Enzymatic digestion of liquid hot water pretreated hybrid poplar

Optimization of biogas production from straw wastes by different pretreatments: Progress, challenges, and prospects

Lignocellulosic biomass (LCB) presents a promising feedstock for carbon management due to enormous potential for achieving carbon neutrality and delivering substantial environmental and economic benefit. Bioenergy derived from LCB accounts for about 10.3 % of the global total energy supply. The generation of bioenergy through anaerobic digestion (AD) in combination with carbon capture and storage, particularly for methane production, provides a cost-effective solution to mitigate greenhouse gas emissions, while concurrently facilitating bioenergy production and the recovery of high-value products during LCB conversion. However, the inherent recalcitrant polymer crystal structure of lignocellulose impedes the accessibility of anaerobic bacteria, necessitating lignocellulosic residue pretreatment before AD or microbial chain elongation. This paper seeks to explore recent advances in pretreatment methods for LCB biogas production, including pulsed electric field (PEF), electron beam irradiation (EBI), freezing-thawing pretreatment, microaerobic pretreatment, and nanomaterials-based pretreatment, and provide a comprehensive overview of the performance, benefits, and drawbacks of the traditional and improved treatment methods. In particular, physical-chemical pretreatment emerges as a flexible and effective option for methane production from straw wastes. The burgeoning field of nanomaterials has provoked progress in the development of artificial enzyme mimetics and enzyme immobilization techniques, compensating for the intrinsic defect of natural enzyme. However, various complex factors, such as economic effectiveness, environmental impact, and operational feasibility, influence the implementation of LCB pretreatment processes. Techno-economic analysis (TEA), life cycle assessment (LCA), and artificial intelligence technologies provide efficient means for evaluating and selecting pretreatment methods. This paper addresses current issues and development priorities for the achievement of the appropriate and sustainable utilization of LCB in light of evolving economic and environmentally friendly social development demands, thereby providing theoretical basis and technical guidance for improving LCB biogas production of AD systems.

Different pre-treatments and kinetic models for bioethanol production from lignocellulosic biomass: A review

Lignocellulosic biomass is the generally explored substrate to produce bioethanol for environmental sustainability due to its availability in abundance. However, the complex network of cellulose-hemicellulose-lignin present in it makes its hydrolysis as a challenging task. To boost the effectiveness of conversion, biomass is pre-treated before enzymatic hydrolysis to alter or destroy its original composition. Enzymes like Cellulases are widely used for breaking down cellulose into fermentable sugars. Enzymatic hydrolysis is a complex process involving many influencing factors such as pH, temperature, substrate concentration. This review presents major four pre-treatment methods used for hydrolysing different substrates under varied reaction conditions along with their mechanism and limitations. A relative comparison of data analysis for most widely studied 10 kinetic models is briefly explained in terms of substrates used to get the brief insight about hydrolysis rates. The summary of pre-treatment methods and hydrolysis rates including cellulase enzyme kinetics will be the value addition for upcoming researchers for optimising the hydrolysis process.

Inhibition of enzyme hydrolysis of cellulose by phenols from hydrothermally pretreated sugarcane straw

Relatively few studies have addressed the characterization of sugarcane straw (SCS) for production of fermentable sugars through enzyme hydrolysis. Straw is a major co-product of the sugarcane harvest in Brazil that has potential to sustainably increase cellulosic feedstocks in Brazil by 50%. Pretreatment of 10% w/v straw with liquid hot water (LHW) at 180 °C for 50 min (severity, S_o, of 4.05), solubilizes hemicellulose, preserves glucan, and generates 4.49 g/L soluble phenolic compounds in the resulting liquid. Extracts from washing pretreated solids with excess hot water followed by acetone resulted in 1.10 and 0.83 g/L phenolics, respectively. Acetone-derived extracts were more inhibitory and decreased glucose yield for enzyme hydrolysis of Solka Floc (a lignin-free cellulose) by 42%. In comparison, pretreated straw washed with hot water or acetone was readily hydrolyzed to 92% and 97% by cellulase enzyme. Hydrothermally treated SCS has the potential to provide a valuable and added source of fermentable sugars suitable for bioprocessing into biofuels and bioproducts when cellulase enzyme inhibitors are removed after pretreatment.

Hydrothermal Liquefaction of Lignocellulosic and Protein-Containing Biomass: A Comprehensive Review

Hydrothermal liquefaction (HTL) is a thermochemical depolymerization technology, also known as hydrous pyrolysis, that transforms wet biomass into biocrude and valuable chemicals at a moderate temperature (usually 200–400 °C) and high pressure (typically 10–25 MPa). In HTL, water acts as a key reactant in HTL activities. Several properties of water are substantially altered as the reaction state gets closer to the critical point of water, which can result in quick, uniform, and effective reactions. The current review covers the HTL of various feedstocks, especially lignocellulosic and high protein-containing feeds with their in-depth information of the chemical reaction mechanisms involved in the HTL. Further, this review gives insight and knowledge about the influencing factors such as biomass pretreatment, process mode, process conditions, etc., which could affect the efficiency of the hydrothermal process and biocrude productivity. In addition, the latest trends, and emerging challenges to HTL are discussed with suitable recommendations.

