Industrial Lignocellulosic Waste Biomasses: Enormous Substrates for Harnessing Enzymes and Bioethanol Productions

1  Introduction

The amount of lignocellulosic biomass supersedes that of any other biomass on earth. Industries worldwide generate millions of tonnes of lignocellulosic waste annually. These wastes can pose environmental challenges if not properly managed. However, managing them can be challenging due to their inherent recalcitrance. Therefore, using them as substrates to produce essential industrial enzymes and biofuels, like bioethanol, is an interesting concept and a sustainable way of valorizing the wastes [1–4]. To improve yields, lignocellulosic biomass is typically pretreated. Several kinds of pretreatment processes, ranging from physical to chemical methods, have been explored in some studies. In some cases, more than one pretreatment technique is applied to adequately deconstruct the complex and recalcitrant structure of lignocellulosic biomass before proceeding to other processes [5–9].

This review, therefore, captures the vast number of lignocellulosic wastes from different industries and their constituents, especially those published within the last decade, as opposed to general lignocellulosic waste biomass or only agricultural lignocellulosic waste biomass already captured in the literature. The great potential of using them to produce microbial lignocellulase enzymes—cellulases, hemicellulases and ligninases—is thoroughly discussed using current literature. The conversion of the produced simple sugars into bioethanol using different microbes, either through separate hydrolysis and fermentation (SHF) or simultaneous saccharification and fermentation (SSF), is presented holistically. The emphasis of this study is primarily on microbial enzymes (crude or purified), rather than commercial enzymes, for the production of fermentable sugars and bioethanol, thereby making this review unique. Furthermore, the challenges and prospects of using industrial lignocellulosic waste biomass are further elucidated to show the possibilities of second-generation bioethanol through industrial waste valorization.

2  Industrial Lignocellulosic Waste Biomass

Lignocellulosic biomass is the most abundant organic biomass on earth. This biomass accounts for a large share of global daily waste, including industrial waste [1,2]. Lignocellulosic waste biomass consists of three major constituents, viz: cellulose, hemicellulose and lignin. The proportions of these constituents depend on the source of the lignocellulosic waste [1,10–12].

Cellulose is an essential constituent of lignocellulosic waste biomass with multiple industrial applications. It is a complex polymer composed of glucose monomers joined together by β-1,4 glycosidic bonds (Fig. 1). Cellulose chains usually form microfibrils, which can be either unordered or highly ordered, resulting in cellulose with amorphous or crystalline structures, respectively [1,13]. Crystalline cellulose is quite rigid and resistant to both biological and chemical hydrolysis, while the amorphous counterpart is less rigid and can easily be hydrolyzed about 3 to 30 times faster than crystalline cellulose [1,14]. In lignocellulose, the cellulose components are usually shielded by hemicelluloses, which also bind to cellulose microfibrils, making them inaccessible and recalcitrant. Cellulosic biomass is also protected by outer layers of lignin polymers, which must first be removed/degraded to reach it (Fig. 2). And this is usually achieved through several kinds of pretreatment techniques [1,14,15].

Figure 1: Chemical structure of cellulose.

Figure 2: Industrial lignocellulosic waste biomasses used in lignocellulases and bioethanol production.

Hemicelluloses also constitute essential components of lignocellulose. They differ from cellulose and have different monomers. These include hexoses, such as glucose, galactose or mannose; pentoses, such as xylose or arabinose; and other methylated derivatives, such as fucose or rhamnose. Furthermore, hemicelluloses could be branched to form more complex structures. Hemicelluloses are the least resistant lignocellulose constituents due to their branched structures [16,17]. Hemicellulose can be categorized into xylans, mannans, β-glucans and galactans (Figs. 3 and 4). Xylans are mainly linear homopolymers with D-xylose linked by β (1 → 4) or β (1 → 3) bonds. However, xylans could also be heteropolymers with other branched monosaccharides, such as D-glucuronic acid or L-arabinose at positions 2 and 3 to form glucuronoxylans or arabinoxylans, respectively. Mannans are either homopolymers or heteropolymers with D-mannose linked by β (1 → 4) bonds or D-mannose linked to D-glucose (i.e., glucomannans), respectively. Mannans could also be branched at position 6 with D-galactose to form galactomannans. β-glucan hemicellulose is made up of a β-glucoside backbone linked by a β (1 → 4) bond, and D-xylose at the sixth position (i.e., xyloglucan) or a mixed structure with β (1 → 4) and β (1 → 3) linked with a D-glucoside backbone called mixed β-glucan. The galactan hemicelluloses are homopolymers with either β (1 → 3) or β (1 → 4) D-galactose backbones connected to mono- or oligosaccharide side chains of L-arabinose or D-galactose at the sixth position [1,17,18] (Figs. 3 and 4).

Figure 3: Chemical structures of hemicelluloses: (a) homoxylan, (b) arabinoxylan, (c) (arabino) glucuronoxylan, (d) O-acetyl-4-O-methylglucuronoxylan, (e) mannan, (f) glucomannan, and (g) O-acetylgalactoglucomannan.

Figure 4: Chemical structures of hemicelluloses: (a) β-(1 → 3, 1 → 4)-glucan, (b) xyloglucan, and (c) arabinogalactan.

The lignin constituent of lignocellulose has no typical structure nor repeatable units like cellulose and hemicellulose. It is usually heterogeneous physically and chemically, with varying monomers. Lignin is synthesized by the combination of different proto-lignins, such as p-coumaryl, sinapyl alcohols and coniferyl, through polycondensation and dehydrogenation by enzymes (Figs. 5 and 6). Different plants have different lignin structures, each containing different proportions of the various proto-lignins. The chemical bonds present in lignins include: arylglycerol-β-ether linkages (β-O-4), pinoresinol (β-β), diarylpropane (β-1), phenylcoumaran (β-5) and biphenyl (5–5) [1,19].

Figure 5: Chemical structures of lignin building blocks: (a) coniferyl alcohol, (b) sinapyl alcohol and (c) p-coumaryl alcohol.

Figure 6: Chemical structure of lignin.

Globally, lignocellulosic wastes comprise a significant portion of industrial and total waste. For example, about 15.7% of lignocellulosic wastes accounts for the total annual waste generated in Europe [1,3]. Industrial lignocellulosic wastes are made of agricultural and forestry (including wood and paper) wastes. Industries involved in the use and/or processing of sugarcane, corn, rice, palm oil, and wheat are among the top producers of lignocellulosic wastes. These industries generate annual waste amounting to millions of tonnes globally. Examples of these wastes include: sugarcane bagasse, corncob, rice straw and husk, oil palm fibres and empty fruit bunches, wheat straw, etc. Closely following the agricultural industry in lignocellulosic waste generation are the wood and paper industries. Sawdust, chippable wastes and wood barks from hard and soft woods are among the top lignocellulosic wastes from the wood industry. The beverage industry, especially the fermentation-based industries, also produces lignocellulosic wastes. For example, beer companies produce a large amount of brewers’ spent grains annually. While black liquor, wood fragments in the form of chips and knots, sludge, waste papers and boards, among others, are generated from the paper industry. Other industries that also generate some levels of lignocellulosic wastes are the textile industries, municipal waste management, and others [11,20,21].

If these industrial lignocellulosic wastes are not properly disposed of, they could pose environmental problems, as some are recalcitrant and may take time to biodegrade. On the other hand, these lignocellulosic wastes could significantly increase the cost of municipal waste management, as they require proper treatment and disposal. However, the microbial valorization of these wastes could lead to a win-win scenario. Wastes could become essential raw materials for producing several important industrial products and metabolites, including enzymes, biofuels, pharmaceuticals, nutraceuticals, biofertilizers, etc. And the substrate is also being simultaneously biodegraded during the process, thereby achieving valorization and waste management [4,11,22]. This is indeed an interesting aspect of circular bio-economy and the zero-waste concept, which aligns with several Sustainable Development Goals, and ultimately preserves the ecosystem.

2.1 Microbial Enzymes for Bioethanol Production from Industrial Lignocellulosic Waste Biomass

Biofuels, such as bioethanol, constitute part of the green fuels that can be generated from the valorization of industrial lignocellulosic wastes. This type of bioethanol is known as a second-generation biofuel. Though lignocellulosic wastes are abundant, their use for bioethanol production poses a significant challenge. The core of which is the hydrolysis of the recalcitrant substrate to produce simple sugars that can be easily fermented. Enzymes are usually used for the hydrolysis/saccharification of lignocellulosic biomass after pretreatments [5,6]. However, the cost of commercial enzymes can increase production costs [5].

Therefore, research has focused on obtaining inexpensive microbial enzymes that can be used in crude or partially purified forms [7]. Separate systems of hydrolysis and fermentation could be used, in which the simple sugars produced are used for fermentation in another bioreactor. In some other cases, SSF is preferred. The SSF technique uses either a single organism that saccharifies and ferments (i.e., consolidated bioethanol process), or consortia of organisms that perform both tasks simultaneously [7,8]. Both techniques have their advantages and disadvantages (some are highlighted in Section 3.1).

Some essential enzymes required in the production of bioethanol from industrial lignocellulosic wastes are cellulases, hemicellulases and ligninases. Several research works have demonstrated the prospects of producing these lignocellulase enzymes by different microorganisms, like bacteria and fungi.

Cellulases, hemicellulases and/or ligninases are important lignocellulosic enzymes needed for the biodegradation of lignocellulosic biomass. Fermentable or reducing sugars are part of the essential products that could be generated from the degradation by these enzymes. These sugars are needed as direct substrates for bioethanol production by microorganisms. Several strategies could be taken to generate simple sugars from industrial lignocellulosic waste biomass (Fig. 7). These include pretreatment and detoxification, or only pretreatment without detoxification, before the use of the lignocellulose. The detoxification process involves removing inhibitory products generated during pretreatment of lignocellulosic substrates. Direct use of lignocellulosic biomass without pretreatment and detoxification is also possible with some microorganisms. However, the actual pathway chosen tends to greatly influence both the yield of fermentable sugars and the eventual production cost.

Figure 7: The routes in the production of fermentable sugars from lignocellulosic biomass.

2.1.1 Cellulases

Cellulase is a group of microbial enzymes that can break down cellulose into disaccharides (i.e., cellobiose) and/or monosaccharides, such as simple sugar (i.e., glucose), through a process known as cellulolysis. These enzymes include endoglucanases, exoglucanases and β-glucosidases [1,23–25].

Some microbes, including bacteria and fungi, have been demonstrated to produce different cellulases in the hydrolysis of lignocellulosic waste biomass. Several species of Bacillus are among the most frequently reported cellulase producers, e.g., B. subtilis, B. siamensis, and B. methylotrophicus [9,26–28]. Other species of filamentous actinobacteria have also been reported as good cellulase-producing strains, e.g., Cellulomonas flavigena, Streptomyces cavourensis, Streptomyces parvus, Streptomyces cavourensis, among others [29,30] (Table 1).

Multiple fungal strains are among the most popular cellulase-producing microbes, having been shown to produce these enzymes in solid-state fermentation. These include yeasts such as Filobasidium aff. oriense, Candida albicans, Barnettozyma californica, Cyberlindnera jardinii, and others [28,31,32]. As well as several moulds, mainly from the Aspergillus genus (especially A. niger and A. flavus), which are quite efficient in producing different cellulases [23,33–37]. Other common cellulase-producing moulds are from the Trichoderma and Fusarium genera [38–44]. The use of the macroscopic fungus, mushroom, in cellulase production has also been reported by several researchers [37,45] (Table 1).

Table 1 gives several industrial lignocellulosic waste biomasses that have been valorized through the production of different microbial cellulases. Most of the reported industrial waste comes from agricultural industries. In some cases, biomass was used as a co-substrate to improve enzyme production [40], while at other times, microbial co-cultures or consortia greatly increased enzyme production [9,26,28,30,31,37].

