Production of high concentrated cellulosic ethanol by acetone/water oxidized pretreated beech wood

The role of CE16 exo-deacetylases in hemicellulolytic enzyme mixtures revealed by the biochemical and structural study of the novel TtCE16B esterase

Acetyl esterases belonging to the carbohydrate esterase family 16 (CE16) is a growing group of enzymes, with exceptional diversity regarding substrate specificity and regioselectivity. However, further insight into the CE16 specificity is required for their efficient biotechnological exploitation. In this work, exo-deacetylase TtCE16B from Thermothelomyces thermophila was heterologously expressed and biochemically characterized. The esterase targets positions O-3 and O-4 of singly and doubly acetylated non-reducing-end xylopyranosyl residues, provided the presence of a free vicinal hydroxyl group at position O-4 and O-3, respectively. Crystal structure of TtCE16B, the first representative among the CE16 enzymes, in apo- and product-bound form, allowed the identification of residues forming the catalytic triad and oxyanion hole, as well as the structural elements related to the enzyme preference for oligomers. The role of TtCE16B in hemicellulose degradation was investigated on acetylated xylan from birchwood and pre-treated beechwood biomass. TtCE16B exhibited complementary activity to commercially available OCE6 acetylxylan esterase. Moreover, it showed synergistic effects with SrXyl43 β-xylosidase. Overall, supplementation of xylan-targeting enzymatic mixtures with both TtCE16B and OCE6 esterases led to a 3-fold or 4-fold increase in xylose release, when using TmXyn10 and TtXyn30A xylanases respectively.

The xylobiohydrolase activity of a GH30 xylanase on natively acetylated xylan may hold the key for the degradation of recalcitrant xylan

Acetyl substitutions are common on the hemicellulosic structures of lignocellulose, which up until recently were known to inhibit xylanase activity. Emerging data, however, suggest that xylanases are able to accommodate acetyl side-groups within their catalytic site. In the present work, a fungal GH30 xylanase from Thermothelomyces thermophila, namely TtXyn30A, was shown to release acetylated xylobiose when acting on pretreated lignocellulosic substrate. The released disaccharides could be acetylated at the 2-OH, 3-OH or both positions of the non-reducing end xylose, but the existence of the acetylation on the reducing end cannot be excluded. The synergy of TtXyn30A with acetyl esterases indicates that particular subsites within its active site cannot tolerate acetylated xylopyranose residues. Molecular docking showed that acetyl group can be accommodated on the 2- or 3-OH position of the non-reducing end xylose, unlike the reducing-end xylose (subsite -1), where only 3-OH decoration can be accommodated. Such insight into the catalytic activity of TtXyn30A could contribute to a better understanding of its biological role and thus lead to a more sufficient biotechnological utilization.

Evaluation of oxy-organosolv pretreatment on lignin extraction from wheat straw

To develop a characteristic "Lignin-first" strategy, the oxy-organosolv delignification processes under mild conditions were comprehensively investigated. Results showed that lignin yield could achieve about 50 % under the optimum process conditions of ethanol concentration 80 %, temperature 90 °C, liquid to wheat straw ratio 25:1 for powdery-scale substrates, which was 65.0 % higher than that for rod-scale substrates under the same conditions. The lignin structural and carbohydrate component results demonstrated the employment of oxygen induced great quantities of lignin dissolving out on the premise of little carbohydrate component (<1 %) and lignin structural (mainly β-O-4 units) changes. Moreover, based on the molecular weight and polydiversity comparison results, the aqueous oxygen could transfer homogeneously in mild organosolv system and result in lignin degradation uniformly. Besides, the employment of oxygen assisted in not only extending the massive lignin removal stage to 30 min and 50 min for P-OEEL and R-OEEL respectively, but also boost the delignification rate with comparison to P-EL and R-EL. Lastly, the excellent anti-oxidant properties of lignin from oxy-organosolv process were demonstrated by scavenging DPPH and ABTS radicals. The economic calculations showed that the cost for lignin production were about 1.58USD/g lignin from powdery-scale wheat straw, providing a competitive route for high-value utilize waste biomass.

