Enzymatic hydrolysis is limited by biomass-water interactions at high-solids: improved performance through substrate modifications

Enhancing enzymatic saccharification yields of cellulose at high solid loadings by combining different LPMO activities

BACKGROUND

The polysaccharides in lignocellulosic biomass hold potential for production of biofuels and biochemicals. However, achieving efficient conversion of this resource into fermentable sugars faces challenges, especially when operating at industrially relevant high solid loadings. While it is clear that combining classical hydrolytic enzymes and lytic polysaccharide monooxygenases (LPMOs) is necessary to achieve high saccharification yields, exactly how these enzymes synergize at high solid loadings remains unclear.

RESULTS

An LPMO-poor cellulase cocktail, Celluclast 1.5 L, was spiked with one or both of two fungal LPMOs from Thermothielavioides terrestris and Thermoascus aurantiacus, TtAA9E and TaAA9A, respectively, to assess their impact on cellulose saccharification efficiency at high dry matter loading, using Avicel and steam-exploded wheat straw as substrates. The results demonstrate that LPMOs can mitigate the reduction in saccharification efficiency associated with high dry matter contents. The positive effect of LPMO inclusion depends on the type of feedstock and the type of LPMO and increases with the increasing dry matter content and reaction time. Furthermore, our results show that chelating free copper, which may leak out of the active site of inactivated LPMOs during saccharification, with EDTA prevents side reactions with in situ generated H2O2 and the reductant (ascorbic acid).

CONCLUSIONS

This study shows that sustaining LPMO activity is vital for efficient cellulose solubilization at high substrate loadings. LPMO cleavage of cellulose at high dry matter loadings results in new chain ends and thus increased water accessibility leading to decrystallization of the substrate, all factors making the substrate more accessible to cellulase action. Additionally, this work highlights the importance of preventing LPMO inactivation and its potential detrimental impact on all enzymes in the reaction.

A critical assessment on scalable technologies using high solids loadings in lignocellulose biorefinery: challenges and solutions

The pretreatment and the enzymatic saccharification are the key steps in the extraction of fermentable sugars for further valorization of lignocellulosic biomass (LCB) to biofuels and value-added products via biochemical and/or chemical conversion routes. Due to low density and high-water absorption capacity of LCB, the large volume of water is required for its processing. Integration of pretreatment, saccharification, and co-fermentation has succeeded and well-reported in the literature. However, there are only few reports on extraction of fermentable sugars from LCB with high biomass loading (>10% Total solids-TS) feasible to industrial reality. Furthermore, the development of enzymatic cocktails can overcome technology hurdles with high biomass loading. Hence, a better understanding of constraints involved in the development of technology with high biomass loading can result in an economical and efficient yield of fermentable sugars for the production of biofuels and bio-chemicals with viable titer, rate, and yield (TRY) at industrial scale. The present review aims to provide a critical assessment on the production of fermentable sugars from lignocelluloses with high solid biomass loading. The impact of inhibitors produced during both pretreatment and saccharification has been elucidated. Moreover, the limitations imposed by high solid loading on efficient mass transfer during saccharification process have been elaborated.

Optimization of Wheat Straw Conversion into Microbial Lipids by Lipomyces tetrasporus DSM 70314 from Bench to Pilot Scale

Microbial lipids are renewable platforms for several applications including biofuels, green chemicals, and nutraceuticals that can be produced from several residual carbon sources. Lignocellulosic biomasses are abundant raw materials for the production of second-generation sugars with conversion yields depending on the quality of the hydrolysates and the metabolic efficiency of the microorganisms. In the present work, wheat straw pre-treated by steam explosion and enzymatically hydrolysed was converted into microbial lipids by Lipomyces tetrasporus DSM 70314. The preliminary optimization of the enzymatic hydrolysis was performed at the bench scale through the response surface methodology (RSM). The fermentation medium and set-up were optimized in terms of the nitrogen (N) source and carbon-to-nitrogen (C/N) ratio yielding to the selection of soy flour as a N source and C/N ratio of 160. The bench scale settings were scaled-up and further optimized at the 10 L-scale and finally at the 50 L pilot scale bioreactor. Process optimization also included oxygen supply strategies. Under optimized conditions, a lipid concentration of 14.8 gL−1 was achieved corresponding to a 23.1% w/w lipid yield and 67.4% w/w lipid cell content. Oleic acid was the most abundant fatty acid with a percentage of 57%. The overall process mass balance was assessed for the production of biodiesel from wheat straw.