Some other factors/techniques have helped increase microbial cellulase production. The importance of mass transfer in solid-state fermentation has been shown to affect cellulase production by moulds. Some studies have reported the use of specialized bioreactors to exploit this concept. For example, Grajales et al. [55] demonstrated that cellulase activity from Myceliophthora thermophila, using sugarcane bagasse and wheat bran, was increased to 49.12 U/mL using a rotary drum bioreactor compared to 47.78 U/mL obtained from the static condition. Similarly, endoglucanase, exoglucanase, and β-glucosidase were reported to increase by 1.27-, 1.18-, and 1.06-fold in tray fermentation [33].

Some of the produced cellulases (or the microorganisms producing them directly) were used to hydrolyze/saccharify several lignocellulosic waste biomasses to produce simple sugars and other products with interesting results (Table 1). Very high levels of reducing/simple sugars (over 100 g/L) were reported in some of the studies. For example, 200 g/L from wheat straw [33], 149.25 and 253.00 mg/g from brewer’s spent grains [27,32], 338 mg/mL from pea peel [34], 288.06 mg/L from rice straw and soyabean residues [48], among others. In some cases of consolidated bioethanol production or where co-cultures/consortia were used with ethanol-producing ability, some level of ethanol was obtained [28,43,46] (Table 1).

2.1.2 Hemicellulases

Hemicellulases are a group of catalytic enzymes produced by microorganisms that can biodegrade hemicelluloses into their monomers, such as xylose, arabinose, mannose, etc. The different kinds of hemicellulases include: xylanases, mannanases, galactanases, xyloglucanases and arabinofuranoidases, which break xylans, mannans, galactans, xyloglucans and arabinofuranosidases, respectively [56,57].

Just like cellulase production, several microorganisms have been reported to produce hemicellulases. These include bacteria, yeasts, moulds and mushrooms. Bacteria such as Bacillus trypoxylicola, Amycolatopsis sp., and Cellulomonas flavigena have been reported to produce hemicellulases from several industrial lignocellulosic waste substrates, like Agba wood, sugarcane bagasse/sorghum husk and sugarcane bagasse, respectively [24,29,58]. Other lignocellulose substrates that have been explored in xylanase and/or mannanase productions are coffee husks, sawdust, brewer’s spent grains, rice straw and husk, waste cotton wool, wheat bran, empty fruit branches, amongst others (Table 2).

Furthermore, moulds such as several species of Aspergillus, Trichoderma harzanium, Lichtheimia ramose, Trametes lactinea, etc., have been demonstrated to synthesize hemicellulases in several studies (Table 2). Some mushrooms have also been used alone or in co-culture with several moulds in hemicellulase production [37,59,60]. High xylanase and mannanase activities have been achieved in some studies using microbes, as presented in Table 2. Different kinds of pretreatments—alkaline, acidic and several physical techniques—were used on some of the substrates to delignify them before the enzymatic hydrolysis or enzyme production [9,29,38,47,58].

Several recent works on microbial hemicellulase production or hydrolysis using industrial lignocellulosic waste substrates are presented in Table 2. The different hemicellulases (mostly xylanases) produced and their activities are also recorded. Some studies also reported products of enzymatic hydrolysis/saccharification, such as reducing/simple/total sugars, from the lignocellulosic waste biomasses [9,23,29,58].

2.1.3 Ligninases

Ligninases or lignin-modifying enzymes are a family of microbial catalytic enzymes that degrade lignin. They include: laccases, lignin peroxidase (LiP), manganese peroxidase (MnP), and others [59,64–67]. Fungi, especially moulds and mushrooms, are the major producers of ligninase, although some studies have shown that bacteria can also produce ligninase [9,44,56,65,68,69]. Examples of these fungi include Trametes versicolor, Trametes lactinea, Fusarium verticillioides, Aspergillus fumigatus, among others (Table 3).

Several lignocellulosic waste biomasses have been shown to be good substrates for producing different ligninases—laccase, MnP and LiP—using either solid-state or submerged fermentation. These include: olive mill waste, sawdust, walnut shell and straw, coffee husk, rice straw, sugarcane bagasse, wheat straw, etc. (Table 3).

A number of these ligninases, the microorganisms that produce them, and the lignocellulosic waste substrates used have been summarized in Table 3. The hydrolysis products of some of the studies have also been captured in the table.

3  Bioethanol Production from Industrial Lignocellulosic Waste Biomass

The unsustainability of fossil fuels and the current global concern about climate change, driven by greenhouse gases, such as carbon dioxide, have necessitated the search for eco-friendly and sustainable energy sources [72–74]. Biomass energy sources, such as bioethanol, derived from lignocellulosic waste, could be a good substitute, as they generate a net-zero carbon footprint and support adequate waste management [75,76]. Some common industrial sources of lignocellulosic waste that could be channeled into bioethanol production include: the agricultural and forestry industries; the food and beverage industry, including fermentation-based companies like breweries; and the paper and textile industries.

3.1 Bioethanol Production from Agricultural-Based Industrial Lignocellulosic Wastes

Second-generation ethanol involves the production of bioethanol from agricultural-based industrial lignocellulosic waste, and encompasses multiple essential steps, such as pretreatment, saccharification, fermentation, distillation, dehydration and rectification [77,78]. The operations and enzymes required to carry out these steps make it a high-cost technology [7].

Several waste biomasses are generated by agricultural-based industries, including both food- and non-food-based crop fractions. The crops that produce the largest lignocellulosic biomass and are universally cultivated are wheat, rice, sugarcane and maize. These produce over 5300 million tons of dry biomass every year [7]. Most agricultural waste residues, like rice straw, wheat bran, cotton stalk, sugarcane bagasse, and wheat straw, are lignocellulosic biomass consisting mainly of hemicellulose, cellulose and lignin. However, wheat straw contains a substantial amount of protein and pectin, whereas rice straw contains more silica [79]. Other lignocellulosic wastes that are produced in large quantities by agricultural industries include corncobs, oil palm fibers and empty fruit bunches.

Various agricultural lignocellulosic industrial wastes and residues have been used in the production of bioethanol through enzymatic hydrolysis [6]. These crops are more available for bioethanol production due to their short-term harvest rotation [6,80]. Pseudostem and rachis of banana waste were utilized as a source of biomass for bioethanol production. Enzymatic hydrolysis yielded ~100 g/L glucose concentration in optimized conditions using glucanase of 14.9 and 16.0 FPU/g at solid loading of 15.1% and 17.6%, respectively. The ethanol yields resulted in 74% and 87% of the maximum attainable ethanol yield, respectively [81]. Potato peel has also been investigated in bioethanol production, where enzymatic hydrolysis was steered with a combination of commercial cellulase and alpha-amylase from Bacillus sp. This led to a saccharification efficiency of 40.13% and ethanol yields of 0.30 g/g consumed sugars (SSF) and 0.40 g/g consumed sugars (SHF), corresponding to 78% of the maximum theoretical ethanol [82] (Table 4).

Since the application of commercial enzymes has been shown to increase production cost, the use of enzyme-producing microorganisms is a better alternative. Aspergillus flavus and Aspergillus niger, known as high-cellulase producers, were used to produce raw sugar from lignocellulosic biomass, which was subsequently used for bioethanol. This gave ethanol concentrations of 11.73 mg/mL (A. flavus) and 71.39 mg/mL (A. niger) from coffee pulp, and concentrations of 68.91 mg/mL (A. niger) and 7.81 mg/mL (A. flavus) from wheat bran [83] (Table 4). Another study evaluated rice husk for its potential for ethanol production using cellulase produced by Thermobifida fusca (Table 4). Optimized conditions resulted in 124.60 g/L of reducing sugar, with 100% consumption when fermented with a consortium of Kluyveromyces marxianus and Saccharomyces cerevisiae, yielding an ethanol concentration of 55.57 g/L [84].

The rate-determining steps that influence the overall process efficacy in the production of bioethanol include: pretreatment, enzymatic hydrolysis and fermentation [85]. High bioethanol yields can be achieved with the appropriate combination of each step in the process, leading to a cost-effective process [86]. The key challenge in bioethanol production is the pretreatment of the lignocellulosic biomass feedstock [86,87]. The lignin and cellulose matrix enclosed by hemicellulose must be fractured during pretreatment to reduce cellulose crystallinity and increase the portion of amorphous cellulose available for enzymatic attack. The pretreatment step comprises the chemical, physical, physicochemical and biological methods [16,17,86].

Pretreatment techniques can improve cellulose access to enzymes during enzymatic hydrolysis by reducing cellulose crystallinity and removing lignin to a large extent, so that the time of hydrolysis and enzyme loading will be minimized [88,89]. Black tea waste was used as a biomass source for bioethanol production. Physical boiling pretreatment method was used to release more fermentable sugars, which were converted to bioethanol using the yeasts, Brettanomyces claussenii and Zygosaccharomyces bailii [90].

Enzymatic hydrolysis is the most common method of hydrolysis and is applied to the pretreated lignocellulosic biomass. It is a very mild process, requires low maintenance cost and produces high yields [7,86]. It is well-suited to various pretreatment techniques, but toxic inhibitors that are poisonous to enzymes (or the microorganisms producing the enzymes) must be removed by detoxification when chemical or physicochemical pretreatment precedes enzymatic hydrolysis [86,91]. On the other hand, enzymatic hydrolysis could be expensive, especially when using commercial enzymes, and this might affect the cost of hydrolyzing lignocellulosic biomass. Also, due to the very sensitive nature of most enzymes, culture conditions such as temperature, pH, salinity, and others, must be adequately monitored and controlled to prevent enzyme inactivation and denaturation. This could also increase the cost of hydrolysis [7,86,91].

The lignocellulosic biomass pretreatment processes, such as lignin removal, hemicellulose solubility, hydrolysis duration, and enzyme loading, play a significant part in hydrolysis efficiency [6]. Enzymatic hydrolysis is a key step in converting cellulose in pretreated biomass to glucose. Enzymes are the most commonly used for the hydrolysis of cellulose and hemicellulose, although acid and alkali pretreatments can also be applied [92]. The conversion of cellulose, which is stable with a crystalline structure and capable of resisting depolymerization, is accomplished by the enzyme cellulase under slight conditions, like pH (4.5–5.0) and temperature (40°C–50°C) [6,86]. Hemicellulose, in contrast, can be hydrolyzed more effortlessly than cellulose as a result of its amorphous structure, which is more accessible. Two categories of enzymes are required for efficient hydrolysis, namely, depolymerizing core enzymes (1-4-mannosidases, xylan 1,4-xylosidases, endo-1,4-xylanases and endo-1,4-mannanases) capable of cleaving the backbone, and de-branching or auxiliary enzymes (acetyl xylan esterase, glucuronidase, L-arabinofuranosidase, ferulic acid esterase, and p-coumaric acid esterase) capable of eliminating side chains that pose steric hindrances to core enzymes. These help amplify the overall production of fermentable sugars gotten from lignocellulosic biomass [75].

The fermentation process involves the use of microorganisms to metabolize the fermentable sugars liberated from enzyme hydrolysis to generate ethyl alcohol as well as other byproducts [93]. Fermentation can be performed separately or concurrently with enzyme hydrolysis [94,95]. The integration procedure for bioethanol production involves three major approaches. They are SHF, SSF and simultaneous saccharification and co-fermentation (SSCF). The SHF technique entails conducting enzymatic hydrolysis and fermentation steps independently under optimal settings [96,97]. The SSF approach involves carrying out enzyme hydrolysis and fermentation at the same time using the same equipment. This tends to enrich the rate of hydrolysis, yields, and product concentrations. Then, the SSCF technique involves conducting hydrolysis and fermentation at the same time, in the same equipment, with concurrent co-fermentation of hexose and pentose sugars using multiple microorganisms, thereby increasing ethanol yield [98]. Simultaneous production of glucose by enzymes and its conversion to ethanol, as in SSF and SSCF, reduces sugar buildup in the system, and lowers cost as well as the processing time [7,8,98]. They also minimize contamination, increase ethanol productivity and evade cellulase inhibition by sugars, which is seen in the SHF process [7,8]. For example, Song et al. [99] demonstrated that ethanol production from fringe wood using the SSF method yielded a higher ethanol concentration (15.2 g/L) and a better conversion yield (83.1%) than the SHF method, which gave 14.8 g/L ethanol at 80.7% conversion yield.