High-pressure microwave-assisted pretreatment of softwood, hardwood and non-wood biomass using different solvents in the production of cellulosic ethanol

BACKGROUND

Pretreatment is an indispensable stage of the preparation of lignocellulosic biomass with key significance for the effectiveness of hydrolysis and the efficiency of the production of cellulosic ethanol. A significant increase in the susceptibility of the raw material to further degradation can be attained as a result of effective delignification in high-pressure conditions. With this in mind, a method of high-pressure pretreatment using microwave radiation and various solvents (water, 40% w/v NaCS, 1% v/v H₂SO₄, 1% w/v NaOH or 60% v/v EtOH with an addition of 1% v/v H₂SO₄) was developed, enabling the acquisition of biomass with an increased susceptibility to the process of enzymatic hydrolysis. The medium obtained in this way can be used for the production of cellulosic ethanol via high-gravity technology (lignocellulosic media containing from 15 to 20% dry weight of biomass). For every type of biomass (pine chips, beech chips and wheat straw), a solvent was selected to be used during the pretreatment, guaranteeing the acquisition of a medium highly susceptible to the process of enzymatic hydrolysis.

RESULTS

The highest efficiency of the hydrolysis of biomass, amounting to 71.14 ± 0.97% (glucose concentration 109.26 ± 3.49 g/L) was achieved for wheat straw subjected to microwave-assisted pretreatment using 40% w/v NaCS. Fermentation of this medium produced ethanol concentration at the level of 53.84 ± 1.25 g/L. A slightly lower effectiveness of enzymatic hydrolysis (62.21 ± 0.62%) was achieved after high-pressure microwave-assisted pretreatment of beech chips using 1% w/v NaOH. The hydrolysate contained glucose in the concentration of 91.78 ± 1.91 g/L, and the acquired concentration of ethanol after fermentation amounted to 49.07 ± 2.06 g/L. In the case of pine chips, the most effective delignification was achieved using 60% v/v EtOH with the addition of 1% v/v H₂SO₄, but after enzymatic hydrolysis, the concentration of glucose in hydrolysate was lower than in the other raw materials and amounted to 39.15 ± 1.62 g/L (the concentration of ethanol after fermentation was ca. 19.67 ± 0.98 g/L). The presence of xylose and galactose was also determined in the obtained fermentation media. The highest initial concentration of these carbohydrates (21.39 ± 1.44 g/L) was observed in beech chips media after microwave-assisted pretreatment using NaOH. The use of wheat straw after pretreatment using EtOH with an addition of 1% v/v H₂SO₄ for the preparation of fermentation medium, results in the generation of the initial concentration of galactose and xylose at the level of 19.03 ± 0.38 g/L.

CONCLUSION

The achieved results indicate a high effectiveness of the enzymatic hydrolysis of the biomass subjected to high-pressure microwave-assisted pretreatment. The final effect depends on the combined use of correctly selected solvents for the different sources of lignocellulosic biomass. On the basis of the achieved results, we can say that the presented method indicates a very high potential in the area of its use for the production of cellulosic ethanol involving high-gravity technology.

Oxidative pretreatment of lignocellulosic biomass for enzymatic hydrolysis: Progress and challenges

Deconstruction of cell wall structure is important for biorefining of lignocellulose to produce various biofuels and chemicals. Oxidative delignification is an effective way to increase the enzymatic digestibility of cellulose. In this work, the current research progress on conventional oxidative pretreatment including wet oxidation, alkaline hydrogen peroxide, organic peracids, Fenton oxidation, and ozone oxidation were reviewed. Some recently developed novel technologies for coupling pretreatment and direct biomass-to-electricity conversion with recyclable oxidants were also introduced. The primary mechanism of oxidative pretreatment to enhance cellulose digestibility is delignification, especially in alkaline medium, thus eliminating the physical blocking and non-productive adsorption of enzymes by lignin. However, the cost of oxidative delignification as a pretreatment is still too expensive to be applied at large scale at present. Efforts should be made particularly to reduce the cost of oxidants, or explore valuable products to obtain more revenue.

