Enhanced lignin removal and enzymolysis efficiency of grass waste by hydrogen peroxide synergized dilute alkali pretreatment

Structure changes of lignin and their effects on enzymatic hydrolysis for bioethanol production: a focus on lignin modification

Enzymatic hydrolysis contributes to obtaining fermentable sugars using pretreated lignocellulose materials for bioethanol generation. Unfortunately, the pretreatment of lignocellulose causes low substrate enzymatic hydrolysis, which is due to the structure changes of lignin to produce main phenolic by-products and non-productive cellulase adsorption. It is reported that modified lignin enhances the speed of enzymatic hydrolysis through single means to decrease the negative effects of fermentation inhibitors or non-productive cellulase adsorption. However, a suitable modified lignin should be selected to simultaneously reduce the fermentation inhibitors concentration and non-productive cellulase adsorption for saving resources and maximizing the enzymatic hydrolysis productivity. Meanwhile, the adsorption micro-mechanisms of modified lignin with fermentation inhibitors and cellulase remain elusive. In this review, different pretreatment effects toward lignin structure, and their impacts on subsequent enzymatic hydrolysis are analyzed. The main modification methods for lignin are presented. Density functional theory is used to screen suitable modification methods for the simultaneous reduction of fermentation inhibitors and non-productive cellulase adsorption. Lignin-fermentation inhibitors and lignin-cellulase interaction mechanisms are discussed using different advanced analysis techniques. This article addresses the gap in previous reviews concerning the application of modified lignin in the enhancement of bioethanol production. For the first time, based on existing studies, this work posits the hypothesis of applying theoretical simulations to screen efficient modified lignin-based adsorbents, in order to achieve a dual optimization of the detoxification and saccharification processes. We aim to improve the integrated lignocellulose transformation procedure for the effective generation of cleaner bioethanol.

A novel seawater hydrothermal-deep eutectic solvent pretreatment enhances the production of fermentable sugars and tailored lignin nanospheres from Pinus massoniana

Lignocellulose biorefinery depended on effective pretreatment strategies is of great significance for solving the current global crisis of ecosystem and energy security. This study proposes a novel approach combining seawater hydrothermal pretreatment (SHP) and microwave-assisted deep eutectic solvent (MD) pretreatment to achieve an effective fractionation of Pinus massoniana into high value-added products. The results indicated that complex ions (Mg2+, Ca2+, and Cl-) in natural seawater served as Lewis acids and dramatically promoted the depolymerization of mannose and xylan into oligosaccharides with 40.17 % and 75.43 % yields, respectively. Subsequent MD treatment realized a rapid and effective lignin fractionation (~90 %) while retaining cellulose. As a result, the integrated pretreatment yielded ~85 % of enzymatic glucose, indicating an eightfold increase compared with untreated pine. Because of the increased hydrophobicity induced by the formation of acyl groups during MD treatment, uniform lignin nanospheres were successfully recovered from the DES. It exhibited low dispersibility (PDI = 2.23), small molecular weight (1889 g/mol), and excellent oxidation resistance (RSI = 5.94), demonstrating promising applications in functional materials. The mechanism of lignin depolymerization was comprehensively elucidated via FTIR, 2D-HSQC NMR, and GPC analyses. Overall, this study provides a novel and environmentally friendly strategy for lignocellulose biorefinery and lignin valorization.

Enhancing biomass conservation and enzymatic hydrolysis of sweet sorghum bagasse by combining pretreatment with ensiling and NaOH

Lignocellulosic pretreatment is an important stage in biomass utilization, which usually requires high input. In this study, a low-cost method using combined ensiling and NaOH was developed for lignocellulosic pretreatment. Sweet sorghum bagasse (SSB) was ensiled for 21 days and then treated with diluted NaOH (0%, 1%, and 2%) for fermentation. The results showed that the application of Lactobacillus plantarum (L) reduced fermentation losses of the silages, mainly low water-soluble carbohydrate (WSC) and ammonia nitrogen loss. Meanwhile, the application of Lactobacillus plantarum and ensiling enzyme (LE) promoted lignocellulosic degradation, as evidenced by low neutral detergent fiber (NDF), acid detergent fiber (ADF), lignin (ADL), and hemicellulosic (HC) contents. The dominant bacterial genera were Lactobacillus, uncultured_bacterium_f_Enterobacteriaceae, and Pantoea after silage, which corresponded to the higher lactic acid and acetic contents and lower pH. The reducing sugar yields of SSB increased after combined pretreatment of silage and NaOH and were further enhanced by the 2% NaOH application, as evidenced by the high reducing sugar yield and microstructure damage, especially in the L-2% NaOH group and the LE-2% NaOH group, in which the reducing sugar yields were 87.99 and 94.45%, respectively, compared with those of the no additive control (CK)-0 NaOH group. Therefore, this study provides an effective method for SSB pretreatment to enhance biomass conservation.

Unveiling the potential of novel recyclable deep eutectic solvent pretreatment: Effective separation of lignin from poplar hydrolyzed residue

To effectively convert the fermentable sugars present in lignocellulosic biomass into biofuels and additional value-added products, it is crucial to remove lignin from the biomass. With the intention of expeditiously remove lignin from poplar wood and improve cellulose saccharification, an innovative ternary deep eutectic solvent (DES) benzyl triethyl ammonium chloride-ethylene glycol-FeCl3 (T-EG-F) was studied for the pretreatment of poplar hydrolyzed residue (PHR). The results revealed that following T-EG-F DES pretreatment at 130 °C for 4 h, the lignin removal rate reached 91.88 %. The effect of DES on PHR and regenerated lignin was comprehensively investigated using X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), Thermogravimetric (TG) and other characterization methods, providing valuable insights into the mechanism of this innovative biomass pretreatment. Moreover, there was a significant improvement in the enzyme digestibility of the DES pretreatment residue. At 48 h, the enzyme load of 30 FPU/g cellulose achieved a remarkable enzyme digestibility of 97.31 %, and this value exhibited a notable increase of 6.56 times compared to the untreated poplar sample. In addition, the T-EG-F could be recycled and reused. This study demonstrates that the potential of T-EG-F DES pretreatment as a green and efficient method for lignin dissociation from lignocellulosic biomass, offering a promising approach for biomass component separation.