The effectiveness of the conversion to bioethanol can be accomplished by the optimization of certain parameters, like enzyme loading, solid loading, time of hydrolysis, shaking speed, concentration of inhibitors and the effect of additives [6,75]. Solid loading increases sugar yield (80–100 g/L). However, very high solid loading can lead to viscosity, which affects enzyme efficiency due to poor mixing and impaired heat and mass transfer [92,100]. Similarly, increasing enzyme loading enhances saccharification efficiency, which provides a high glucose yield [92,100,101]. A high solid loading (20%) of hydrogen peroxide and phosphoric acid-pretreated wheat straw substrate gave a maximum glucose concentration of 164.9 g/L at an ethanol conversion rate of 64% with low enzyme input [102].

The shaking speed at an optimal level is required to guarantee optimal mass and heat transfer that translates into elevated glucose yield, but too high a speed causes shearing that can destroy enzymes (and even affect the microbes producing the enzymes), while a lower speed leads to poor mixing and reduced monosugar yields [6,103]. The long time required for complete hydrolysis limits ethanol production on a commercial scale, but the use of engineered microbes or their enzyme cocktails can shorten hydrolysis time [104].

The concentration of inhibitors that were generated during pretreatment can delay or halt enzyme hydrolysis. Therefore, performing detoxification before or during hydrolysis is critical to the process or selecting suitable pretreatment techniques that produce a limited amount of inhibitors [105].

Additives can enhance glucose yield in hydrolysis by obstructing the interactions between enzymes and lignin. This intensifies positive substrate-enzyme communications and recovers operability of cellulose hydrolysis. Examples of such additives are polymer-based polyethylene glycol (PEG), microbial surfactants (red quinoa saponins), non-catalytic protein [bovine serum albumin (BSA)], novel chemical surfactants or non-ionic surfactants (Triton X 100, Tween 80) [92,103,106]. Enzyme hydrolysis of wheat straw and sugarcane bagasse was studied using commercial cellulase, and the addition of Tween 20 to sugarcane bagasse (0.5%) and wheat straw (5.0 g/L) improved hydrolysis yield by decreasing enzyme loading and time of hydrolysis [103].

3.2 Bioethanol Production from Fermentation-Based Industrial Lignocellulosic Wastes

The beverage and/or fermentation industries generate some amount of lignocellulosic waste biomass that could be valorized to bioethanol. This plant-based biomass is an important source of raw materials for many fermentation-based industries [1,3,109]. Industrial waste includes both organic and inorganic wastes, and can harm the environment if not properly disposed of; therefore, its management is crucial [110]. The reusability of industrial by-products as raw materials for ethanol production contributes to resource efficiency by reducing energy consumption, lowering greenhouse gas emissions and curbing excessive landfill waste. The valorization of industrial waste supports circular production practices and sustainability [111].

Some of these fermentation-based industries include: breweries, wineries, distilleries, vinegar industries, etc. Lignocellulosic wastes generated from these industries are grape pomace, vine shoots, brewer’s spent grains, spent hops, trubs, cereal and grain husks/remains, sugar beets, sugarcane bagasse, grape and berry peels and remains, and others [1,111–113].

Table 5 presents some fermentation-based industrial wastes, their lignocellulosic composition, pretreatment process and ethanol yield using microorganisms. Ethanol yields of over 20% were reported in some of the studies [77,113–115]. Industrial lignocellulosic wastes that were reported include grape and apple pomace, brewer’s spent grain, and different distillages, among others.

3.3 Bioethanol Production from Forestry-Based Industrial Lignocellulosic Wastes

Another type of lignocellulosic waste is forestry-based feedstocks, which consist of woody materials used to produce bioethanol. This can be categorized into softwood and hardwood. Softwoods have low density and can grow more frequently. Examples include spruce, fir, pine, cypress, etc. While hardwoods are angiosperms that grow slowly, they are generally deciduous. Examples include mahogany, beech, cottonwood, maple, oak, poplar, and others [123]. Unlike softwood, which is often used in construction and in the paper and manufacturing industries, hardwood cannot be recycled for use in these industries despite the huge amount of waste generated annually from forest thinning [99,123]. This is due to the presence of short fibers and knots in hardwood [99]. Hence, the need to recycle the discarded hardwood as a biomass source for the potential generation of bioethanol as a result of its high cellulose (45%–50%) and hemicellulose (25%–35%) contents, which increase its hydrolysis into fermentable sugars with fewer enzymes and also increase productivity [99].

Cottonwood is considered the most appropriate wood for bioethanol production because of its advantages, such as high productivity, potential for restoration, a large number of clones, and uniformity in planting material [124]. Several forestry-based feedstocks contain more lignin and less ash, making them attractive substrates for improved bioethanol conversion [125]. However, this high lignin content can make them more challenging to process than agricultural residues. New delignification procedures, such as deep eutectic solvents and ionic liquid pretreatment, have been shown to degrade lignin structure in forest-based residues [126,127].

The hydrogen peroxide-acetic acid (HPAC) pretreatment method has been reported to efficiently eliminate lignin from numerous biomasses by reacting to generate peracetic acid (PAA), which breaks down lignin into monomeric phenolic compounds, thereby converting the side chains of propane into either a carboxyl or hydroxyl group [128,129]. Research conducted by Song et al. [99] evaluated the enzyme hydrolysis of HPAC-pretreated fringe (Chionanthus retusus) wood and recorded a significant reduction in lignin, which improved the hydrolysis efficiency by about 2.8 times better than that of the raw resources, contributing to an amplification in the productivity of bioethanol at 14.8 mg/mL (Table 6).

Various forestry-based feedstocks offer several benefits as substrates and as renewable energy sources for bioethanol production. Palm wood was investigated for bioethanol generation. Pretreating this forestry biomass with hydrothermal and chemical methods provided a bioethanol yield of 22.90 g/L after hydrolysis by cellulase secreted by the fungus, T. reesei, with optimized conditions of Artificial Neural Network at an agitation rate of 156 rpm, pH 5, temperature of 45°C, substrate concentration of 8% (v/v) and inoculum size of 3.2% (v/v) [130] (Table 6). Pine needle waste, another forest biomass, was hydrolyzed by a cellulase-producing Bacillus subtilis and a xylanase-producing Bacillus pumilus. This generated 25.64 ± 2.36 mg/mL of fermentable sugars with an ethanol concentration of 17.65 g/L and a fermentation efficiency of 91.22% [131] (Table 6). Another study investigated a mixed softwood sawdust feedstock as a substrate for bioethanol production. Enzymatic hydrolysis was conducted on the pretreated feedstock using the commercial enzyme Cellic Ctec2, and the hydrolysate was fermented with S. cerevisiae. The hydrolysis attained 80% saccharification yield, and about 80% theoretical conversion yield of glucose to ethanol [100].

Several other forestry-based lignocellulosic resources, such as poplar and willow wood, have been described for use in bioethanol production with fermentation efficiencies between 68% and 92% [132,133].

3.4 Bioethanol Production from Other Industrial Lignocellulosic Wastes

The paper and pulp industry is a lignocellulosic waste source that generates global waste (17%) [20,21]. They are linked to wood waste, including bark, unprocessed chips and sawdust. Another waste is the alkali compounds, a blend of sodium hydroxide, sodium sulphite and water-alkali-soluble lignin fragments, called black liquor, used to remove lignin. They are also linked to sludge from water treatment processes and to chemical solvents such as chlorine dioxide, ozone, chlorine and hydrogen peroxide for bleaching the pulp [20,21]. The major waste product from this industry is black liquor, amounting to about 170 million tonnes per year [21]. The composition of black liquor depends on wood type and pulping conditions, with an increased recovery rate of lignin for softwood [136].

Due to the physical and chemical properties of these woody wastes, which have lignocellulosic attributes, they can be widely used in several industries as a source of fuel or energy [137,138]. Some biofuel production using other industrial lignocellulosic biomass is presented in Table 7. Two types of pulp and paper sludge were evaluated for their potential as substrate in bioethanol production using a genetically modified cellulase-producing S. cerevisiae in an SSF process. The ethanol concentration was recorded as 52.7 ± 4.8 g/L for corrugated recycle paper sludge (CR-PS) and 101.8 ± 15.5 g/L for virgin pulp paper sludge (VP-PS). Ethanol yields of 12.0 ± 2.9 g ethanol/100 g dry PS for CR-PS and 22.7 ± 3.4 g ethanol/100 g dry PS for VP-PS were obtained, giving a percentage theoretical value of 89.6 ± 18.2% (CR-PS) and 111.3 ± 16.8% (VP-PS) [139] (Table 7). Farghaly et al. [140] also evaluated the ethanol-generating possibility of paperboard mill sludge using enzymes generated by A. niger, and S. cerevisiae as the fermenting microbe. The maximum glucose yield was noted at 33.2 ± 0.3 g/L, and the highest ethanol yield of 10.97 ± 0.55 g/L at 2% (v/v) HCl (Table 7).

Another industry that produces lignocellulosic waste is the textile industry. Wastes generated from textile industries include pigments, alkalis, textile residues, heavy metals, sludges, organic stabilizers, peroxides, and chemical solvents [20]. Only about 78%–90% of the solid waste is recyclable, yet only about 15%–20% of textile residues are presently being recycled [141]. Among other waste types, only the textile residues comprise lignocellulosic biomass [141]. Cotton gin waste and cotton gin dust, both being textile residues, were analyzed for their ethanol-producing potential using the commercial enzyme Cellic CTec2 and the microorganism, S. cerevisiae. The maximum fermentable sugar concentration was recorded at 47.8 and 42.5 g/L with an ethanol yield of 86.9 ± 1.6 and 87.5 ± 0.7% for cotton gin dust and cotton gin waste, respectively [142]. Another cotton-based textile waste [white T-shirt (WTS), linen and white towel (WT)] was evaluated for its ability to produce ethanol using cellulase produced by T. reesei [143]. Maximum ethanol yields were recorded at 537 mL/kg (WTS), 502 mL/kg (WT), and 487 mL/kg (linen), with glucose conversion yields of 83.5%, 82.3% and 75.7%, respectively, after 24 h of fermentation (Table 7).

Other industrial sources of lignocellulosic biomass, such as food and municipal waste generated from commercial and institutional sources, have also been implicated in bioethanol production. For example, Prasoulas et al. [144] showed that cellulase produced by Fusarium oxysporum F3 produced 33.7 g/L glucose, and when supplemented with commercial glucoamylase, showed an increase in glucose to ~45.0 g/L. The sugar produced was fermented with a blend of S. cerevisiae and F. oxysporum F3, yielding 20.0 g/L (with indigenous enzyme) and 30.0 g/L (with commercial glucoamylase) ethanol (Table 7). Another study on municipal waste using cellulase from B. subtilis gave the highest reducing sugar with biosurfactant addition at 9076.10 μg/mL, and the highest bioethanol production at 60.27 mL/L [145] (Table 7). These industrial lignocellulosic biomasses are considered easiest to use due to their high quality and are less likely to be mixed with other molecules [1].