The Consistency of Yields and Chemical Composition of HTL Bio-Oils from Lignins Produced by Different Preprocessing Technologies

This work evaluates the effect of feedstock type and composition on the conversion of lignin to liquid by solvolysis with formic acid as hydrogen donor (LtL), by analyzing the yields and molecular composition of the liquid products and interpreting them in terms of both the type and the preprocessing of the lignocellulosic biomass using chemometric data analysis. Lignin samples of different types and purities from softwood, hardwood, and grasses (rice straw and corn stover) have been converted to bio-oil, and the molecular composition analyzed and quantified using GC-MS. LtL solvolysis was found to be a robust method for lignin conversion in terms of converting all samples into bio-oils rich in phenolic compounds regardless of the purity of the lignin sample. The bio-oil yields ranged from 24–94 wt.% relative to lignin input and could be modelled well as a function of the elemental composition of the feedstock. On a molecular basis, the softwood-derived bio-oil contained the most guaiacol-derivatives, and syringol was correlated to hardwood. However, the connection between compounds in the bio-oil and lignin origin was less pronounced than the effects of the methods for biomass fractionation, showing that the pretreatment of the biomass dominates both the yield and molecular composition of the bio-oil and must be addressed as a primary concern when utilization of lignin in a biorefinery is planned.

Self-generated peroxyacetic acid in phosphoric acid plus hydrogen peroxide pretreatment mediated lignocellulose deconstruction and delignification

BACKGROUND

Peroxyacetic acid involved chemical pretreatment is effective in lignocellulose deconstruction and oxidation. However, these peroxyacetic acid are usually artificially added. Our previous work has shown that the newly developed PHP pretreatment (phosphoric acid plus hydrogen peroxide) is promising in lignocellulose biomass fractionation through an aggressive oxidation process, while the information about the synergistic effect between H3PO4 and H2O2 is quite lack, especially whether some strong oxidant intermediates is existed. In this work, we reported the PHP pretreatment system could self-generate peroxyacetic acid oxidant, which mediated the overall lignocellulose deconstruction, and hemicellulose/lignin degradation.

RESULTS

The PHP pretreatment profile on wheat straw and corn stalk were investigated. The pathways/mechanisms of peroxyacetic acid mediated-PHP pretreatment were elucidated through tracing the structural changes of each component. Results showed that hemicellulose was almost completely solubilized and removed, corresponding to about 87.0% cellulose recovery with high digestibility. Rather high degrees of delignification of 83.5% and 90.0% were achieved for wheat straw and corn stalk, respectively, with the aid of peroxyacetic acid oxidation. A clearly positive correlation was found between the concentration of peroxyacetic acid and the extent of lignocellulose deconstruction. Peroxyacetic acid was mainly self-generated through H2O2 oxidation of acetic acid that was produced from hemicellulose deacetylation and lignin degradation. The self-generated peroxyacetic acid then further contributed to lignocellulose deconstruction and delignification.

CONCLUSIONS

The synergistic effect of H3PO4 and H2O2 in the PHP solvent system could efficiently deconstruct wheat straw and corn stalk lignocellulose through an oxidation-mediated process. The main function of H3PO4 was to deconstruct biomass recalcitrance and degrade hemicellulose through acid hydrolysis, while the function of H2O2 was to facilitate the formation of peroxyacetic acid. Peroxyacetic acid with stronger oxidation ability was generated through the reaction between H2O2 and acetic acid, which was released from xylan and lignin oxidation/degradation. This work elucidated the generation and function of peroxyacetic acid in the PHP pretreatment system, and also provide useful information to tailor peroxide-involved pretreatment routes, especially at acidic conditions.