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.

Efficient Corncob Biorefinery for Ethanol Initiated by a Novel Pretreatment of Densifying Lignocellulosic Biomass with Sulfuric Acid

Corncob is a potential feedstock for biorefineries to produce cellulosic ethanol and other chemicals. Densifying lignocellulosic biomass with chemicals followed by autoclave (DLCA) has been confirmed an efficient and economical pretreatment method, and it was applied in the present work for conversion of corncob to bioethanol. The dosage of sulfuric acid, solid loading of biomass, and autoclave time for pretreatment were investigated. Enzymatic hydrolysis at 25–35% solids loadings resulted in 91–97% sugar conversions. Fermentation of the resulted hydrolysates went well with the highest ethanol titer reaching 75.71 g/L at 35% solid loading. Simultaneous saccharification and co-fermentation was applied to further improve xylose consumption at high solids loadings and the ethanol titer was enhanced to 82.0 g/L at 35% solid loading with an ethanol yield of 21.67 kg/100 kg corncob. This study demonstrated DLCA provided a highly digestible and highly fermentable corncob for biorefinery.

Bioconversion of food and lignocellulosic wastes employing sugar platform: A review of enzymatic hydrolysis and kinetics

Bioenergy and biochemicals can be sustainably produced through fermentation and anaerobic digestion (AD). However, this bioconversion processes could be more economical if the hydrolysis rates of substrates in bioreactors can be accelerated. In this review, the feasibilities of including enzymatic hydrolysis (EH) in various bioconversion systems were studied to facilitate the biological synergy. The reaction kinetics of EH in bioconversion systems comparing pretreated lignocellulosic biomass (LCB) and food waste (FW) substrates were reviewed. Possible strategies to improve the hydrolysis efficiency were explored, including co-cultivation during enzyme production and replacement of pure enzyme with on-site produced fungal mash during EH. Key insights into improvement of current AD and fermentation technologies were summarized and further formed into suggestions of future directions in techno-economic feasibility of biorefinery using mixture of the first-generation food crop feedstock with FW; and/or co-digestion of FW with LCB.

Declining carbohydrate solubilization with increasing solids loading during fermentation of cellulosic feedstocks by Clostridium thermocellum: documentation and diagnostic tests

Background

For economically viable 2nd generation biofuels, processing of high solid lignocellulosic substrate concentrations is a necessity. The cellulolytic thermophilic anaerobe Clostridium thermocellum is one of the most effective biocatalysts for solubilization of carbohydrate harbored in lignocellulose. This study aims to document the solubilization performance of Clostridium thermocellum at increasing solids concentrations for two lignocellulosic feedstocks, corn stover and switchgrass, and explore potential effectors of solubilization performance.

Results

Monocultures of Clostridium thermocellum demonstrated high levels of carbohydrate solubilization for both unpretreated corn stover and switchgrass. However, fractional carbohydrate solubilization decreases with increasing solid loadings. Fermentation of model insoluble substrate (cellulose) in the presence of high solids lignocellulosic spent broth is temporarily affected but not model soluble substrate (cellobiose) fermentations. Mid-fermentation addition of cells (C. thermocellum) or model substrates did not significantly enhance overall corn stover solubilization loaded at 80 g/L, however cultures utilized the model substrates in the presence of high concentrations of corn stover. An increase in corn stover solubilization was observed when water was added, effectively diluting the solids concentration mid-fermentation. Introduction of a hemicellulose-utilizing coculture partner, Thermoanaerobacterium thermosaccharolyticum, increased the fractional carbohydrate solubilization at both high and low solid loadings. Residual solubilized carbohydrates diminished significantly in the presence of T. thermosaccharolyticum compared to monocultures of C. thermocellum, yet a small fraction of solubilized oligosaccharides of both C₅ and C₆ sugars remained unutilized.