Improving the Biocontrol Potential of Endophytic Bacteria Bacillus subtilis with Salicylic Acid against Phytophthora infestans-Caused Postharvest Potato Tuber Late Blight and Impact on Stored Tubers Quality

Potato (Solanum tuberosum L.) tubers are a highly important food crop in many countries due to their nutritional value and health-promoting properties. Postharvest disease caused by Phytophthora infestans leads to the significant decay of stored potatoes. The main objective of this study was to evaluate the effects of the endophytic bacteria, Bacillus subtilis (strain 10–4), or its combination with salicylic acid (SA), on some resistance and quality traits of stored Ph. infestans-infected potato tubers. The experiments were conducted using hydroponically grown potato mini-tubers, infected prior to storage with Ph. infestans, and then coated with B. subtilis, alone and in combination with SA, which were then stored for six months. The results revealed that infection with Ph. infestans significantly increased tuber late blight incidence (up to 90–100%) and oxidative and osmotic damage (i.e., malondialdehyde and proline) in tubers. These phenomena were accompanied by a decrease in starch, reducing sugars (RS), and total dry matter (TDM) contents and an increase in amylase (AMY) activity. Moreover, total glycoalkaloids (GA) (α-solanine, α-chaconine) notably increased in infected tubers, exceeding (by 1.6 times) permissible safe levels (200 mg/kg FW). Treatments with B. subtilis or its combination with SA decreased Ph. infestans-activated tuber late blight incidence (by 30–40%) and reduced oxidative and osmotic damages (i.e., malondialdehyde and proline) and AMY activity in stored, infected tubers. Additionally, these treatments decreased pathogen-activated GA accumulation and increased ascorbic acid in stored tubers. Thus, the results indicated that endophytic bacteria B. subtilis, individually, and especially in combination with SA, have the potential to increase potato postharvest resistance to late blight and improve tuber quality in long-term storage.

Severity factor kinetic model as a strategic parameter of hydrothermal processing (steam explosion and liquid hot water) for biomass fractionation under biorefinery concept

Hydrothermal processes are an attractive clean technology and cost-effective engineering platform for biorefineries based in the conversion of biomass to biofuels and high-value bioproducts under the basis of sustainability and circular bioeconomy. The deep and detailed knowledge of the structural changes by the severity of biomasses hydrothermal fractionation is scientifically and technological needed in order to improve processes effectiveness, reactors designs, and industrial application of the multi-scale target compounds obtained by steam explosion and liquid hot water systems. The concept of the severity factor [log₁₀ (R_o)] established>30 years ago, continues to be a useful index that can provide a simple descriptor of the relationship between the operational conditions for biomass fractionation in second generation of biorefineries. This review develops a deep explanation of the hydrothermal severity factor based in lignocellulosic biomass fractionation with emphasis in research advances, pretreatment operations and the applications of severity factor kinetic model.

Liquid Hot Water Pretreatment of Lignocellulosic Biomass at Lab and Pilot Scale

Liquid hot water pretreatment is considered to be a promising method for increasing biomass digestibility due to the moderate operational conditions without chemical additions. A necessary step towards the scalability of this pretreatment process is performing pilot plant trials. Upscaling was evaluated with a scaling factor of 500, by using 50 mL in the laboratory and 25 L in a pilot plant batch reactor. Pretreatment times were varied from 30 to 240 min, and temperatures used were 180–188 °C, while applying similar heating profiles at both scales. The initial mass fraction of poplar wood chips ranged from 10% to 16%. Liquid hot water pretreatment at laboratory and pilot scale led to analogous results. The acetic acid analysis of the liquid and solid fractions obtained after pretreatment indicated that complete deacetylation of poplar biomass can be achieved.

Sugar Production from Hybrid Poplar Sawdust: Optimization of Enzymatic Hydrolysis and Wet Explosion Pretreatment

Wet explosion pretreatment of hybrid poplar sawdust (PSD) for the production of fermentable sugar was carried out in the pilot-scale. The effects of pretreatment conditions, such as temperature (170-190 °C), oxygen dosage (0.5-7.5% of dry matter (DM), w/w), residence time (10-30 min), on cellulose and hemicellulose digestibility after enzymatic hydrolysis were ascertained with a central composite design of the experiment. Further, enzymatic hydrolysis was optimized in terms of temperature, pH, and a mixture of CTec2 and HTec2 enzymes (Novozymes). Predictive modeling showed that cellulose and hemicellulose digestibility of 75.1% and 83.1%, respectively, could be achieved with a pretreatment at 177 °C with 7.5% O2 and a retention time of 30 min. An increased cellulose digestibility of 87.1% ± 0.1 could be achieved by pretreating at 190 °C; however, the hemicellulose yield would be significantly reduced. It was evident that more severe conditions were required for maximal cellulose digestibility than that of hemicellulose digestibility and that an optimal sugar yield demanded a set of conditions, which overall resulted in the maximum sugar yield.

The Effect of Endophytic Bacteria Bacillus subtilis and Salicylic Acid on Some Resistance and Quality Traits of Stored Solanum tuberosum L. Tubers Infected with Fusarium Dry Rot

The effect of endophytic Bacillus subtilis (strains 10-4, 26D) and their compositions withsalicylic acid (SA) on some resistance and quality traits of stored potatoes infected with Fusariumdry rot were studied. The experiments were carried out on hydroponically grown Solanumtuberosum L. tubers that were infected before storage with Fusarium oxysporum and coated with B.subtilis 10-4, 26D with and without exogenous SA, and then stored for six months. It has been shownthat 10-4, 26D, 10-4 + SA, and 26D + SA reduced in different levels (up to 30-50%) the incidence ofF. oxysporum-caused dry rot (with the highest effect for 10-4 + SA). SA notably enhanced the positiveeffect of 10-4, while for 26D, such an effect was not observed. All of the tested treatments increasedamylase (AMY) and AMY inhibitors activity in infected tubers, while decreased Fusarium-inducedprotease activity (except in the case of 10-4 + SA, which promoted a slight increase) was revealed.10-4, 26D, and their compositions with SA decreased (in different degrees) the pathogen-causedlipid peroxidation, proline, and reducing sugars accumulation in potatoes after long-term storage.It was also discovered 10-4 and 26D, regardless of SA presence, decrease pathogen-inducedglycoalkaloids α-Solanine and α-Chaconine accumulation and preserved increased levels of starchand total dry matter in infected stored potatoes. The findings indicate endophytic B. subtilis and itscompositions with SA is a promising eco-friendly and bio-safe approach to cope with postharvestdecays of potato during long-term storage; however, when developing preparations-compositionsit should take into account the strain-dependent manner of B. subtilis action together with SA.

