High-concentrated substrate enzymatic hydrolysis of pretreated rice straw with glycerol and aluminum chloride at low cellulase loadings

Co-production of fermentable sugars and highly active lignin from eucalyptus via a mild preprocessing with diethylene glycol and chromic chloride

Eucalyptus was pretreated with diethylene glycol catalyzed by 0.02 mol/L CrCl3 for 10 min, resulting in 91 % delignification and 98 % cellulose recovery, with trace fermentation inhibitors generated. After the mild pretreatment, the accessibility and affinity of cellulase to eucalyptus was enhanced, especially since enzyme adsorption rate increased by 1.6-fold. Therefore, glucose yield of pretreated eucalyptus was 7.9-fold higher than that of untreated eucalyptus after hydrolyzed 48 h, in which the maximum glucose concentration reached 62 g/L from eucalyptus by adding Tween 80. According to the characterization analysis, the structure of the eucalyptus lignin-carbohydrate complexes structure was destroyed during the pretreatment, while lignin fragments was likely reacted with diethylene glycol to form the stabilized aromatic ethers. Moreover, the extracted Deg-lignin exhibited better performances than commercial alkali lignin such as higher fluorescence intensity, less negative surface charge, and lower particle size. The mild pretreatment method with diethylene glycol and CrCl3 provided a promising approach for co-production of fermentable sugars and high activity lignin from lignocellulosic biomass.

Surfactants facilitated glycerol organosolv pretreatment of lignocellulosic biomass by structural modification for co-production of fermentable sugars and highly reactive lignin

This study reported that surfactants could facilitate the organosolv pretreatment of lignocellulosic biomass (LCB) to produce fermentable sugars and highly active lignin. Under the optimized conditions, the surfactant-assisted glycerol organosolv (saGO) pretreatment achieved 80.7% delignification with a retention of 93.4% cellulose and 83.0% hemicellulose. The saGO pretreated substrate exhibited an excellent enzymatic hydrolyzability, achieving 93% of glucose yield from the enzymatic hydrolysis at 48 h. Structural analysis showed that the saGO lignin contained rich β-O-4 bondings with less repolymerization and lower phenolic hydroxyl groups, thus forming highly reactive lignin fragments. The analysis evidenced that the surfactant graft the lignin by structural modification, which was responsible for the excellent substrate hydrolyzability. The co-production of fermentable sugars and organosolv lignin almost recovered a gross energy (87.2%) from LCB. Overall, the saGO pretreatment holds a lot of promise for launching a novel pathway towards lignocellulosic fractionation and lignin valorization.

Enhancing ethylene glycol and ferric chloride pretreatment of rice straw by low-pressure carbon dioxide to improve enzymatic saccharification

Ethylene glycol and ferric chloride pretreatment assisted by low-pressure carbon dioxide (1 MPa CO₂) realized the targeted deconstruction of lignocelluloses at 170 °C for 5 min, achieving 98 % cellulose recovery with removal of 92 % lignin and 90 % hemicellulose. After the pretreatment, the formation of stable platform mono-phenol components would be with the destruction of the lignin-carbohydrate complexes structure, and the surface of rice straw became rough, with a less negative charge and higher specific surface area, while the enzyme adsorption rate increased by 8.1 times. Furthermore, the glucose yield of pretreated straw was remarkably increased by 5.6 times that of the untreated straw, reaching 91 % after hydrolyzed for 48 h. With Tween 80 added in concentrated solid (12 %) hydrolysis at low cellulase loading (3 FPU/g dry substrate), half of the hydrolysis time was shortened than that without Tween 80, with 45 % higher glucose yield.