Production of single cell protein from brewer's spent grain through enzymatic saccharification and fermentation enhanced by ammoniation pretreatment

Brewer's spent grain (BSG) is a major low-value by-product of beer industry. To realize the high value application of BSG, this work proposed a strategy to produce single cell protein (SCP) with oligosaccharide prebiotics from BSG, via ammoniation pretreatment, enzymatic hydrolysis, and fermentation. The optimum conditions of ammoniation pretreatment obtained by response surface method were 11 % ammonia dosage (w/w), 63 °C for 26 h. Suitable enzyme and yeast were screened to enhance the conversion of cellulose and hemicellulose in BSG into sugars and maximize the SCP yield. It was shown that using lignocellulolytic enzyme SP from Penicillium oxalicum and Trichosporon cutaneum, about 310 g of SCP with 80 g of arabinoxylo-oligosaccharides were obtained from 1000 g of BSG. This process is low cost, high efficiency, and easy to implement, which has good industrial application prospects.

Low-temperature pretreatment by AlCl3-catalyzed 1,4-butanediol solution for producing 'ideal' lignin with super-high content of β-O-4 linkages

High contents of internal β-O-4 linkages in lignin are critical for high-yield production of high-value aromatic monomers by depolymerization. However, it remains great challenge due to lack of suitable protection strategy. In this work, a very effective lignin-first strategy was developed to produce ideal lignin with a super high content of β-O-4 linkages (up to 72 %) from poplar, in which the pretreatment was undertaken at low temperatures of 90-130 °C with the use of AlCl3-catalyzed 1, 4-butanediol solution. 2D-HSQC NMR spectra revealed that lignin β-O-4 linkages were protected from etherification of the OH group by 1, 4-butanediol at the α position of lignin aliphatic chains. Besides, the OH groups at the γ position of lignin was also etherified, leading the formation of a structure of Ph-CH=CHCH2O(CH2)4OH. Interestingly, structure protection facilitated the formation of lignin nanoparticles via self-assembly (<100 nm). In addition, it was observed from pyrolysis results that addition of 1, 4-butanediol remarkably protected the structure of lignin by avoiding condensation, promoting the production of aromatics. The cellulose-rich fraction possessed a high cellulose digestibility of 91.64 % by enzymatic hydrolysis at a cellulase dosage of 15 FPU/g cellulose, approximately 6-fold untreated poplar (15.91 %). This low-temperature lignin-first strategy was of great importance for multi-products biorefining lignocellulose because it leads to the production of both lignin with super high content of β-O-4 linkages for depolymerization and highly digestible cellulose for sugar production.

Enhanced bioelectricity generation in thermophilic microbial fuel cell with lignocellulose as an electron donor by resazurin-mediated electron transfer

Microbial fuel cell (MFC) with lignocellulose as an electron donor is considered a sustainable biorefinery. However, low lignocellulose degradation and energy output restrict the scale of application. Herein, the extracellular electron transfer (EET) capacity of Acetivibrio thermocellus DSM 1313 with lignocellulose as substrate was shown to be mediated by the self-produced flavin, and its intracellular electron transfer went through the whole respiratory chain. Thermophilic MFC with resazurin exhibited an increase in the open circuit voltage by 37.78%, and a 2.60 folds increase in power density of 77.85 mW/m2, respectively. Differential pulse voltammetry and electrochemical impedance spectroscopy analysis indicated that resazurin decreased the solution and anode charge transfer resistance, and enhanced the extracellular electrochemical activity. Furthermore, resazurin resulted in a lower redox potential, allowing preferential electron transfer to resazurin rather than flavin. This research establishes a resazurin-mediated thermophilic MFC with lignocellulose as substrate, which provides novel idea on the biomass refinery.

Physicochemical Pretreatment of Vietnamosasa pusilla for Bioethanol and Xylitol Production

The consumption of fossil fuels has resulted in severe environmental consequences, including greenhouse gas emissions and climate change. Therefore, transitioning to alternative energy sources, such as cellulosic ethanol, is a promising strategy for reducing environmental impacts and promoting sustainable low-carbon energy. Vietnamosasa pusilla, an invasive weed, has been recognized as a high potential feedstock for sugar-based biorefineries due to its high total carbohydrate content, including glucan (48.1 ± 0.3%) and xylan (19.2 ± 0.4%). This study aimed to examine the impact of NaOH pretreatment-assisted autoclaving on V. pusilla feedstock. The V. pusilla enzymatic hydrolysate was used as a substrate for bioethanol and xylitol synthesis. After treating the feedstock with varying concentrations of NaOH at different temperatures, the glucose and xylose recovery yields were substantially higher than those of the untreated material. The hydrolysate generated by enzymatic hydrolysis was fermented into bioethanol using Saccharomyces cerevisiae TISTR 5339. The liquid byproduct of ethanol production was utilized by Candida tropicalis TISTR 5171 to generate xylitol. The results of this study indicate that the six- and five-carbon sugars of V. pusilla biomass have great potential for the production of two value-added products (bioethanol and xylitol).