Effect of Lignin Content on Cellulolytic Saccharification of Liquid Hot Water Pretreated Sugarcane Bagasse

Lignin contributes to the rigid structure of the plant cell wall and is partially responsible for the recalcitrance of lignocellulosic materials to enzymatic digestion. Overcoming this recalcitrance is one the most critical issues in a sugar-flat form process. This study addresses the effect of low lignin sugarcane bagasse on enzymatic hydrolysis after liquid hot water pretreatment at 190 °C and 20 min (severity factor: 3.95). The hydrolysis of bagasse from a sugarcane line selected for a relatively low lignin content, gave an 89.7% yield of cellulose conversion to glucose at 40 FPU/g glucan versus a 68.3% yield from a comparably treated bagasse from the high lignin bred line. A lower enzyme loading of 5 FPU/g glucan (equivalent to 3.2 FPU/g total solids) resulted in 31.4% and 21.9% conversion yields, respectively, for low and high lignin samples, suggesting the significance of lignin content in the saccharification process. Further increases in the enzymatic conversion of cellulose to glucose were achieved when the bagasse sample was pre-incubated with a lignin blocking agent, e.g., bovine serum albumin (50 mg BSA/g glucan) at 50 °C for 1 h prior to an actual saccharification. In this work, we have demonstrated that even relatively small differences in lignin content can result in considerably increased sugar production, which supports the dissimilarity of bagasse lignin content and its effects on cellulose digestibility. The increased glucose yields with the addition of BSA helped to decrease the inhibition of non-productive absorption of cellulose enzymes onto lignin and solid residual lignin fractions.

Lignin-Enzyme Interactions in the Hydrolysis of Lignocellulosic Biomass

Lignin is central to overcoming recalcitrance in the enzyme hydrolysis of lignocellulose. While the term implies a physical barrier in the cell wall structure, there are also important biochemical components that direct interactions between lignin and the hydrolytic enzymes that attack cellulose in plant cell walls. Progress toward a deeper understanding of the lignin synthesis pathway - and the consistency between a range of observations over the past 40 years in the very extensive literature on cellulose hydrolysis - is resulting in advances in reducing a major impediment to cellulose conversion: the cost of enzymes. This review addresses lignin and its role in the hydrolysis of hardwood and other lignocellulosic residues.

Hydrolysis of untreated lignocellulosic feedstock is independent of S-lignin composition in newly classified anaerobic fungal isolate, Piromyces sp. UH3-1

BACKGROUND

Plant biomass is an abundant but underused feedstock for bioenergy production due to its complex and variable composition, which resists breakdown into fermentable sugars. These feedstocks, however, are routinely degraded by many uncommercialized microbes such as anaerobic gut fungi. These gut fungi express a broad range of carbohydrate active enzymes and are native to the digestive tracts of ruminants and hindgut fermenters. In this study, we examine gut fungal performance on these substrates as a function of composition, and the ability of this isolate to degrade inhibitory high syringyl lignin-containing forestry residues.

RESULTS

We isolated a novel fungal specimen from a donkey in Independence, Indiana, United States. Phylogenetic analysis of the Internal Transcribed Spacer 1 sequence classified the isolate as a member of the genus Piromyces within the phylum Neocallimastigomycota (Piromyces sp. UH3-1, strain UH3-1). The isolate penetrates the substrate with an extensive rhizomycelial network and secretes many cellulose-binding enzymes, which are active on various components of lignocellulose. These activities enable the fungus to hydrolyze at least 58% of the glucan and 28% of the available xylan in untreated corn stover within 168 h and support growth on crude agricultural residues, food waste, and energy crops. Importantly, UH3-1 hydrolyzes high syringyl lignin-containing poplar that is inhibitory to many fungi with efficiencies equal to that of low syringyl lignin-containing poplar with no reduction in fungal growth. This behavior is correlated with slight remodeling of the fungal secretome whose composition adapts with substrate to express an enzyme cocktail optimized to degrade the available biomass.

CONCLUSIONS

Piromyces sp. UH3-1, a newly isolated anaerobic gut fungus, grows on diverse untreated substrates through production of a broad range of carbohydrate active enzymes that are robust to variations in substrate composition. Additionally, UH3-1 and potentially other anaerobic fungi are resistant to inhibitory lignin composition possibly due to changes in enzyme secretion with substrate. Thus, anaerobic fungi are an attractive platform for the production of enzymes that efficiently use mixed feedstocks of variable composition for second generation biofuels. More importantly, our work suggests that the study of anaerobic fungi may reveal naturally evolved strategies to circumvent common hydrolytic inhibitors that hinder biomass usage.

Biomass augmentation through thermochemical pretreatments greatly enhances digestion of switchgrass by Clostridium thermocellum

BACKGROUND

The thermophilic anaerobic bacterium Clostridium thermocellum is a multifunctional ethanol producer, capable of both saccharification and fermentation, that is central to the consolidated bioprocessing (CBP) approach of converting lignocellulosic biomass to ethanol without external enzyme supplementation. Although CBP organisms have evolved efficient machinery for biomass deconstruction, achieving complete solubilization requires targeted approaches, such as pretreatment, to prepare recalcitrant biomass feedstocks for further biological digestion. Here, differences between how C. thermocellum and fungal cellulases respond to senescent switchgrass prepared by four different pretreatment techniques revealed relationships between biomass substrate composition and its digestion by the two biological approaches.

