Organosolv pretreatment of lignocellulosic biomass for enzymatic hydrolysis

Production of ethanol by bioconversion of lignocellulosic biomass has attracted much interest in recent years. However, the pretreatment process for increasing the enzymatic digestibility of cellulose has become a key step in commercialized production of cellulosic ethanol. During the last decades, many pretreatment processes have been developed for decreasing the biomass recalcitrance, but only a few of them seem to be promising. From the point of view for integrated utilization of lignocellulosic biomass, organosolv pretreatment provides a pathway for biorefining of biomass. This review presents the progress of organosolv pretreatment of lignocellulosic biomass in recent decades, especially on alcohol, organic acid, organic peracid and acetone pretreatments, and corresponding action mechanisms. Evaluation and prospect of organosolv pretreatment were performed. Finally, some recommendations for future investigation of this pretreatment method were given.     

Enhanced enzymatic hydrolysis of rapeseed straw by popping pretreatment for bioethanol production

The objective of this study was to find a pretreatment process that enhances enzymatic conversion of biomass to sugars. Rapeseed straw was pretreated by two processes: a wet process involving wet milling plus a popping treatment, and a dry process involving popping plus dry milling. The effects of the pretreatments were studied both in terms of structural and compositional changes and change in susceptibility to enzymatic hydrolysis. After application of the wet and dry processes, the amounts of cellulose and xylose in the straw were 37-38% and 14-15%, respectively, compared to 31% and 12% in untreated counterparts. In enzymatic hydrolysis performance, the wet process presented the best glucose yield, with a 93.1% conversion, while the dry process yielded 69.6%, and the un-pretreated process yielded <20%. Electron microscopic studies of the straw also showed a relative increase in susceptibility to enzymatic hydrolysis with pretreatment.     

常压甘油自催化预处理麦草浓醪发酵纤维素乙醇

目前纤维素乙醇成本偏高的根本原因在于没有达到淀粉质乙醇发酵水平的"三高"(高浓度、高转化率和高效率)指标,提高水解糖液浓度和避免发酵抑制物来实现浓醪发酵,是解决问题的关键。文中以常压甘油自催化预处理麦草为底物,尝试采用不同发酵策略,探讨其浓醪发酵产纤维素乙醇的可行性。在优化培养条件(15%底物浓度,加酶量30 FPU/g干底物,温度37℃,接种量10%)下同步糖化发酵72 h,纤维素乙醇产量为31.2 g/L,转化率为73%,发酵效率0.43 g/(L·h);采用半同步(预酶解24 h)糖化发酵72 h,纤维素乙醇浓度达到33.7 g/L,转化率为79%,发酵效率为0.47 g/(L·h),其中(半)同步糖化发酵中90%以上纤维素已被糖化水解用于发酵;采用分批补料式半同步糖化发酵,补料到基质浓度相当于30%,发酵72 h时纤维素乙醇产量达到51.2 g/L,转化率为62%,发酵效率为0.71 g/(L·h)。在所有浓醪发酵中乙酸不足3 g/L,无糠醛和羟甲基糠醛等发酵抑制物。以上结果表明,常压甘油自催化预处理木质纤维素基质适用于纤维素乙醇发酵;分批补料式半同步糖化发酵策略可用来进行浓醪纤维素乙醇发酵;未来工作中提高基质纯度和强化酶解产糖是浓醪纤维素乙醇达到"三高"指标的关键。     

Enhanced enzymatic hydrolysis of wheat straw by aqueous glycerol pretreatment

Considering the practical technology-economy of glycerol processing from oleochemicals industry, the ensuing work was proposed to further explore the atmospheric aqueous glycerol autocatalytic organosolv pretreatment (AAGAOP) to improve the enzymatic hydrolysis of lignocellulosic biomass. With the liquid-solid ratio of 20 g g(-1) at 220 degrees C for 3h, the AAGAOP enabled wheat straw to remove approximately 70% hemicelluloses and approximately 65% lignin, with approximately 98% cellulose retention. The pretreated fiber was achieved with approximately 90% of the enzymatic hydrolysis yield after 48 h. At oven-drying, dehydration was likely to cause the hornification of fiber, which was responsible for the low enzymatic hydrolysis of dried fiber. With SEM observations, the AAGAOP disrupted wheat straw into thin and fine fibrils, with a small average size and more surface area. The AAGAOP technique, as a novel strategy, enhanced the enzymatic hydrolysis of lignocellulosic biomass by removing the chemically compositional barrier and altering the physically structural impediment.     