4  Challenges and Prospects in the Use of Industrial Lignocellulosic Wastes for Enzymes and Bioethanol Production

One of the greatest challenges of using any lignocellulosic biomass (including those from industries) for bioethanol production, and the greatest drawback of second-generation bioethanol production, is the recalcitrant nature of the substrate and the need for pretreatments. The outer protective lignin and the crystalline hemicellulose layers usually shield the cellulose polymers in lignocellulose, which greatly hinders the applications of this biomass (Fig. 2) [1,14–16,147]. Several pretreatment methods, including physical, chemical and enzymatic approaches or combinations of them, have been demonstrated in some works to delignify biomass and aid the release of cellulose from various polymer shields. Pretreatment has also been reported to aid the access of hemicellulase enzymes to the hemicellulose constituents of the biomass [87,148,149].

However, these pretreatments come with their own challenges. Aside from the potential increase in production costs, chemical pretreatments often generate inhibitory by-products. These include furfural, formic and acetic acids, among others. At some concentrations, these compounds can be toxic and/or inhibit both microorganisms and their enzymes needed in the biorefinery process. Some studies have demonstrated that detoxification after pretreatment can remove inhibitory compounds, thereby facilitating better microbial hydrolysis and fermentation (Fig. 7). Notwithstanding, detoxification processes could also increase production costs, especially in large-scale bioethanol or lignocellulase enzyme production [150,151]. The use of pretreatment methods that release little or no inhibitory substances, or the isolation/selection and genetic modification of microorganisms that can withstand these inhibitory substances, or that hydrolyze or saccharify lignocellulosic biomass without pretreatment, is critical to addressing this challenge [4]. Such microbes are gaining relevance in bioethanol refinery technologies, including consolidated bioethanol process, where pretreatment and/or detoxification are not core necessities in bioethanol production from lignocellulose, thereby reducing production costs [4,43,152,153].

Another reason that may require detoxification is contamination of industrial lignocellulosic biomass with other chemicals. Since these waste biomasses are products of industrial processes, the possibility that they are laden with toxic industrial chemicals could pose another challenge. Detoxification of the lignocellulosic biomass could be done before use. However, getting or modifying microorganisms or enzymes that can thrive in such contamination is a more cost-effective approach than the costly detoxification [1,4].

The application of genetically engineered microbes for lignocellulase enzymes and bioethanol production from lignocellulosic waste biomass has not only helped develop microbes that can withstand inhibitory substances from pretreatment, but has also greatly improved enzyme and bioethanol yields. Several Escherichia coli strains (especially the BL21) have been transformed to express cellulases [154–156], xylanase [157] and other enzymes. Some other microorganisms that have been modified to improve lignocellulase enzyme production include: B. subtilis [158], Neurospora crassa [159], Trichoderma afroharzianum [160], among others. Furthermore, modified single organisms that can hydrolyze lignocellulosic biomass and ferment the resulting simple sugars (including hexoses and pentoses) have also been reported [8,40,43,46,98]. These have ultimately helped reduce the cost of producing lignocellulase enzymes and bioethanol.

Another serious challenge in lignocellulose applications in biofuel production is the cost of enzymatic hydrolysis. Emphasis on in-house crude enzyme production partly addresses this challenge. To further reduce cost, techniques that encourage enzyme and/or microbe reusability have been developed. One such approach is immobilization of enzymes and microbes. Studies have shown that immobilization in various carriers can enable the recycling of enzymes and microbes with retained efficacy for multiple cycles. This therefore, eliminates the time and cost of preparing fresh cultures [161]. However, this method may not be efficient for certain processes involving certain microbes, such as filamentous fungi. It could also affect mass transfer efficiency, and might easily introduce contamination into subsequent cycles. Estimating and standardizing immobilized microbes and enzymes can also be challenging. In microbial ecology, the fear that microbes may subtly evolve with different cycles is yet another consideration. Other technologies that could be adopted for reusability and optimized yield include applications of nanotechnology [52,162–164], genetic modifications of enzymes and microbes [155–157,159] and integrated biorefinery techniques [4,165].

5  Conclusion

No doubt, the increasing global population and industrialization will continue to increase the volume of industrial lignocellulosic waste generated. This increase in human population will also drive higher energy demand, which, if met solely from fossil fuels, would not be sustainable and eco-friendly. Converting these industrial lignocellulosic waste biomasses into essential enzymes and bioethanol, or other green biomass fuels, will simultaneously valorize the waste into commercial products while addressing waste management concerns. The role of microorganisms in this valorization process is sine qua non. As “living” biorefineries, microbes are paving the way for the production of essential bio-products, including fermentable sugars and bioethanol, from lignocellulosic waste biomass by circumventing conventional chemical pretreatment and/or detoxification processes. This will ultimately lead to the harnessing of green fuels and the achievement of several sustainable development goals.

Acknowledgement: Not applicable.

Funding Statement: The authors received no specific funding for this study.

Author Contributions: The authors confirm contributions to the paper as follows: Conceptualization: Chukwuemeka Samson Ahamefule. Writing—original draft preparation: Chukwuemeka Samson Ahamefule, Chidimma Osilo and Jennifer O. Unachukwu. Writing—review and editing: Blessing C. Ahamefule, Stella N. Madueke, Chidimma Osilo and Chukwuemeka Samson Ahamefule. Supervision: Chukwuemeka Samson Ahamefule. All authors reviewed and approved the final version of the manuscript.

Availability of Data and Materials: Not applicable.

Ethics Approval: Not applicable.

Conflicts of Interest: The authors declare no conflicts of interest.

References

1. Cazier EA, Pham TN, Cossus L, Abla M, Ilc T, Lawrence P. Exploring industrial lignocellulosic waste: sources, types, and potential as high-value molecules. Waste Manag. 2024;188:11–38. doi:10.1016/j.wasman.2024.07.029. [Google Scholar] [CrossRef]

2. Zoghlami A, Paës G. Lignocellulosic biomass: understanding recalcitrance and predicting hydrolysis. Front Chem. 2019;7:874. doi:10.3389/fchem.2019.00874. [Google Scholar] [CrossRef]

3. Adewuyi A. Underutilized lignocellulosic waste as sources of feedstock for biofuel production in developing countries. Front Energy Res. 2022;10:741570. doi:10.3389/fenrg.2022.741570. [Google Scholar] [CrossRef]

4. Ahamefule CS, Osilo C, Ahamefule BC, Madueke SN, Moneke AN. Simultaneous production of biofuel from agricultural wastes and bioremediation of the waste substrates: a review. Curr Res Microb Sci. 2024;7(4):100305. doi:10.1016/j.crmicr.2024.100305. [Google Scholar] [CrossRef]

5. Basera P, Chakraborty S, Sharma N. Lignocellulosic biomass: insights into enzymatic hydrolysis, influential factors, and economic viability. Discov Sustain. 2024;5(1):311. doi:10.1007/s43621-024-00543-5. [Google Scholar] [CrossRef]

6. Vasić K, Knez Ž, Leitgeb M. Bioethanol production by enzymatic hydrolysis from different lignocellulosic sources. Molecules. 2021;26(3):753. doi:10.3390/molecules26030753. [Google Scholar] [CrossRef]

7. Carrillo-Nieves D, Rostro Alanís MJ, de la Cruz Quiroz R, Ruiz HA, Iqbal HMN, Parra-Saldívar R. Current status and future trends of bioethanol production from agro-industrial wastes in Mexico. Renew Sustain Energy Rev. 2019;102(2):63–74. doi:10.1016/j.rser.2018.11.031. [Google Scholar] [CrossRef]

8. Chohan NA, Aruwajoye GS, Sewsynker-Sukai Y, Gueguim Kana EB. Valorisation of potato peel wastes for bioethanol production using simultaneous saccharification and fermentation: process optimization and kinetic assessment. Renew Energy. 2020;146(7):1031–40. doi:10.1016/j.renene.2019.07.042. [Google Scholar] [CrossRef]

9. Kamsani N, Radzi ZM, Salleh MM, Yahya A, Chong CS. Solid state fermentation-based pretreatment of sawdust for reducing sugar production: the role of Aspergillus sp. A1 and Bacillus sp. B1 co-culture. Waste Biomass Valorization. 2026;17(3):1587–606. doi:10.1007/s12649-025-03212-4. [Google Scholar] [CrossRef]

10. Ghaemi F, Abdullah LC, Ariffin H. Lignocellulose structure and the effect on nanocellulose production. In: Lignocellulose for future bioeconomy. Amsterdam, The Netherlands: Elsevier; 2019. p. 17–30. doi:10.1016/b978-0-12-816354-2.00002-5. [Google Scholar] [CrossRef]

11. Blasi A, Verardi A, Lopresto CG, Siciliano S, Sangiorgio P. Lignocellulosic agricultural waste valorization to obtain valuable products: an overview. Recycling. 2023;8(4):61. doi:10.3390/recycling8040061. [Google Scholar] [CrossRef]

12. Bhardwaj AK, Thakur B, Tripathi SK, Turabayev ME, Aljulaih AA, Kharlamova TS, et al. Recent advances in valorizing agricultural waste: a sustainable approach. Waste Biomass Valorization. 2025;183(2):42. doi:10.1007/s12649-025-03419-5. [Google Scholar] [CrossRef]

13. McNamara JT, Morgan JLW, Zimmer J. A molecular description of cellulose biosynthesis. Annu Rev Biochem. 2015;84(1):895–921. doi:10.1146/annurev-biochem-060614-033930. [Google Scholar] [CrossRef]

14. Gao Q, Guo LN, Li SH, Wu W, Ding JW, Xu HJ, et al. Biodegradation mechanism of cellulose, hemicellulose, and lignin in bacteria-dominant aerobic composting from agricultural biomass waste: a review. Carbohydr Polym Technol Appl. 2025;11(2):100879. doi:10.1016/j.carpta.2025.100879. [Google Scholar] [CrossRef]

15. Jung SJ, Kim SH, Chung IM. Comparison of lignin, cellulose, and hemicellulose contents for biofuels utilization among 4 types of lignocellulosic crops. Biomass Bioenergy. 2015;83(7):322–7. doi:10.1016/j.biombioe.2015.10.007. [Google Scholar] [CrossRef]

16. Rao J, Lv Z, Chen G, Peng F. Hemicellulose: structure, chemical modification, and application. Prog Polym Sci. 2023;140:101675. doi:10.1016/j.progpolymsci.2023.101675. [Google Scholar] [CrossRef]

17. Gautam D, Rana V, Sharma S, Kumar Walia Y, Kumar K, Umar A, et al. Hemicelluloses: a review on extraction and modification for various applications. ChemistrySelect. 2025;10(24):e06050. doi:10.1002/slct.202406050. [Google Scholar] [CrossRef]

18. Yoshimi Y, Tryfona T, Dupree P. Structure, modification pattern, and conformation of hemicellulose in plant biomass. J Appl Glycosci. 2024;72(1):7201301. doi:10.5458/jag.7201301. [Google Scholar] [CrossRef]

19. Erfani Jazi M, Narayanan G, Aghabozorgi F, Farajidizaji B, Aghaei A, Ali Kamyabi M, et al. Structure, chemistry and physicochemistry of lignin for material functionalization. SN Appl Sci. 2019;1(9):1094. doi:10.1007/s42452-019-1126-8. [Google Scholar] [CrossRef]

20. Millati R, Cahyono RB, Ariyanto T, Azzahrani IN, Putri RU, Taherzadeh MJ. Agricultural, industrial, municipal, and forest wastes. In: Sustainable resource recovery and zero waste approaches. Amsterdam, The Netherlands: Elsevier; 2019. p. 1–22. doi:10.1016/b978-0-444-64200-4.00001-3. [Google Scholar] [CrossRef]