RESULTS

Alamo switchgrass was pretreated using hydrothermal, dilute acid, dilute alkali, and co-solvent-enhanced lignocellulosic fractionation (CELF) pretreatments to produce solids with varying glucan, xylan, and lignin compositions. C. thermocellum achieved highest sugar release and metabolite production from de-lignified switchgrass prepared by CELF and dilute alkali pretreatments demonstrating greater resilience to the presence of hemicellulose sugars than fungal enzymes. 100% glucan solubilization and glucan plus xylan release from switchgrass were achieved using the CELF-CBP combination. Lower glucan solubilization and metabolite production by C. thermocellum was observed on solids prepared by dilute acid and hydrothermal pretreatments with higher xylan removal from switchgrass than lignin removal. Further, C. thermocellum (2% by volume inoculum) showed ~ 48% glucan solubilization compared to < 10% through fungal enzymatic hydrolysis (15 and 65 mg protein/g glucan loadings) of unpretreated switchgrass indicating the effectiveness of C. thermocellum's cellulosome. Overall, C. thermocellum performed equivalent to 65 and better than 15 mg protein/g glucan fungal enzymatic hydrolysis on all substrates except CELF-pretreated substrates. CELF pretreatments of switchgrass produced solids that were highly digestible regardless of whether C. thermocellum or fungal enzymes were chosen.

CONCLUSIONS

The unparalleled comprehensive nature of this work with a comparison of four pretreatment and two biological digestion techniques provides a strong platform for future integration of pretreatment with CBP. Lignin removal had a more positive impact on biological digestion of switchgrass than xylan removal from the biomass. However, the impact of switchgrass structural properties, including cellulose, hemicellulose, and lignin characterization, would provide a better understanding of lignocellulose deconstruction.

Temperature dependent cellulase adsorption on lignin from sugarcane bagasse

Extents of adsorption of cellulolytic enzymes on lignin, derived from sugarcane bagasse, were an inverse function of incubation temperature and varied with type of lignin extraction. At 45 °C, lignin derived from acid hydrolyzed liquid hot water pretreated bagasse completely adsorbed cellulolytic enzymes from Trichoderma reesei within 90 min. Lignin derived from enzyme hydrolyzed liquid hot water pretreated bagasse adsorbed only 60% of T. reesei endoglucanase, exoglucanase and β-glucosidase activities. β-Glucosidase from Aspergillus niger was not adsorbed. At 30 °C, adsorption of all of the enzymes was minimal and enzyme hydrolysis at 30 °C approached that at 45 °C after 168 h. Hence, temperature provided an approach to decrease loss of enzyme activity by reducing enzyme adsorption on lignin. This helps to explain why simultaneous saccharification and fermentation (SSF) and consolidated bioprocessing (CBP), both carried out at 30-32 °C, could offer viable options for mitigating lignin-derived inhibition effects.

Physico-Chemical Conversion of Lignocellulose: Inhibitor Effects and Detoxification Strategies: A Mini Review

A pretreatment of lignocellulosic biomass to produce biofuels, polymers, and other chemicals plays a vital role in the biochemical conversion process toward disrupting the closely associated structures of the cellulose-hemicellulose-lignin molecules. Various pretreatment steps alter the chemical/physical structure of lignocellulosic materials by solubilizing hemicellulose and/or lignin, decreasing the particle sizes of substrate and the crystalline portions of cellulose, and increasing the surface area of biomass. These modifications enhance the hydrolysis of cellulose by increasing accessibilities of acids or enzymes onto the surface of cellulose. However, lignocellulose-derived byproducts, which can inhibit and/or deactivate enzyme and microbial biocatalysts, are formed, including furan derivatives, lignin-derived phenolics, and carboxylic acids. These generation of compounds during pretreatment with inhibitory effects can lead to negative effects on subsequent steps in sugar flat-form processes. A number of physico-chemical pretreatment methods such as steam explosion, ammonia fiber explosion (AFEX), and liquid hot water (LHW) have been suggested and developed for minimizing formation of inhibitory compounds and alleviating their effects on ethanol production processes. This work reviews the physico-chemical pretreatment methods used for various biomass sources, formation of lignocellulose-derived inhibitors, and their contributions to enzymatic hydrolysis and microbial activities. Furthermore, we provide an overview of the current strategies to alleviate inhibitory compounds present in the hydrolysates or slurries.

An integrated process to produce bio-ethanol and xylooligosaccharides rich in xylobiose and xylotriose from high ash content waste wheat straw

A bio-refinery process of wheat straw pulping solid residue (waste wheat straw, WWS) was established by combining prewashing and liquid hot water pretreatment (LHWP). The results showed that employing a prewashing step prior to the LHWP remarkably improved enzymatic glucose yields from 39.7% to 76.6%. Moreover, after 96h simultaneous saccharification and fermentation (SSF), identical ethanol yields of 0.41g/g-cellulose were obtained despite varied solid loadings (5-30%). Beyond ethanol, enzymatic post-hydrolysis of the prehydrolyzate effectively increased xylobiose and xylotriose yields from 15mg/g-WWS and 14mg/g-WWS to 53mg/g-WWS and 20mg/g-WWS, respectively. For mass balance, about 10.9tons raw WWS will be consumed to produce 1ton ethanol, in addition to producing 614.8kg xylooligosaccharides (XOS) containing 334.3kg xylobiose and 124.8kg xylotriose. The results demonstrated that the integrated process for the WWS bio-refinery is promising, based on value-adding co-production in addition to robust ethanol yields.

