lactic acid as an alternative precursor to biodegradable polylactic acid (PLA) has gained considerable interest in recent years (López-Gómez et al. 2019). Based on a low price of PLA ($0.4 to $9 per kg based on purity and seller), it would be a candidate to replace many other plastic materials (Rajendran and Han 2022). Conversely, global demand for lactic acid has exceeded 1.0 million metric tonnes by 2020 with a projected skyrocketing market value of $2.1 billion in 2016 to $9.8 billion by 2025 (Manandhar and Shah 2020). The current market price of lactic acid is comparatively high at $1.3 to $4.0 per kg (Mazzoli 2020). According to the Markets and Markets report 2020, the global market of bio-based lactic acid was worth USD 1.1 billion in 2020. It is forecasted to grow to USD 2.1 billion by 2025 at a compound annual growth rate (CAGR) of 12.8% (Marketsandmarkets.com 2020).

In recent years, several major companies from different countries, e.g., NatureWorks LLC (United States), Archer Daniels Midland Company (United States), Corbion-Purac (The Netherlands), and Galactic S. A. (Belgium) are emerging and actively engaged in commercializing bio-lactic acid-based products as new commodities (Cubas-Cano et al. 2020). The market of Corbion-Purac is comprised of 74.2% net sales of bakery products, 23.4% of biochemical supplies, and 2.4% others across all of its production plants in the USA, Netherlands, Spain, Brazil, and Thailand (Hörhammer et al. 2014). Nature Works LLC is one of the leading companies in lactic acid production with the brand Ingeo^TM. It can accommodate 140,000 metric tons of Ingeo^TM biopolymer for various applications, e.g., safe food packaging, health and personal care, home appliances and fused deposition modeling (FDM) 3D printing filament (Oliveira et al. 2018b).

Presently, sugarcane dominates the global lactic acid feedstock with the largest revenue (39.0%) in 2020, followed by yeast extract and other raw feedstocks, e.g., cassava, sweet sorghum, potato, wheat, rye-barley, rice, xylan, and galactan. Given the numerous carbon sources from crop sugars that are used for lactic acid production, and despite promising pure lactic acid productivity, the majority of the carbon sources have issues with the cost, seasonality, and by-products formation, as well as competing with food availability (Oliveira et al. 2018b). The issues have a major effect on the overall production cost of lactic acid; to meet the growing demand of lactic acid products, the overall production costs must be less than $0.8 per kg and the market price should decrease by half the current rate (Ajala et al. 2020). The cost of carbon feedstocks makes up 40% to 70% of the total production cost of lactic acid production (López-Gómez et al. 2019). Hence, in the long run of cost-effective lactic acid production, lignocellulosic material is an ideal sugar feedstock alternative due to its low price, sustainability, and non-food abundance, hence achieving food security (Abdel-Rahman et al. 2021). As oil palm biomass, palm kernel cakes (PKC) are relatively abundant and contain most of the needed macro- and micronutrients for the fermentation with microbes (Riyadi et al. 2017).

As compared to other oil palm biomasses, PKC are easily hydrolysable since they contain a low amount of lignins and cellulose and are primarily made up of linear mannan polymers of (1🡪4) linked β-D-mannopyranose and galactomannan with (1🡪6) linked α-D- galactopyranose side groups. Hence, the additional cost for pretreatment can be reduced in terms of readying the lignocellulose for enzyme digestibility, and effectively reduce the total production cost of lactic acid (Raita et al. 2016). To date, PKC has been used to produce the following bio-based products, in the increasing ranking order of economic value: biofuel, biohydrogen, biobutanol, bioadhesive, prebiotic poultry diet feed, and fine chemicals (Stegmann et al. 2020). However, limited focus has been directed towards the highest economic-value bioproducts from PKC. Hence, the usage of PKC as a potential feedstock for a higher economical value product, i.e., lactic acid in this review, not only brings novelty to fully exploit the beneficial nutrients in PKC, but also elevates the image of locally-harvested biomass, promotes a circular economy and sustainability, and generates a larger national revenue.

POTENTIAL COMMODITY APPLICATIONS

Bio-based lactic acid is one of the most promising building-block chemicals for the production of secondary conversion chemicals, e.g., acrylic acid, alkyl lactates, propylene glycol, acetaldehyde, 2,3-pentane dione, dioxanes, polyesters, lactide, and polymers (Manandhar and Shah 2020). The secondary chemicals derived from lactic acid, including green solvents, poly-acrylate, specialty chemical intermediates, poly acrylamide, and phthalate polyesters, have been widely used as precursors in various sectors, e.g., safe food supply, environment, industrial, recreation, housing, transportation, textiles, health, and hygiene applications, as summarized in Fig. 1 (Werpy and Petersen 2004).

Fig. 1. Applications of bio-based lactic acid

There is also a substantial demand for bio-based lactic acid in the manufacturing of polylactic acid (PLA) polymer (Nagarajan et al. 2020). Among all bio-based commodities, PLA is a fast-rising product with a forecasted CAGR of 16.62% in 2025 (Chandel et al. 2020). Polylactic acid polymer is bio-degradable, compostable, and biocompatible. In addition, PLA is widely used in many applications including eco-friendly packages, beverages bottle, prosthetic devices, sutures, and internal drug packaging.

BIO-BASED LACTIC ACID BIOCATALYSTS

Bio-based lactic acid is a primary product of homofermentative glucose metabolism via the Embden–Meyerhof (EM) pathway, which requires glucose-6P isomerase, 6-phosphofructokinase, and fructose-bisphosphate aldolase (Eiteman and Ramalingam 2015). A single mole of glucose yields two moles of pyruvate, which then accepts electrons to form two moles of bio-based lactic acid in a reaction catalyzed via lactate dehydrogenase (LDH) (Hatti-Kaul et al. 2018). Bio-based lactic acid can be produced from biomass feedstocks via aerobically catabolizing starch into lactic acid with the aid of amylolytic fungi, Rhizopus oryzae and Rhizopus arrhizus, as biocatalysts. Nevertheless, amylolytic fungi have poor lactic acid productivity due to a poor reaction rate in limited mass transfer. Therefore, bio-based lactic acid is primarily produced through the anaerobic processing of carbohydrate substances via homolactic microorganisms, e.g., various modified strains of lactic acid-producing bacteria (Juodeikiene et al. 2016). In comparison with aerobic processing, anaerobic fermentation could afford high yields even with less energy requirement. However, anaerobes require specialized media and apparatus along with meticulous skills and techniques, especially for cultivation, which makes them economically unattractive (Huang and Tang 2007). Hence, a novel breakthrough in the lactic acid production has been made using facultative anaerobic A. succinogenes 130Z in a dual-phase (aerobic-anaerobic) fermentation strategy, which afforded productivity of 22-fold higher than in normal anaerobic conditions (Li et al. 2010). The potential characteristics, advantages, and disadvantages of the different lactic acid-producing microorganisms are summarized in Table 1.

Four primary species of bacteria are involved in the production of lactic acid: lactic acid bacteria (LAB), Bacillus sp., Escherichia coli, and Corynebacterium glutamicum (Abdel-Rahman et al. 2013). It is estimated that approximately 90% of global lactic acid production stems from LAB fermentation (Sauer et al. 2008). However, findings on the potential rumen type bacteria, Actinobacillus succinogenes, as a high-titer lactic-acid-producing bacteria are limited, as it has been predominantly used in the production of succinic acid.

Bacterial Strains

Lactobacillus sp.

Lactic acid bacteria (LAB) can be categorized into homofermentative and heterofermentative strains. Homofermentative LAB has an aldolase enzyme expressing gene and produces lactic acid as the primary product of glycolysis by converting one glucose molecule into two lactic acid molecules while releasing two ATP molecules. Conversely, heterofermentative LAB utilizes a phosphoketolase pathway to convert a single xylose molecule into one lactic acid and one ethanol or acetic acid molecule (Juturu and Wu 2016). The metabolic pathway of LAB is summarized in Fig 2.

Table 1. The Potential, Characteristics, Advantages, and Disadvantages of Bio-Based Lactate Producers

Fig. 2. The metabolism pathway of LAB

Lactobacillus sp., belonging to the LAB category, has a variety of genera with more than 150 different species. It is a facultatively anaerobic, Gram-positive, catalase-negative, immotile, and non-spore-forming bacilli (Mejia-Gomez et al. 2020). It has diversified catabolic activities from different environments including the gastro-intestinal tract, soil, and decayed matter. These findings contribute to the suitability of Lactobacillus sp. as a candidate for the conversion of second-generation lignocellulosic feedstocks into value-added metabolites (Boguta et al. 2014). A previous study by Lin et al. (2020) reported a titer of 15.02 g/L ± 0.80 g/L lactic acid after fermenting a synergy of 6% L. acidophilus and L. plantarum inocula in a medium of seaweed hydrolysate (10.54 g/L ± 1.29 g/L) for longer than 24 h at a temperature of 37 °C. Another study found that the simultaneous saccharification and fermentation of L. pentosus in banana flesh hydrolysate containing 56 g/L of reducing sugars yielded 50 g/L of lactic acid (Azaizeh et al. 2020).

Bacillus sp.