21. Haile A, Gelebo GG, Tesfaye T, Mengie W, Mebrate MA, Abuhay A, et al. Pulp and paper mill wastes: utilizations and prospects for high value-added biomaterials. Bioresour Bioprocess. 2021;8(1):35. doi:10.1186/s40643-021-00385-3. [Google Scholar] [CrossRef]

22. Gil A. Current insights into lignocellulose related waste valorization. Chem Eng J Adv. 2021;8:100186. doi:10.1016/j.ceja.2021.100186. [Google Scholar] [CrossRef]

23. Kaur J, Chugh P, Soni R, Soni SK. A low-cost approach for the generation of enhanced sugars and ethanol from rice straw using in-house produced cellulase-hemicellulase consortium from A. niger P-19. Bioresour Technol Rep. 2020;11(173):100469. doi:10.1016/j.biteb.2020.100469. [Google Scholar] [CrossRef]

24. Kshirsagar SD, Saratale GD, Saratale RG, Govindwar SP, Oh MK. An isolated Amycolatopsis sp. GDS for cellulase and xylanase production using agricultural waste biomass. J Appl Microbiol. 2016;120(1):112–25. doi:10.1111/jam.12988. [Google Scholar] [CrossRef]

25. Srivastava N, Elgorban AM, Mishra PK, Marraiki N, Alharbi AM, Ahmad I, et al. Enhance production of fungal cellulase cocktail using cellulosic waste. Environ Technol Innov. 2020;19:100949. doi:10.1016/j.eti.2020.100949. [Google Scholar] [CrossRef]

26. Asiri M, Singh T, Mohammad A, Al Ali A, Alqahtani A, Saeed M, et al. Bacterial cellulase production via co-fermentation of paddy straw and Litchi waste and its stability assessment in the presence of ZnMg mixed-phase hydroxide-based nanocomposite derived from Litchi chinensis seeds. Int J Biol Macromol. 2023;238(1):124284. doi:10.1016/j.ijbiomac.2023.124284. [Google Scholar] [CrossRef]

27. Choińska-Pulit A, Sobolczyk-Bednarek J, Łaba W. Cellulolytic potential of newly isolated alcohol-tolerant Bacillus methylotrophicus. Materials. 2025;18(14):3256. doi:10.3390/ma18143256. [Google Scholar] [CrossRef]

28. Rathod BG, Poosarla VG, Rajagopalan G, Kumari A, Shivshetty N. Bioethanol production from fruit waste using a microbial co-culture of Bacillus siamensis F2 and Candida albicans GP1. Biofuels Bioprod Bioref. 2025;19(6):1915–30. doi:10.1002/bbb.2792. [Google Scholar] [CrossRef]

29. Rojas-Rejón ÓA, Poggi-Varaldo HM, Ramos-Valdivia AC, Ponce-Noyola T, Cristiani-Urbina E, Martínez A, et al. Enzymatic saccharification of sugar cane bagasse by continuous xylanase and cellulase production from Cellulomonas flavigena PR-22. Biotechnol Prog. 2016;32(2):321–6. doi:10.1002/btpr.2213. [Google Scholar] [CrossRef]

30. Mawad AMM, Al-Turk AIM, Mohamed NH, Almutrafy A, Alhujaily A, Saleem N, et al. Biodegradation of lignocellulosic wastes by thermotolerant cellulolytic actinomycetal consortium isolated from Uhud Mountain, Madinah, Saudi Arabia. Biodegradation. 2025;36(5):78. doi:10.1007/s10532-025-10171-z. [Google Scholar] [CrossRef]

31. Cerda A, Mejías L, Gea T, Sánchez A. Cellulase and xylanase production at pilot scale by solid-state fermentation from coffee husk using specialized consortia: the consistency of the process and the microbial communities involved. Bioresour Technol. 2017;243(9):1059–68. doi:10.1016/j.biortech.2017.07.076. [Google Scholar] [CrossRef]

32. Wagner E, Ceriani Nakamurakare E, Robles CA, Carmaran CC, Rojas NL. An unexplored enzymatic source for Brewer’s spent grain valorization: the role of wood-substrate beetle-associated fungi and endophytes. Biomass Bioenergy. 2025;197:107788. doi:10.1016/j.biombioe.2025.107788. [Google Scholar] [CrossRef]

33. Singh A, Bajar S, Devi A, Bishnoi NR. Adding value to agro-industrial waste for cellulase and xylanase production via solid-state bioconversion. Biomass Convers Biorefin. 2023;13(9):7481–90. doi:10.1007/s13399-021-01503-z. [Google Scholar] [CrossRef]

34. Mostafa FA, Wehaidy HR, Sharaf S, El-Hennawi HM, Mahmoud SA, Saleh SAA. Aspergillus awamori MK788209 cellulase: production, statistical optimization, pea peels saccharification and textile applications. Microb Cell Fact. 2024;23(1):11. doi:10.1186/s12934-023-02286-w. [Google Scholar] [CrossRef]

35. Dias LM, dos Santos BV, Albuquerque CJB, Baeta BEL, Pasquini D, Baffi MA. Biomass sorghum as a novel substrate in solid-state fermentation for the production of hemicellulases and cellulases by Aspergillus niger and A. fumigatus. J Appl Microbiol. 2018;124(3):708–18. doi:10.1111/jam.13672. [Google Scholar] [CrossRef]

36. Santos GB, de Sousa Francisco Filho Á, Rêgo da Silva Rodrigues J, Rodrigues de Souza R. Cellulase production by Aspergillus niger using urban lignocellulosic waste as substrate: evaluation of different cultivation strategies. J Environ Manag. 2022;305:114431. doi:10.1016/j.jenvman.2022.114431. [Google Scholar] [CrossRef]

37. Martins IMNBR, Gomes LSVO, Pasquini D, Baffi MA. Production and characterization of cellulases and hemicellulases from a consortium between Pleurotus ostreatus and Aspergillus niger cultured in agro-industrial wastes. Res Soc Dev. 2021;10(10):e396101019020. doi:10.33448/rsd-v10i10.19020. [Google Scholar] [CrossRef]

38. Kumar Ramamoorthy N, Sambavi TR, Renganathan S. A study on cellulase production from a mixture of lignocellulosic wastes. Process Biochem. 2019;83:148–58. doi:10.1016/j.procbio.2019.05.006. [Google Scholar] [CrossRef]

39. Sasaki C, Matsuura K, Omasa T. Cellulase production on easy-to-handle solid media containing agricultural waste and its application for enzymatic hydrolysis of cellulosic biomass. Biomass Convers Biorefin. 2024;14(22):27955–65. doi:10.1007/s13399-022-03518-6. [Google Scholar] [CrossRef]

40. Dong M, Wang S, Xu F, Xiao G, Bai J. Efficient utilization of waste paper as an inductive feedstock for simultaneous production of cellulase and xylanase by Trichoderma longiflorum. J Clean Prod. 2021;308(1):127287. doi:10.1016/j.jclepro.2021.127287. [Google Scholar] [CrossRef]

41. Dey P, Chakrabortty S, Haldar D, Sowmya A, Rangarajan V, Ruiz HA. Kinetic model supported improved and optimized submerged production strategy of cellulase enzyme from newspaper waste biomass. Bioprocess Biosyst Eng. 2022;45(8):1281–95. doi:10.1007/s00449-022-02741-9. [Google Scholar] [CrossRef]

42. Asgher M, Wahab A, Bilal M, Nasir Iqbal HM. Lignocellulose degradation and production of lignin modifying enzymes by Schizophyllum commune IBL-06 in solid-state fermentation. Biocatal Agric Biotechnol. 2016;6(6):195–201. doi:10.1016/j.bcab.2016.04.003. [Google Scholar] [CrossRef]

43. Nait M’Barek H, Arif S, Taidi B, Hajjaj H. Consolidated bioethanol production from olive mill waste: wood-decay fungi from central Morocco as promising decomposition and fermentation biocatalysts. Biotechnol Rep. 2020;28:e00541. doi:10.1016/j.btre.2020.e00541. [Google Scholar] [CrossRef]

44. Geethanjali PA, Gowtham HG, Jayashankar M. Biodegradation potential of indigenous litter dwelling ligninolytic fungi on agricultural wastes. Bull Natl Res Cent. 2020;44(1):173. doi:10.1186/s42269-020-00426-5. [Google Scholar] [CrossRef]

45. Vichitraka A, Somboon P, Tantratian S, Onmankhong J, Sirisomboon P, Pornchaloempong P, et al. Application of baby corn husk as a biological sustainable feedstock for the production of cellulase and xylanase by Lentinus squarrosulus Mont. Bioresour Technol Rep. 2023;21(87):101341. doi:10.1016/j.biteb.2023.101341. [Google Scholar] [CrossRef]

46. Kumari S, Kumar A, Kumar R. A cold-active cellulase produced from Exiguobacterium sibiricum K1 for the valorization of agro-residual resources. Biomass Convers Biorefin. 2023;13(16):14777–87. doi:10.1007/s13399-022-03031-w. [Google Scholar] [CrossRef]

47. Garcia NFL, da Silva Santos FR, Bocchini DA, da Paz MF, Fonseca GG, Leite RSR. Catalytic properties of cellulases and hemicellulases produced by Lichtheimia ramosa: potential for sugarcane bagasse saccharification. Ind Crops Prod. 2018;122:49–56. doi:10.1016/j.indcrop.2018.05.049. [Google Scholar] [CrossRef]

48. Boondaeng A, Keabpimai J, Trakunjae C, Vaithanomsat P, Srichola P, Niyomvong N. Cellulase production under solid-state fermentation by Aspergillus sp. IN5: parameter optimization and application. Heliyon. 2024;10(5):e26601. doi:10.1016/j.heliyon.2024.e26601. [Google Scholar] [CrossRef]

49. Yousef NMH, Mawad AMM. Characterization of thermo/halo stable cellulase produced from halophilic Virgibacillus salarius BM-02 using non-pretreated biomass. World J Microbiol Biotechnol. 2022;39(1):22. doi:10.1007/s11274-022-03446-7. [Google Scholar] [CrossRef]

50. Jain L, Kurmi AK, Agrawal D. Conclusive selection of optimal parameters for cellulase production by Talaromyces verruculosus IIPC 324 under SSF via saccharification of acid-pretreated sugarcane bagasse. Biofuels. 2021;12(1):61–9. doi:10.1080/17597269.2018.1449063. [Google Scholar] [CrossRef]

51. Ritika, Pant S, Prakash A, Vundavilli PR, Khadanga KC, Kuila A, et al. Concomitant inhibitor-tolerant cellulase and xylanase production towards sustainable bioethanol production by Zasmidiumcellare CBS 146.36. Fuel. 2024;375(41):132593. doi:10.1016/j.fuel.2024.132593. [Google Scholar] [CrossRef]

52. Srivastava N, Mohammad A, Pal DB, Srivastava M, Alshahrani MY, Ahmad I, et al. Enhancement of fungal cellulase production using pretreated orange peel waste and its application in improved bioconversion of rice husk under the influence of nickel cobaltite nanoparticles. Biomass Convers Biorefin. 2024;14(5):6687–96. doi:10.1007/s13399-022-03070-3. [Google Scholar] [CrossRef]

53. Sethi BK, Das AS, Satpathy A, Behera BC. Ethanol production by a cellulolytic fungus Aspergillus terreus NCFT 4269.10 using agro-waste as a substrate. Biofuels. 2017;8(2):207–13. doi:10.1080/17597269.2016.1221296. [Google Scholar] [CrossRef]

54. Pimentel DC, de Souza JB, Ventorim RZ, Alfenas RF, Alfenas AC, Guimarães VM, et al. Evaluation of lignocellulolytic fungal enzymes for Eucalyptus wood degradation. Int Biodeterior Biodegrad. 2024;193(5):105830. doi:10.1016/j.ibiod.2024.105830. [Google Scholar] [CrossRef]