Advances and perspectives on mass transfer and enzymatic hydrolysis in the enzyme-mediated lignocellulosic biorefinery: A review

Enzymatic hydrolysis is a critical process for the cellulase-mediated lignocellulosic biorefinery to produce sugar syrups that can be converted into a whole range of biofuels and biochemicals. Such a process operating at high-solid loadings (i.e., scarcely any free water or roughly ≥ 15% solids, w/w) is considered more economically feasible, as it can generate a high sugar concentration at low operation and capital costs. However, this approach remains restricted and incurs "high-solid effects", ultimately causing the lower hydrolysis yields with increasing solid loadings. The lack of available water leads to a highly viscous system with impaired mixing that exhibits strong transfer resistance and reaction limitation imposed on enzyme action. Evidently, high-solid enzymatic hydrolysis involves multi-scale mass transfer and multi-phase enzyme reaction, and thus requires a synergistic perspective of transfer and biotransformation to assess the interactions among water, biomass components, and cellulase enzymes. Porous particle characteristics of biomass and its interface properties determine the water form and distribution state surrounding the particles, which are summarized in this review aiming to identify the water-driven multi-scale/multi-phase bioprocesses. Further aided by the cognition of rheological behavior of biomass slurry, solute transfer theories, and enzyme kinetics, the coupling effects of flow-transfer-reaction are revealed under high-solid conditions. Based on the above basic features, this review lucidly explains the causes of high-solid hydrolysis hindrances, highlights the mismatched issues between transfer and reaction, and more importantly, presents the advanced strategies for transfer and reaction enhancements from the viewpoint of process optimization, reactor design, as well as enzyme/auxiliary additive customization.

Bioethanol Production from Lignocellulosic Biomass-Challenges and Solutions

Regarding the limited resources for fossil fuels and increasing global energy demands, greenhouse gas emissions, and climate change, there is a need to find alternative energy sources that are sustainable, environmentally friendly, renewable, and economically viable. In the last several decades, interest in second-generation bioethanol production from non-food lignocellulosic biomass in the form of organic residues rapidly increased because of its abundance, renewability, and low cost. Bioethanol production fits into the strategy of a circular economy and zero waste plans, and using ethanol as an alternative fuel gives the world economy a chance to become independent of the petrochemical industry, providing energy security and environmental safety. However, the conversion of biomass into ethanol is a challenging and multi-stage process because of the variation in the biochemical composition of biomass and the recalcitrance of lignin, the aromatic component of lignocellulose. Therefore, the commercial production of cellulosic ethanol has not yet become well-received commercially, being hampered by high research and production costs, and substantial effort is needed to make it more widespread and profitable. This review summarises the state of the art in bioethanol production from lignocellulosic biomass, highlights the most challenging steps of the process, including pretreatment stages required to fragment biomass components and further enzymatic hydrolysis and fermentation, presents the most recent technological advances to overcome the challenges and high costs, and discusses future perspectives of second-generation biorefineries.

High-solids loading processing for an integrated lignocellulosic biorefinery: Effects of transport phenomena and rheology - A review

This review aims to present an analysis and discussion on the processing of lignocellulosic biomass in terms of biorefinery concept and circular bioeconomy operating at high solids lignocellulosic (above 15% [w/w]) at the pretreatment, enzymatic hydrolysis stage, and fermentation strategy for an integrated lignocellulosic bioprocessing. Studies suggest high solids concentration enzymatic hydrolysis for improved sugars yields and methods to overcome mass transport constraints. Rheological and computational fluid dynamics models of high solids operation through evaluation of mass and momentum transfer limitations are presented. Also, the review paper explores operational feeding strategies to obtain high ethanol concentration and conversion yield, from the hydrothermal pretreatment and investigates the impact of mass load over the operational techniques. Finally, this review contains a brief overview of some of the operations that have successfully scaled up and implemented high-solids enzymatic hydrolysis in terms of the biorefinery concept.