Pretreatment and enzymatic process modification strategies to improve efficiency of sugar production from sugarcane bagasse

Pretreatment and enzymatic hydrolysis play a critical role in the economic production of sugars and fuels from lignocellulosic biomass. In this study, we evaluated diverse pilot-scale pretreatments and different post-pretreatment strategies for the production of fermentable sugars from sugarcane bagasse. For the pretreatment of bagasse at pilot-scale level, steam explosion without catalyst and combination of sulfuric and oxalic acids at low and high loadings were used. Subsequently, to enhance the efficiency of enzymatic hydrolysis of the pretreated bagasse, three different post-pretreatment process schemes were investigated. In the first scheme (Scheme 1), enzymatic hydrolysis was conducted on the whole pretreated slurry, without treatments such as washing or solid–liquid separation. In the second scheme (Scheme 2), the pretreated slurry was first pressure filtered to yield a solid and liquid phase. Following filtration, the separated liquid phase was remixed with the solid wet cake to generate slurry, which was then subsequently used for enzymatic hydrolysis. In the third scheme (Scheme 3), the pretreated slurry was washed with more water and filtered to obtain a solid and liquid phase, in which only the former was subjected to enzymatic hydrolysis. A 10 % higher enzymatic conversion was obtained in Scheme 2 than Scheme 1, while Scheme 3 resulted in only a 5–7 % increase due to additional washing unit operation and solid–liquid separation. Dynamic light scattering experiments conducted on post-pretreated bagasse indicate decrease of particle size due to solid–liquid separation involving pressure filtration provided increased the yield of C6 sugars. It is anticipated that different process modification methods used in this study before the enzymatic hydrolysis step can make the overall cellulosic ethanol process effective and possibly cost effective.

Green methods of lignocellulose pretreatment for biorefinery development

Lignocellulosic biomass is the most abundant, low-cost, bio-renewable resource that holds enormous importance as alternative source for production of biofuels and other biochemicals that can be utilized as building blocks for production of new materials. Enzymatic hydrolysis is an essential step involved in the bioconversion of lignocellulose to produce fermentable monosaccharides. However, to allow the enzymatic hydrolysis, a pretreatment step is needed in order to remove the lignin barrier and break down the crystalline structure of cellulose. The present manuscript is dedicated to reviewing the most commonly applied "green" pretreatment processes used in bioconversion of lignocellulosic biomasses within the "biorefinery" concept. In this frame, the effects of different pretreatment methods on lignocellulosic biomass are described along with an in-depth discussion on the benefits and drawbacks of each method, including generation of potentially inhibitory compounds for enzymatic hydrolysis, effect on cellulose digestibility, and generation of compounds toxic for the environment, and energy and economic demand.

Prewashing enhances the liquid hot water pretreatment efficiency of waste wheat straw with high free ash content

The effect of prewashing process prior to the liquid hot water (LHW) pretreatment of high free ash content waste wheat straw (WWS) was investigated. It was found that prewashing process decreased the ash content of WWS greatly, from 29.48% to 9.82%. This contributed to the lower pH value of prehydrolyzate and higher xylan removal in the following LHW pretreatment. More importantly, the prewashing process effectively increased the cellulose enzymatic hydrolysis efficiency of pretreated WWS, from 53.04% to 84.15%. The acid buffering capacity (ABC) and cation exchange capacity (CEC) of raw and prewashed WWS were examined. The majority of free ash removal from WWS by prewashing resulted in the decrease of the ABC of the WWS from 211.74 to 61.81mmol/pH-kg, and potentially enhancing the efficiency of the follow-up LHW pretreatment.

Effect of phenolic compounds from pretreated sugarcane bagasse on cellulolytic and hemicellulolytic activities

This work shows both cellulases and hemicellulases are inhibited and deactivated by water-soluble and acetone extracted phenolics from sugarcane bagasse pretreated at 10% (w/v) for 30 min in liquid hot water at 180 or 200°C. The dissolved phenolics in vacuum filtrate increased from 1.4 to 2.4 g/L as temperature increased from 180 to 200°C. The suppression of cellulose and hemicellulose hydrolysis by phenolics is dominated by deactivation of the β-glucosidase or β-xylosidase components of cellulase and hemicellulase enzyme by acetone extract at 0.2-0.65 mg phenolics/mg enzyme protein and deactivation of cellulases and hemicellulases by the water soluble components in vacuum filtrate at 0.05-2mg/mg. Inhibition was a function of the type of enzyme and the manner in which the phenolics were extracted from the bagasse.

Liquid hot water pretreatment of lignocellulosic biomass for bioethanol production accompanying with high valuable products

Pretreatment is an essential prerequisite to overcome recalcitrance of biomass and enhance the ethanol conversion efficiency of polysaccharides. Compared with other pretreatment methods, liquid hot water (LHW) pretreatment not only reduces the downstream pressure by making cellulose more accessible to the enzymes but minimizes the formation of degradation products that inhibit the growth of fermentative microorganisms. Herein, this review summarized the improved LHW process for different biomass feedstocks, the decomposition behavior of biomass in the LHW process, the enzymatic hydrolysis of LHW-treated substrates, and production of high value-added products and ethanol. Moreover, a combined process producing ethanol and high value-added products was proposed basing on the works of Guangzhou Institute of Energy Conversion to make LHW pretreatment acceptable in the biorefinery of cellulosic ethanol.