Conclusion

Diminishing fractional carbohydrate solubilization with increasing substrate loading was observed for C. thermocellum-mediated solubilization and fermentation of unpretreated lignocellulose feedstocks. Results of experiments involving spent broth addition do not support a major role for inhibitors present in the liquid phase. Mid-fermentation addition experiments confirm that C. thermocellum and its enzymes remain capable of converting model substrates during the middle of high solids lignocellulose fermentation. An increase in fractional carbohydrate solubilization was made possible by (1) mid-fermentation solid loading dilutions and (2) coculturing C. thermocellum with T. thermosaccharolyticum, which ferments solubilized hemicellulose. Incomplete utilization of solubilized carbohydrates suggests that a small fraction of the carbohydrates is unaffected by the extracellular carbohydrate-active enzymes present in the culture.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02110-4.

Macroalgae (Ulva reticulata) derived biohydrogen recovery through mild surfactant induced energy and cost efficient dispersion pretreatment technology

Currently identification of alternate fuel is the key area of research under progress to overcome the depletion of fossil fuels, meet the domestic and industrial requirements. Generation of hydrogen, which is a clean fuel gas can solve various environmental related problems. Extensive research is being carried out to increase production of hydrogen through different substrates. This study aims to increase the production of hydrogen from Ulva reticulata (a macroalgal biomass). Initially, the biomass is pretreated mechanically with disperser and a biosurfactant, namely rhamnolipid in order to increase the solubilization of the biomass. The rate of COD liquefaction increased from 14% to 25% with the addition of biosurfactant to the macroalgal biomass, which is further treated mechanically using a disperser. The disperser rotor speed of 12,000 rpm and the specific energy input of 1175 kJ/kg TS (Total Solids) with the disintegration time of 30 min and biosurfactant dosage of 0.075 g/g TS were considered as the optimum parameters for the effective liquefaction of the macroalgal biomass. Approximately 3500 mg/L of the biopolymers were released after the combinative pretreatment (using disperser and biosurfactant). About 80 mL biohydrogen/g COD (Chemical Oxygen Demand) was generated when the biomass was pretreated with both the disperser and biosurfactant while the biomass pretreated with the disperser alone generated just 30 mL biohydrogen/g COD and the untreated biomass generated 5 mL biohydrogen/g COD. Thus, it can be concluded that Ulva reticulata can be utilized effectively to generate biohydrogen.

Impact of particle size reduction on high gravity enzymatic hydrolysis of steam-exploded wheat straw

Economically feasible bioethanol production from lignocellulosic biomass requires solid loadings ≥ 15% dry matter (DM, w/w). However, increased solid loadings can lead to process difficulties, which are characterized by high apparent slurry viscosity, insufficient substrate mixing and limited water availability, resulting in reduced final glucose yields. To overcome these limitations, this study focused on enzymatic hydrolysis of 10–35% DM solid loadings with steam-exploded wheat straw in two different particle sizes. At solid loadings of 20 and 25% DM small particle size of ≤ 2.5 mm yielded 16.9 ± 1.1% and 10.2 ± 1.4% increased final glucose concentrations compared to large particle size of 30 ± 20 mm. Small particle size also positively influenced slurry viscosity and, therefore, miscibility. As a key finding of this investigation, high gravity enzymatic hydrolysis with solid loadings of 30–35% DM was indeed successfully employed when wheat straw was applied in small particle size. Here, the highest final glucose yield was achieved with 127.9 ± 4.9 g L−1 at 35% DM solid loading. An increase in the solid loading from 10 to 35% DM in small particle size experiments resulted in a 460% increase in the final glucose concentration.