Conversion of organosolv pretreated hardwood biomass into 5-hydroxymethylfurfural (HMF) by combining enzymatic hydrolysis and isomerization with homogeneous catalysis

BACKGROUND

Over the last few years, valorization of lignocellulosic biomass has been expanded beyond the production of second-generation biofuels to the synthesis of numerous platform chemicals to be used instead of their fossil-based counterparts. One such well-researched example is 5-hydroxymethylfurfural (HMF), which is preferably produced by the dehydration of fructose. Fructose is obtained by the isomerization of glucose, which in turn is derived by the hydrolysis of cellulose. However, to avoid harsh reaction conditions with high environmental impact, an isomerization step towards fructose is necessary, as fructose can be directly dehydrated to HMF under mild conditions. This work presents an optimized process to produce fructose from beechwood biomass hydrolysate and subsequently convert it to HMF by employing homogeneous catalysis.

RESULTS

The optimal saccharification conditions were identified at 10% wt. solids loading and 15 mg enzyme/g_solids, as determined from preliminary trials on pure cellulose (Avicel® PH-101). Furthermore, since high rate glucose isomerization to fructose requires the addition of sodium tetraborate, the optimum borate to glucose molar ratio was determined to 0.28 and was used in all experiments. Among 20 beechwood solid pulps obtained from different organosolv pretreatment conditions tested, the highest fructose production was obtained with acetone (160 °C, 120 min), reaching 56.8 g/100 g pretreated biomass. A scale-up hydrolysis in high solids (25% wt.) was then conducted. The hydrolysate was subjected to isomerization eventually leading to a high-fructose solution (104.5 g/L). Dehydration of fructose to HMF was tested with 5 different catalysts (HCl, H₃PO₄, formic acid, maleic acid and H-mordenite). Formic acid was found to be the best one displaying 79.9% sugars conversion with an HMF yield and selectivity of 44.6% and 55.8%, respectively.

CONCLUSIONS

Overall, this work shows the feasibility of coupling bio- and chemo-catalytic processes to produce HMF from lignocellulose in an environmentally friendly manner. Further work for the deployment of biocatalysts for the oxidation of HMF to its derivatives could pave the way for the emergence of an integrated process to effectively produce biobased monomers from lignocellulose.

Recent advances in the valorization of plant biomass

Plant biomass is a highly abundant renewable resource that can be converted into several types of high-value-added products, including chemicals, biofuels and advanced materials. In the last few decades, an increasing number of biomass species and processing techniques have been developed to enhance the application of plant biomass followed by the industrial application of some of the products, during which varied technologies have been successfully developed. In this review, we summarize the different sources of plant biomass, the evolving technologies for treating it, and the various products derived from plant biomass. Moreover, the challenges inherent in the valorization of plant biomass used in high-value-added products are also discussed. Overall, with the increased use of plant biomass, the development of treatment technologies, and the solution of the challenges raised during plant biomass valorization, the value-added products derived from plant biomass will become greater in number and more valuable.

OxiOrganosolv: A novel acid free oxidative organosolv fractionation for lignocellulose fine sugar streams

The valorization of lignocellulosic biomass towards the production of value-added products requires an efficient pretreatment/fractionation step. In this work we present a novel, acid-free, mildly oxidative organosolv delignification process -OxiOrganosolv- which employs oxygen gas to depolymerize and remove lignin. The results demonstrate that the OxiOrganosolv process achieved lignin removal as high as 97% in a single stage, with a variety of solvents; it was also efficient in delignifying both beechwood (hardwood) and pine (softwood), a task in which organosolv pretreatments have failed in the past. Minimal amounts of sugar degradation products were detected, while cellulose recovery was ~100% in the solid pulp. Enzymatic hydrolysis of pulps showed >80 wt% cellulose conversion to glucose. Overall, the OxiOrganosolv pretreatment has significant advantages, including high delignification efficiency of hardwood and softwood biomass, absence of acid homogeneous catalysis and all corresponding challenges involved, and close to zero losses of sugars to degradation products.