Current Pretreatment Technologies for the Development of Cellulosic Ethanol and Biorefineries

Lignocellulosic materials, such as forest, agriculture, and agroindustrial residues, are among the most important resources for biorefineries to provide fuels, chemicals, and materials in such a way to substitute for, at least in part, the role of petrochemistry in modern society. Most of these sustainable biorefinery products can be produced from plant polysaccharides (glucans, hemicelluloses, starch, and pectic materials) and lignin. In this scenario, cellulosic ethanol has been considered for decades as one of the most promising alternatives to mitigate fossil fuel dependence and carbon dioxide accumulation in the atmosphere. However, a pretreatment method is required to overcome the physical and chemical barriers that exist in the lignin-carbohydrate composite and to render most, if not all, of the plant cell wall components easily available for conversion into valuable products, including the fuel ethanol. Hence, pretreatment is a key step for an economically viable biorefinery. Successful pretreatment method must lead to partial or total separation of the lignocellulosic components, increasing the accessibility of holocellulose to enzymatic hydrolysis with the least inhibitory compounds being released for subsequent steps of enzymatic hydrolysis and fermentation. Each pretreatment technology has a different specificity against both carbohydrates and lignin and may or may not be efficient for different types of biomasses. Furthermore, it is also desirable to develop pretreatment methods with chemicals that are greener and effluent streams that have a lower impact on the environment. This paper provides an overview of the most important pretreatment methods available, including those that are based on the use of green solvents (supercritical fluids and ionic liquids).

Investigation of the pellets produced from sugarcane bagasse during liquid hot water pretreatment and their impact on the enzymatic hydrolysis

In the process of liquid hot water (LHW) pretreatment, there are numbers of pellets formed on the lignocellulosic surface. The characteristics and effect of pellets on the enzymatic hydrolysis of LHW-treated sugarcane bagasse (SCB) were investigated. After SCB was treated with LHW at 180°C, the pellets deposited on the surface of solid residues were extracted gently with 1% sodium hydroxide (NaOH) solution. They were composed of 81.0% lignin, 7.0% glucan, and 3.2% xylan. The LHW pretreatment solution (PS) was sprayed to the filter paper, and the pellets were observed on its surface. Fourier transform infrared spectroscopy (FTIR) data showed that lignin was also the main component of the PS pellets. The effect of the pellets on enzymatic hydrolysis was chiefly attributed to the steric hindrance, not the cellulase adsorption. The structural characteristics of LHW-treated SCB might play a more important role in influencing the enzymatic hydrolysis than the pellets.

Liquid hot water pretreatment on different parts of cotton stalk to facilitate ethanol production

To investigate pretreatment demand for different parts of biomass, cotton stalk was separated into stem, branch and boll shell, which were treated by liquid hot water pretreatment (LHWP) with severity from 2.77 to 4.42. Based on weight loss (WL, w/w) mainly caused by hemicellulose removal, it was found that boll shell (WL, 46.93%) was more sensitive to LHWP than stem (WL, 38.85%). Although ethanol yield of 18.3, 16.27 and 21.08g/100g was achieved from stem, branch and boll shell with pretreatment severity at 4.42, ratio of ethanol yield to pretreatment energy input for particular parts was different. For boll shell and branch, the maximum ratio of ethanol yield to energy input were 1.37 and 1.33g ethanolkJ(-1) with severity at 4.34, while it was 1.20 for stem at 3.66. This indicates that different pretreatment demands for different parts of plants should be considered in order to save pretreatment energy input.

Chemical and structural changes associated with Cu-catalyzed alkaline-oxidative delignification of hybrid poplar

BACKGROUND

Alkaline hydrogen peroxide pretreatment catalyzed by Cu(II) 2,2'-bipyridine complexes has previously been determined to substantially improve the enzymatic hydrolysis of woody plants including hybrid poplar as a consequence of moderate delignification. In the present work, cell wall morphological and lignin structural changes were characterized for this pretreatment approach to gain insights into pretreatment outcomes and, specifically, to identify the extent and nature of lignin modification.

RESULTS

Through TEM imaging, this catalytic oxidation process was shown to disrupt cell wall layers in hybrid poplar. Cu-containing nanoparticles, primarily in the Cu(I) oxidation state, co-localized with the disrupted regions, providing indirect evidence of catalytic activity whereby soluble Cu(II) complexes are reduced and precipitated during pretreatment. The concentration of alkali-soluble polymeric and oligomeric lignin was substantially higher for the Cu-catalyzed oxidative pretreatment. This alkali-soluble lignin content increased with time during the catalytic oxidation process, although the molecular weight distributions were unaltered. Yields of aromatic monomers (including phenolic acids and aldehydes) were found to be less than 0.2 % (wt/wt) on lignin. Oxidation of the benzylic alcohol in the lignin side-chain was evident in NMR spectra of the solubilized lignin, whereas minimal changes were observed for the pretreatment-insoluble lignin.

CONCLUSIONS

These results provide indirect evidence for catalytic activity within the cell wall. The low yields of lignin-derived aromatic monomers, together with the detailed characterization of the pretreatment-soluble and pretreatment-insoluble lignins, indicate that the majority of both lignin pools remained relatively unmodified. As such, the lignins resulting from this process retain features closely resembling native lignins and may, therefore, be amenable to subsequent valorization.

Validation of a novel sequential cultivation method for the production of enzymatic cocktails from Trichoderma strains

The development of new cost-effective bioprocesses for the production of cellulolytic enzymes is needed in order to ensure that the conversion of biomass becomes economically viable. The aim of this study was to determine whether a novel sequential solid-state and submerged fermentation method (SF) could be validated for different strains of the Trichoderma genus. Cultivation of the Trichoderma reesei Rut-C30 reference strain under SF using sugarcane bagasse as substrate was shown to be favorable for endoglucanase (EGase) production, resulting in up to 4.2-fold improvement compared with conventional submerged fermentation. Characterization of the enzymes in terms of the optimum pH and temperature for EGase activity and comparison of the hydrolysis profiles obtained using a synthetic substrate did not reveal any qualitative differences among the different cultivation conditions investigated. However, the thermostability of the EGase was influenced by the type of carbon source and cultivation system. All three strains of Trichoderma tested (T. reesei Rut-C30, Trichoderma harzianum, and Trichoderma sp INPA 666) achieved higher enzymatic productivity when cultivated under SF, hence validating the proposed SF method for use with different Trichoderma strains. The results suggest that this bioprocess configuration is a very promising development for the cellulosic biofuels industry.