Study on the effect of enzymatic treatment of tobacco on HnB cigarettes and microbial succession during fermentation

Starch and cellulose are the fundamental components of tobacco, while their excessive content will affect the quality of tobacco. Enzymatic treatment with different enzymes is a promising method to modulate the chemical composition and improve the sensory quality of tobacco leaves. In this study, enzymatic treatments, such as amylase, cellulase, and their mixed enzymes, were used to improve tobacco quality, which could alter the content of total sugar, reducing sugar, starch, and cellulose in tobacco leaves. The amylase treatment changed surface structure of tobacco leaves, increased the content of neophytadiene in tobacco by 16.48%, and improved the total smoking score of heat-not-burn (HnB) cigarette products by 5.0 points compared with the control. The Bacillus, Rubrobacter, Brevundimonas, Methylobacterium, Stenotrophomonas, Acinetobacter, Pseudosagedia-chlorotica, and Sclerophora-peronella were found to be significant biomarkers in the fermentation process by LEfSe analysis. The Basidiomycota and Agaricomycetes were significantly correlated with aroma and flavor, taste, and total score of HnB. The results showed that microbial community succession occurred due to amylase treatment, which promoted the formation of aroma compounds, and regulated the chemical composition of tobacco, and improved tobacco quality during tobacco fermentation. This study provides a method for enzymatic treatment to upgrade the quality of tobacco raw materials, thereby improving the quality of HnB cigarettes, and the potential mechanism is also revealed by chemical composition and microbial community analysis. KEY POINTS: Enzymatic treatment can change the chemical composition of tobacco leaves. The microbial community was significantly affected by enzymatic treatment. The quality of HnB cigarettes was significantly improved by amylase treatment.

Tobacco microbial screening and application in improving the quality of tobacco in different physical states

The first-cured tobacco contains macromolecular substances with negative impacts on tobacco products quality, and must be aged and fermented to mitigate their effects on the tobacco products quality. However, the natural fermentation takes a longer cycle with large coverage area and low economic efficiency. Microbial fermentation is a method to improve tobacco quality. The change of chemical composition of tobacco during the fermentation is often correlated with shapes of tobacco. This study aimed to investigate the effects of tobacco microorganisms on the quality of different shapes of tobacco. Specifically, Bacillus subtilis B1 and Cytobacillus oceanisediminis C4 with high protease, amylase, and cellulase were isolated from the first-cured tobacco, followed by using them for solid-state fermentation of tobacco powder (TP) and tobacco leaves (TL). Results showed that strains B1 and C4 could significantly improve the sensory quality of TP, enabling it to outperform TL in overall texture and skeleton of tobacco products during cigarette smoking. Compared with the control, microbial fermentation could increase reducing sugar; regulate protein, starch, and cellulose, reduce nicotine, improve total aroma substances, and enable the surface of fermented TP and TL to be more loose, wrinkled, and porous. Microbial community analysis indicated that strains B1 and C4 could change the native structure of microbial community in TP and TL. LEfSe analysis revealed that the potential key biomarkers in TP and TL were Bacilli, Pseudonocardia, Pantoea, and Jeotgalicoccus, which may have cooperative effects with other microbial taxa in improving tobacco quality. This study provides a theoretical basis for improving tobacco fermentation process for better cigarettes quality.

Optimization of lignin extraction from bamboo by ultrasound-assisted organosolv pretreatment

For a sustainable biorefinery, reduction in the recalcitrance of lignocellulosic biomass is very crucial for the efficient utilization of each fraction. The present work investigated an integrated pretreatment method to recover high-quality lignin along with the cellulose-rich pulp. An optimization study employing response surface methodology investigated the synergistic effects of ultrasound and organosolv pretreatment from Bambusa tulda (bamboo). The optimal condition (180 °C, 55 min, and 30 min sonication) resulted in 65.81 ± 2.40% of lignin yield with 95.37 ± 1.17% purity. A reduction in 7.85% yield and 1.54% purity of lignin with organosolv pretreatment highlighted the efficacy of sonication in lignin extraction. Ultrasound resulted in homolytic cleavage of the lignin-carbohydrate bond that enhanced delignification and increase the cellulose crystallinity. NMR, FTIR, GPC, and TGA of lignin suggested the superiority of sonication in maintaining lignin quality. A significant amount of β-O-4 linkages in extracted lignin is favorable for its subsequent valorization.

Nature-inspired pretreatment of lignocellulose - Perspective and development

As sustainability gains increasing importance in addition to cost-effectiveness as a criterion for evaluating engineering systems and practices, biological processes for lignocellulose pretreatment have attracted growing attention. Biological systems such as white and brown rot fungi and wood-consuming insects offer fascinating examples of processes and systems built by nature to effectively deconstruct plant cell walls under environmentally benign and energy-conservative environments. Research in the last decade has resulted in new knowledge that advanced the understanding of these systems, provided additional insights into these systems' functional mechanisms, and demonstrated various applications of these processes. The new knowledge and insights enable the adoption of a nature-inspired strategy aiming at developing technologies that are informed by the biological systems but superior to them by overcoming the inherent weakness of the natural systems. This review discusses the nature-inspired perspective and summarizes related advancements, including the evolution from biological systems to nature-inspired processes, the features of biological pretreatment mechanisms, the development of nature-inspired pretreatment processes, and future perspective. This work aims to highlight a different strategy in the research and development of novel lignocellulose pretreatment processes and offer some food for thought.

Review of chemical pretreatment of lignocellulosic biomass using low-liquid and low-chemical catalysts for effective bioconversion

Chemical pretreatment of lignocellulosic biomass (LCB) is essential for effective biological conversion in subsequent steps to produce biofuels or biochemicals. For effective pretreatment, high lignin content and its recalcitrant nature of LCB are major factors influencing bioconversion, especially lignin is known to be effectively solubilized by alkaline, organic, and deep eutectic solvents, ionic liquids, while hemicellulose is effectively dissolved by various acid catalysts and organic solvents. Depending on the pretreatment method/catalyst used, different pretreatment process scheme should be applied with different amounts of catalyst and water inputs to achieve a satisfactory effect. In addition, the amount of processing water required in the following processes such as washing, catalyst recovery, and conditioning after pretreatment is critical factor for scale-up (commercialization). In this review, the amount of catalyst and/or water used, and the effect of pretreatment, properties of the products, and recovery of liquid are also discussed.