Analysis of water content in wood material through 1D and 2D 1 H NMR relaxometry: Application to the determination of the dry mass of wood

There is an increasing interest on wood as it is an environmentally sustainable product (e.g., biodegradable and renewable). Thus, an accurate characterisation of wood properties is of extreme importance as they define the kind of application for which each type of wood can be used. For instance, dry mass of wood is a key parameter itself and is needed to calculate Moisture Content (MC) of wood, which is correlated to its physical properties. Due to the limitations of commonly used drying methods, preliminary work has shown the potential of ¹ H NMR to measure dry mass of wood, but it has never been validated. Here, we performed a critical analysis of 1D and 2D ¹ H NMR relaxometry methods for obtaining the dry mass of wood, and we compared their performance to three commonly used drying methods. This showed that commonly used drying methods do not remove all water from wood. Moreover, we are able to classify them accordingly to their performance. In addition, we showed that MC values obtained by ¹ H NMR relaxometry methods are higher (up to 20%) than values from commonly used drying methods. This empathises the importance of accurate values of dry mass of wood and the utility of ¹ H NMR relaxometry on wood sciences. When comparing both NMR relaxometry methods, 2D should provide the more accurate results, but 1D measurements would also be a recommended choice as they are faster than 2D and their results clearly overcome commonly used drying methods in a noninvasive and nondestructive manner.

Effect of solid loading on the behaviour of pectin-degrading enzymes

BACKGROUND

Pectin plays a role in the recalcitrance of plant biomass by affecting the accessibility of other cell wall components to enzymatic degradation. Elimination of pectin consequently has a positive impact on the saccharification of pectin-rich biomass. This work thus focused on the behaviour of different pectin-degrading enzymes in the presence of low (5%) to high (35%) solid loading of lemon peel.

RESULTS

High solid loading of lemon peel affected pectin solubilisation differently depending on the pectinase used. Pectin lyase was less sensitive to a reduction of water content than was a mixture of endopolygalacturonase and pectin methylesterase, regardless of whether or not the latter's mode of action is processive or not. Marked changes in water mobility were observed along with enzymatic degradation depending on the enzyme used. However, the pectin lyase resulted in less pronounced shifts in water distribution than polygalacturonase-pectin methylesterase mixtures. At similar pectin concentration, pectin solutions hindered the diffusion of hydrolases more than the solid substrate. This can be attributed to the high viscosity of the highly concentrated pectin solutions while the solid substrate may provide continuous diffusion paths through pores.

CONCLUSIONS

The increase in solid substrate loading reduced the efficiency of pectin-degrading enzymes catalysing hydrolysis more significantly than those catalysing β-elimination. LF-NMR experiments highlighted the impact of solid loading on water mobility. Compared to other enzymes and whatever the solid loading, pectin lyase led to longer relaxation times linked with the most destructuration of the solid substrate. This new information could benefit the biorefinery processing of pectin-rich plant material when enzymes are used in the treatment.

Efficient Co-Utilization of Biomass-Derived Mixed Sugars for Lactic Acid Production by Bacillus coagulans Azu-10

Lignocellulosic and algal biomass are promising substrates for lactic acid (LA) production. However, lack of xylose utilization and/or sequential utilization of mixed-sugars (carbon catabolite repression, CCR) from biomass hydrolysates by most microorganisms limits achievable titers, yields, and productivities for economical industry-scale production. This study aimed to design lignocellulose-derived substrates for efficient LA production by a thermophilic, xylose-utilizing, and inhibitor-resistant Bacillus coagulans Azu-10. This strain produced 102.2 g/L of LA from 104 g/L xylose at a yield of 1.0 g/g and productivity of 3.18 g/L/h. The CCR effect and LA production were investigated using different mixtures of glucose (G), cellobiose (C), and/or xylose (X). Strain Azu-10 has efficiently co-utilized GX and CX mixture without CCR; however, total substrate concentration (>75 g/L) was the only limiting factor. The strain completely consumed GX and CX mixture and homoferemnatively produced LA up to 76.9 g/L. On the other hand, fermentation with GC mixture exhibited obvious CCR where both glucose concentration (>25 g/L) and total sugar concentration (>50 g/L) were the limiting factors. A maximum LA production of 50.3 g/L was produced from GC mixture with a yield of 0.93 g/g and productivity of 2.09 g/L/h. Batch fermentation of GCX mixture achieved a maximum LA concentration of 62.7 g/L at LA yield of 0.962 g/g and productivity of 1.3 g/L/h. Fermentation of GX and CX mixture was the best biomass for LA production. Fed-batch fermentation with GX mixture achieved LA production of 83.6 g/L at a yield of 0.895 g/g and productivity of 1.39 g/L/h.