There are several advantages of using Bacillus sp. to lower the production cost of lactic acid. Unlike other LAB species, Bacillus sp. requires simple media and cheaper nitrogenenous sources including (NH₄)₂SO₄ and corn steep liquor as their primary nutrient source (Juturu and Wu 2016). They are also heat-labile and can produce lactic acid under high thermal fermentation conditions (greater than or equal to 50 °C), hence removing additional sterilization costs (Aulitto et al. 2017). They can consume both pentose and hexose sugars, thus maximizing the utilization of all sugars from the lignocellulose (Pleissner et al. 2016). However, the species lacks the native genetic tools to metabolize glucose and xylose simultaneously; hence, the metabolism of sugars is inhibited with a carbon catabolite repression (CCR) and thus reduces the output of lactic acid (Li et al. 2017). Another study by Jiang et al. (2016) incorporated inhibitor tolerant mutagenesis into B. coagulans GKN316 to produce 45.39 g/L with an 83.15% yield of lactic acid from corn stover hydrolysate. These Bacillus species include Bacillus coagulans, Bacillus stearothermophilus, Bacillus licheniformis, and Bacillus subtilis (Abdel-Rahman et al. 2013).

Escherichia coli (E. coli)

Escherichia coli strains can rapidly metabolize hexose and pentose sugars, require a minimal demand of nutrients, and are easy to engineer in order to enhance product yield and productivity. Thus, they can be a better cell factory for lactic acid production. Wild-type E. coli generates ethanol and multiple organic acids, e.g., lactic acid, acetic acid, succinic acid, and formic acid, as a replacement to the reducing equivalents in glycolysis (Abdel-Rahman et al. 2013). The three native glycolytic pathways in E. coli are the Embden–Meyerhof–Parnas pathway (EMPP), the oxidative pentose phosphate pathway (OPPP), and the Entner Doudoroff pathway (EDP) (Hollinshead et al. 2016). Two molecules of pyruvates, namely ATP and NADH, are released for every glucose molecule catalyzed by ten enzymatic reactions in the EMP pathway. The second half of the EMPP further converts phosphoenolpyruvate to pyruvate, which serves as a precursor for the formation of succinate, lactate, formate, acetate, and ethanol during anaerobic conditions (as shown in Fig. 3) (Wang et al. 2019). The OPPP is an oxidative pathway for the synthesis of NADPH. The EDP is only activated in the presence of gluconate. Therefore, the metabolism of E. coli most likely depends on the EMP and OPP pathways (Hollinshead et al. 2016).

Fig. 3. The metabolism pathways of E. coli: EMP pathway: Embden–Meyerhof–Parnas pathway; and OPP pathway: oxidative pentose phosphate pathway

The capability of E. coli to survive in both aerobiosis and anaerobiosis, as well as fluctuating oxygen conditions has led to easy manipulations of its metabolism and distortions of repeated functions, which have reportedly caused improved performance under anaerobic conditions (Finn et al. 2017). Evolution engineering has been underway to improve the anaerobic growth of E. coli for the escalation of lactic acid production. This results in a better-evolved, more effective strain as a platform for the anaerobic yield of organic acids. Collaborative works between systematic and classical molecular biology have found that a mutation of sucD and ilvG, coupled with manipulations in an early stage of the evolution of RNA transcription and DNA replication machinery genes, have yielded a prominent effect on the anaerobic growth of E. coli W3110. A study by Wang et al. (2019) demonstrated the competency of an evolutionary engineered E. coli WE269 by successfully yielding 96.5% more lactate than a wild-type strain.

Actinobacillus succinogenes

Actinobacillus succinogenes is a Gram-negative bacteria generally classified as a safe, non-pathogenic biosafety level 1 (BSL-1) microorganism by the DSMZ and ATCC (Chiang et al. 2018). It is an immotile, non-sporulating, facultative anaerobic, pleomorphic, coccobacilli-shaped, capnophilic bacteria, and falls taxonomically under the Pasteurellaceae family (Wang et al. 2019). Actinobacillus succinogenes is able to consume a wide spectrum of carbon sources, e.g., glucose, xylose, arabinose, mannose, galactose, fructose, sucrose, lactose, cellobiose, mannitol, maltose, glycerol, and sucrose, under anaerobic conditions (Luthfi et al. 2017). This is due to the conditions of its natural ecological habitat, i.e., the bovine rumen, which contains an abundance of different carbon sources (Dessie et al. 2018). It is worthy of note that A. succinogenes is one of the most promising succinate and lactate hosts, due to its robustness in tolerating environments with high organic acid concentrations and other potential growth suppressants (Jiang et al. 2014; Chiang et al. 2018; Dessie et al. 2018; Wang et al. 2019). A few studies have recently found that succinic and lactic acid can be produced by A. succinogenes in high yields (50% to 87% and 96% to 97%, respectively) (Wang et al. 2014). This high lactic acid productivity can be achieved using dual phase cultivation of the strain to shift the carbon flux distribution into lactate (Zhang et al. 2019). The presence of oxygen at the beginning of the fermentation promotes cell growth and cell viability in first phase and stimulates the formation of pyruvate and lactate in the latter phase of anaerobic cultivation (Li et al. 2010). Moreover, lactate dehydrogenase (LDH) activity also increased by 18-fold in dual phase fed-batches modes cultivation, resulting in 32-fold increase in lactic acid production with a final concentration of 135.6 g/L ± 0.14 g/L (Wang et al. 2016). Hence, A. succinogenes is a promising bioproducer for lactic acid, even with no genetic modification.

Figure 4 depicts the metabolic pathway of A. succinogenes. The metabolism in the cell relies on the uptake of glucose via permease. The glucose is phosphorylated by the phosphoenolpyruvate dependant phosphotransferase system and hexokinase activity to form glucose-6-phosphate. The metabolic conversion of glucose-6-phosphate into phosphoenolpyruvate (PEP) takes place in the glycolytic pathway and OPPP (McKinlay et al. 2007). The anaerobic carbon flux of A. succinogenes utilizes PEP to split the reaction into two branches. The first branch produces lactate, acetate, ethanol, and the formate-forming C3 pathway, while the second branch leads into the succinate-forming C4 pathway (McKinlay and Vieille 2008). The lactate-forming C3 pathway is controlled by the pyruvate formate lyase 1-activating protein (pflA) and 6-lactate dehydrogenase (encoded LDH) and results in the formation of lactic acid (Zhang et al. 2019). The 6-lactate dehydrogenase is the NADH-dependent catabolic enzyme that serves to reduce pyruvate for producing lactic acid (Novy et al. 2018; Chroumpi et al. 2020).

Fig. 4. Metabolism pathway of A. succinogenes: LDH: lactate dehydrogenase

PERSPECTIVE OF PALM BIOMASS FROM MALAYSIA

Agricultural byproducts comprising of lignocellulosic materials are a promising feedstock for the production of lactic acid because they are abundant, inexpensive, renewable, sustainable, environmentally friendly, and can potentially reduce environmental pollution (Azaizeh et al. 2020). Moreover, since lignocellulose biomass is non-edible and is a source of fermentable sugars, it is highly suited for the fermentation of various biochemicals, e.g., bio-based lactic acid, via microorganisms. It is a good alternative, as it reduces the competition between the demand for resources required in the production of lactic acid and those used for global food and feed (Aulitto et al. 2017). The primary component of lignocellulosic biomass is a cell wall comprised of 30% to 60% cellulose, 20% to 40% hemicellulose, 15% to 25% lignin, and other extractives (Nanda et al. 2015; Bukhari et al. 2020b).

Malaysia is one of the leading countries involved in promoting biomass usage (Luthfi et al. 2017). The annual rate of biomass generation in Malaysia averages at 80 million tonnes. It exceeded 100 million tonnes in 2020 and is expected to rise to 1200 million tonnes by 2035 (Yatim et al. 2017; How et al. 2019). The various types of biomasses used are rice husks, oil palm biomass, sago biomass, timber residue, coconut trunk fibers, sugar cane residues, and other forms of agricultural residues (Yatim et al. 2017). As one of the largest producers of palm oil in the world, Malaysia stores an ample amount of oil palm biomass, which remain mostly untapped. The oil palm biomass including empty fruit bunches (EFB), oil palm trunks (OPT), mesocarp fibres (MF), oil palm fronds (OPF), and palm kernel shell (PKS) have high calorific values. Hence, they can be reutilized in the production of food, pharmaceutical constituents, high-quality chemicals, biofuel, polymers, and bioenergy via multiple techniques from a biorefinery processing perspective (Sadhukhan et al. 2018; Hassan et al. 2019).

Palm Kernel Cake as an Emerging Biorefinery Feedstock

The palm oil industry in Malaysia has a total of 457 actively operating palm oil mills with an entire processing capacity of 116.81 million tons of fresh fruit bunches (FFB) per year (Parveez et al. 2021). This is made up of 20% crude palm oil and 80% oil palm lignocellulosic solid biomass on a wet basis (Azman et al. 2016; Raita et al. 2016). The oil palm residues primarily contain 23% empty fruit bunches, 28% palm oil mill effluents, 7% PKC, 15% oil palm mesocarp fibers, and 6% palm kernel shells, as illustrated in Fig. 5 (Onoja et al. 2019). The PKC is the residue of palm fruit after extracting the oil, which makes up approximately 2% to 7% of the total oil palm biomass (Onoja et al. 2019; Jana and Kango 2020). Malaysia produced 2.59 million tons of PKC in 2019 and the figure is expected to grow bigger in the near future (Halim et al. 2020).

Fig. 5. The schematic diagram of oil palm components with their weight percentage on a wet basis

Palm kernel cakes, though inexpensive, plentiful, and hold a vast potential for biochemical conversions, remain ineffectively utilized (Asri et al. 2020). Presently, the utilization of PKC primarily focuses on husbandry feed additives, but has minimal nutrient and amino acid contents, low protein digestibility, and is rough-textured (Oladokun et al. 2016; Alshelmani et al. 2017). As can be seen in Fig. 6, pharmaceutical and fine chemical products are categorized under the highest value biomass applications. This is followed by food and feed products as well as polymers and eco-friendly materials. Energy and fuels are categorized under the lowest-value biomass application in the bio-based product value pyramid (as shown in Fig. 6) (Stegmann et al. 2020).