Pretreatment Strategies to Enhance Enzymatic Hydrolysis and Cellulosic Ethanol Production for Biorefinery of Corn Stover

There is a rising interest in bioethanol production from lignocellulose such as corn stover to decrease the need for fossil fuels, but most research mainly focuses on how to improve ethanol yield and pays less attention to the biorefinery of corn stover. To realize the utilization of different components of corn stover in this study, different pretreatment strategies were used to fractionate corn stover while enhancing enzymatic digestibility and cellulosic ethanol production. It was found that the pretreatment process combining dilute acid (DA) and alkaline sodium sulfite (ASS) could effectively fractionate the three main components of corn stover, i.e., cellulose, hemicellulose, and lignin, that xylose recovery reached 93.0%, and that removal rate of lignin was 85.0%. After the joint pretreatment of DA and ASS, the conversion of cellulose at 72 h of enzymatic hydrolysis reached 85.4%, and ethanol concentration reached 48.5 g/L through fed-batch semi-simultaneous saccharification and fermentation (S-SSF) process when the final concentration of substrate was 18% (w/v). Pretreatment with ammonium sulfite resulted in 83.8% of lignin removal, and the conversion of cellulose and ethanol concentration reached 86.6% and 50 g/L after enzymatic hydrolysis of 72 h and fed-batch S-SSF, respectively. The results provided a reference for effectively separating hemicellulose and lignin from corn stover and producing cellulosic ethanol for the biorefinery of corn stover.

Bioprocess development of 2, 3-butanediol production using agro-industrial residues

The valorization of agricultural and industrial wastes for fuel and chemical production benefits environmental sustainability. 2, 3-Butanediol (2,3-BDO) is a value-added platform chemical covering many industrial applications. Since the global market is increasing drastically, production rates have to increase. In order to replace the current petroleum-based 2,3-BDO production, renewable feedstock's ability has been studied for the past few decades. This study aims to find an improved bioprocess for producing 2,3-BDO from agricultural and industrial residues, consequently resulting in a low CO₂ emission bioprocess. For this, screening of 13 different biomass samples for hydrolyzable sugars has been done. Alkali pretreatment has been performed with the processed biomass and enzyme hydrolysis performed using commercial cellulase. Among all biomass hydrolysate oat hull and spruce bark biomass could produce the maximum amount of total reducing sugars. Later oat hull and spruce bark biomass with maximum hydrolyzable sugars have been selected for submerged fermentation studies using Enterobacter cloacae SG1. After fermentation, 37.59 and 26.74 g/L of 2,3-BDO was obtained with oat hull and spruce bark biomass, respectively. The compositional analysis of each step of biomass processing has been performed and changes in each component have been evaluated. The compositional analysis has revealed that biomass composition has changed significantly after pretreatment and hydrolysis leading to a remarkable release of sugars which can be utilized by bacteria for 2,3-BDO production. The results have been found to be promising, showing the potential of waste biomass residues as a low-cost raw material for 2,3-BDO production and thus a new lead in an efficient waste management approach for less CO₂ emission.

Effect of Ammoniated Fiber Explosion Combined with H2O2 Pretreatment on the Hydrogen Production Capacity of Herbaceous and Woody Waste

An appropriate pretreatment process is an important part of the preparation of biomass energy from agricultural and forestry waste. Compared to physical and chemical pretreatments alone, the combined ammoniated fiber explosion (AFEX) + hydrogen peroxide (H₂O₂) pretreatment process can significantly improve the lignin degradation rate and saccharification efficiency, thus improving the hydrogen production capacity during medium-temperature dark fermentation. This study showed that the combined pretreatment increased the saccharification efficiency of herbaceous, hardwood, and softwood biomass by 58.7, 39.5, and 20.6% and the corresponding gas production reached 145.49, 80.75, and 57.52 mL/g, respectively. In addition, X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy showed that AFEX + H₂O₂ disrupted the structure of the feedstock and was more favorable for lignin removal. Soluble metabolites indicated that AFEX + H₂O₂ pretreatment enhanced the butyrate metabolic pathway of the substrate and biohydrogen generation and increased the levels of extracellular polymers and microbial community structure.

Promising seawater hydrothermal combining electro-assisted pretreatment for corn stover valorization within a biorefinery concept

In this study, for the first time, seawater hydrothermal (SH) pretreatment combining subsequent electrogenerated alkaline hydrogen peroxide (EAHP) pretreatment was proposed to achieve an effective fractionation of corn stover into high value-added products. During SH pretreatment, complex ions in natural seawater (Mg²⁺, Ca²⁺ and Cl^-) were used to promote depolymerization of xylan into xylo-oligosaccharides with 49.37% yield (190 °C,40 min), 18.52% higher than that of deionized water. Subsequent EAHP treatment not only provided a green and economical way to produce hydrogen peroxide but also synchronously realized satisfied delignification (94.91%). The integrated pretreatment resulted in 91.16% of glucose yield, which was about 5.6 times more than that of unpretreated corn stover. In addition, the recovered lignin fraction which has a potential application in functional materials were investigated by FTIR, 2D-HSQC NMR and GPC. In short, this work provided a novel and environmentally-friendly strategy for biorefinery-based fractionation of corn stover.

Xylose recovery and bioethanol production from sugarcane bagasse pretreated by mild two-stage ultrasonic assisted dilute acid

Pretreatment can improve biomass biodegradability. Here, a novel sugarcane bagasse (SCB) pretreatment process based on two-stage ultrasonic assisted dilute H₂SO₄ (TUDA) under mild conditions was reported. After optimization, the pretreatment was shown to significantly degrade hemicellulose (92.40%) and remove lignin (57.41%) of SCB, leading to reduction of inhibitors and an ethanol fermentation efficiency of 93.37% by SSCF under cellulase 10 FPU/g SCB and 30% pretreated SCB loading. Physical characterization revealed that two-stage ultrasonic could better disrupt SCB than traditional ultrasonic by amplifying the collapse effect and synergistically promoting lignin removal through dilute H₂SO₄. Furthermore, xylose was also effectively recovered from pretreatment supernatant by biochar derived from bagasse. This study established a simple and efficient pretreatment process for high value-added recycling of SCB from solid residue to pretreatment liquid.