Effect of liquid hot water pre-treatment on sugarcane press mud methane yield

Sugarcane press mud was pretreated by liquid hot water (LHW) at different temperatures (140-210 °C) and pre-treatment times (5-20 min) in order to assess the effects on the chemical oxygen demand (COD) solubilisation, inhibitors formation and methane yield. The experimental results showed that a high degree of biomass solubilisation was possible using LHW. Higher methane yields were obtained at lower severities (log(Ro) = 2.17-2.77) with (i) mild temperatures (140-150 °C) and long contact times (12.5 min, 20 min) or (ii) mild temperatures (175 °C) with short contact time (2 min). The highest increase in methane yield (up to 63%) compared to the untreated press mud was found at 150 °C for 20 min. At temperatures of 200 °C and 210 °C, low methane efficiency was attributed to the possible formation of refractory compounds through the Maillard reaction.

Pretreatment methods for bioethanol production

Lignocellulosic biomass, such as wood, grass, agricultural, and forest residues, are potential resources for the production of bioethanol. The current biochemical process of converting biomass to bioethanol typically consists of three main steps: pretreatment, enzymatic hydrolysis, and fermentation. For this process, pretreatment is probably the most crucial step since it has a large impact on the efficiency of the overall bioconversion. The aim of pretreatment is to disrupt recalcitrant structures of cellulosic biomass to make cellulose more accessible to the enzymes that convert carbohydrate polymers into fermentable sugars. This paper reviews several leading acidic, neutral, and alkaline pretreatments technologies. Different pretreatment methods, including dilute acid pretreatment (DAP), steam explosion pretreatment (SEP), organosolv, liquid hot water (LHW), ammonia fiber expansion (AFEX), soaking in aqueous ammonia (SAA), sodium hydroxide/lime pretreatments, and ozonolysis are intensively introduced and discussed. In this minireview, the key points are focused on the structural changes primarily in cellulose, hemicellulose, and lignin during the above leading pretreatment technologies.

Novel pretreatment of steam explosion associated with ammonium chloride preimpregnation

Improving nitrogen content and enhancing enzymatic hydrolysis are key processes involved in cellulosic ethanol production. Steam explosion (SE) associated with NH4Cl preimpregnation was carried out to investigate effects of the pretreatment on nitrogen content, enzymatic digestibility, and ethanol production. Results showed that nitrogen content in pretreated samples increased, which can be used as nitrogen resource for ethanol fermentation. The highest glucose yield of sample pretreated by 1.4MPa SE with 90g/l NH4Cl preimpregnation was 62.64%, which was 2.1 and 0.2 times higher than that of untreated sample and 1.4MPa SE pretreated sample, respectively. Ethanol yield of sample pretreated by 1.1MPa SE with 135g/l NH4Cl preimpregnation resulted in 1.93 and 0.69 times higher than that of untreated sample and 1.1MPa SE pretreated sample, respectively. This novel pretreatment improved nitrogen content and enhanced enzymatic digestibility under mild conditions, and could be recommended to further industrial application.

Bioethanol from poplar: a commercially viable alternative to fossil fuel in the European Union

BACKGROUND

The European Union has made it a strategic objective to develop its biofuels market in order to minimize greenhouse gas (GHG) emissions, to help mitigate climate change and to address energy insecurity within the transport sector. Despite targets set at national and supranational levels, lignocellulosic bioethanol production has yet to be widely commercialized in the European Union. Here, we use techno-economic modeling to compare the price of bioethanol produced from short rotation coppice (SRC) poplar feedstocks under two leading processing technologies in five European countries.

RESULTS

Our evaluation shows that the type of processing technology and varying national costs between countries results in a wide range of bioethanol production prices (€0.275 to 0.727/l). The lowest production prices for bioethanol were found in countries that had cheap feedstock costs and high prices for renewable electricity. Taxes and other costs had a significant influence on fuel prices at the petrol station, and therefore the presence and amount of government support for bioethanol was a major factor determining the competitiveness of bioethanol with conventional fuel. In a forward-looking scenario, genetically engineering poplar with a reduced lignin content showed potential to enhance the competitiveness of bioethanol with conventional fuel by reducing overall costs by approximately 41% in four out of the five countries modeled. However, the possible wider phenotypic traits of advanced poplars needs to be fully investigated to ensure that these do not unintentionally negate the cost savings indicated.

CONCLUSIONS

Through these evaluations, we highlight the key bottlenecks within the bioethanol supply chain from the standpoint of various stakeholders. For producers, technologies that are best suited to the specific feedstock composition and national policies should be optimized. For policymakers, support schemes that benefit emerging bioethanol producers and allow renewable fuel to be economically competitive with petrol should be established. Finally, for researchers, better control over plant genetic engineering and advanced breeding and its consequential economic impact would bring valuable contributions towards developing an economically sustainable bioethanol market within the European Union.