Novel Routes in Transformation of Lignocellulosic Biomass to Furan Platform Chemicals: From Pretreatment to Enzyme Catalysis

The constant depletion of fossil fuels along with the increasing need for novel materials, necessitate the development of alternative routes for polymer synthesis. Lignocellulosic biomass, the most abundant carbon source on the planet, can serve as a renewable starting material for the design of environmentally-friendly processes for the synthesis of polyesters, polyamides and other polymers with significant value. The present review provides an overview of the main processes that have been reported throughout the literature for the production of bio-based monomers from lignocellulose, focusing on physicochemical procedures and biocatalysis. An extensive description of all different stages for the production of furans is presented, starting from physicochemical pretreatment of biomass and biocatalytic decomposition to monomeric sugars, coupled with isomerization by enzymes prior to chemical dehydration by acid Lewis catalysts. A summary of all biotransformations of furans carried out by enzymes is also described, focusing on galactose, glyoxal and aryl-alcohol oxidases, monooxygenases and transaminases for the production of oxidized derivatives and amines. The increased interest in these products in polymer chemistry can lead to a redirection of biomass valorization from second generation biofuels to chemical synthesis, by creating novel pathways to produce bio-based polymers.

Interaction of enzymes with lignocellulosic materials: causes, mechanism and influencing factors

For the production of biofuel (bioethanol), enzymatic adsorption onto a lignocellulosic biomass surface is a prior condition for the enzymatic hydrolysis process to occur. Lignocellulosic substances are mainly composed of cellulose, hemicellulose and lignin. The polysaccharide matrix (cellulose and hemicellulose) is capable of producing bioethanol. Therefore, lignin is removed or its concentration is reduced from the adsorption substrates by pretreatments. Selected enzymes are used for the production of reducing sugars from cellulosic materials, which in turn are converted to bioethanol. Adsorption of enzymes onto the substrate surface is a complicated process. A large number of research have been performed on the adsorption process, but little has been done to understand the mechanism of adsorption process. This article reviews the mechanisms of adsorption of enzymes onto the biomass surfaces. A conceptual adsorption mechanism is presented which will fill the gaps in literature and help researchers and industry to use adsorption more efficiently. The process of enzymatic adsorption starts with the reciprocal interplay of enzymes and substrates and ends with the establishment of molecular and cellular binding. The kinetics of an enzymatic reaction is almost the same as that of a characteristic chemical catalytic reaction. The influencing factors discussed in detail are: surface characteristics of the participating materials, the environmental factors, such as the associated flow conditions, temperature, concentration, etc. Pretreatment of lignocellulosic materials and optimum range of shear force and temperature for getting better results of adsorption are recommended.

A new synergistic relationship between xylan-active LPMO and xylobiohydrolase to tackle recalcitrant xylan

BACKGROUND

Hemicellulose accounts for a significant part of plant biomass, and still poses a barrier to the efficient saccharification of lignocellulose. The recalcitrant part of hemicellulose is a serious impediment to the action of cellulases, despite the use of xylanases in the cellulolytic cocktail mixtures. However, the complexity and variety of hemicelluloses in different plant materials require the use of highly specific enzymes for a complete breakdown. Over the last few years, new fungal enzymes with novel activities on hemicelluloses have emerged. In the present study, we explored the synergistic relationships of the xylan-active AA14 lytic polysaccharide monooxygenase (LPMO), PcAA14B, with the recently discovered glucuronoxylan-specific xylanase TtXyn30A, of the (sub)family GH30_7, displaying xylobiohydrolase activity, and with commercial cellobiohydrolases, on pretreated natural lignocellulosic substrates.

RESULTS

PcAA14B and TtXyn30A showed a strong synergistic interaction on the degradation of the recalcitrant part of xylan. PcAA14B was able to increase the release of xylobiose from TtXyn30A, showing a degree of synergism (DS) of 3.8 on birchwood cellulosic fibers, and up to 5.7 on pretreated beechwood substrates. The increase in activity was dose- and time- dependent. A screening study on beechwood materials pretreated with different methods showed that the effect of the PcAA14B-TtXyn30A synergism was more prominent on substrates with low hemicellulose content, indicating that PcAA14B is mainly active on the recalcitrant part of xylan, which is in close proximity to the underlying cellulose fibers. Simultaneous addition of both enzymes resulted in higher DS than sequential addition. Moreover, PcAA14B was found to enhance cellobiose release from cellobiohydrolases during hydrolysis of pretreated lignocellulosic substrates, as well as microcrystalline cellulose.