Recent advances of lignin valorization techniques toward sustainable aromatics and potential benchmarks to fossil refinery products

Aromatic compounds are important fuels and key chemical precursors for organic synthesis, however the current aromatics market are mainly relying on fossil resources which will eventually contribute to carbon emissions. Lignin has been recognized as a drop-in substitution to conventional aromatics, with its values gradually realized after tremendous research efforts in the recent five years. To facilitate the development of a possible lignin economics, this study overviewed the recent advances of various biorefinery techniques and the remaining challenging for lignin valorization. Starting with recent discovery of unexplored lignin structures, the potential functions of lignin related chemical structures were emphasized. The important breakthrough of lignin-first pretreatment, catalytic lignin depolymerization, and the high value products with possible benchmark with modern aromatics were reviewed with possible future targets. Possible retrofit of conventional petroleum refinery for lignin products were also introduced and hopefully paving a way to progressively migrate the industry towards carbon neutrality.

A review on recent developments in hydrodynamic cavitation and advanced oxidative processes for pretreatment of lignocellulosic materials

Environmental problems due to utilization of fossil-derived materials for energy and chemical generation has prompted the use of renewable alternative sources, such as lignocellulose biomass (LB). Indeed, the production of biomolecules and biofuels from LB is among the most important current research topics aiming to development a sustainable bioeconomy. Yet, the industrial use of LB is limited by the recalcitrance of biomass, which impairs the hydrolysis of the carbohydrate fractions. Hydrodynamic cavitation (HC) and Advanced Oxidative Processes (AOPs) has been proposed as innovative pretreatment strategies aiming to reduce process time and chemical inputs. Therefore, the underlying mechanisms, procedural strategies, influence on biomass structure, and research gaps were critically discussed in this review. The performed discussion can contribute to future developments, giving a wide overview of the main involved aspects.

Combination strategy of laccase pretreatment and rhamnolipid addition enhance ethanol production in simultaneous saccharification and fermentation of corn stover

The effects of laccase pretreatment and surfactant addition in the simultaneous saccharification and fermentation (SSF) of corn stover by engineered Saccharomyces cerevisiae were studied. Surfactants Tween-80, tea saponin and rhamnolipid improved ethanol production in SSF, among which the biosurfactant rhamnolipid reached the highest ethanol yield. At the 6 d in SSF, the ethanol content of addition rhamnolipid of laccase pretreatment corn stover (Lac-CS) and Lac-CS reached 0.73 g/L and 0.56 g/L, which was 2.32 folds and 1.54 folds higher than the control of 0.22 g/L, respectively. These findings suggested that the combination of laccase pretreatment and rhamnolipid addition further improve ethanol production. GC-MS, composition of corn stover, protein concentration of supernatant and glucose content studies were executed to explore the mechanism of combination strategy of laccase pretreatment and rhamnolipid addition enhance ethanol production. This study provides guidance for the application of laccase and surfactant in bioethanol production.

A review on physico-chemical delignification as a pretreatment of lignocellulosic biomass for enhanced bioconversion

Effective pretreatment of lignocellulosic biomass (LCB) is one of the most important steps in biorefinery, ensuring the quality and commercial viability of the overall bioprocess. Lignin recalcitrance in LCB is a major bottleneck in biological conversion as the polymerization of lignin with hemicellulose hinders enzyme accessibility and further bioconversion to fuels and chemicals. Therefore, there is a need to delignify LCB to ease further bioprocessing. The efficiency of delignification, quality and quantity of the desired products, and generation of inhibitors depend upon the type of pretreatment employed. This review summarizes different single and integrated physicochemical pretreatments for delignification. Additionally, conditions required for effective delignification and the advantages and drawbacks of each method were evaluated. Advances in overcoming the recalcitrance of residual lignin to saccharification and the methods to recover lignin after delignification are also discussed. Efficient lignin recovery and valorization strategies provide an avenue for the sustainable lignocellulose biorefinery.

Combination of hydrothermal and chemi-mechanical pretreatments to enhance enzymatic hydrolysis of poplar branches and insights on cellulase adsorption

An integration of different pretreatments is important to overcome recalcitrance and realize efficient bioconversion of lignocellulosic biomass. This study aims at the effects of combination of hydrothermal pretreatment and different chemi-mechanical pretreatments on enzymatic hydrolysis, and understanding the enzymes adsorption mechanism. The combination of hydrothermal and chemi-mechanical pretreatments effectively improved the enzymatic hydrolysis of poplar substrates, in which the enzymatic hydrolysis of substrates pretreated by hydrothermal pretreatment + Fenton pretreatment + mechanical refining (HFM) was the highest (92.39% of glucose conversion yield, and 20.88 g/L of glucose concentration). The substrates' main characteristics were obviously changed after combined pretreatments, such as swelling ability and specific surface area of substrates were increased. The Langmuir adsorption model (R² > 0.98) and pseudo second-order adsorption kinetic model (R²≈1) were well suitable to describe the adsorption of enzymes on substrates, meanwhile the adsorption mechanism was summarized.