Therefore, the redistribution of PKC to meet the needs of competing bio-based applications can be achieved by reutilizing the PKC in the production of valuable lactic acid. Pretreated PKC has been recently incorporated with enzymatic hydrolysis as feedstock to produce biobutanol, biohydrogen, bioethanol, prebiotic, poultry diet feed additives, bioadhesive, and biofuels (Cerveró et al. 2010; Shukor et al. 2013; Shukor et al. 2014; Azman et al. 2016; Jahromi et al. 2016; Shukor et al. 2016; Tafsin et al. 2017; Sari et al. 2018).

Fig. 6. The value pyramid of bio-based products

These interests emerge due to the potentially low lignin contents in PKC coupled with the richness in polysaccharides, including cellulose (11.6% glucan), hemicellulose (35.2% mannan, 2.6% xylan), and other fermentable C6 sugars (approximately 50%) made of glucose and mannose, which can be further utilized by most of the fermenting microbes (Azman et al. 2016; Jahromi et al. 2016). The primary carbohydrates in PKC are naturally in the form of mannan, glucomannan, galactomannan, and galacto-glucomannan (Kalidas et al. 2017). Similar to hemicellulose, the mannans in PKC are compact and crystallized, yet are still easily hydrolysable due to the presence of a highly linear β-1,4-linked mannose backbone and a low amount of single α-1,6-linked galactose side groups (Li et al. 2018).

The fermentable sugars content in PKC is as high as the sugar content found in empty fruit bunches (EFB) and oil palm trunk (OPT), which are 49% to 56% and 68.9% of the carbohydrates, respectively, which were reportedly used as a carbon feedstock in previous lactic acid production (Sitompul et al. 2014; Eom et al. 2015; Juturu and Wu 2018). Hence, PKC has a great potential to be one of the most promising emerging feedstocks for lactic acid production (Asri et al. 2020). Furthermore, PKC contains most of the macro- and micronutrients required for a cost-effective basal medium for fermentation, including 22000 mg/kg of protein, 6190.9 mg/kg of potassium, 533.5 mg/kg of calcium, 382.5 mg/kg of sodium, 310.4 mg/kg of iron, 161.6 mg/kg of manganese, and 0.84 mg/kg of phosphorus (Shukor et al. 2016). Comprising low level of lignin and cellulose, PKC only requires mild pretreatment with lesser formation of inhibitory by-products for superior purity/optical purity of lactic acid.

PRETREATMENT OF PALM KERNEL CAKES

Pretreatment refers to the disruption of the recalcitrant structure of lignins to expose the underlying carbohydrate components for enzyme accessibility (Haldar and Purkait 2021). A good pretreatment method should be cheap, eco-friendly, and with minimal microorganism inhibitor formation (Cheah et al. 2020). The delignification of a lignocellulosic biomass in the pretreatment phase serves to increase the total surface area of the carbohydrates and therefore the simple sugar yield by fragmenting the size of the biomass and decrystalizing the cellulose structure (Rezania et al. 2020). Table 2 summarizes a variety of pretreatment and hydrolysis methods used in previous works for PKC recovery.

Table 2. Pretreatment and Hydrolysis Methods of Palm Kernel Cakes (PKC) Reported in Previous Studies

The common pretreatment methods found in previous studies can be effectively classified as physical, chemical, or physicochemical based on their operational mechanisms. Physical methods tend to fractionate the structure of the biomass, reducing the particle size and allowing for biomass susceptibility to enzyme digestibility. Methods that fall under this category include grinding, ball milling, microwave-assisted pretreatment, and electron beam irradiation (Anu et al. 2020).

Fig. 7. The process flow including a variety of pretreatments and the hydrolysis of PKC for bio-lactic acid production

However, these methods require high power and energy and can result in inefficient sugar recovery (Anu et al. 2020; Haldar and Purkait 2021). The demerits of the physical methods have thus given rise to the advent of cheaper, faster, and more effective chemical and physicochemical pretreatments. Besides eliminating the lignins, the content of hemicellulose and cellulose recovery can be maximized. These methods include acid, alkaline, hydrothermal, and steam explosion (Ranjithkumar et al. 2017; Rahmati et al. 2020). The process flow, which includes a variety of pretreatments and the hydrolysis of PKC, of bio-lactic acid production is simplified in Fig. 7.

Alkaline Pretreatment

Alkaline pretreatment is a vastly studied pretreatment technique that enhances enzyme digestibility by increasing the surface area of carbohydrates for enzyme digestibility. The application of bases under low pressure and temperature conditions allows them to penetrate the lignocellulose structure, cleave the ester bonds between the hemicellulose and lignin, solubilize the lignin, remove the acetyl groups, and swell the cellulose to achieve an increased surface area (Bensah and Mensah 2013). The process results in a solid portion of cellulose and a liquid stream comprising of solubilized lignins, hemicelluloses, and intact inorganic chemicals. Despite its longer retention time, it is efficient in the delignification of low lignin lignocellulosic biomasses (Guragain et al. 2016; Xu et al. 2016). Commonly used bases include sodium hydroxide, calcium hydroxide, potassium hydroxide, and ammonium hydroxide (Aftab et al. 2019).

Among these bases, sodium hydroxide emerges as the most effective base due to its ability to dissociate in different materials (Baruah et al. 2018). The usage of only 1% sodium hydroxide in the pretreatment yielded 83% reducing sugars in corn stover, 78% in brown-midrib (BMR) sorghum stalks, 75% in sorghum stalks, 65% in switchgrass, and 58% in poplar biomass (Guragain et al. 2016). When applied to PKC, the total reducing sugars obtained from a 1% sodium hydroxide pretreatment at a temperature of 80 °C for 1 h resulted in 2.15 g/L of total sugars with no growth of C. acetobutylicum YM1, i.e., no butanol formation throughout the 72 h of fermentation (Al-Tabib et al. 2017). It was reported that the Clostridium growth was inhibited by the presence of phenolic compounds released from the delignification of the PKC during the alkaline pretreatment. However, a neutralization step is needed prior to fermentation to remove the inhibitory phenolic compounds, thus increasing the overall production cost of a biochemical (Xu et al. 2016). Hence, alkaline pretreatment seems to be less effective for the cellulose and hemicellulose recovery in PKC.

Hydrothermal Pretreatment

Hydrothermal pretreatment is considered an ideal alternative for the pretreatment of lignocellulosic biomass in large industrial-scale processing because it is safer, more environmentally friendly, energy-saving, has a low capital cost, and contributes to a lower formation of inhibitory compounds (Rizal et al. 2018; Padilha et al. 2019). Hydrothermal pretreatment involves the use of hot water and steam explosion to break down the recalcitrant structures of lignocellulosic biomass while hydrolyzing the cellulose and degrading the hemicellulose and lignin (Ewanick and Bura 2010; Ahmad et al. 2018).

The use of hot water entails a temperature of 170 to 230 °C and a pressure of greater than 5 MPa for a minimum of 20 min with ionizing hydronium ion as a catalyst to depolymerize the hemicellulose (Ali et al. 2020). Steam explosion involves heating the lignocellulosic materials to a temperature between 160 and 290 °C, at a pressure of 20 bar to 50 bar for 20 min to 40 min (Ahmed et al. 2019).

Due to a sudden drop in pressure, steam explosion causes the autohydrolysis of hemicellulose through the explosion effect, hence improving the enzyme digestibility of the cellulose-rich materials (Raita et al. 2016). Autohydrolysis is a form of high-pressured steam pretreatment by heating lignocellulosic materials at a temperature of 121 °C for 20 min, which releases a blackish-brown fraction of cellulose-rich materials and leaves behind hemicellulose and solubilized lignin (Debiagi et al. 2020).

The effectiveness of hydrothermal pretreatment in terms of releasing phenols from lignins while yielding reducing sugars is indicated by the severity factor, log R_o. This is associated with the synergistic reactions between the temperature and time (Zakaria et al. 2015). A study by Mi et al. (2016) found that pretreatment of PKC with hot water at a temperature of 180 °C for 20 min yielded 12.5% reducing sugars without furfural formation. The same study also noted that a similar yield of reducing sugars was obtained when PKC was pretreated with hot steam autohydrolysis at temperatures higher than 121 °C for 20 min. Another study reported that subjecting PKC to a steam explosion at a temperature of 163 °C at 6.5 bar for 20 min resulted in a 15.69% yield of reducing sugars with furfural formation (Raita et al. 2016). No considerable difference in the total sugar yield was noted between the raw, untreated PKC, and pretreated PKC after autohydrolysis at a temperature of 126 °C for 11 min (Cerveró et al. 2010). The same study also demonstrated that untreated PKC and pretreated PKC via hot steam autohydrolysis yielded similar amounts of reducing sugars and butanol. It should be noted that pretreating PKC by heating it at a temperature of 180 °C for 10 min will yield a lower amount of reducing sugars compared to untreated PKC due to the degradation of sugars at high temperatures via the Maillard reaction (Al-Tabib et al. 2017). A case in point, Tafsin et al. (2017) found that the autohydrolysis of PKC at a temperature of 121 °C for 15 min recovered as low as 0.91% to 3.25% of the total sugars. Previous studies applied hydrothermal pretreatment followed by enzymatic hydrolysis on PKC to obtain an optimum sugars recovery range of 28.34% to 43.43%, as depicted in Table 2 (Cerveró et al. 2010; Jahromi et al. 2016; Mi et al. 2016; Raita et al., 2016). In general, the hydrothermal pretreatment of PKC is cost-effective, energy saving, and relatively mild, but the sugars recovery is found to be considerably low compared to the alkaline pretreatment; hence, the sugar recovery has to be further optimized via enzymatic hydrolysis.