55. Grajales LM, Wang H, Casciatori FP, Thoméo JC. Intensified rotary drum bioreactor for cellulase production from agro-industrial residues by solid-state cultivation. Chem Eng Process Process Intensif. 2025;210:110223. doi:10.1016/j.cep.2025.110223. [Google Scholar] [CrossRef]

56. Neelkant KS, Shankar K, Jayalakshmi SK, Sreeramulu K. Optimization of conditions for the production of lignocellulolytic enzymes by Sphingobacterium sp. ksn-11 utilizing agro-wastes under submerged condition. Prep Biochem Biotechnol. 2019;49(9):927–34. doi:10.1080/10826068.2019.1643735. [Google Scholar] [CrossRef]

57. Bilal M, Wang Z, Cui J, Ferreira LFR, Bharagava RN, Iqbal HMN. Environmental impact of lignocellulosic wastes and their effective exploitation as smart carriers—a drive towards greener and eco-friendlier biocatalytic systems. Sci Total Environ. 2020;722(4):137903. doi:10.1016/j.scitotenv.2020.137903. [Google Scholar] [CrossRef]

58. Onyema VE, Ezugwu AL, Ezike TC, Chilaka FC. Enzymatic biodegradation of a mannan and galactoglucomannan extracted from African rose wood and Agba wood by hemicellulase from Bacillus trypoxylicola. Discov Bact. 2024;1(1):4. doi:10.1007/s44351-024-00006-2. [Google Scholar] [CrossRef]

59. de Oliveira Rodrigues P, Gurgel LVA, Pasquini D, Badotti F, Góes-Neto A, Baffi MA. Lignocellulose-degrading enzymes production by solid-state fermentation through fungal consortium among Ascomycetes and Basidiomycetes. Renew Energy. 2020;145:2683–93. doi:10.1016/j.renene.2019.08.041. [Google Scholar] [CrossRef]

60. Naidu Y, Siddiqui Y, Idris AS. Comprehensive studies on optimization of ligno-hemicellulolytic enzymes by indigenous white rot hymenomycetes under solid-state cultivation using agro-industrial wastes. J Environ Manage. 2020;259:110056. doi:10.1016/j.jenvman.2019.110056. [Google Scholar] [CrossRef]

61. Taddia A, Brandaleze GN, Boggione MJ, Bortolato SA, Tubio G. An integrated approach to the sustainable production of xylanolytic enzymes from Aspergillus niger using agro-industrial by-products. Prep Biochem Biotechnol. 2020;50(10):979–91. doi:10.1080/10826068.2020.1777425. [Google Scholar] [CrossRef]

62. Ravichandra K, Balaji R, Devarapalli K, Cheemalamarri C, Poda S, Banoth L, et al. Impact of structure and composition of different sorghum xylans as substrates on production of xylanase enzyme by Aspergillus fumigatus RSP-8. Ind Crops Prod. 2022;188(2):115660. doi:10.1016/j.indcrop.2022.115660. [Google Scholar] [CrossRef]

63. Ali S, Noor P, Ahmad MU, Khan QF, William K, Liaqat I, et al. Kinetics of cellulase-free endo xylanase hyper-synthesis by Aspergillus Niger using wheat bran as a potential solid substrate. BMC Biotechnol. 2024;24(1):69. doi:10.1186/s12896-024-00895-w. [Google Scholar] [CrossRef]

64. Tišma M, Jurić A, Bucić-Kojić A, Panjičko M, Planinić M. Biovalorization of brewers’ spent grain for the production of laccase and polyphenols: biovalorization of brewers’ spent grain. J Inst Brew. 2018;124(2):182–6. doi:10.1002/jib.479. [Google Scholar] [CrossRef]

65. Ramarajan R, Manohar CS. Biological pretreatment and bioconversion of agricultural wastes, using ligninolytic and cellulolytic fungal consortia. Biorem J. 2017;21(2):89–99. doi:10.1080/10889868.2017.1282937. [Google Scholar] [CrossRef]

66. Biko ODV, Viljoen-Bloom M, van Zyl WH. Microbial lignin peroxidases: applications, production challenges and future perspectives. Enzyme Microb Technol. 2020;141:109669. doi:10.1016/j.enzmictec.2020.109669. [Google Scholar] [CrossRef]

67. Janusz G, Pawlik A, Sulej J, Świderska-Burek U, Jarosz-Wilkołazka A, Paszczyński A. Lignin degradation: microorganisms, enzymes involved, genomes analysis and evolution. FEMS Microbiol Rev. 2017;41(6):941–62. doi:10.1093/femsre/fux049. [Google Scholar] [CrossRef]

68. Feng Q, Guo G, Yan H, Li F. Effect of pretreatment on solid-state fermentation of natural lignocellulose by Aspergillus fumigatus G-13. Environ Prog Sustain Energy. 2022;41(6):e13914. doi:10.1002/ep.13914. [Google Scholar] [CrossRef]

69. Yuliana T, Komara DZ, Saripudin GLU, Subroto E, Safitri R. Potential of lignocellulosic waste for laccase production by Trametes versicolor under submerged fermentation. Pak J Biol Sci. 2021;24(6):699–705. doi:10.3923/pjbs.2021.699.705. [Google Scholar] [CrossRef]

70. de Cassia Pereira Scarpa J, Paganini Marques N, Alves Monteiro D, Martins GM, de Paula AV, Boscolo M, et al. Saccharification of pretreated sugarcane bagasse using enzymes solution from Pycnoporus sanguineus MCA 16 and cellulosic ethanol production. Ind Crops Prod. 2019;141(2):111795. doi:10.1016/j.indcrop.2019.111795. [Google Scholar] [CrossRef]

71. Munir N, Asghar M, Tahir IM, Riaz M, Bilal M, Ali Shah SM. Utilization of agro-wastes for production of ligninolytic enzymes in liquid state fermentation by Phanerochaete chrysosporium-Ibl-03. Int J Chem Biochem Sci. 2015;7:9–14. [Google Scholar]

72. Kyei SK, Boateng HK, Frimpong AJ. Renewable energy innovations: fulfilling SDG targets. Clean Energy. 2025;9(2):190–203. doi:10.1093/ce/zkae109. [Google Scholar] [CrossRef]

73. Trégarot E, D’Olivo JP, Botelho AZ, Cabrito A, Cardoso GO, Casal G, et al. Effects of climate change on marine coastal ecosystems—a review to guide research and management. Biol Conserv. 2024;289:110394. doi:10.1016/j.biocon.2023.110394. [Google Scholar] [CrossRef]

74. Ali F, Dawood A, Hussain A, Alnasir MH, Khan MA, Butt TM, et al. Fueling the future: biomass applications for green and sustainable energy. Discov Sustain. 2024;5(1):156. doi:10.1007/s43621-024-00309-z. [Google Scholar] [CrossRef]

75. Broda M, Yelle DJ, Serwańska K. Bioethanol production from lignocellulosic biomass—challenges and solutions. Molecules. 2022;27(24):8717. doi:10.3390/molecules27248717. [Google Scholar] [CrossRef]

76. Mignogna D, Szabó M, Ceci P, Avino P. Biomass energy and biofuels: perspective, potentials, and challenges in the energy transition. Sustainability. 2024;16(16):7036. doi:10.3390/su16167036. [Google Scholar] [CrossRef]

77. Favaro L, Cagnin L, Basaglia M, Pizzocchero V, van Zyl WH, Casella S. Production of bioethanol from multiple waste streams of rice milling. Bioresour Technol. 2017;244:151–9. doi:10.1016/j.biortech.2017.07.108. [Google Scholar] [CrossRef]

78. Mansouri A, Rihani R, Laoufi AN, Özkan M. Production of bioethanol from a mixture of agricultural feedstocks: biofuels characterization. Fuel. 2016;185:612–21. doi:10.1016/j.fuel.2016.08.008. [Google Scholar] [CrossRef]

79. Sarkar N, Ghosh SK, Bannerjee S, Aikat K. Bioethanol production from agricultural wastes: an overview. Renew Energy. 2012;37(1):19–27. doi:10.1016/j.renene.2011.06.045. [Google Scholar] [CrossRef]

80. Limayem A, Ricke SC. Lignocellulosic biomass for bioethanol production: current perspectives, potential issues and future prospects. Prog Energy Combust Sci. 2012;38(4):449–67. doi:10.1016/j.pecs.2012.03.002. [Google Scholar] [CrossRef]

81. Guerrero AB, Ballesteros I, Ballesteros M. The potential of agricultural banana waste for bioethanol production. Fuel. 2018;213:176–85. doi:10.1016/j.fuel.2017.10.105. [Google Scholar] [CrossRef]

82. Ben Atitallah I, Antonopoulou G, Ntaikou I, Alexandropoulou M, Nasri M, Mechichi T, et al. On the evaluation of different saccharification schemes for enhanced bioethanol production from potato peels waste via a newly isolated yeast strain of Wickerhamomyces anomalus. Bioresour Technol. 2019;289:121614. doi:10.1016/j.biortech.2019.121614. [Google Scholar] [CrossRef]

83. Alabdalall AH, Almutari AA, Aldakeel SA, Albarrag AM, Aldakheel LA, Alsoufi MH, et al. Bioethanol production from lignocellulosic biomass using Aspergillus niger and Aspergillus flavus hydrolysis enzymes through immobilized S. cerevisiae. Energies. 2023;16(2):823. doi:10.3390/en16020823. [Google Scholar] [CrossRef]

84. Echa C, Ekpenyong M, Edeghor U, Ubi D, Edet P, Itam D, et al. Saccharification and co-fermentation of lignocellulosic biomass by a cockroach-gut bacterial symbiont and yeast cocktail for bioethanol production. BMC Biotechnol. 2024;24(1):102. doi:10.1186/s12896-024-00932-8. [Google Scholar] [CrossRef]

85. Parisutham V, Kim TH, Lee SK. Feasibilities of consolidated bioprocessing microbes: from pretreatment to biofuel production. Bioresour Technol. 2014;161:431–40. doi:10.1016/j.biortech.2014.03.114. [Google Scholar] [CrossRef]

86. Saini JK, Saini R, Tewari L. Lignocellulosic agriculture wastes as biomass feedstocks for second-generation bioethanol production: concepts and recent developments. 3 Biotech. 2015;5(4):337–53. doi:10.1007/s13205-014-0246-5. [Google Scholar] [CrossRef]

87. Riaz S, John AJ, Samuel MS, Ethiraj S. Recent developments and emerging methodologies in the pre-treatment of lignocellulosic biomass. Sustain Chem Environ. 2025;11:100285. doi:10.1016/j.scenv.2025.100285. [Google Scholar] [CrossRef]

88. Sun S, Sun S, Cao X, Sun R. The role of pretreatment in improving the enzymatic hydrolysis of lignocellulosic materials. Bioresour Technol. 2016;199:49–58. doi:10.1016/j.biortech.2015.08.061. [Google Scholar] [CrossRef]

89. Mohanty B, Abdullahi II. Bioethanol production from lignocellulosic waste—a review. Biosci Biotech Res Asia. 2016;13(2):1153–61. doi:10.13005/bbra/2146. [Google Scholar] [CrossRef]

90. Indira D, Das B, Bhawsar H, Moumita S, Johnson EM, Balasubramanian P, et al. Investigation on the production of bioethanol from black tea waste biomass in the seawater-based system. Bioresour Technol Rep. 2018;4(5):209–13. doi:10.1016/j.biteb.2018.11.003. [Google Scholar] [CrossRef]

91. Ibrahim MF, Ramli N, Kamal Bahrin E, Abd-Aziz S. Cellulosic biobutanol by clostridia: challenges and improvements. Renew Sustain Energy Rev. 2017;79:1241–54. doi:10.1016/j.rser.2017.05.184. [Google Scholar] [CrossRef]