Glycerol organosolv pretreatment can unlock lignocellulosic biomass for production of fermentable sugars: Present situation and challenges

The complex structure of lignocellulosic biomass forms the recalcitrance to prevent the embedded holo-cellulosic sugars from undergoing the biodegradation. Therefore, a pretreatment is often required for an efficient enzymatic lignocellulosic hydrolysis. Recently, glycerol organosolv (GO) pretreatment is revealed potent in selective deconstruction of various lignocellulosic biomass and effective improvement of enzymatic hydrolysis. Evidently, the GO pretreatment is capable to modify the structure of dissolved components by glycerolysis, i.e., by trans-glycosylation onto glyceryl glycosides and by hydroxylation grafting onto glyceryl lignin. Such modifications tend to protect these main components against excessive degradation, which can be mainly responsible for the obviously less fermentation inhibitors arising in the GO pretreatment. This pretreatment can provide opportunities for valorization of emerging lignocellulosic biorefinery with production of value-added biochemicals. Recent advances in GO pretreatment of lignocellulosic biomass followed by enzymatic hydrolysis are reviewed, and perspectives are made for addressing remaining challenges.

Intensification of sugar production by using Tween 80 to enhance metal-salt catalyzed pretreatment and enzymatic hydrolysis of sugarcane bagasse

In this study, different metal-salt catalyzed pretreatment was presented to disorganize the obstinate structure by eliminating the majority of hemicellulose, fractional of lignin, and improve the enzymatic saccharification of sugarcane bagasse. With the accession of Tween 80 during enzymolysis, all metal-salt pretreated substrates presented higher glucose yields, especially for CuCl₂. Furthermore, Tween 80 was added to the pretreatment, enhancing the elimination of hemicellulose and lignin, decreasing the degradation of sugars to inhibitors, and presenting superior performance on improving glucose yield. In addition, the maximum glucose yield of 88.0% was achieved by using Tween 80 concomitantly with AlCl₃ pretreatment and enzymolysis. It was also found that adding Tween 80 during pretreatment or/and enzymolysis after 24 h could liberate the similar glucose without Tween 80 after 72 h. However, the enhancement of Tween 80 at 6 h was higher than that at 72 h.

Efficiency enhancement of a new cellulase cocktail at low enzyme loading for high solid digestion of alkali catalyzed atmospheric glycerol organosolvent pre-treated sugarcane bagasse

The acquisition during biomass saccharification of elevated levels of fermentable sugars with lower cellulase concentration is central to ensuring an economically viable and industrially relevant hydrolytic process. Thus, using a new cellulase preparation (LT4) at low cellulase loading (2 mg protein/g dried substrate), this study assessed the possible boosting effect of integrating accessory enzymes and additives on high-solids hydrolysis of sugarcane bagasse via fed-batch feeding. Hydrolysis which commenced with initial 8% solids loading and subsequent substrate feeding of 4% solids at 6 h, 18 h, and 24 h respectively, proved optimal for the 20% high-solids saccharification producing 158 g/L total sugars and 83% glucose yield after 72 h with the combined optimized additives and accessory enzymes. The results obtained indicate that the integration of accessory enzymes and additives offers a benignant approach to minimizing the enzyme load and cost of high solids saccharification of lignocellulosic heteropolymers while also boosting enzyme hydrolytic performance.

Emerging trends in high-solids enzymatic saccharification of lignocellulosic feedstocks for developing an efficient and industrially deployable sugar platform

For the techno-commercial success of any lignocellulosic biorefinery, the cost-effective production of fermentable sugars for the manufacturing of bio-based products is indispensable. High-solids enzymatic saccharification (HSES) is a straightforward approach to develop an industrially deployable sugar platform. Economic incentives such as reduced capital and operational expenditure along with environmental benefits in the form of reduced effluent discharge makes this strategy more lucrative for exploitation. However, HSES suffers from the drawback of non-linear and disproportionate sugar yields with increased substrate loadings. To overcome this bottleneck, researchers tend to perform HSES at high enzyme loadings. Nonetheless, the production costs of cellulases are one of the key contributors that impair the entire process economics. This review highlights the relentless efforts made globally to attain a high-titer of sugars and their fermentation products by performing efficient HSES at low cellulase loadings. In this context, technical innovations such as advancements in new pretreatment strategies, next-generation cellulase cocktails, additives, accessory enzymes, novel reactor concepts and enzyme recycling studies are especially showcased. This review further covers new insights, learnings and prospects in the area of lignocellulosic bioprocessing.