Enhanced enzymatic saccharification of sorghum straw by effective delignification via combined pretreatment with alkali extraction and deep eutectic solvent soaking

Hydrogen bond donor (HBD) in ChCl-based deep eutectic solvent (DESs) had significant influence on the Sorghum straw (SS) pretreatment. Lactic acid (LAC) was chosen as the appropriate HBD for preparing ChCl-based DES to pretreat Sorghum straw (SS). Furthermore, sequential pretreatment with dilute sodium hydroxide (0.75 wt%) for 1 h at 121 °C and ChCl:LAC soaking at 140 °C for 40 min was applied to pretreat SS for removing lignin (78.4%) and xylan (67.6%). Hydrolysis for 72 h, the reducing sugar yield reached 94.9%. Moreover, relationships of delignification and xylan removal with saccharification were explored after pretreatment. Finally, the fermentability of SS-hydrolysates was verified by bioethanol fermentation by S. cerevissiae with the yield of 0.45 g ethanol/g glucose. No significant inhibition was observed on ethanol fermentation. Obviously, establishment of high-efficient combination pretreatment with alkali extraction and ChCl:LAC soaking was successfully demonstrated for enhancing enzymatic saccharification of SS.

Cutaneotrichosporon oleaginosus: A Versatile Whole-Cell Biocatalyst for the Production of Single-Cell Oil from Agro-Industrial Wastes

Cutaneotrichosporon oleaginosus is an oleaginous yeast with several favourable qualities: It is fast growing, accumulates high amounts of lipids and has a very broad substrate spectrum. Its resistance to hydrolysis by-products makes it a promising biocatalyst for custom tailored microbial oils. C. oleaginosus can accumulate up to 60 wt.% of its biomass as lipids. This species is able to grow by using several compounds as a substrate, such as acetic acid, biodiesel-derived glycerol, N-acetylglucosamine, lignocellulosic hydrolysates, wastepaper and other agro-industrial wastes. This review is focused on state-of-the-art innovative and sustainable biorefinery schemes involving this promising yeast and second- and third-generation biomasses. Moreover, this review offers a comprehensive and updated summary of process strategies, biomass pretreatments and fermentation conditions for enhancing lipid production by C. oleaginosus as a whole-cell biocatalyst. Finally, an overview of the main industrial applications of single-cell oil is reported together with future perspectives.

Synergism of sweeping frequency ultrasound and deep eutectic solvents pretreatment for fractionation of sugarcane bagasse and enhancing enzymatic hydrolysis

Sugarcane bagasse (SCB) is an abundant agricultural waste in China and the conversion of the waste into plethora of useful resources is very vital. To achieve this, fractionation of the waste is highly important in the biomass biorefinery. The present study aims at investigating the synergistic role of deep eutectic solvents (DES) with sweeping frequency ultrasound (SFUS) and fixed frequency ultrasound (FFUS) in the fractionation of SCB to enhance the enzymatic saccharification process. Therefore, the effects of ultrasound (US) and DES conditions on the pretreatment efficiency were investigated. Under optimum SCB pretreatment conditions, FFUS (40 kHz, 60 min) + DES (choline chloride (ChCl)-lactic acid (LA), 120 °C, 3 h) and SFUS (40 kHz, 60 min) + DES (ChCl-LA, 120 °C, 3 h), the lignin removal rates were 80.13 and 85.62%, respectively. The hemicellulose removal rates were 78.08 and 90.46%, respectively; and the contents of glucose, xylose and arabinose in the liquid fractions after FFUS + DES pretreatment were 7.07, 17.95 and 3.01%, respectively. However, the yield of glucose, xylose, and cellobiose after enzymatic hydrolysis of the SFUS + DES pretreated SCB were 86.76, 38.68, and 20.76%. Analytical studies revealed that the SFUS + DES pretreatment can effectively destroy the ultrastructure of SCB and reduce the crystallinity of cellulose. Furthermore, the mechanism of pretreatment with SFUS + DES was proposed, which confirmed the excellent performance of SFUS + DES. Thus, the application of SFUS + DES pretreatment was able to improve the removal of lignin and hemicellulose from SCBs.

Improvement of Enzymatic Glucose Conversion from Chestnut Shells through Optimization of KOH Pretreatment

Worldwide, about one-third of food produced for human consumption is wasted, which includes byproducts from food processing, with a significant portion of the waste still being landfilled. The aim of this study is to convert chestnut shells (CNSs) from food processing into a valuable resource through bioprocesses. Currently, one of the highest barriers to bioprocess commercialization is low conversion of sugar from biomass, and KOH pretreatment was suggested to improve enzymatic digestibility (ED) of CNS. KOH concentration of 3% (w/w) was determined as a suitable pretreatment solution by a fundamental experiment. The reaction factors including temperature, time and solid/liquid (S/L) ratio were optimized (77.1 g/L CNS loading at 75 °C for 2.8 h) by response surface methodology (RSM). In the statistical model, temperature and time showed a relatively significant effect on the glucan content (GC) and ED, but S/L ratio was not. GC and ED of the untreated CNS were 45.1% and 12.7%, respectively. On the other hand, GC and ED of pretreated CNS were 83.2% and 48.4%, respectively, and which were significantly improved by about 1.8-fold and 3.8-fold compared to the control group. The improved ED through the optimization is expected to contribute to increasing the value of byproducts generated in food processing.

Effect of Pretreatments on the Enzymatic Hydrolysis of High-Yield Bamboo Chemo-Mechanical Pulp by Changing the Surface Lignin Content

Hydrogen peroxide chemo-mechanical pulp (APMP), sulfonated chemo-mechanical pulp (SCMP), and chemical thermomechanical pulp (CTMP) were used as raw materials to explore the effects of hydrogen peroxide (HP), Fenton pretreatment (FP), and ethanol pretreatment (EP) on the enzymatic hydrolysis of high-yield bamboo mechanical pulp (HBMP). The surface lignin distribution and contents of different HBMPs were determined using confocal laser scanning microscopy (CLSM) and X-ray photoelectron spectroscopy (XPS). The correlation between the surface lignin and the enzymatic hydrolysis of HBMP was also investigated. The residue of enzymatic hydrolysis was used to adsorb methylene blue (MB). The results showed that the cracks and fine fibers on the surface of APMP, SCMP, and CTMP increased after FP, when compared to HP and EP. The total removal content of hemicellulose and lignin in SCMP after FP was higher than with HP and EP. Compared to SCMP, the crystallinity increased by 15.4%, and the surface lignin content of Fenton-pretreated SCMP decreased by 11.7%. The enzymatic hydrolysis efficiency of HBMP after FP was higher than with HP and EP. The highest enzymatic hydrolysis of Fenton-pretreated SCMP was 49.5%, which was higher than the enzymatic hydrolysis of Fenton-pretreated APMP and CTMP. The removal rate of MB reached 94.7% after the adsorption of the enzymatic hydrolysis residue of SCMP. This work provides an effective approach for a high value-added utilization of high-yield bamboo pulp.