Novel sustainable alternatives for the fashion industry: A method of chemically recycling waste textiles via acid hydrolysis

The fashion industry has a considerable environmental impact, especially due to the increased generation of waste textiles as a result of fast fashion business models. Although fiber-to-fiber recycling processes are being developed, such a process is in reality a downcycling process, in which the mechanical properties of the textile fibers are impoverished with each cycle. Thus, new alternatives are required to completely close the fashion loop through chemically recycling textile fibers unfit for other types of recycling or resale due to their poor quality. We have evaluated the possibility of using acid hydrolysis to directly depolymerize the cotton fibers in waste textiles to produce a glucose solution, which could subsequently be used for the production of chemicals or fuels. Although a one-step procedure with sulfuric acid was unable to deliver high glucose production, it was possible to achieve a glucose yield over 90% through a two-step procedure, in which concentrated and dilute sulfuric acid were combined to exploit the benefits of both concentrations. Glucose concentrations around 40 g/L were achieved by increasing the solids loading in the two-step process, which might be sufficiently high for the fermentation of the solution into high-value products. Thus, this study demonstrates that it would be possible to chemically recycle (cellulose-based) waste textiles via acid hydrolysis, which, if correctly designed, could avoid the need to use enzymes to achieve high conversion efficiencies.

Enzymatic degradation of maize shoots: monitoring of chemical and physical changes reveals different saccharification behaviors

BACKGROUND

The recalcitrance of lignocellulosics to enzymatic saccharification has been related to many factors, including the tissue and molecular heterogeneity of the plant particles. The role of tissue heterogeneity generally assessed from plant sections is not easy to study on a large scale. In the present work, dry fractionation of ground maize shoot was performed to obtain particle fractions enriched in a specific tissue. The degradation profiles of the fractions were compared considering physical changes in addition to chemical conversion.

RESULTS

Coarse, medium and fine fractions were produced using a dry process followed by an electrostatic separation. The physical and chemical characteristics of the fractions varied, suggesting enrichment in tissue from leaves, pith or rind. The fractions were subjected to enzymatic hydrolysis in a torus reactor designed for real-time monitoring of the number and size of the particles. Saccharification efficiency was monitored by analyzing the sugar release at different times. The lowest and highest saccharification yields were measured in the coarse and fine fractions, respectively, and these yields paralleled the reduction in the size and number of particles. The behavior of the positively- and negatively-charged particles of medium-size fractions was contrasted. Although the amount of sugar release was similar, the changes in particle size and number differed during enzymatic degradation. The reduction in the number of particles proceeded faster than that of particle size, suggesting that degradable particles were degraded to the point of disappearance with no significant erosion or fragmentation. Considering all fractions, the saccharification yield was positively correlated with the amount of water associated with [5-15 nm] pore size range at 67% moisture content while the reduction in the number of particles was inversely correlated with the amount of lignin.

CONCLUSION

Real-time monitoring of sugar release and changes in the number and size of the particles clearly evidenced different degradation patterns for fractions of maize shoot that could be related to tissue heterogeneity in the plant. The biorefinery process could benefit from the addition of a sorting stage to optimise the flow of biomass materials and take better advantage of the heterogeneity of the biomass.