Severity factor coefficients for subcritical liquid hot water pretreatment of hardwood chips

Single stage and multi-stage liquid hot water pretreatments of mixed hardwood pinchips were investigated at various severities (log R0  = 3.65-4.81) to assess the efficiencies of the pretreatments with respect to achieving high pentose sugar yields and improved enzymatic digestibility of pretreated cellulose. We investigate the effect of pretreatment parameters that is, temperature, and time, as expressed in the severity factor, on the recovery of sugars and hydrolyzability of pretreated cellulose. We find the severity factor, in its widely used form, is an incomplete measure for evaluating the pretreatment efficiencies and predicting overall sugar yields when pretreatment temperatures above 200°C are used. Corrections to the severity factor and its correlation to the measured pretreatment responses (% xylan solubilization, xylan recovery as fermentable sugars, cellulose enzymatic digestibility) indicate a greater influence of temperature on the pretreatment efficiencies than predicted by the commonly used severity factor. A low temperature, long residence time is preferred for hemicellulose dissolution during the pretreatment since the condition favors oligosaccharide and monomeric sugar formation over sugar degradation. On the contrary, high cellulose hydrolyzability is achieved with a high temperature (>200°C), high severity pretreatment when pretreatment is followed by enzyme hydrolysis. In multi-stage pretreatment, the first low-severity pretreatment is optimized for solubilizing fast-hydrolyzing hemicellulose while minimizing formation of furans. The subsequent pretreatment is carried out at over 200°C to recover the difficult-to-hydrolyze hemicellulose fraction as well as to increase susceptibility of pretreated cellulose to enzymes. High recovery (>92%) of hemicellulose-derived pentose sugars and enhanced enzymatic hydrolysis of pretreated cellulose (where >80% glucose yield results with 20 FPU = 32 mg protein/g glucan or 10-13 mg/g initial hardwood) are achieved by applying a multi-stage pretreatment. This work shows how the severity equation may be used to obtain a single characteristic curve that correlate xylan solubilization and enzymatic cellulose hydrolysis as a function of severity at pretreatment temperatures up to 230°C.

Surface characterizations of bamboo substrates treated by hot water extraction

Environment Scanning Electron Microscopy (ESEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface morphology and chemical changes on both the interior and exterior surface of bamboo (Dendrocalamopsis oldhami) substrates treated by hot water extraction. ESEM results showed the visible changes between exterior and interior surface of the treated substrates, in where spherical droplets did not extensively appear on both the surfaces at start of the pretreatment; nevertheless the droplets formation on the exterior surface occurred more rapidly than that of the interior surface. Results from XPS examination that the increase of C1 (C-C, C-H) concentration and decrease of O/C ratio and O1 (C=O) concentration of the samples on the both surfaces further demonstrated that both surfaces consisted of increasing amount of lignin as the extraction continued, especially for exterior surface. The O/C ratios finally reached to a level-off value with exterior surface 0.34 and interior surface 0.37.

Fractionation of cellulase and fermentation inhibitors from steam pretreated mixed hardwood

The purpose of liquid hot water and steam pretreatment of wood is to fractionate hemicelluloses, partially solubilize lignin, and enhance enzyme hydrolysis of cellulose. The pretreatment also solubilizes sugar oligomers, lignin-derived phenolic compounds, acetic acid, and furan derivatives that inhibit cellulase enzymes and/or impede fermentation of hydrolysates by yeasts. This work extends knowledge of the relative contribution of identified inhibitors, and the effect of temperature on their release when pretreated materials are washed and filtered with hot water. Dramatic yield improvements occur when polymeric or activated carbon adsorbs and removes inhibitors. By desorbing, recovering, and characterizing adsorbed molecules we found phenolic compounds were strong inhibitors of enzyme hydrolysis and fermentation of concentrated filtrates by Saccharomyces cerevisiae wine yeast NRRL Y-1536 or xylose fermenting yeast 424A (LNH-ST). These data show that separation of inhibitors from pretreatment liquid will be important in achieving maximal enzyme activity and efficient fermentations.

Liquid hot water pretreatment of sugarcane bagasse and its comparison with chemical pretreatment methods for the sugar recovery and structural changes

Liquid hot water (LHW), dilute hydrochloric acid (HCl) and dilute sodium hydroxide (NaOH) were applied to sugarcane bagasse (SB). Application of the same analytical methods and material balance approaches facilitated meaningful comparisons of glucose and xylose yields from combined pretreatment and enzymatic hydrolysis. All pretreatments enhanced sugar recovery from pretreatment and subsequent enzymatic hydrolysis substantially compared to untreated sugarcane bagasse. Adding Tween80 in the enzymatic hydrolysis process increased the conversion level of glucan/xylan by 0.3-fold, especially for the low pH pretreatment where more lignin was left in the solids. The total sugar recovery from sugarcane bagasse with the coupled operations of pretreatment and 72 h enzymatic digestion reached 71.6% for LHW process, 76.6% for HCl pretreatment and 77.3% for NaOH pretreatment. Different structural changes at the plant tissue, cellular, and cell wall levels might be responsible for the different enzymatic digestibility. Furthermore, a combined LHW and aqueous ammonia pretreatment was proposed to reduce energy input and enhance the sugar recovery.

Rapid selection and identification of Miscanthus genotypes with enhanced glucan and xylan yields from hydrothermal pretreatment followed by enzymatic hydrolysis

BACKGROUND

Because many Miscanthus genotypes can be cultivated with relatively high productivity and carbohydrate content, Miscanthus has great potential as an energy crop that can support large scale biological production of biofuels.

RESULTS

In this study, batch hydrothermal pretreatment at 180°C for 35 min followed by enzymatic hydrolysis was shown to give the highest total sugar yields for Miscanthus x giganteus cv. Illinois planted in Illinois. High throughput pretreatment at 180°C for 35 min and 17.5 min followed by co-hydrolysis in a multi-well batch reactor identified two varieties out of 80 that had significantly higher sugar yields from pretreatment and enzymatic hydrolysis than others. The differences in performance were then related to compositions of the 80 varieties to provide insights into desirable traits for Miscanthus that enhance sugar yields.

CONCLUSIONS

High throughput pretreatment and co-hydrolysis (HTPH) rapidly identified promising genotypes from a wide range of Miscanthus genotypes, including hybrids of Miscanthus sacchariflorus/M. sinensis and Miscanthus lutarioriparius, differentiating the more commercially promising species from the rest. The total glucan plus xylan content in Miscanthus appeared to influence both mass and theoretical yields, while lignin and ash contents did not have a predictable influence on performance.