Acid Hydrolysis

The recalcitrant structure of crystallized cellulose and ligno-polysaccharide linkages have given rise to a variety of severe treatment methods. Amongst these, acid hydrolysis has been the most favorable chemical pretreatment (Rahmati et al. 2020). Acid hydrolysis destroys the linkages of lignins and depolymerizes the hemicelluloses to yield high recovery of monomer sugars. This technique can be performed with mineral acids (hydrochloric acid, sulphuric acid, phosphoric acid, nitric acid) as well as organic acids (oxalic acid, acetic acid, formic acid, propionic acid) (Rezania et al. 2020). Pretreatment via use of mineral acids can be further subcategorized into concentrated and diluted acid methods. Concentrated acid methods involve the use of greater than 30% acid. These methods are less likely to be applied, due to their costly operational procedures, highly corrosive nature, and formation of inhibitory byproducts, e.g., phenolic compounds, aldehydes, 5-hydroxymethylfurfural (HMF), and furfural (Jönsson and Martín 2016).

Many studies have demonstrated that applying dilute acid hydrolysis (less than 10% acid) on a wide spectrum of lignocellulose biomasses results in high sugar monomer recovery and improved hemicellulose structure in terms of enzyme digestibility (Chen et al. 2017). A study by Agbor et al. (2011) reported that subjecting biomass to a 0.1% dilute sulphuric acid pretreatment resulted in a greater than 80% glucose yield. In addition, a 1.5% dilute sulphuric acid pretreatment on Bermuda grass and rye straw before they were subjected to enzymatic hydrolysis yielded 19.71% and 22.93% of reducing sugars, respectively (Ranjithkumar et al. 2017). A study by Azman et al. (2016) demonstrated that the usage of 7% (v/v) dilute sulphuric acid in the pretreatment of PKC released 27.75 g/L of mannose, 2.75 g/L of glucose, and 2.60 g/L of xylose. Another study found that the use of 2% dilute sulphuric acid in PKC treatment yielded 30 g/L of reducing sugars containing 3.78 g/L of glucose and 26.22 g/L of mannose (Al-Tabib et al. 2017).

Even though the use of dilute mineral acid is a cheaper technique, it is still corrosive and forms inhibitory compounds; the demerits are similar to its concentrated counterpart (Ahmad et al. 2018). Due to the above issues, organic acids, e.g., oxalic acid, acetic acid, and propionic acid, have emerged as environmentally friendly alternatives (Baruah et al. 2018). These organic acids are effective in hydrolyzing a wide spectrum of carbohydrate feedstock over different ranges of temperatures and pHs, are less toxic, provides no inhibition during glycolysis, and are odorless because of their multiple pKa values (Ranjithkumar et al. 2017). Furthermore, inhibitory compounds, e.g., HMF and furfural, are found as trace elements in the organic acid-pretreated hydrolysate (Bukhari et al. 2020). Oxalic acid was demonstrated by a study and found that 92% glucose can be recovered from sugarcane bagasse after being pretreated via the microwave-oxalic acid method (Solihat et al. 2020). In an experiment studying the effects of hydrochloric acid, maleic acid, oxalic acid, and sulfuric acid on the pretreatment of sugarcane bagasse, it was found that the oxalic acid pretreatment yielded the highest maximum percentage yield of 95.7% xylose (Yan et al. 2018). In addition, the same observations were seen in a study by Bukhari et al. (2020), which found that the use of oxalic acid yielded 67% xylose, making it the most effective organic acid for the acid hydrolysis of oil palm trunk biomass (OPTB). In a similar study, the acid hydrolysis performance of oxalic acid, formic acid, and citric acid were tested on OPTB with xylose recoveries of 61.2%, 20.1%, and 12.9%, respectively. The excellent performance of oxalic acid is due to the release of H⁺ from its dicarboxylic structure, which has superior selectivity for the hydrolysis of hemicellulose into monomer sugars, is less toxic, and does not inhibit the glycolysis of bacteria in the latter fermentation process (Song et al. 2020). Oxalic acid has been used in various biomass pretreatments. However, few findings demonstrate the effectiveness of oxalic acid and other organic acids for the pretreatment of PKC. To date, only dilute sulphuric acid and acetic acid have been deployed on PKC, which produced a hydrolysate rich in mannose for subsequent biochemical production (Azman et al. 2016; Tafsin et al. 2017). Therefore, using oxalic acid pretreatment in PKC seems to be a good tradeoff between high sugars recovery yields (mainly mannan) and avoiding the formation of inhibitory compounds. Judging from the aforementioned aspects, oxalic acid pretreatment is preferred for the processing of PKC to achieve desirable purity and yield of lactic acid without requiring enzymes or additional solvents.

Enzymatic Hydrolysis

Enzymatic hydrolysis is a key performer in subsequent steps to ensure the highest possible concentration of fermentable sugars (Peinemann and Pleissner 2020). It degrades β-mannan in PKC and releases sugar-rich hydrolysate monomers as a carbon source for further use in the fermentation process (Shukor et al. 2016; Sathitkowitchai et al. 2018). The hydrolysis of mannan involves four types of enzymes: endo(1→4)-β-mannanase, exo(1→4)-β-mannanase or β-mannosidase, β-glucosidase, and α-galactosidase. Endo(1→4)-β-mannanase depolymerizes mannan, galactomannan, and glucomannan into oligomers, so that β-mannosidase can cleave them into mannobiose. β-glucosidase and α-galactosidase then catalyze the mannobiose into mannose monomers (Aguzey et al. 2020; Peinemann and Pleissner 2020).

The efficiency of mannan-rich PKC hydrolysis has improved with the development of enzyme cocktails that aim to improve the saccharification and stability efficiency through the reduction of end-product inhibitors and lignin binding (Østby et al. 2020). Cellic CTec2 from Novozymes, a core enzyme cocktail, is known to exhibit a high efficiency for the saccharification of cellulose and hemicellulose, thus optimizing operational cost-efficiency for converting pretreated PKC into fermentable sugars (Malgas and Pletschke 2020). Malgas and Pletschke (2020) found that the synergistic effects between mannanase and Cellic CTec2 during mannan hydrolysis increased the fermentable sugar yield by 1.3-fold. Another study also found that enzymatically hydrolyzing 5% pretreated PKC with the synergistic effects of mannanase and Cellic CTec2 at a temperature of 50 °C and a pH of at 5250 rpm for 72 h afforded the highest fermentable sugar yield of 28.34 g/L ± 0.06 g/L (Raita et al. 2016).

Saccharification performance can be further maximized by optimizing the enzyme loading, pH, and temperature of the reaction medium, the substrate concentration, and the duration of incubation (Shukor et al. 2016). The longer the incubation time, the higher the yield of reducing sugars. However, the yield of sugar will begin to decrease after the optimum incubation time, due to the formation of inhibitory products that degrade the reducing sugars (Solihat et al. 2020).

FERMENTATION STRATEGY FOR LACTIC ACID PRODUCTION

As discussed in the previous sections of the paper, the biochemical conversion of lignocellulosic sugars into organic acids and metabolites includes pretreatment, enzymatic or acid hydrolysis, and fermentation steps. The resulting products and the overall production performance depend on the fermenting microorganisms (Komesu et al. 2017). Numerous fermentation strategies are underway to produce high titer lactic acid using different types of biomasses and lactic acid-bacteria, as summarized in Table 3. It is worth mentioning that a high titer of lactic acid is obtained during fermentation using Actinobacillus succinogenes by benefiting from its incomplete TCA cycle (Zhang et al. 2019).

A study has demonstrated that fed-batch fermentation can produce lactic acid at 1.65 times greater yield than its batch counterpart (Chen et al. 2020). Aeration, another factor in the process of fermentation, plays a critical role in maximizing the yield and productivity of lactic acid. It was reported that the early introduction of oxygen-rich aeration followed by deoxygenated air sparged in the dual-phase cultivation strategy promoted a 22-fold increase in the titers of lactic acid (Li et al. 2010). The bi-stage fermentation enhances the C3-lactate formation flux, suppresses the C4-succinate formation flux, and enhances the activity of LDH in Actinobacillus succinogenes 18-fold without the need for genetic modification (Zhang et al. 2019). The initial aerobic cultivation promotes cell growth and viability, while subsequent anaerobic cultivations enhance the high titers of lactic acid (Wang et al. 2016).

Table 3. Different Biomass Substrates with the Concentration, Yield, and Productivity of Lactic Acid Fermentation Using Different Microbes

PALM KERNEL CAKE AS A POTENTIAL FEEDSTOCK FOR LACTIC ACID PRODUCTION

Palm kernel cake is low cost and locally available material in Malaysia, but its application is not fully satisfactorily acceptable in animal feeding, as it has low palatability and low nutritional value due to the presence of non-starch polysaccharides (NSPs) and being comprised of 15% to 17% crude protein and 16% crude fibre. It is also lacking in a few essential amino acids, e.g., lysine and methionine (Azizi et al. 2021). To date, the usage of PKC covers only 30% of ruminants feed and are still dependent on the cost of other protein supplements and the cost of balancing the amino acid proportions. Even though comprehensive studies are underway on assessing new strategies to improvise the nutritive value of PKC as animal feed, such as supplementation with enzymes, extrusion, and conversion through fermentation (Alshelmani et al. 2021), higher cost and energy will be required for the feed formulation development. As a result, the feed industry in Malaysia is still heavily dependent on imported feeds, which accounts for RM10 billion per year. According to the Department of Statistics Malaysia (2021), approximately 2.53 million tons of the PKC in Malaysia, which accounts for USD 31 million, is exported to India, China, the European Union (EU), and the Philippines (Parveez et al. 2021). Approximately 90% of PKC exports are absorbed by the EU bloc, especially Germany and the Netherlands, as components in their livestock feed formulations (Aspar 2001). The abundant availability of PKC in Malaysia from 42 plants crushers with a total capacity of 7.17 million tons of PKC in 2020 should be effectively exploited by reallocating the utilization of PKC to produce higher economical value products in the form of fine chemicals (Parveez et al. 2021). Bio-based lactic acid is currently in the spotlight as a fine-chemical with fast growing application and market demand. Therefore, taking this opportunity to bring in a bigger revenue to the country, process improvements in the biorefinery of PKC allows the industry to move towards the use of greener technology, which aligns with the current needs of the growing global population, modern urbanization, economical development, and a new sustainable living environment.