Current and novel approaches to downstream processing of microalgae: A review

Biotechnological application of microalgae cultures at large scale has significant potential in the various fields of biofuels, food and feed, cosmetic, pharmaceutic, environmental remediation and water treatment. Despite this great potential application, industrialisation of microalgae culture and valorisation is still faced with serious remaining challenges in culture scale-up, harvesting and extraction of target molecules. This review presents a general summary of current techniques for harvesting and extraction of biomolecules from microalgae, their relative merits and potential for industrial application. The cell wall composition and its impact on microalgae cell disruption is discussed. Additionally, more recent progress and promising experimental methods and studies are summarised that would allow the reader to further investigate the state of the art. A final survey of energetic assessments of the different techniques is also made. Bead milling and high-pressure homogenisation seem to give clear advantages in terms of target high value compounds extraction from microalgae, with enzyme hydrolysis as a promising emerging technique. Future industrialisation of microalgae for high scale biotechnological processing will require the establishment of universal comparison-standards that would enable easy assessment of one technique against another.

Investigating the role of mechanics in lignocellulosic biomass degradation during hydrolysis: Part II

Lignocellulose breakdown in biorefineries is facilitated by enzymes and physical forces. Enzymes degrade and solubilize accessible lignocellulosic polymers, primarily on fiber surfaces, and make fibers physically weaker. Meanwhile physical forces acting during mechanical agitation induce tearing and cause rupture and attrition of the fibers, leading to liquefaction, that is, a less viscous hydrolysate that can be further processed in industrial settings. This study aims at understanding how mechanical agitation during enzymatic saccharification can be used to promote fiber attrition. The effects of reaction conditions, such as substrate and enzyme concentration on fiber attrition rate and hydrolysis yield were investigated. To gain insight into the fiber attrition mechanism, enzymatic hydrolysis was compared to hydrolysis by use of hydrochloric acid. Results show that fiber attrition depends on several factors concerning reactor design and operation including drum diameter, rotational speed, mixing schedule, and concentrations of fibers and enzymes. Surprisingly, different fiber attrition patterns during enzymatic and acid hydrolysis were found for similar mixing schedules. Specifically, for tumbling mixing, slow continuous mixing appears to function better than faster, intermittent mixing even for the same total number of drum revolutions. The findings indicate that reactor design and operation as well as hydrolysis conditions are key to process optimization and that detailed insights are needed to obtain fast liquefaction without sacrificing saccharification yields.

Enzyme-Mediated Lignocellulose Liquefaction Is Highly Substrate-Specific and Influenced by the Substrate Concentration or Rheological Regime

The high viscosities/yield stresses of lignocellulose slurries makes their industrial processing a significant challenge. However, little is known regarding the degree to which liquefaction and its enzymatic requirements are specific to a substrate's physicochemical and rheological properties. In the work reported here, the substrate- and rheological regime-specificities of liquefaction of various substrates were assessed using real-time in-rheometer viscometry and offline oscillatory rheometry when hydrolyzed by combinations of cellobiohydrolase (Trichoderma reesei Cel7A), endoglucanase (Humicola insolens Cel45A), glycoside hydrolase (GH) family 10 xylanase, and GH family 11 xylanase. In contrast to previous work that has suggested that endoglucanase activity dominates enzymatic liquefaction, all of the enzymes were shown to have at least some liquefaction capacity depending on the substrate and reaction conditions. The contribution of individual enzymes was found to be influenced by the rheological regime; in the concentrated regime, the cellobiohydrolase outperformed the endoglucanase, achieving 2.4-fold higher yield stress reduction over the same timeframe, whereas the endoglucanase performed best in the semi-dilute regime. It was apparent that the significant differences in rheology and liquefaction mechanisms made it difficult to predict the liquefaction capacity of an enzyme or enzyme cocktail at different substrate concentrations.