Potential of time-stepping stochastic models as tools for guiding the design and operation of processes for the enzymatic hydrolysis of polysaccharides - A review

Processes for the enzymatic hydrolysis of polysaccharides in biorefineries are becoming increasingly important. The complex network of reactions involved in polysaccharide hydrolysis can be described by stochastic models that advance in steps of time. Such models have the potential to be important tools for guiding process design and operation, and several have been developed over the last two decades. We evaluate these models. Many of the current stochastic models for the hydrolysis of colloidal polysaccharides use empirical parameters that have no recognized biological meaning. Only one model uses classical parameters of enzyme kinetics, namely specificity constants and saturation constants. Recent stochastic models for the hydrolysis of insoluble cellulose give valuable insights into the molecular-level phenomenon that limit hydrolysis rates. We conclude that, if stochastic models of enzymatic polysaccharide hydrolysis are to become widely used tools for guiding process development, then further improvements are required.

Enzymatic conversion of pretreated lignocellulosic biomass: A review on influence of structural changes of lignin

The demands of energy sustainability drive efforts to bio-chemical conversion of biomass into biofuels through pretreatment, enzymatic hydrolysis, and microbial fermentation. Pretreatment leads to significant structural changes of the complex lignin polymer that affect yield and productivity of the enzymatic conversion of lignocellulosic biomass. Structural changes of lignin after pretreatment include functional groups, inter unit linkages and compositions. These changes influence non-productive adsorption of enzyme on lignin through hydrophobic interaction and electrostatic interaction as well as hydrogen bonding. This paper reviews the relationships between structural changes of lignin and enzymatic hydrolysis of pretreated lignocellulosic biomass. The formation of pseudo-lignin during dilute acid pretreatment is revealed, and their negative effect on enzymatic hydrolysis is discussed.

Sequential pretreatment of sugarcane bagasse by alkali and organosolv for improved delignification and cellulose saccharification by chimera and cellobiohydrolase for bioethanol production

Sequential pretreatments for sugarcane bagasse (scb) by NaOH followed by organosolv under mild conditions were evaluated for cellulose recovery and dilignification. The best-optimized sequential pretreatment of scb was obtained at 10% (w/v) of raw scb loading at 1% (w/v) NaOH (50 °C, 2 h) followed by treatment with organosolv (85%, v/v phosphoric acid, 50 °C, 1 h) with chilled acetone. This sequentially pretreated scb showed cellulose recovery, 66.1% (w/w) and delignification, 83.2% (w/w). NaOH or organosolv pretreated scb showed lower cellulose recovery 47.4% (w/w) or 54.5% (w/w) with lower delignification, 61% (w/w) or 56% (w/w), respectively. Pretreated solid residue of sequentially pretreated scb was enzymatically saccharified by chimera (β-glucosidase and endoglucanase, CtGH1-L1-CtGH5-F194A) and cellobiohydrolase (CtCBH5A) cloned from Clostridium thermocellum. Enzymatic hydrolysate of best sequentially pretreated scb gave total reducing sugar (TRS) yield, 230 mg/g and glucose yield, 137 mg/g pretreated scb. Only organosolv pretreated scb gave TRS yield, 112.5 mg/g and glucose yield, 72 mg/g of pretreated scb. Thus, sequentially pretreated scb resulted in 37% higher enzymatic digestibility than only orgnaosolv pretreated scb. Higher enzymatic digestibility was supported by higher crystallinity index CrI (45%) than those obtained with only organosolv pretreated (38%) or raw scb (25%). Field Emission Scanning Electron Microscope (FESEM) and Fourier-transform infrared (FT-IR) analyses showed enhanced cellulose exposure in sequentially pretreated scb. Preliminary investigation of bioethanol production at small scale by separate hydrolysis and fermentation (SHF) of enzymatic hydrolysate from best sequentially pretreated scb by Saccharomyces cerevisiae gave maximum ethanol yield of 0.42 g/g of glucose.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-020-02600-y.

Improved Glucose Recovery from Sicyos angulatus by NaOH Pretreatment and Application to Bioethanol Production

As greenhouse gases and environmental pollution become serious, the demand for alternative energy such as bioethanol has rapidly increased, and a large supply of biomass is required for bioenergy production. Lignocellulosic biomass is the most abundant on the planet and a large part of it, the second-generation biomass, has the advantage of not being a food resource. In this study, Sicyos angulatus, known as an invasive plant (harmful) species, was used as a raw material for bioethanol production. In order to improve enzymatic hydrolysis, S. angulatus was pretreated with different NaOH concentration at 121 °C for 10 min. The optimal NaOH concentration for the pretreatment was determined to be 2% (w/w), and the glucan content (GC) and enzymatic digestibility (ED) were 46.7% and 55.3%, respectively. Through NaOH pretreatment, the GC and ED of S. angulatus were improved by 2.4-fold and 2.5-fold, respectively, compared to the control (untreated S. angulatus). The hydrolysates from S. angulatus were applied to a medium for bioethanol fermentation of Saccharomyces cerevisiae K35. Finally, the maximum ethanol production was found to be 41.3 g based on 1000 g S. angulatus, which was 2.4-fold improved than the control group.

