NC State
Abd Rahim, N., Indera Luthfi, A. A., Mohamed Abdul, P., Mad Jahim, J., and Bukhari, N. A. (2022). "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," BioResources, 17(2), 3781-3809.


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.

Download PDF

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 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.

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:


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; 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).


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% ( 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 IngeoTM. It can accommodate 140,000 metric tons of IngeoTM 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.


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 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


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).

C:\Users\aishah313\Downloads\Lactate metabolic pathway_auto_x2 (2).jpg

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 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