CONCLUSIONS

The results of the present study revealed a new synergistic relationship not only among two recently discovered xylan-active enzymes, the LPMO PcAA14B, and the GH30_7 glucuronoxylan-active xylobiohydrolase TtXyn30A, but also among PcAA14B and cellobiohydrolases. We hypothesize that PcAA14B creates free ends in the xylan polymer, which can be used as targets for the action of TtXyn30A. The results are of special importance for the design of next-generation enzymatic cocktails, able to efficiently remove hemicelluloses, allowing complete saccharification of cellulose in plant biomass.

Valorizing Waste Lignocellulose-Based Furniture Boards by Phosphoric Acid and Hydrogen Peroxide (Php) Pretreatment for Bioethanol Production and High-Value Lignin Recovery

Three typical waste furniture boards (fiberboard, chipboard, and blockboard) were pretreated with phosphoric acid and hydrogen peroxide (PHP). The fractionation process of these feedstocks was attempted in order to harvest the cellulose-rich fraction for enzymatic hydrolysis and bioethanol conversion; further, lignin recovery was also considered in this process. The results indicated that 78.9–91.2% of the cellulose was recovered in the cellulose-rich fraction. The decreased crystallinity, which promoted the water retention capacity and enzyme accessibility, contributed greatly to the excellent hydrolysis performance of the cellulose-rich fraction. Therefore, rather high cellulose–glucose conversions of 83.3–98.0% were achieved by hydrolyzing the pretreated furniture boards, which allowed for harvesting 208–241 g of glucose from 1.0 kg of feedstocks. Correspondingly, 8.1–10.4 g/L of ethanol were obtained after 120 h of simultaneous saccharification and fermentation. The harvested lignin exhibited abundant carboxyl –OH groups (0.61–0.67 mmol g−1). In addition, approximately 15–26 g of harvested oligosaccharides were integrated during PHP pretreatment. It was shown that PHP pretreatment is feasible for these highly recalcitrant biomass board materials, which can diversify the bioproducts used in the integrated biorefinery concept.

Acetone/water oxidation of corn stover for the production of bioethanol and prebiotic oligosaccharides

Ethanol production at high-gravity promise to achieve concentrations over the threshold for an economical distillation process and concurrently reduce water consumption. However, a persisting limitation is the poor mass transfer conditions resulting in low ethanol yields and concentrations. Hereby, the combination of an acetone/water oxidation pretreatment process (AWO) with a liquefaction/saccharification step, using a free-fall mixer, before simultaneous saccharification and fermentation (SSF) can realize ethanol concentrations of up to ca. 74 g/L at a solids content of 20 wt%. The free-fall mixer achieved a biomass slurry viscosity reduction by 87% after only 2 h of enzymatic saccharification, indicating the efficiency of the mixing system. Furthermore, the direct enzymatic treatment of AWO pretreated corn stover (CS) by a GH11 recombinant xylanase, led to the production of xylooligosaccharides (XOS) with prebiotic potential and the removal of insoluble fibers of hemicellulose improved the glucose release of AWOCS by 22%.

Bioethanol production from wheat straw by phosphoric acid plus hydrogen peroxide (PHP) pretreatment via simultaneous saccharification and fermentation (SSF) at high solid loadings

Phosphoric acid plus hydrogen peroxide (PHP) pretreatment was employed on wheat straw for ethanol conversion by simultaneous saccharification and fermentation (SSF) at high loadings. Results showed solid loading of PHP-pretreated wheat straw can be greatly promoted to 20%. Although more enzyme input improved ethanol conversion significantly, it still can be potentially reduced to 10-20 mg protein/g cellulose. Increasing yeast input also promoted ethanol conversion, however, the responses were not significant. Response surface method was employed to optimize SSF conditions with the strategy of maximizing ethanol conversion and concentration and minimizing enzyme and yeast input. Results indicated that ethanol conversion of 88.2% and concentration of 69.9 g/L were obtained after 120 h SSF at solid loading of 15.3%, and CTec2 enzyme and yeast were in lower input of 13.2 mg protein/g cellulose and 1.0 g/L, respectively. Consequently, 15.5 g ethanol was harvested from 100 g wheat straw in the optimal conditions.