92. Robak K, Balcerek M. Current state-of-the-art in ethanol production from lignocellulosic feedstocks. Microbiol Res. 2020;240:126534. doi:10.1016/j.micres.2020.126534. [Google Scholar] [CrossRef]

93. Abdu Yusuf A, Inambao FL. Bioethanol production techniques from lignocellulosic biomass as alternative fuel: a review. Int J Adv Res Eng Technol. 2019;10(3):259–88. doi:10.34218/ijaret.10.3.2019.026. [Google Scholar] [CrossRef]

94. Jahnavi G, Prashanthi GS, Sravanthi K, Rao LV. Status of availability of lignocellulosic feed stocks in India: biotechnological strategies involved in the production of Bioethanol. Renew Sustain Energy Rev. 2017;73:798–820. doi:10.1016/j.rser.2017.02.018. [Google Scholar] [CrossRef]

95. Paulova L, Patakova P, Branska B, Rychtera M, Melzoch K. Lignocellulosic ethanol: technology design and its impact on process efficiency. Biotechnol Adv. 2015;33(6):1091–107. doi:10.1016/j.biotechadv.2014.12.002. [Google Scholar] [CrossRef]

96. Tavva SSMD, Deshpande A, Durbha SR, Palakollu VAR, Goparaju AU, Yechuri VR, et al. Bioethanol production through separate hydrolysis and fermentation of Parthenium hysterophorus biomass. Renew Energy. 2016;86:1317–23. doi:10.1016/j.renene.2015.09.074. [Google Scholar] [CrossRef]

97. Chow TJ, Su HY, Tsai TY, Chou HH, Lee TM, Chang JS. Using recombinant Cyanobacterium (Synechococcus elongatus) with increased carbohydrate productivity as feedstock for bioethanol production via separate hydrolysis and fermentation process. Bioresour Technol. 2015;184(1):33–41. doi:10.1016/j.biortech.2014.10.065. [Google Scholar] [CrossRef]

98. Mohd Azhar SH, Abdulla R, Jambo SA, Marbawi H, Gansau JA, Mohd Faik AA, et al. Yeasts in sustainable bioethanol production: a review. Biochem Biophys Rep. 2017;10:52–61. doi:10.1016/j.bbrep.2017.03.003. [Google Scholar] [CrossRef]

99. Song Y, Cho EJ, Park CS, Oh CH, Park BJ, Bae HJ. A strategy for sequential fermentation by Saccharomyces cerevisiae and Pichia stipitis in bioethanol production from hardwoods. Renew Energy. 2019;139:1281–9. doi:10.1016/j.renene.2019.03.032. [Google Scholar] [CrossRef]

100. Abdou Alio M, Tugui OC, Rusu L, Pons A, Vial C. Hydrolysis and fermentation steps of a pretreated sawmill mixed feedstock for bioethanol production in a wood biorefinery. Bioresour Technol. 2020;310:123412. doi:10.1016/j.biortech.2020.123412. [Google Scholar] [CrossRef]

101. Du J, Li Y, Zhang H, Zheng H, Huang H. Factors to decrease the cellulose conversion of enzymatic hydrolysis of lignocellulose at high solid concentrations. Cellulose. 2014;21(4):2409–17. doi:10.1007/s10570-014-0301-8. [Google Scholar] [CrossRef]

102. Qiu J, Ma L, Shen F, Yang G, Zhang Y, Deng S, et al. Pretreating wheat straw by phosphoric acid plus hydrogen peroxide for enzymatic saccharification and ethanol production at high solid loading. Bioresour Technol. 2017;238:174–81. doi:10.1016/j.biortech.2017.04.040. [Google Scholar] [CrossRef]

103. Chen YA, Zhou Y, Qin Y, Liu D, Zhao X. Evaluation of the action of Tween 20 non-ionic surfactant during enzymatic hydrolysis of lignocellulose: pretreatment, hydrolysis conditions and lignin structure. Bioresour Technol. 2018;269:329–38. doi:10.1016/j.biortech.2018.08.119. [Google Scholar] [CrossRef]

104. Fenila F, Shastri Y. Optimal control of enzymatic hydrolysis of lignocellulosic biomass. Resour Effic Technol. 2016;2:S96–S104. doi:10.1016/j.reffit.2016.11.006. [Google Scholar] [CrossRef]

105. Moreno AD, Ibarra D, Alvira P, Tomás-Pejó E, Ballesteros M. A review of biological delignification and detoxification methods for lignocellulosic bioethanol production. Crit Rev Biotechnol. 2015;35(3):342–54. doi:10.3109/07388551.2013.878896. [Google Scholar] [CrossRef]

106. Wei W, Jin Y, Wu S, Yuan Z. Improving corn stover enzymatic saccharification via ferric chloride catalyzed dimethyl sulfoxide pretreatment and various additives. Ind Crops Prod. 2019;140:111663. doi:10.1016/j.indcrop.2019.111663. [Google Scholar] [CrossRef]

107. Ibrahim NA, Majeed HH, Abid RA, Alsultan GA, Mijan NA, Lee HV, et al. Ethanol production from lignocellulosic waste materials: kinetics and optimization studies. RSC Adv. 2025;15(32):26091–103. doi:10.1039/d5ra02272j. [Google Scholar] [CrossRef]

108. Pantoja GV, de Oliveira JAR. Chemical analysis and antioxidant capacity of the stages of lignocellulosic ethanol production from Amazonian fruit industrial waste. Fermentation. 2024;10(10):496. doi:10.3390/fermentation10100496. [Google Scholar] [CrossRef]

109. Ojo AO. An overview of lignocellulose and its biotechnological importance in high-value product production. Fermentation. 2023;9(11):990. doi:10.3390/fermentation9110990. [Google Scholar] [CrossRef]

110. Zielińska M, Bułkowska K, Mikucka W. Valorization of distillery stillage for bioenergy production: a review. Energies. 2021;14(21):7235. doi:10.3390/en14217235. [Google Scholar] [CrossRef]

111. Fernandes PCB, Silva J. Brewing by-products: source, nature, and handling in the dawn of a circular economy age. Biomass. 2025;5(3):49. doi:10.3390/biomass5030049. [Google Scholar] [CrossRef]

112. Mangione R, Simões R, Pereira H, Catarino S, Ricardo-da-Silva J, Miranda I, et al. Potential use of grape stems and pomaces from two red grapevine cultivars as source of oligosaccharides. Processes. 2022;10(9):1896. doi:10.3390/pr10091896. [Google Scholar] [CrossRef]

113. Mikucka W, Zielińska M. Distillery stillage: characteristics, treatment, and valorization. Appl Biochem Biotechnol. 2020;192(3):770–93. doi:10.1007/s12010-020-03343-5. [Google Scholar] [CrossRef]

114. Alonso-Riaño P, Amândio MST, Xavier AMRB, Beltrán S, Sanz MT. Subcritical water as pretreatment technique for bioethanol production from brewer’s spent grain within a biorefinery concept. Polymers. 2022;14(23):5218. doi:10.3390/polym14235218. [Google Scholar] [CrossRef]

115. Dong J, Fakhari M, Ban L, Polhemus K, Roji Shehu M, Doustkhahvajari F, et al. High-efficiency conversion of corn bran to ethanol at 150 L scale. Bioresour Technol. 2024;408:131216. doi:10.1016/j.biortech.2024.131216. [Google Scholar] [CrossRef]

116. Pawar SS, Cavan P, Bankar S. Production of bio-ethanol from biomass (press mud). Int J Adv Res Ideas Innov Technol. 2017;3(3):1550–5. [Google Scholar]

117. Vučurović D, Bajić B, Vučurović V, Jevtić-Mučibabić R, Dodić S. Bioethanol production from spent sugar beet pulp—process modeling and cost analysis. Fermentation. 2022;8(3):114. doi:10.3390/fermentation8030114. [Google Scholar] [CrossRef]

118. Devi N, Cruz-Martins N, Malik T, Kumar V, Islam MT, Verma R, et al. Phytochemical profiling and anti-bacterial activity of red delicious apple pomace with integrated hydrolysis to obtain fermentable sugars: a biorefinery approach. Food Sci Nutr. 2025;13(7):e70543. doi:10.1002/fsn3.70543. [Google Scholar] [CrossRef]

119. Chaudhary A, Hussain Z, Aihetasham A, El-Sharnouby M, Abdul Rehman R, Azmat Ullah Khan M, et al. Pomegranate peels waste hydrolyzate optimization by response surface methodology for bioethanol production. Saudi J Biol Sci. 2021;28(9):4867–75. doi:10.1016/j.sjbs.2021.06.081. [Google Scholar] [CrossRef]

120. Antunes FAF, Freitas JBF, Prado CA, Castro-Alonso MJ, Diaz-Ruiz E, Mera AE, et al. Valorization of corn cobs for xylitol and bioethanol production through column reactor process. Energies. 2023;16(13):4841. doi:10.3390/en16134841. [Google Scholar] [CrossRef]

121. Cripwell R, Favaro L, Rose SH, Basaglia M, Cagnin L, Casella S, et al. Utilisation of wheat bran as a substrate for bioethanol production using recombinant cellulases and amylolytic yeast. Appl Energy. 2015;160(C):610–7. doi:10.1016/j.apenergy.2015.09.062. [Google Scholar] [CrossRef]

122. Mikulski D, Kłosowski G. Microwave-assisted dilute acid pretreatment in bioethanol production from wheat and rye stillages. Biomass Bioenergy. 2020;136(2):105528. doi:10.1016/j.biombioe.2020.105528. [Google Scholar] [CrossRef]

123. Carević I, Baričević A, Štirmer N, Šantek Bajto J. Correlation between physical and chemical properties of wood biomass ash and cement composites performances. Constr Build Mater. 2020;256:119450. doi:10.1016/j.conbuildmat.2020.119450. [Google Scholar] [CrossRef]

124. Zabed H, Sahu JN, Suely A, Boyce AN, Faruq G. Bioethanol production from renewable sources: current perspectives and technological progress. Renew Sustain Energy Rev. 2017;71:475–501. doi:10.1016/j.rser.2016.12.076. [Google Scholar] [CrossRef]

125. Stolarski MJ, Krzyżaniak M, Łuczyński M, Załuski D, Szczukowski S, Tworkowski J, et al. Lignocellulosic biomass from short rotation woody crops as a feedstock for second-generation bioethanol production. Ind Crops Prod. 2015;75:66–75. doi:10.1016/j.indcrop.2015.04.025. [Google Scholar] [CrossRef]

126. El Akkari M, Ben Fradj N, Gabrielle B, Njakou Djomo S. Spatially-explicit environmental assessment of bioethanol from Miscanthus and switchgrass in France. Clean Circ Bioecon. 2023;6(3):100059. doi:10.1016/j.clcb.2023.100059. [Google Scholar] [CrossRef]

127. Liu J, Wang C, Zhao X, Yin F, Yang H, Wu K, et al. Bioethanol production from corn straw pretreated with deep eutectic solvents. Electron J Biotechnol. 2023;62(6):27–35. doi:10.1016/j.ejbt.2022.12.004. [Google Scholar] [CrossRef]

128. Song Y, Wi SG, Kim HM, Bae HJ. Cellulosic bioethanol production from Jerusalem artichoke (Helianthus tuberosus L.) using hydrogen peroxide-acetic acid (HPAC) pretreatment. Bioresour Technol. 2016;214(13):30–6. doi:10.1016/j.biortech.2016.04.065. [Google Scholar] [CrossRef]

129. Wi SG, Cho EJ, Lee DS, Lee SJ, Lee YJ, Bae HJ. Lignocellulose conversion for biofuel: a new pretreatment greatly improves downstream biocatalytic hydrolysis of various lignocellulosic materials. Biotechnol Biofuels. 2015;8(1):228. doi:10.1186/s13068-015-0419-4. [Google Scholar] [CrossRef]