High Productivity Ethanol from Solid-State Fermentation of Steam-Exploded Corn Stover Using Zymomonas mobilis by N2 Periodic Pulsation Process Intensification

Solid-state fermentation, featured by water-saving, eco-friendly and high concentration product, is a promising technology in lignocellulosic ethanol industry. However, in solid-state fermentation system, large gas content inside the substrate directly leads to high oxygen partial pressure and inhibits ethanol fermentation. Z. mobilis can produce ethanol from glucose near the theoretical maximum value, but this ethanol yield would be greatly decreased by high oxygen partial pressure during solid-state fermentation. In this study, we applied N₂ periodic pulsation process intensification (NPPPI) to ethanol solid-state fermentation, which displaced air with N₂ and provided a proper anaerobic environment for Z. mobilis. Based on the water state distribution, the promotion effects of NPPPI on low solid loading and solid-state fermentation were analyzed to confirm the different degrees of oxygen inhibition in ethanol solid-state fermentation. During the simultaneous saccharification solid-state fermentation, the NPPPI group achieved 45.29% ethanol yield improvement and 30.38% concentration improvement compared with the control group. NPPPI also effectively decreased 58.47% of glycerol and 84.24% of acetic acid production and increased the biomass of Z. mobilis. By coupling the peristaltic enzymatic hydrolysis and fed-batch culture, NPPPI made the ethanol yield and concentration reach 80.11% and 55.06 g/L, respectively, in solid-state fermentation.

Constraints and advances in high-solids enzymatic hydrolysis of lignocellulosic biomass: a critical review

The industrial production of sugar syrups from lignocellulosic materials requires the conduction of the enzymatic hydrolysis step at high-solids loadings (i.e., with over 15% solids [w/w] in the reaction mixture). Such conditions result in sugar syrups with increased concentrations and in improvements in both capital and operational costs, making the process more economically feasible. However, this approach still poses several technical hindrances that impact the process efficiency, known as the "high-solids effect" (i.e., the decrease in glucan conversion yields as solids load increases). The purpose of this review was to present the findings on the main limitations and advances in high-solids enzymatic hydrolysis in an updated and comprehensive manner. The causes for the rheological limitations at the onset of the high-solids operation as well as those influencing the "high-solids effect" will be discussed. The subject of water constraint, which results in a highly viscous system and impairs mixing, and by extension, mass and heat transfer, will be analyzed under the perspective of the limitations imposed to the action of the cellulolytic enzymes. The "high-solids effect" will be further discussed vis-à-vis enzymes end-product inhibition and the inhibitory effect of compounds formed during the biomass pretreatment as well as the enzymes' unproductive adsorption to lignin. This review also presents the scientific and technological advances being introduced to lessen high-solids hydrolysis hindrances, such as the development of more efficient enzyme formulations, biomass and enzyme feeding strategies, reactor and impeller designs as well as process strategies to alleviate the end-product inhibition. We surveyed the academic literature in the form of scientific papers as well as patents to showcase the efforts on technological development and industrial implementation of the use of lignocellulosic materials as renewable feedstocks. Using a critical approach, we expect that this review will aid in the identification of areas with higher demand for scientific and technological efforts.

Pretreatment for biorefineries: a review of common methods for efficient utilisation of lignocellulosic materials

The implementation of biorefineries based on lignocellulosic materials as an alternative to fossil-based refineries calls for efficient methods for fractionation and recovery of the products. The focus for the biorefinery concept for utilisation of biomass has shifted, from design of more or less energy-driven biorefineries, to much more versatile facilities where chemicals and energy carriers can be produced. The sugar-based biorefinery platform requires pretreatment of lignocellulosic materials, which can be very recalcitrant, to improve further processing through enzymatic hydrolysis, and for other downstream unit operations. This review summarises the development in the field of pretreatment (and to some extent, of fractionation) of various lignocellulosic materials. The number of publications indicates that biomass pretreatment plays a very important role for the biorefinery concept to be realised in full scale. The traditional pretreatment methods, for example, steam pretreatment (explosion), organosolv and hydrothermal treatment are covered in the review. In addition, the rapidly increasing interest for chemical treatment employing ionic liquids and deep-eutectic solvents are discussed and reviewed. It can be concluded that the huge variation of lignocellulosic materials makes it difficult to find a general process design for a biorefinery. Therefore, it is difficult to define "the best pretreatment" method. In the end, this depends on the proposed application, and any recommendation of a suitable pretreatment method must be based on a thorough techno-economic evaluation.