The valorization of municipal grass waste for the extraction of cellulose nanocrystals

The study reports on the valorization of municipal grass waste (MGW) for the extraction of cellulose nanocrystals (CNCs), as an eco-friendly and sustainable low-cost precursor for cellulose nanomaterial production. The raw MGW was subjected to boiling in water pretreatment, and alkali and bleaching treatments for the extraction of cellulose fibers, followed by isolation of the CNCs through a conventional acid hydrolysis technique. Fourier transform infrared spectroscopy was used to analyze the cellulose fibers extracted while scanning electron microscopy and transmission electron microscopy images confirmed the presence of cellulose fibers and CNCs, respectively. The chemical composition of MGW was ascertained through the TAPPI-222 om-02 standard for lignin content and determination of α-cellulose. The diameters of CNCs are in the range of 5–15 nm with the length ranging from 100 nm to 500 nm, while a crystallinity index of 58.2% was determined from X-ray diffraction analysis. The production of CNCs from MGW is an avenue to convert green waste into a value-added product, in addition to reducing the volume of cumulative waste in the environment.

The production of CNCs from MGW is an avenue to convert green waste into a value-added product.

Physicochemical Properties and Lignin Degradation of Thermal-Pretreated Oil Palm Empty Fruit Bunch

Oil palm empty fruit bunches (EFB) are recoverable lignocellulosic biomass serving as feedstock for biofuel production. The major hurdle in producing biofuel from biomass is the abundance of embedded recalcitrant lignin. Pretreatment is a key step to increase the accessibility of enzymes to fermentable sugars. In this study, thermal pretreatments at moderate temperatures ranging from 150 °C to 210 °C, at different durations (30–120 min) and EFB particle sizes (1–10 mm), were employed to maximize lignin degradation. Observation through a scanning electron microscope (SEM) revealed disruptions in EFB structure and the removal of silica bodies and other impurities upon thermal pretreatment. Remarkable changes on the elemental contents and functional groups occurred, as was evident from the energy dispersive X-ray (EDX) and Fourier transform infrared (FTIR) analyses. The smallest EFB size yielded higher lignin degradation—about 2.3-fold and 1.2-fold higher—than the biggest and moderate tested EFB sizes, indicating a smaller particle size provides a higher surface area for bioreaction. Furthermore, applying a longer duration of treatment and a higher temperature enhanced lignin degradation by up to 58%. This study suggests that moderate thermal treatment could enhance lignin degradation by altering the physicochemical structure of EFB, which is beneficial in improving biofuel production.

Current state-of-the-art in ethanol production from lignocellulosic feedstocks

The renewable lignocellulosic biomass is a sustainable feedstock for the production of bioethanol, which shows the potential to replace fossil fuels. Due to the recalcitrant structure of plant cell wall made of cellulose, hemicellulose, and lignin, the biomass conversion process requires the use of efficient pretreatment process before enzymatic hydrolysis and fermentation to degrade the crystallinity of cellulose fibres and to remove lignin from biomass. Proper pretreatment techniques, economical production of cellulolytic enzymes, and effective fermentation of glucose and xylose in the presence of inhibitors are key challenges for the viable production of bioethanol. Although new strains capable of fermenting xylose are being designed, they are often not resistant to toxic compounds in hydrolysates. This paper provides an in-depth review of lignocellulosic bioethanol production via biochemical route, focusing on the most widely used pretreatment technologies and key operational conditions of enzymatic hydrolysis and fermentation considering sugar/ethanol yields. In addition, this review examines the relevant detoxification strategies for the removal of toxic substances and the importance of immobilization. The review also indicates potential usage of engineered microorganisms to improve glucose and xylose fermentation, cellulolytic enzymes production, and response to stress conditions.

Designing Efficient Processes for Sustainable Bioethanol and Bio-Hydrogen Production from Grass Lawn Waste

The effect of thermal, acid and alkali pretreatment methods on biological hydrogen (BHP) and bioethanol production (BP) from grass lawn (GL) waste was investigated, under different process schemes. BHP from the whole pretreatment slurry of GL was performed through mixed microbial cultures in simultaneous saccharification and fermentation (SSF) mode, while BP was carried out through the C5yeast Pichia stipitis, in SSF mode. From these experiments, the best pretreatment conditions were determined and the efficiencies for each process were assessed and compared, when using either the whole pretreatment slurry or the separated fractions (solid and liquid), the separate hydrolysis and fermentation (SHF) or SSF mode, and especially for BP, the use of other yeasts such as Pachysolen tannophilus or Saccharomyces cerevisiae. The experimental results showed that pretreatment with 10 gH₂SO₄/100 g total solids (TS) was the optimum for both BHP and BP. Separation of solid and liquid pretreated fractions led to the highest BHP (270.1 mL H₂/g TS, corresponding to 3.4 MJ/kg TS) and also BP (108.8 mg ethanol/g TS, corresponding to 2.9 MJ/kg TS) yields. The latter was achieved by using P. stipitis for the fermentation of the hydrolysate and S. serevisiae for the solid fraction fermentation, at SSF.

Recent developments in the application of kraft pulping alkaline chemicals for lignocellulosic pretreatment: Potential beneficiation of green liquor dregs waste

Lignocellulosic waste has offered a cost-effective and food security-wise substrate for the generation of biofuels and value-added products. However, its recalcitrant properties necessitate pretreatment. Of the various pretreatment methods, alkaline techniques have gained prominence as efficient catalysts. The kraft pulping industry represents a major hub for the generation of white, black and green liquor alkaline solutions during the paper making process. Despite its well-known significance in the kraft pulping process, green liquor (GL) has been widely applied for lignocellulosic pretreatment. Recently, green liquor dregs (GLD), an alkaline waste generated from the kraft pulping industry has piqued interest. Therefore, this review outlines the general flow of the kraft pulping process and the alkaline chemicals derived. In addition, the extensively studied GL for lignocellulosic pretreatment is discussed. Subsequently, the potential beneficiation of GLD for lignocellulosic pretreatment is presented. Furthermore, the challenges and prospects of lignocellulosic pretreatments are highlighted.