A comparative study of the enzymatic hydrolysis of batch organosolv-pretreated birch and spruce biomass

A shift towards a sustainable and green society is vital to reduce the negative effects of climate change associated with increased CO₂ emissions. Lignocellulosic biomass is both renewable and abundant, but is recalcitrant to deconstruction. Among the methods of pretreatment available, organosolv (OS) delignifies cellulose efficiently, significantly improving its digestibility by enzymes. We have assessed the hydrolysability of the cellulose-rich solid fractions from OS-pretreated spruce and birch at 2% w/v loading (dry matter). Almost complete saccharification of birch was possible with 80 mg enzyme preparation/g_solids (12 FPU/g_solids), while the saccharification yield for spruce was only 70%, even when applying 60 FPU/g_solids. As the cellulose content is enriched by OS, the yield of glucose was higher than in their steam-exploded counterparts. The hydrolysate was a transparent liquid due to the absence of phenolics and was also free from inhibitors. OS pretreatment holds potential for use in a large-scale, closed-loop biorefinery producing fuels from the cellulose fraction and platform chemicals from the hemicellulose and lignin fractions respectively.

Acid Assisted Organosolv Delignification of Beechwood and Pulp Conversion towards High Concentrated Cellulosic Ethanol via High Gravity Enzymatic Hydrolysis and Fermentation

Background: Future biorefineries will focus on converting low value waste streams to chemical products that are derived from petroleum or refined sugars. Feedstock pretreatment in a simple, cost effective, agnostic manner is a major challenge. Methods: In this work, beechwood sawdust was delignified via an organosolv process, assisted by homogeneous inorganic acid catalysis. Mixtures of water and several organic solvents were evaluated for their performance. Specifically, ethanol (EtOH), acetone (AC), and methyl- isobutyl- ketone (MIBK) were tested with or without the use of homogeneous acid catalysis employing sulfuric, phosphoric, and oxalic acids under relatively mild temperature of 175 °C for one hour. Results: Delignification degrees (DD) higher than 90% were achieved, where both AC and EtOH proved to be suitable solvents for this process. Both oxalic and especially phosphoric acid proved to be good alternative catalysts for replacing sulfuric acid. High gravity simultaneous saccharification and fermentation with an enzyme loading of 8.4 mg/g_solids at 20 wt.% initial solids content reached an ethanol yield of 8.0 w/v%. Conclusions: Efficient delignification combining common volatile solvents and mild acid catalysis allowed for the production of ethanol at high concentration in an efficient manner.

Ethanol production from mixtures of sugarcane bagasse and Dioscorea composita extracted residue with high solid loading

Various mixing ratios of alkali pretreated sugarcane bagasse and starch-rich waste Dioscorea composita hemls extracted residue (DER) were evaluated via simultaneous saccharification and fermentation (SSF) with 12% (w/w) solid loading, and the mixture ratio of 1:1 achieved the highest ethanol concentration and yield. When the solid loading was increased from 12% to 32%, the ethanol concentration was increased to 72.04 g/L, whereas the ethanol yield was reduced from 84.40% to 73.71%. With batch feeding and the addition of 0.1% (w/v) Tween 80, the final ethanol concentration and yield of SSF at 34% loading were 82.83 g/L and 77.22%, respectively. Due to the integration with existing starch-based ethanol industry, the co-fermentation is expected to be a competitive alternative form for cellulosic ethanol production.

Optimization of cellulolytic enzyme components through engineering Trichoderma reesei and on-site fermentation using the soluble inducer for cellulosic ethanol production from corn stover

BACKGROUND

Cellulolytic enzymes produced by Trichoderma reesei are widely studied for biomass bioconversion, and enzymatic components vary depending on different inducers. In our previous studies, a mixture of glucose and disaccharide (MGD) was developed and used to induce cellulase production. However, the enzymatic profile induced by MGD is still not defined, and further optimization of the enzyme cocktail is also required for efficient ethanol production from lignocellulosic biomass.