CONCLUSIONS

1. Malaysia is one of the few countries with an untapped and underutilized pool of renewable resources in the form of oil palm biomass.
2. The palm kernel cake (PKC) is identified as an excellent feedstock for the production of bio-based lactic acid, due to its abundance, low costs, and high amount of fermentable sugars.
3. This review also analyzes criteria in the selection of promising fermentative microorganisms for the effective conversion of fermentable sugars into bio-based lactic acid.

ACKNOWLEDGMENTS

The authors acknowledge the profound financial support from the Ministry of Higher Education, Malaysia through the Research University Grant (No. GGPM-2021-005). The authors wish to gratefully acknowledge the financial support by Yayasan Sime Darby through the UKM-Yayasan Sime Darby Chair for Sustainability Postgraduate Scholarship.

REFERENCES CITED

Abdel-Rahman, M. A., Tashiro, Y., and Sonomoto, K. (2013). “Recent advances in lactic acid production by microbial fermentation processes,” Biotechnology Advances 31(6), 877-902. DOI: 10.1016/j.biotechadv.2013.04.002

Abdel-Rahman, M. A., Hassan, S. E.-D., Alrefaey, H. M. A., and Elsakhawy, T. (2021). “Efficient co-utilization of biomass-derived mixed sugars for lactic acid production by Bacillus coagulans Azu-10,” Fermentation 7(1), 1-17. DOI: 10.3390/fermentation7010028

Aftab, M. N., Iqbal, I., Riaz, F., Karadag, A., and Tabatabaei, M. (2019). “Different pretreatment methods of lignocellulosic biomass for use in biofuel production,” in: Biomass for Bioenergy-Recent Trends and Future Challenges. A. El-Fatah Abomohra (ed.), InTechOpen Limited, London.

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

Aguzey, H. A., Gao, Z. H., Wu, H. H., Cheng, G. L., Wu, Z. M., Chen, J. H., and Niu, Z. L. (2020). “Enzymatic treatment of palm kernel cake improves intestinal health, gut microbiota and immune system of broilers,” Journal of Animal and Plant Sciences 30(3), 517-526. DOI: 10.36899/JAPS.2020.3.0061

Ahmad, F., Silva, E. L., and Varesche, M. B. A. (2018). “Hydrothermal processing of biomass for anaerobic digestion – A review,” Renewable and Sustainable Energy Reviews 98, 108-124. DOI: 10.1016/j.rser.2018.09.008

Ahmad, F. B., Zhang, Z., Doherty, W. O. S., and O’Hara, I. M. (2019). “The outlook of the production of advanced fuels and chemicals from integrated oil palm biomass biorefinery,” Renewable and Sustainable Energy Reviews 109, 386-411. DOI: 10.1016/j.rser.2019.04.009

Ahmed, B., Aboudi, K., Tyagi, V. K., Álvarez-Gallego, C. J., Fernández-Güelfo, L. A., Romero-García, L. I., and Kazmi, A. A. (2019). “Improvement of anaerobic digestion of lignocellulosic biomass by hydrothermal pretreatment,” Applied Sciences 9(18), 1-17. DOI: 10.3390/app9183853

Ajala, E. O., Olonade, Y. O., Ajala, M. A., and Akinpelu, G. S. (2020). “Lactic acid production from lignocellulose – A review of major challenges and selected solutions,” ChemBioEng Reviews 7(2), 1-13. DOI: 10.1002/cben.201900018

Ali, N., Zhang, Q., Liu, Z.-Y., Li, F.-L., Lu, M., and Fang, X.-C. (2020). “Emerging technologies for the pretreatment of lignocellulosic materials for bio-based products,” Applied Microbiology and Biotechnology 104(2), 455-473. DOI: 10.1007/s00253-019-10158-w

Alshelmani, M. I., Loh, T. C., Foo, H. L., Sazili, A. Q., and Lau, W. H. (2017). “Effect of solid state fermentation on nutrient content and ileal amino acids digestibility of palm kernel cake in broiler chickens,” Indian Journal of Animal Sciences 87(9), 1135-1140.

Al-Tabib, A. I., Al-Shorgani, N. K., Hasan, H. A., Hamid, A. A., and Kali, N. S. (2017). “Production of acetone, butanol, and ethanol (ABE) by Clostridium acetobutylicum YM1 from pretreated palm kernel cake in batch culture fermentation,” BioResources 12(2), 3371-3386. DOI: 10.15376/biores.12.2.3371-3386

Anu, Kumar, A., Rapoport, A., Kunze, G., Kumar, S., Singh, D., and Singh, B. (2020). “Multifarious pretreatment strategies for the lignocellulosic substrates for the generation of renewable and sustainable biofuels: A review,” Renewable Energy 160, 1228-1252. DOI: 10.1016/j.renene.2020.07.031

Aspar, H. M. (2001). “Malaysian palm kernel cake as animal feed,” Palm Oil Developments 34, 4-6.

Asri, N. M., Muhialdin, B. J., Zarei, M., and Saari, N. (2020). “Low molecular weight peptides generated from palm kernel cake via solid state lacto-fermentation extend the shelf life of bread,” LWT 134, 1-11. DOI: 10.1016/j.lwt.2020.110206

Aulitto, M., Fusco, S., Bartolucci, S., Franzén, C. J., and Contursi, P. (2017). “Bacillus coagulans MA-13: A promising thermophilic and cellulolytic strain for the production of lactic acid from lignocellulosic hydrolysate,” Biotechnology for Biofuels 10(1), 1-15. DOI: 10.1186/s13068-017-0896-8

Azaizeh, H., Tayeh, H. N. A., Schneider, R., Klongklaew, A., and Venus, J. (2020). “Production of lactic acid from carob, banana and sugarcane lignocellulose biomass,” Molecules 25(13), 1-14. DOI: 10.3390/molecules25132956

Azizi, M. N., Loh, T. C., Foo, H. L., and Chung, E. L. T. (2021). “Is palm kernel cake a suitable alternative feed ingredient for poultry?,” Animals 11(2), 1-15. DOI: 10.3390/ani11020338

Azman, N. F., Abdeshahian, P., Kadier, A., Shukor, H., Al-Shorgani, N. K. N., Hamid, A. A., and Kalil, M. S. (2016). “Utilization of palm kernel cake as a renewable feedstock for fermentative hydrogen production,” Renewable Energy 93, 700-708. DOI: 10.1016/j.renene.2016.03.046

Baruah, J., Nath, B. K., Sharma, R., Kumar, S., Deka, R. C., Baruah, D. C., and Kalita, E. (2018). “Recent trends in the pretreatment of lignocellulosic biomass for value-added products,” Frontiers in Energy Research 6, 1-19. DOI: 10.3389/fenrg.2018.00141

Bensah, E. C., and Mensah, M. (2013). “Chemical pretreatment methods for the production of cellulosic ethanol: Technologies and innovations,” International Journal of Chemical Engineering 2013, 1-22. DOI: 10.1155/2013/719607

Boguta, A. M., Bringel, F., Martinussen, J., and Jensen, P. R. (2014). “Screening of lactic acid bacteria for their potential as microbial cell factories for bioconversion of lignocellulosic feedstocks,” Microbial Cell Factories 13(1), 1-16. DOI: 10.1186/s12934-014-0097-0

Bukhari, N. A., Jahim, J. M., Loh, S. K., Nasrin, A. B., Harun, S., and Abdul, P. M. (2020a). “Organic acid pretreatment of oil palm trunk biomass for succinic acid production,” Waste and Biomass Valorization 11(10), 5549-5559. DOI: 10.1007/s12649-020-00953-2

Bukhari, N. A., Loh, S. K., Luthfi, A. A. I., Nasrin, A. B., Abdul, P. M., Harun, S., and Jahim, J. M. (2020b). “Oil palm trunk biomass pretreatment with oxalic acid and its effect on enzymatic digestibility and fermentability,” Materials Today: Proceedings 42, 119-123. DOI: 10.1016/j.matpr.2020.10.26

Cerveró, J. M., Skovgaard, P. A., Felby, C., Sørensen, H. R., and Jørgensen, H. (2010). “Enzymatic hydrolysis and fermentation of palm kernel press cake for production of bioethanol,” Enzyme and Microbial Technology 46(3-4), 177-184. DOI: 10.1016/j.enzmictec.2009.10.012

Chandel, A. K., Garlapati, V. K., Kumar, S. P. J., Hans, M., Singh, A. K., and Kumar, S. (2020). “The role of renewable chemicals and biofuels in building a bioeconomy,” Biofuels, Bioproducts and Biorefining 14(4), 830-844. DOI: 10.1002/bbb.2104

Cheah, W. Y., Sankaran, R., Show, P. L., Ibrahim, T. N. B. T., Chew, K. W., Culaba, A., and Chang, J.-S. (2020). “Pretreatment methods for lignocellulosic biofuels production: Current advances, challenges and future prospects,” Biofuel Research Journal 7(1), 1115-1127. DOI: 10.18331/BRJ2020.7.1.4