Comparison of Corn Stover Pretreatments with Lewis Acid Catalyzed Choline Chloride, Glycerol and Choline Chloride-Glycerol Deep Eutectic Solvent

Herein, corn stover (CS) was pretreated by less corrosive lewis acid FeCl₃ acidified solutions of neat and aqueous deep eutectic solvent (DES), aqueous ChCl and glycerol at 120 °C for 4 h with single FeCl₃ pretreatment as control. It was unexpected that acidified solutions of both ChCl and glycerol were found to be more efficient at removing lignin and xylan, leading to higher enzymatic digestibility of pretreated CS than acidified DES. Comparatively, acidified ChCl solution exhibited better pretreatment performance than acidified glycerol solution. In addition, 20 wt% water in DES dramatically reduced the capability of DES for delignification and xylan removal and subsequent enzymatic cellulose saccharification of pretreated CS. Correlation analysis showed that enzymatic saccharification of pretreated CS was highly correlated to delignification and cellulose crystallinity, but lowly correlated to xylan removal. Recyclability experiments of different acidified pretreatment solutions showed progressive decrease in the pretreatment performance with increasing recycling runs. After four cycles, the smallest decrease in enzymatic cellulose conversion (22.07%) was observed from acidified neat DES pretreatment, while the largest decrease (43.80%) was from acidified ChCl pretreatment. Those findings would provide useful information for biomass processing with ChCl, glycerol and ChCl-glycerol DES.

Highly efficient synthesis of biodiesel catalyzed by a cellulose@hematite-zirconia nanocomposite

The depletion of fossil fuels calls for the development of renewable alternatives such as biodiesel and has inspired much research on catalysts for the production of biodiesel through the esterification of biomass-derived materials. Herein, a green heterogeneous catalyst for highly efficient biodiesel synthesis was fabricated from rice straw-derived cellulose, hematite, and zirconia and was shown to contain porous irregularly shaped α-Fe2O3-ZrO2 composites (average particle size = 42.5 nm) evenly distributed on the nanocellulose surface. The optimal catalyst (nanocellulose:α-Fe2O3-ZrO2 = 2:1, w/w) afforded biodiesel in a yield of 92.50% and with specifications close to those prescribed by international standards. This catalyst could be reused for up to five cycles without a marked activity loss, with the biodiesel yield in the fifth cycle equaling 80.0%. The developed nanocomposite holds great promise for cutting the costs of biodiesel production, as it is derived from biodegradable raw materials and is renewable, non-corrosive, easy to handle, and green. In addition, the large-scale discharge of this catalyst after use does not pose a hazard to the environment.

Alleviating product inhibition of Trichoderma reesei cellulase complex with a product-activated mushroom endoglucanase

Product inhibition of cellulase is a challenging issue in industrial processes. Here, we introduced a product-activated mushroom cellulase, PaCel3A from Polyporus arcularius, into Trichoderma reesei. The filter paper activity, carboxymethyl cellulase activity, and saccharification efficiency (substrate: pretreated rice straw, PRS) of transformants increased significantly with this enzyme (by 18.4-26.8%, 13.8-22.8%, and 17.0%, respectively). A mutant of PaCel3A, PaCel3AM, obtained based on B-factor analysis, saturated mutagenesis, and residual activity assay, showed improved thermostability. The PRS saccharification efficiency using the cellulase complex from T. reesei transformants overexpressing pacel3am increased by 56.4%-63.0%. In addition, the T. reesei cellulase complex obtained by adding the purified recombinant PaCel3AM from T. reesei (rCel3aM-tr) to hydrolyze PRS resulted in increased reducing sugar yields at all sampling points, outperforming the cellulase complexes without rCel3aM-tr. These results suggest that introducing product-activated cellulase genes is a simple and feasible method to alleviate the product inhibition of cellulase.