Mild Acid-Catalyzed Atmospheric Glycerol Organosolv Pretreatment Effectively Improves Enzymatic Hydrolyzability of Lignocellulosic Biomass

Conventional atmospheric glycerol organosolv pretreatment is energy-intensive with the requirement of long time and/or high temperature. Herein, acid-catalyzed atmospheric glycerol organosolv (ac-AGO) pretreatment was developed under a mild condition to modify the sugarcane bagasse structure for improving enzymatic hydrolyzability. Using single factor and central composite design experiments, ac-AGO pretreatment was optimized at 200 °C for 15 min with 0.06% H₂SO₄ addition, wherein the hemicellulose and lignin removal rates were 82 and 52%, respectively, with extremely high cellulose retention of 98%. The ac-AGO-pretreated substrate exhibited good enzymatic hydrolyzability at a modest cellulase loading, affording a 70% glucose yield after 72 h. Multiple analysis tools were used to correlate the hydrolyzability of the substrate with its structural features. The results indicated that the mild ac-AGO pretreatment can modify the lignocellulosic biomass structure to achieve good hydrolyzability, mainly resulting in significant hemicellulose removal.

Smart fermentation engineering for butanol production: designed biomass and consolidated bioprocessing systems

There is a renewed interest in acetone-butanol-ethanol (ABE) fermentation from renewable substrates for the sustainable and environment-friendly production of biofuel and platform chemicals. However, the ABE fermentation is associated with several challenges due to the presence of heterogeneous components in the renewable substrates and the intrinsic characteristics of ABE fermentation process. Hence, there is a need to select optimal substrates and modify their characteristics suitable for the ABE fermentation process or microbial strain. This "designed biomass" can be used to establish the consolidated bioprocessing systems. As there are very few reports on designed biomass, the main objectives of this review are to summarize the main challenges associated with ABE fermentation from renewable substrates and to introduce feasible strategies for designing the substrates through pretreatment and hydrolysis technologies as well as through the establishment of consolidated bioprocessing systems. This review offers new insights on improving the efficiency of ABE fermentation from designed renewable substrates.

Bioethanol Production from UK Seaweeds: Investigating Variable Pre-treatment and Enzyme Hydrolysis Parameters

This study describes the method development for bioethanol production from three species of seaweed. Laminaria digitata, Ulva lactuca and for the first time Dilsea carnosa were used as representatives of brown, green and red species of seaweed, respectively. Acid thermo-chemical and entirely aqueous (water) based pre-treatments were evaluated, using a range of sulphuric acid concentrations (0.125-2.5 M) and solids loading contents (5-25 % [w/v]; biomass: reactant) and different reaction times (5-30 min), with the aim of maximising the release of glucose following enzyme hydrolysis. A pre-treatment step for each of the three seaweeds was required and pre-treatment conditions were found to be specific to each seaweed species. Dilsea carnosa and U. lactuca were more suited with an aqueous (water-based) pre-treatment (yielding 125.0 and 360.0 mg of glucose/g of pre-treated seaweed, respectively), yet interestingly non pre-treated D. carnosa yielded 106.4 g g^-1 glucose. Laminaria digitata required a dilute acid thermo-chemical pre-treatment in order to liberate maximal glucose yields (218.9 mg glucose/g pre-treated seaweed). Fermentations with S. cerevisiae NCYC2592 of the generated hydrolysates gave ethanol yields of 5.4 g L^-1, 7.8 g L^-1 and 3.2 g L^-1 from D. carnosa, U. lactuca and L. digitata, respectively. This study highlighted that entirely aqueous based pre-treatments are effective for seaweed biomass, yet bioethanol production alone may not make such bio-processes economically viable at large scale.