RESULTS

In this study, cellulolytic enzymes produced by T. reesei Rut C30 using MGD and alkali-pretreated corn stover (APCS) as inducer were compared. Cellular secretome in response to each inducer was analyzed, which revealed a similar enzyme profile. However, significant difference in the content of cellulases and xylanase was detected. Although MGD induction enhanced β-glucosidase production, its activity was still not sufficient for biomass hydrolysis. To overcome such a disadvantage, aabgl1 encoding β-glucosidase in Aspergillus aculeatus was heterologously expressed in T. reesei Rut C30 under the control of the pdc1 promoter. The recombinant T. reesei PB-3 strain showed an improved β-glucosidase activity of 310 CBU/mL in the fed-batch fermentation, 71-folds higher than that produced by the parent strain. Meanwhile, cellulase activity of 50 FPU/mL was detected. Subsequently, the crude enzyme was applied for hydrolyzing corn stover with a solid loading of 20% through separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation, respectively, for ethanol production. Better performance was observed in the SHF process, through which a total of 119.9 g/L glucose was released within 12 h for concomitant ethanol production of 54.2 g/L.

CONCLUSIONS

The similar profile of cellulolytic enzymes was detected under the induction of MGD and APCS, but higher amount of cellulases was present in the crude enzyme induced by MGD. However, β-glucosidase activity induced by MGD was not sufficient for hydrolyzing lignocellulosic biomass. High titers of cellulases and β-glucosidase were achieved simultaneously by heterologous expression of aabgl1 in T. reesei and fed-batch fermentation through feeding MGD. We demonstrated that on-site cellulase production by T. reesei PB-3 has a potential for efficient biomass saccharification and ethanol production from lignocellulosic biomass.

A novel hybrid organosolv: steam explosion method for the efficient fractionation and pretreatment of birch biomass

BACKGROUND

The main role of pretreatment is to reduce the natural biomass recalcitrance and thus enhance saccharification yield. A further prerequisite for efficient utilization of all biomass components is their efficient fractionation into well-defined process streams. Currently available pretreatment methods only partially fulfill these criteria. Steam explosion, for example, excels as a pretreatment method but has limited potential for fractionation, whereas organosolv is excellent for delignification but offers poor biomass deconstruction.

RESULTS

In this article, a hybrid method combining the cooking and fractionation of conventional organosolv pretreatment with the implementation of an explosive discharge of the cooking mixture at the end of pretreatment was developed. The effects of various pretreatment parameters (ethanol content, duration, and addition of sulfuric acid) were evaluated. Pretreatment of birch at 200 °C with 60% v/v ethanol and 1% w/w_biomass H₂SO₄ was proven to be the most efficient pretreatment condition yielding pretreated solids with 77.9% w/w cellulose, 8.9% w/w hemicellulose, and 7.0 w/w lignin content. Under these conditions, high delignification of 86.2% was demonstrated. The recovered lignin was of high purity, with cellulose and hemicellulose contents not exceeding 0.31 and 3.25% w/w, respectively, and ash to be < 0.17% w/w in all cases, making it suitable for various applications. The pretreated solids presented high saccharification yields, reaching 68% at low enzyme load (6 FPU/g) and complete saccharification at high enzyme load (22.5 FPU/g). Finally, simultaneous saccharification and fermentation (SSF) at 20% w/w solids yielded an ethanol titer of 80 g/L after 192 h, corresponding to 90% of the theoretical maximum.

CONCLUSIONS

The novel hybrid method developed in this study allowed for the efficient fractionation of birch biomass and production of pretreated solids with high cellulose and low lignin contents. Moreover, the explosive discharge at the end of pretreatment had a positive effect on enzymatic saccharification, resulting in high hydrolyzability of the pretreated solids and elevated ethanol titers in the following high-gravity SSF. To the best of our knowledge, the ethanol concentration obtained with this method is the highest so far for birch biomass.