A cellulose degrading bacterial strain used to modify rice straw can enhance Cu(II) removal from aqueous solution

The development of lignocellulose-based adsorbents for the removal of heavy metals from wastewater has attracted much recent attention. In this work, a high-yield cellulose bacterial strain Comamonas testosteroni FJ17 was evaluated for its capacity to modify rice straw towards increased Cu(II) removal. For optimum modification time (45.5 h), inoculum concentration (1.25%), and rice straw dose (12.6 g L^-1) the optimized adsorption capacity was 28.4 mg g^-1. After strain FJ17 modification the equilibrium adsorption percentage of rice straw for Cu(II) increased from 6.6 to 27.4% at an initial concentration of 100 mg L^-1. This increase was attributed to an increase in rice straw surface modification, leading to improved adsorption ability. SEM-EDS indicated that, following strain FJ17 treatment, the surface of the rice straw became more disintegrated and the specific surface area consequentially increased from 1.9 to 3.7 m² g^-1. FTIR analysis also showed new functional groups (carbonyl) appearing, and CC and CH₃CR functionality being enhanced after biomodification. Functional groups associated with the benzene ring, silicified polymer and carbohydrates were all involved in the adsorption process. Adsorption of Cu was well described by the Freundlich isotherm model (R² > 0.98) where adsorption was endothermic with potential for both chemical and physical interactions to coexist. Reusability experiments showed that the removal efficiency of Cu(II) decreased from 96.9 to 73.2% after five cycles. Overall C.testosteroni-treated rice straw had significant potential as a heavy metal biosorbent.

Scale-up of two-step acid-catalysed glycerol pretreatment for production of oleaginous yeast biomass from sugarcane bagasse by Rhodosporidium toruloides

Two-step dilute acid and acid-catalysed glycerol pretreatment was developed to maximise sugar yield from sugarcane bagasse. At the laboratory scale, dilute acid pretreatment at 130 °C followed by acid-catalysed glycerol pretreatment at 170 °C led to a total sugar (C5 + C6) yield of 82%, 31% higher than that from one-step acid-catalysed glycerol pretreatment. At the pilot scale, the two-step dilute acid and acid-catalysed glycerol pretreatment led to a maximum sugar yield of 74%, 13% higher than that from one-step pretreatment with 52% reduction in glycerol usage. The enzymatic hydrolysate containing glucose and residual glycerol were used to produce microbial oils by a Rhodosporidium toruloides strain. A fed-batch cultivation strategy led to the production of 44.8 g/L cell mass, including 26.6 g/L oil, 8.6 g/L protein and 12.7 mg/L carotenoid. The cell mass and oil yields were 19% higher than those from batch cultivation as feedstock inhibition and catabolite repression were alleviated.

Microbial lipid production from rice straw hydrolysates and recycled pretreated glycerol

Microbial lipids were produced by both rice straw hydrolysates and recycled pretreated glycerol. First, lipid fermentation of glucose via Cryptococcus curvatus was optimized by response surface methodology. Variables were selected by Plackett-Burman design, and optimized by central composite design, achieving 4.9 g/L total lipid and 0.16 g/g lipid yield, and increased further as glucose increased from 30 to 50 g/L. Secondly, after pretreatment, 72% lignin of rice straw was removed with glucose yield increased by 2.4 times to 74% at 20% substrate and 3 FPU/g. Subsequently, its hydrolysates produced high total lipid (8.8 g/L) and lipid yield (0.17 g/g). Finally, recycled glycerol reached the maximum total lipid of 7.2 g/L and high lipid yield of 0.16 g/g. Based on the calculation, 2.9 g total lipid would be produced from 1 g rice straw and the recycled glycerol, with a similar composition to soybean oil.

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.