130. Raja Sathendra E, Baskar G, Praveenkumar R, Gnansounou E. Bioethanol production from palm wood using Trichoderma reesei and Kluveromyces marxianus. Bioresour Technol. 2019;271(4):345–52. doi:10.1016/j.biortech.2018.09.134. [Google Scholar] [CrossRef]

131. Dwivedi D, Rathour RK, Sharma V, Rana N, Bhatt AK, Bhatia RK. Co-fermentation of forest pine needle waste biomass hydrolysate into bioethanol. Biomass Convers Biorefin. 2024;14(7):8829–41. doi:10.1007/s13399-022-02896-1. [Google Scholar] [CrossRef]

132. Ziegler-Devin I, Menana Z, Chrusciel L, Chalot M, Bert V, Brosse N. Steam explosion pretreatment of willow grown on phytomanaged soils for bioethanol production. Ind Crops Prod. 2019;140(3):111722. doi:10.1016/j.indcrop.2019.111722. [Google Scholar] [CrossRef]

133. Zhu J, Chen L, Gleisner R, Zhu JY. Co-production of bioethanol and furfural from poplar wood via low temperature (≤90°C) acid hydrotropic fractionation (AHF). Fuel. 2019;254:115572. doi:10.1016/j.fuel.2019.05.155. [Google Scholar] [CrossRef]

134. Söderström J, Pilcher L, Galbe M, Zacchi G. Two-step steam pretreatment of softwood with SO2 impregnation for ethanol production. In: Biotechnology for fuels and chemicals. Totowa, NJ, USA: Humana Press; 2002. p. 5–21. doi:10.1007/978-1-4612-0119-9_1. [Google Scholar] [CrossRef]

135. Yuan Z, Li G, Wei W, Wang J, Fang Z. A comparison of different pre-extraction methods followed by steam pretreatment of bamboo to improve the enzymatic digestibility and ethanol production. Energy. 2020;196(1):117156. doi:10.1016/j.energy.2020.117156. [Google Scholar] [CrossRef]

136. Jardim JM, Hart PW, Lucia LA, Jameel H, Chang HM. The effect of the kraft pulping process, wood species, and pH on lignin recovery from black liquor. Fibers. 2022;10(2):16. doi:10.3390/fib10020016. [Google Scholar] [CrossRef]

137. Simão L, Hotza D, Raupp-Pereira F, Labrincha JA, Montedo ORK. Wastes from pulp and paper mills—a review of generation and recycling alternatives. Cerâmica. 2018;64(371):443–53. doi:10.1590/0366-69132018643712414. [Google Scholar] [CrossRef]

138. Sarkar R, Kurar R, Gupta AK, Mudgal A, Gupta V. Use of paper mill waste for brick making. Cogent Eng. 2017;4(1):1405768. doi:10.1080/23311916.2017.1405768. [Google Scholar] [CrossRef]

139. van Dyk J, Görgens JF, van Rensburg E. Enhanced ethanol production from paper sludge waste under high-solids conditions with industrial and cellulase-producing strains of Saccharomyces cerevisiae. Bioresour Technol. 2024;394(5):130163. doi:10.1016/j.biortech.2023.130163. [Google Scholar] [CrossRef]

140. Farghaly A, Elsamadony M, Ookawara S, Tawfik A. Bioethanol production from paperboard mill sludge using acid-catalyzed bio-derived choline acetate ionic liquid pretreatment followed by fermentation process. Energy Convers Manag. 2017;145:255–64. doi:10.1016/j.enconman.2017.05.004. [Google Scholar] [CrossRef]

141. Stanescu MD. State of the art of post-consumer textile waste upcycling to reach the zero waste milestone. Environ Sci Pollut Res. 2021;28(12):14253–70. doi:10.1007/s11356-021-12416-9. [Google Scholar] [CrossRef]

142. Fockink DH, Maceno MAC, Ramos LP. Production of cellulosic ethanol from cotton processing residues after pretreatment with dilute sodium hydroxide and enzymatic hydrolysis. Bioresour Technol. 2015;187:91–6. doi:10.1016/j.biortech.2015.03.096. [Google Scholar] [CrossRef]

143. Cho EJ, Lee YG, Song Y, Kim HY, Nguyen DT, Bae HJ. Converting textile waste into value-added chemicals: an integrated bio-refinery process. Environ Sci Ecotechnol. 2023;15:100238. doi:10.1016/j.ese.2023.100238. [Google Scholar] [CrossRef]

144. Prasoulas G, Gentikis A, Konti A, Kalantzi S, Kekos D, Mamma D. Bioethanol production from food waste applying the multienzyme plasma at laboratory and bench-scale levels and its application as a starter culture in a meat product. Fermentation. 2020;6(2):39. doi:10.3390/FERMENTATION6020039. [Google Scholar] [CrossRef]

145. Hosny M, El-Sheshtawy HS. Effect of biosurfactant on hydrolysis of municipal waste by cellulases producing bacteria for bioethanol production. Curr Res Green Sustain Chem. 2022;5(1):100294. doi:10.1016/j.crgsc.2022.100294. [Google Scholar] [CrossRef]

146. Shafiei M, Karimi K, Taherzadeh MJ. Pretreatment of spruce and oak by N-methylmorpholine-N-oxide (NMMO) for efficient conversion of their cellulose to ethanol. Bioresour Technol. 2010;101(13):4914–8. doi:10.1016/j.biortech.2009.08.100. [Google Scholar] [CrossRef]

147. Bueno Moron J, van Klink GPM, Gruter GM. Lignocellulose saccharification: historical insights and recent industrial advancements towards 2nd generation sugars. RSC Sustain. 2025;3(3):1170–211. doi:10.1039/d4su00600c. [Google Scholar] [CrossRef]

148. Copenhaver K, Li K, Wang L, Lamm M, Zhao X, Korey M, et al. Pretreatment of lignocellulosic feedstocks for cellulose nanofibril production. Cellulose. 2022;29(9):4835–76. doi:10.1007/s10570-022-04580-z. [Google Scholar] [CrossRef]

149. Kululo WW, Habtu NG, Abera MK, Sendekie ZB, Fanta SW, Yemata TA. Advances in various pretreatment strategies of lignocellulosic substrates for the production of bioethanol: a comprehensive review. Discov Appl Sci. 2025;7(5):476. doi:10.1007/s42452-025-06748-1. [Google Scholar] [CrossRef]

150. Jain S, Kumar S. Advances and challenges in pretreatment technologies for bioethanol production: a comprehensive review. Sustain Chem Clim Action. 2024;5(3):100053. doi:10.1016/j.scca.2024.100053. [Google Scholar] [CrossRef]

151. Kim KH, Yoo CG. Challenges and perspective of recent biomass pretreatment solvents. Front Chem Eng. 2021;3:785709. doi:10.3389/fceng.2021.785709. [Google Scholar] [CrossRef]

152. Buraimoh OM, Ogunyemi AK, Isanbor C, Aina OS, Amund OO, Ilori MO, et al. Sustainable generation of bioethanol from sugarcane wastes by Streptomyces coelicolor strain COB KF977550 isolated from a tropical estuary. Sci Afr. 2021;11(12):e00709. doi:10.1016/j.sciaf.2021.e00709. [Google Scholar] [CrossRef]

153. Sanjivkumar M, Brindhashini A, Deivakumari M, Palavesam A, Immanuel G. Investigation on saccharification and bioethanol production from pretreated agro-residues using a mangrove associated actinobacterium Streptomyces variabilis (MAB3). Waste Biomass Valorization. 2018;9(6):969–84. doi:10.1007/s12649-017-9886-0. [Google Scholar] [CrossRef]

154. Wang Z, Tang H, Li Y, Yang B, Liang X, Gong H, et al. Characterization and synergistic activity of heterologously expressed microbial-derived endoglucanase and bifunctional cellulase on wheat straw. Int J Biol Macromol. 2024;282(11):137485. doi:10.1016/j.ijbiomac.2024.137485. [Google Scholar] [CrossRef]

155. Bature I, Liang Z, Wu X, Yang F, Yang Y, Dong P, et al. Isolation, cloning, and characterization of a novel GH5 cellulase from yak rumen metagenome for enhanced lignocellulose hydrolysis in biofuel production and ruminant feed utilization. Enzyme Microb Technol. 2025;191(5):110737. doi:10.1016/j.enzmictec.2025.110737. [Google Scholar] [CrossRef]

156. Arya M, Chauhan G, Verma U, Sharma M. Cloning, heterologous expression and purification of the novel thermo-alkalistable cellulase from Geobacillus sp. TP-3 and its molecular characterisation. Beni Suef Univ J Basic Appl Sci. 2024;13(1):36. doi:10.1186/s43088-024-00495-9. [Google Scholar] [CrossRef]

157. Wang S, Han H, Dong J, Li M, Wang Q, Wang L, et al. A novel GH11 β-1, 4-xylanase from Fusarium verticillioides: its eukaryotic expression, biochemical characterization and synergistic effect with cellulase on lignocellulosic biomass degradation. Int J Biol Macromol. 2025;305(Pt 1):141169. doi:10.1016/j.ijbiomac.2025.141169. [Google Scholar] [CrossRef]

158. Liu S, Li Y, Quan L, Liu HX, Luo Y, Wang YZ. Enhancing cellulase biosynthesis of Bacillus subtilis Z2 by regulating intracellular NADH level. iScience. 2025;28(5):112341. doi:10.1016/j.isci.2025.112341. [Google Scholar] [CrossRef]

159. Chen Y, Sun H, Chen H, Wu J, Huang J, Jiang X, et al. Enhancing cellulase production in Neurospora crassa through combined deletion of the phospholipase D-encoding gene pla-7 and modulation of transcription factor CLR-2 expression. Int J Biol Macromol. 2025;307(5):141944. doi:10.1016/j.ijbiomac.2025.141944. [Google Scholar] [CrossRef]

160. Askari H, Soleimanian-Zad S, Kadivar M, Shahbazi S. Creating a novel genetic diversity of Trichoderma afroharzianum by γ-radiation for xylanase-cellulase production. Heliyon. 2024;10(7):e28349. doi:10.1016/j.heliyon.2024.e28349. [Google Scholar] [CrossRef]

161. Rodríguez-Restrepo YA, Orrego CE. Immobilization of enzymes and cells on lignocellulosic materials. Environ Chem Lett. 2020;18(3):787–806. doi:10.1007/s10311-020-00988-w. [Google Scholar] [CrossRef]

162. Singh R, Ali Saati A, Faidah H, Bantun F, Jalal NA, Haque S, et al. Prospects of microbial cellulase production using banana peels wastes for antimicrobial applications. Int J Food Microbiol. 2023;388(42):110069. doi:10.1016/j.ijfoodmicro.2022.110069. [Google Scholar] [CrossRef]

163. Hussain S, Yasin MT, Ahmad K, Khan S, Ahmad R, Khan J, et al. Enhancement effect of AgO nanoparticles on fermentative cellulase activity from thermophilic Bacillus subtilis Ag-PQ. J Genet Eng Biotechnol. 2023;21(1):151. doi:10.1186/s43141-023-00619-1. [Google Scholar] [CrossRef]

164. Singh A, Kim BS. Nanoparticle-assisted bioethanol production from various lignocellulosic biomass. Int J Energy Res. 2025;2025(1):1937931. doi:10.1155/er/1937931. [Google Scholar] [CrossRef]

165. Ahamefule CS, Osilo C, Ahamefule BC, Ogbonna JC. Biofuel-integrated routes. In: Value-added products from algae. Berlin/Heidelberg, Germany: Springer; 2023. p. 191–229. doi:10.1007/978-3-031-42026-9_8. [Google Scholar] [CrossRef]