Chen, H., Liu, J., Chang, X., Chen, D., Xue, Y., Liu, P., Lin, H., and Han, S. (2017). “A review on the pretreatment of lignocellulose for high-value chemicals,” Fuel Processing Technology 160, 196-206. DOI: 10.1016/j.fuproc.2016.12.007

Chen, P.-T., Hong, Z.-S., Cheng, C.-L., Ng, I.-S., Lo, Y.-C., Nagarajan, D., and Chang, J.-S. (2020). “Exploring fermentation strategies for enhanced lactic acid production with polyvinyl alcohol-immobilized Lactobacillus plantarum 23 using microalgae as feedstock,” Bioresource Technology 308, 1-13. DOI: 10.1016/j.biortech.2020.123266

Chiang, Y.-S., Kuo, Y.-Y., and Li, S.-Y. (2018). “A novel method for preparing high purity Actinobacillus succinogenes stock and its long-term acid production in a packed bed reactor,” Bioresource Technology Reports 2, 62-68. DOI: 10.1016/j.biteb.2018.04.004

Chroumpi, T., Mäkelä, M. R., and Vries, R. P. d. (2020). “Engineering of primary carbon metabolism in filamentous fungi,” Biotechnology Advances 43, 1-23. DOI: 10.1016/j.biotechadv.2020.107551

Costa, S., Summa, D., Semeraro, B., Zappaterra, F., Rugiero, I., and Tamburini, E. (2020). “Fermentation as a strategy for bio-transforming waste into resources: Lactic acid production from agri-food residues,” Fermentation 7(1), 1-12. DOI: 10.3390/fermentation7010003

Cubas-Cano, E., Venus, J., González-Fernández, C., and Tomás-Pejó, E. (2020). Assessment of different Bacillus coagulans strains for L-lactic acid production from defined media and gardening hydrolysates: Effect of lignocellulosic inhibitors,” Journal of Biotechnology 323, 9-16. DOI: 10.1016/j.jbiotec.2020.07.017

Debiagi, F., Madeira, T. B., Nixdorf, S. L., and Mali, S. (2020). “Pretreatment efficiency using autoclave high-pressure steam and ultrasonication in sugar production from liquid hydrolysates and access to the residual solid fractions of wheat bran and oat hulls,” Applied Biochemistry and Biotechnology 190(1), 166-181. DOI: 10.1007/s12010-019-03092-0

Dessie, W., Xin, F., Zhang, W., Jiang, Y., Wu, H., Ma, J., and Jiang, M. (2018). “Opportunities, challenges, and future perspectives of succinic acid production by Actinobacillus succinogenes,” Applied Microbiology and Biotechnology 102(23), 9893-9910. DOI: 10.1007/s00253-018-9379-5

Din, N. A. S., Lim, S. J., Maskat, M. Y., Abd Mutalib, S., and Zaini, N. A. M. (2021). “Lactic acid separation and recovery from fermentation broth by ion-exchange resin: A review,” Bioresources and Bioprocessing, 8(1), 1-23. DOI: 10.1186/s40643-021-00384-4

Eiteman, M. A., and Ramalingam, S. (2015). “Microbial production of lactic acid. Biotechnology Letters 37(5), 955-972. DOI: 10.1007/s10529-015-1769-5

Eom, I.-Y., Oh, Y.-H., Park, S. J., Lee, S.-H., and Yu, J.-H. (2015). “Fermentative L-lactic acid production from pretreated whole slurry of oil palm trunk treated by hydrothermolysis and subsequent enzymatic hydrolysis,” Bioresource Technology 185, 143-149. DOI: 10.1016/j.biortech.2015.02.060

Ewanick, S., and Bura, R. (2010). “Hydrothermal pretreatment of lignocellulosic biomass,” in: Bioalcohol Production: Biochemical Conversion of Lignocellulosic Biomass, K. Waldron (ed.), Woodhead Publishing, Sawston, United Kingdom, pp. 3-23.

Finn, T. J., Shewaramani, S., Leahy, S. C., Janssen, P. H., and Moon, C. D. (2017). “Dynamics and genetic diversification of Escherichia coli during experimental adaptation to an anaerobic environment,” PeerJ 1-29. DOI: 10.7717/peerj.3244

Food and Drug Administration (2015). “Lactic acid,” Code of Federal Regulations Title 21. Food and Drug Administration (FDA), United States.

Guragain, Y. N., Wang, D., and Vadlani, P. V. (2016). “Appropriate biorefining strategies for multiple feedstocks: Critical evaluation for pretreatment methods, and hydrolysis with high solids loading,” Renewable Energy 96(Part A), 832-842. DOI: 10.1016/j.renene.2016.04.099

Haldar, D., and Purkait, M. K. (2021). “A review on the environment-friendly emerging techniques for pretreatment of lignocellulosic biomass: Mechanistic insight and advancements,” Chemosphere 264(Part 2), 1-16. DOI: 10.1016/j.chemosphere.2020.128523

Halim, R. M., Ramli, R., Mat, C. R. C., Hadi, N. A., Othman, M. F., and Aziz, A. A. (2020). “Quality improvement of palm kernel cake as broiler feed using pre-cleaning system,” Journal of Oil Palm Research 33(3), 458-472. DOI: 10.21894/jopr.2020.0112

Hassan, N., Idris, A., and Akhtar, J. J. J. K. (2019). “Overview on bio-refinery concept in Malaysia: Potential high value-added products from palm oil biomass,” J. Kejuruter 2, 113-124.

Hatti-Kaul, R., Chen, L., Dishisha, T., and Enshasy, H. E. (2018). “Lactic acid bacteria: From starter cultures to producers of chemicals,” FEMS Microbiology Letters 365(20), 1-20. DOI: 10.1093/femsle/fny213

Hollinshead, W. D., Rodriguez, S., Martin, H. G., Wang, G., Baidoo, E. E. K., Sale, K. L., Keasling, J. D., Mukhopadhyay, A., and Tang, Y. J. (2016). “Examining Escherichia coli glycolytic pathways, catabolite repression, and metabolite channeling using Δpfk mutants,” Biotechnology for Biofuels 9(1), 1-13. DOI: 10.1186/s13068-016-0630-y

Hörhammer, H. S., Treasure, T. H., Gonzalez, R. W., and Heiningen, A. R. P. v. (2014). “Larch biorefinery: Technical and economic evaluation,” Industrial and Engineering Chemistry Research 53(3), 1206-1213. DOI: 10.1021/ie403653j

How, B. S., Ngan, S. L., Hong, B. H., Lam, H. L., Ng, W. P. Q., Yusup, S., Ghani, W. A. W. A. K., Kansha, Y., Chan, Y. H., Cheah, K. W., et al. (2019). “An outlook of Malaysian biomass industry commercialisation: Perspectives and challenges,” Renewable and Sustainable Energy Reviews 113, 1-19. DOI: 10.1016/j.rser.2019.109277

Huang, W. C., and Tang, I. C. (2007). “Bacterial and yeast cultures–process characteristics, products, and applications,” in: Bioprocessing for value-added products from renewable resources, S.T. Yang (ed.), Elsevier B.V., Amsterdam, Netherlands, pp. 185-223.

Idrees, M., Adnan, A., and Qureshi, F. A. (2013). “Optimization of sulfide/sulfite pretreatment of lignocellulosic biomass for lactic acid production,” BioMed Research International 2013, 1-12. DOI: 10.1155/2013/934171

Ilmén, M., Koivuranta, K., Ruohonen, L., Rajgarhia, V., Suominen, P., and Penttilä, M. (2013). “Production of L-lactic acid by the yeast Candida sonorensis expressing heterologous bacterial and fungal lactate dehydrogenases,” Microbial Cell Factories 12(1), 1-15. DOI: 10.1186/1475-2859-12-53

Jahromi, M. F., Liang, J. B., Abdullah, N., Goh, Y. M., Ebrahimi, R., and Shokryazdan, P. (2016). “Extraction and characterization of oligosaccharides from palm kernel cake as prebiotic,” BioResources 11(1), 674-695. DOI: 10.15376/biores.11.1.674-695

Jana, U. K., and Kango, N. (2020). “Characteristics and bioactive properties of mannooligosaccharides derived from agro-waste mannans,” International Journal of Biological Macromolecules 149, 931-940. DOI: 10.1016/j.ijbiomac.2020.01.304

Jiang, M., Dai, W., Xi, Y., Wu, M., Kong, X., Ma, J., Zhang, M., Chen, K., and Wei, P. (2014). “Succinic acid production from sucrose by Actinobacillus succinogenes NJ113,” Bioresource Technology 153, 327-332. DOI: 10.1016/j.biortech.2013.11.062

Jiang, T., Qiao, H., Zheng, Z., Chu, Q., Li, X., Yong, Q., and Ouyang, J. (2016). “Lactic acid production from pretreated hydrolysates of corn stover by a newly developed Bacillus coagulans strain,” PLoS ONE 11(2), 1-13. DOI: 10.1371/journal.pone.0149101

Jönsson, L. J., and Martín, C. (2016). “Pretreatment of lignocellulose: Formation of inhibitory by-products and strategies for minimizing their effects,” Bioresource Technology 199, 103-112. DOI: 10.1016/j.biortech.2015.10.009

Juodeikiene, G., Zadeike, D., Bartkiene, E., Bartkevics, V., Dikiy, A., and Shumilina, E. (2016). “Potential of an exploitation of acid-tolerant antimicrobial microorganisms evolving enzyme systems for the utilization of dairy by-products and lignocellulosic biomass to lactic acid,” Frontiers in Bioengineering and Biotechnology 4, 1-4. DOI: 10.3389/fbioe.2016.00092

Juturu, V., and Wu, J. C. (2016). “Microbial production of lactic acid: The latest development,” Critical Reviews in Biotechnology 36(6), 967-977. DOI: 10.3109/07388551.2015.1066305

Juturu, V., and Wu, J. C. (2018). “Production of high concentration of L-lactic acid from oil palm empty fruit bunch by thermophilic Bacillus coagulans JI12,” Biotechnology and Applied Biochemistry 65(2), 145-149. DOI: 10.1002/bab.1567

Kalidas, N. R., Saminathan, M., Ismail, I. S., Abas, F., Maity, P., Islam, S. S., Manshoor, N., and Shaari, K. (2017). “Structural characterization and evaluation of prebiotic activity of oil palm kernel cake mannanoligosaccharides,” Food Chemistry 234, 348-355. DOI: 10.1016/j.foodchem.2017.04.159

Komesu, A., Oliveira, J. A. R. d., Martins, L. H. d. S., Maciel, M. R. W., and Filho, R. M. (2017). “Lactic acid manufacture,” BioResources 12(2), 4364-4383. DOI: 10.15376/biores.12.2.4364-4383

Li, Q., Wang, D., Song, Z., Zhou, W., Wu, Y., Xing, J., and Su, Z. (2010). “Dual-phase fermentation enables Actinobacillus succinogenes 130Z^T to be a potential role for high-level lactate production from the bioresource,” Bioresource Technology 101(19), 7665-7667. DOI: 10.1016/j.biortech.2010.04.058

Li, Y., Yi, P., Liu, J., Yan, Q., and Jiang, Z. (2018). “High-level expression of an engineered β-mannanase (mRmMan5A) in Pichia pastoris for manno-oligosaccharide production using steam explosion pretreated palm kernel cake,” Bioresource Technology 256, 30-37. DOI: 10.1016/j.biortech.2018.01.138

Lin, H.-T. V., Huang, M.-Y., Kao, T.-Y., Lu, W.-J., Lin, H.-J., and Pan, C.-L. (2020). “Production of lactic acid from seaweed hydrolysates via lactic acid bacteria fermentation,” Fermentation 6(1), 12-14. DOI: 10.3390/FERMENTATION6010037

López-Gómez, J. P., Alexandri, M., Schneider, R., and Venus, J. (2019). “A review on the current developments in continuous lactic acid fermentations and case studies utilising inexpensive raw materials,” Process Biochemistry 79, 1-10. DOI: 10.1016/j.procbio.2018.12.012

Luthfi, A. A. I., Jahim, J. M., Harun, S., Tan, J. P., and Mohammad, A. W. (2016). “Biorefinery approach towards greener succinic acid production from oil palm frond bagasse,” Process Biochemistry 51(10), 1527-1537. DOI: 10.1016/j.procbio.2016.08.011

Luthfi, A. A. I., Manaf, S. F. A., Illias, R. M., Harun, S., Mohammad, A. W., and Jahim, J. M. (2017). “Biotechnological route for sustainable succinate production utilizing oil palm frond and kenaf as potential carbon sources,” Applied Microbiology and Biotechnology 101(8), 3055-3075. DOI: 10.1007/s00253-017-8210-z

Malgas, S., and Pletschke, B. I. (2020). “Combination of CTec2 and GH5 or GH26 endo-mannanases for effective lignocellulosic biomass degradation,” Catalysts 10(10), 1-8. DOI: 10.3390/catal10101193

Manandhar, A., and Shah, A. (2020). “Techno-economic analysis of bio-based lactic acid,” Processes 8(2), 1-14. DOI: 10.3390/pr8020199

Marketsandmarkets.com (2020). “Lactic acid market by application – Global forecast to 2025,” https://www.marketsandmarkets.com/Market-Reports/polylacticacid-387.html. Accessed 23 December 2021

Martinez, F. A. C., Balciunas, E. M., Salgado, J. M., González, J. M. D., Converti, A., and Oliveira, R. P. d. S. (2013). “Lactic acid properties, applications and production: A review,” Trends in Food Science and Technology 30(1), 70-83. DOI: 10.1016/j.tifs.2012.11.007

Mazzoli, R. (2020). “Metabolic engineering strategies for consolidated production of lactic acid from lignocellulosic biomass,” Biotechnology and Applied Biochemistry 67(1), 61-72. DOI: 10.1002/bab.1869

McKinlay, J. B., and Vieille, C. (2008). “¹³C-metabolic flux analysis of Actinobacillus succinogenes fermentative metabolism at different NaHCO₃ and H₂ concentrations,” Metabolic Engineering 10(1), 55-68. DOI: 10.1016/j.ymben.2007.08.004

McKinlay, J. B., Shachar-Hill, Y., Zeikus, J. G., and Vieille, C. (2007). “Determining Actinobacillus succinogenes metabolic pathways and fluxes by NMR and GC-MS analyses of ¹³C-labeled metabolic product isotopomers,” Metabolic Engineering 9(2), 177-192. DOI: 10.1016/j.ymben.2006.10.006

Mejia-Gomez, C. E., and Balcázar, N. (2020). “Isolation, characterisation and continuous culture of Lactobacillus spp. and its potential use for lactic acid production from whey,” Food Science and Technology 40(4), 1021-1028. DOI: 10.1590/fst.29619

Mi, S., Li, H., Li, S., and Han, Y. (2016). “The synergism of hot water pretreatment and enzymatic hydrolysis in depolymerization of lignocellulosic content of palm kernel cake,” Journal of Molecular Catalysis B: Enzymatic 134(Part A), 37-42. DOI: 10.1016/j.molcatb.2016.09.004

Nagarajan, D., Nandini, A., Dong, C.-D., Lee, D.-J., and Chang, J. S. (2020). “Lactic acid production from renewable feedstocks using poly(vinyl alcohol)-immobilized Lactobacillus plantarum 23,” Industrial and Engineering Chemistry Research 59(39), 17156-17164. DOI: 10.1021/acs.iecr.0c01422

Nanda, S., Azargohar, R., Dalai, A. K., and Kozinski, J. A. (2015). “An assessment on the sustainability of lignocellulosic biomass for biorefining,” Renewable and Sustainable Energy Reviews 50, 925-941. DOI: 10.1016/j.rser.2015.05.058

Novy, V., Brunner, B., and Nidetzky, B. (2018). “L-lactic acid production from glucose and xylose with engineered strains of Saccharomyces cerevisiae: Aeration and carbon source influence yields and productivities,” Microbial Cell Factories 17(1), 1-11. DOI: 10.1186/s12934-018-0905-z

Oladokun, A. A., Rahman, W. A., and Suparjo, N. M. (2016). “Prospect of maximising palm kernel cake utilization for livestock and poultry in Malaysia: A review,” Journal of Biology, Agriculture and Heathcare 6(13), 107-113.

Oliveira, R. A. d., Komesu, A., Rossell, C. E. V., and Filho, R. M. (2018a). “Challenges and opportunities in lactic acid bioprocess design—From economic to production aspects,” Biochemical Engineering Journal 133, 219-239. DOI: 10.1016/j.bej.2018.03.003

Oliveira, R. A., Komesu, A., Vaz Rossell, C. E., Wolf Maciel, M. R., and Maciel Filho, R. (2018b). “Evaluation of hybrid short path evaporation to concentrate lactic acid and sugars from fermentation,” BioResources 13(2), 2187-2203.

Onoja, E., Chandren, S., Razak, F. I. A., Mahat, N. A., and Wahab, R. A. (2019). “Oil palm (Elaeis guineensis) biomass in Malaysia: The present and future prospects,” Waste and Biomass Valorization 10(8), 2099-2117. DOI:10.1007/s12649-018-0258-1

Østby, H., Hansen, L. D., Horn, S. J., Eijsink, V. G. H., and Várnai, A. (2020). “Enzymatic processing of lignocellulosic biomass: Principles, recent advances and perspectives,” Journal of Industrial Microbiology and Biotechnology 47(9-10), 623-657. DOI: 10.1007/s10295-020-02301-8

Padilha, C. E. d. A., Nogueira, C. d. C., Filho, M. A. O., Sousa Júnior, F. C. d., Assis, C. F. d., Souza, D. F. d. S., Oliveira, J. A. d., and Santos, E. S. d. (2019). “Fractionation of green coconut fiber using sequential hydrothermal/alkaline pretreatments and Amberlite XAD-7HP resin,” Journal of Environmental Chemical Engineering 7(6), 1-9. DOI: 10.1016/j.jece.2019.103474

Parveez, G. K. A., Tarmizi, A. H. A., Sundram, S., Loh, S. K., Ong-Abdullah, M., Palam, K. D. P., Salleh, K. M., Ishak, S. M., and Idris, Z. (2021). “Oil palm economic performance in Malaysia and R&D progress in 2020,” Journal of Oil Palm Research 33(2), 181-214. DOI: 10.21894/jopr.2021.0026

Pateraki, C., Patsalou, M., Vlysidis, A., Kopsahelis, N., Webb, C., Koutinas, A. A., and Koutinas, M. (2016). “Actinobacillus succinogenes: Advances on succinic acid production and prospects for development of integrated biorefineries,” Biochemical Engineering Journal 112, 285-303. DOI: 10.1016/j.bej.2016.04.005

Peinemann, J. C., and Pleissner, D. (2020). “Continuous pretreatment, hydrolysis, and fermentation of organic residues for the production of biochemicals,” Bioresource Technology 295, 1-8. DOI: 10.1016/j.biortech.2019.122256

Pleissner, D., Neu, A.-K., Mehlmann, K., Schneider, R., Puerta-Quintero, G. I., and Venus, J. (2016). “Fermentative lactic acid production from coffee pulp hydrolysate using Bacillus coagulans at laboratory and pilot scales,” Bioresource Technology 218, 167-173. DOI: 10.1016/j.biortech.2016.06.078

Rahmati, S., Doherty, W., Dubal, D., Atanda, L., Moghaddam, L., Sonar, P., Hessel, V., and Ostrikov, K. (2020). “Pretreatment and fermentation of lignocellulosic biomass: Reaction mechanisms and process engineering,” Reaction Chemistry and Engineering 5(11), 2017-2047. DOI: 10.1039/d0re00241k

Rajendran, N., and Han, J. (2022). “Integrated polylactic acid and biodiesel production from food waste: Process synthesis and economics,” Bioresource Technology 343, article no. 126119. DOI: 10.1016/j.biortech.2021.126119

Raita, M., Ibenegbu, C., Champreda, V., and Leak, D. J. (2016). “Production of ethanol by thermophilic oligosaccharide utilising Geobacillus thermoglucosidasius TM242 using palm kernel cake as a renewable feedstock,” Biomass and Bioenergy 95, 45-54. DOI: 10.1016/j.biombioe.2016.08.015

Ranjithkumar, M., Ravikumar, R., Sankar, M. K., Kumar, M. N., and Thanabal, V. (2017). “An effective conversion of cotton waste biomass to ethanol: A critical review on pretreatment processes,” Waste and Biomass Valorization 8(1), 57-68. DOI: 10.1007/s12649-016-9563-8

Rezania, S., Oryani, B., Cho, J., Talaiekhozani, A., Sabbagh, F., Hashemi, B., Rupani, P. F., and Mohammadi, A. A. (2020). “Different pretreatment technologies of lignocellulosic biomass for bioethanol production: An overview,” Energy 199, 1-14. DOI: 10.1016/j.energy.2020.117457

Riyadi, F. A., Alam, M. Z., Salleh, M. N., and Salleh, H. M. (2017). “Optimization of thermostable organic solvent-tolerant lipase production by thermotolerant Rhizopus sp. using solid-state fermentation of palm kernel cake,” 3 Biotech 7(5), 1-11. DOI: 10.1007/s13205-017-0932-1

Rizal, N. F. A. A., Ibrahim, M. F., Zakaria, M. R., Abd-Aziz, S., Yee, P. L., and Hassan, M. A. (2018). “Pre-treatment of oil palm biomass for fermentable sugars production,” Molecules 23(6), 1-14. DOI: 10.3390/molecules23061381

Sadhukhan, J., Martinez-Hernandez, E., Murphy, R. J., Ng, D. K. S., Hassim, M. H., Ng, K. S., Kin, W. Y., Jaye, I. F. M., Hang, M. Y. L. P., and Andiappan, V. (2018). “Role of bioenergy, biorefinery and bioeconomy in sustainable development: Strategic pathways for Malaysia,” Renewable and Sustainable Energy Reviews 81, 1966-1987. DOI: 10.1016/j.rser.2017.06.007

Sari, Y. W., Silviana, M. M., Kurniati, M., and Budiman, I. (2018). “Valorization of palm kernel cake as bioadhesive for particle board,” IOP Conference Series: Earth and Environmental Science 187(1), 1-7. DOI: 10.1088/1755-1315/187/1/012008

Sathitkowitchai, W., Nitisinprasert, S., and Keawsompong, S. (2018). “Improving palm kernel cake nutrition using enzymatic hydrolysis optimized by Taguchi method,” 3 Biotech 8(10), 1-7. DOI: 10.1007/s13205-018-1433-6

Sauer, M., Porro, D., Mattanovich, D., and Branduardi, P. (2008). “Microbial production of organic acids: Expanding the markets,” Trends in Biotechnology 26(2), 100-108. DOI: 10.1016/j.tibtech.2007.11.006

Shukor, H., Kalil, M. S., Anuar, N., Hamid, A. A., and Hanan, A. (2013). “Small review: Strategies for palm kernel cake (PKC) as a new potential substrate in biofuel production,” International Journal on Advanced Science, Engineering and Information Technology 3(3), 248-255. DOI: 10.18517/ijaseit.3.3.329

Shukor, H., Al-Shorgani, N. K. N., Abdeshahian, P., Hamid, A. A., Anuar, N., Rahman, N. A., Isa, M. H. M., and Kalil, M. S. (2014). “Biobutanol production from palm kernel cake (PKC) using Clostridium saccharoperbutylacetonicum N1-4 in batch culture fermentation,” BioResources 9(3), 5325-5338. DOI: 10.15376/biores.9.3.5325-5338

Shukor, H., Abdeshahian, P., Al-Shorgani, N. K. N., Hamid, A. A., Rahman, N. A., and Kalil, M. S. (2016). “Enhanced mannan-derived fermentable sugars of palm kernel cake by mannanase-catalyzed hydrolysis for production of biobutanol,” Bioresource Technology 218, 257-264. DOI: 10.1016/j.biortech.2016.06.084

Sitompul, J. P., Simangunsong, R. F., Asrizal, A. A., Alisyahbana, H., Lee, H. W., and Rasrendra, C. B. (2014). “Catalytic conversion of empty fruit bunch of palm oil for producing lactic acid,” Procedia Chemistry 9, 88-93. DOI: 10.1016/j.proche.2014.05.011

Solihat, N. N., Hermiati, E., and Adi, D. T. N. (2020). “Microwave assisted dilute organic acid pre-treatment of oil palm empty fruit bunch to improve enzyme accessibility,” IOP Conference Series: Materials Science and Engineering 935(1), 1-15. DOI: 10.1088/1757-899X/935/1/012046

Song, L., Wang, R., Jiang, J., Xu, J., and Gou, J. (2020). “Stepwise separation of poplar wood in oxalic acid/water and γ-valerolactone/water systems,” RSC Advances 10(19), 11188-11199. DOI: 10.1039/d0ra01163k

Stegmann, P., Londo, M., and Junginger, M. (2020). “The circular bioeconomy: Its elements and role in European bioeconomy clusters,” Resources, Conservation and Recycling: X 6, 1-17. DOI: 10.1016/j.rcrx.2019.100029

Tafsin, M., Hanafi, N. D., and Yusraini, E. (2017). “Extraction process of palm kernel cake as a source of mannan for feed additive on poultry diet,” IOP Conference Series: Earth and Environmental Science 65(1), 1-6. DOI: 10.1088/1755-1315/65/1/012020

Wang, C., Li, Q., Wang, D., and Xing, J. (2014). “Improving the lactic acid production of Actinobacillus succinogenes by using a novel fermentation and separation integration system,” Process Biochemistry 49(8), 1245-1250. DOI: 10.1016/j.procbio.2014.04.009

Wang, Z., Xiao, W., Zhang, A., Ying, H., Chen, K., and Ouyang, P. (2016). “Potential industrial application of Actinobacillus succinogenes NJ113 for pyruvic acid production by microaerobic fermentation,” 32, 1-7. DOI: 10.1007/s11814-016-0168-5

Wang, B., Zhang, X., Yu, X., Cui, Z., Wang, Z., Chen, T., and Zhao, X. (2019). “Evolutionary engineering of Escherichia coli for improved anaerobic growth in minimal medium accelerated lactate production,” Applied Microbiology and Biotechnology 103(5), 2155-2170. DOI: 10.1007/s00253-018-09588-9

Werpy, T., and Petersen, G. (2004). “Top Value Added Chemicals from Biomass Volume I – Results of Screening for Potential Candidates from Sugars and Synthesis Gas (DOE/GO-102004-1992), National Renewable Energy Laboratory, Golden, CO.

Xu, H., Li, B., and Mu, X. (2016). “Review of alkali-based pretreatment to enhance enzymatic saccharification for lignocellulosic biomass conversion,” Industrial & Engineering Chemistry Research 55(32), 8691-8705. DOI: 10.1021/acs.iecr.6b01907

Yan, Y., Zhang, C., Lin, Q., Wang, X., Cheng, B., Li, H., and Ren, J. (2018). “Microwave-assisted oxalic acid pretreatment for the enhancing of enzyme hydrolysis in the production of xylose and arabinose from bagasse,” Molecules 23(4), 1-13. DOI: 10.3390/molecules23040862

Yatim, P., Lin, N. S., Lam, H. L., and Choy, E. A. (2017). “Overview of the key risks in the pioneering stage of the Malaysian biomass industry,” Clean Technologies and Environmental Policy 19(7), 1825-1839. DOI: 10.1007/s10098-017-1369-2

Ye, L., Hudari, M. S. B., Zhou, X., Zhang, D., Li, Z., and Wu, J. C. (2013). “Conversion of acid hydrolysate of oil palm empty fruit bunch to L-lactic acid by newly isolated Bacillus coagulans JI12,” Applied Microbiology and Biotechnology 97(11), 4831-4838. DOI: 10.1007/s00253-013-4788-y

Zakaria, M. R., Hirata, S., and Hassan, M. A. (2015). “Hydrothermal pretreatment enhanced enzymatic hydrolysis and glucose production from oil palm biomass,” Bioresource Technology 176, 142-148. DOI: 10.1016/j.biortech.2014.11.027

Zhang, W., Yang, Q., Wu, M., Liu, H., Zhou, J., Dong, W., Ma, J., Jiang, M., and Xin, F. (2019). “Metabolic regulation of organic acid biosynthesis in Actinobacillus succinogenes,” Frontiers in Bioengineering and Biotechnology 7, 1-10. DOI: 10.3389/fbioe.2019.00216

Article submitted: December 3, 2021; Peer review completed: February 12, 2022; Revised version received and accepted: April 9, 2022; Published: April 14, 2022.

DOI: 10.15376/biores.17.2.Rahim