Two-stage fungal biopulping for improved enzymatic hydrolysis of wood

A novel two-stage, whole organism fungal biopulping method was examined for increasing the yield of enzymatic hydrolysis of wood into soluble glucose. Liriodendron tulipifera wood chips (1 g) were exposed to liquid culture suspensions of white rot (Ceriporiopsis subvermispora) or brown rot (Postia placenta) fungi and incubated at 28 °C, either alone in single-stage 30 day (one fungal species applied) or two-stage 60 day (both fungal species applied in alternative succession) treatments. Fungi grew in all treatments, but did not significantly decrease the percent carbohydrate content of the wood. Two-stage treatments differed significantly in mass loss depending on order of exposure, suggesting additive or inhibitory fungal interactions occurred. Treatments consisting of C. subvermispora followed by P. placenta exhibited 6 ± 0.5% mass loss and increased the yield of enzymatic hydrolysis by 67-119%. This significant hydrolysis improvement suggests that fungal biopulping technologies could support commercial lignocellulosic ethanol production efforts if further developed.     

The effect of high intensity mixing on the enzymatic hydrolysis of concentrated cellulose fiber suspensions

Enzymatic hydrolysis of lignocellulosic biomass in a high shear environment was examined. The conversion of cellulose to glucose in samples mixed in a torque rheometer producing shear flows similar to those found in twin screw extruders was greater than that of unmixed samples. In addition, there is a synergistic effect of mixing and enzymatic hydrolysis; mixing increases the rate of cellulose conversion while the increased conversion facilitates mixing. The synergy appears to result in part from particle size reduction, which is more significant when hydrolysis occurs during intense mixing.     

Dilute acid pretreatment, enzymatic saccharification and fermentation of wheat straw to ethanol

Wheat straw consists of 48.57 ± 0.30% cellulose and 27.70 ± 0.12% hemicellulose on dry solid (DS) basis and has the potential to serve as a low cost feedstock for production of ethanol. Dilute acid pretreatment at varied temperature and enzymatic saccharification were evaluated for conversion of wheat straw cellulose and hemicellulose to monomeric sugars. The maximum yield of monomeric sugars from wheat straw (7.83%, w/v, DS) by dilute H₂SO₄ (0.75%, v/v) pretreatment and enzymatic saccharification (45 °C, pH 5.0, 72 h) using cellulase, β-glucosidase, xylanase and esterase was 565 ± 10 mg/g. Under this condition, no measurable quantities of furfural and hydroxymethyl furfural were produced. The yield of ethanol (per litre) from acid pretreated enzyme saccharified wheat straw (78.3 g) hydrolyzate by recombinant Escherichia coli strain FBR5 was 19 ± 1 g with a yield of 0.24 g/g DS. Detoxification of the acid and enzyme treated wheat straw hydrolyzate by overliming reduced the fermentation time from 118 to 39 h in the case of separate hydrolysis and fermentation (35 °C, pH 6.5), and increased the ethanol yield from 13 ± 2 to 17 ± 0 g/l and decreased the fermentation time from 136 to 112 h in the case of simultaneous saccharification and fermentation (35 °C, pH 6.0).     

Optimization of organic acid pretreatment of wheat straw

The objective of this study was to determine the effectiveness of different organic acids (maleic, succinic, and oxalic acid) on enzymatic hydrolysis and fermentation yields of wheat straw. It was also aimed to optimize the process conditions (temperature, acid concentration, and pretreatment time) by using response surface methodology (RSM). In line with this objective, the wheat straw samples were pretreated at three different temperatures (170, 190, and 210°C), acid concentrations (1%, 3%, and 5%) and pretreatment time (10, 20, and 30 min). The findings show that at extreme pretreatment conditions, xylose was solubilized in liquid phase, causing an increase in cellulose and lignin content of biomass. Enzymatic hydrolysis experiments revealed that maleic and oxalic acids were quite effective at achieving high sugar yields (>90%) from wheat straw. In contrast, the highest sugar yields were 50–60%, when the samples were pretreated with succinic acid, indicating that succinic acid was not as effective. The optimum process conditions for maleic acid were, 210°C, 1.08% acid concentration, and 19.8 min; for succinic acid 210°C, 5% acid concentration, and 30 min; for oxalic acid 210°C, 3.6% acid concentration, and 16.3 min. The ethanol yields obtained at optimum conditions were 80, 79, and 59% for maleic, oxalic and succinic acid, respectively. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1487–1493, 2016     

Improving enzymatic hydrolysis of wheat straw using ionic liquid 1-ethyl-3-methyl imidazolium diethyl phosphate pretreatment

This study aims to establish a cellulose pretreatment process using ionic liquids (ILs) for efficient enzymatic hydrolysis. The IL 1-ethyl-3-methyl imidazolium diethyl phosphate ([EMIM]DEP) was selected in view of its low viscous and the potential of accelerating enzymatic hydrolysis, and it could be recyclable. The yield of reducing sugars from wheat straw pretreated with this IL at 130 °C for 30 min reached 54.8% after being enzymatically hydrolyzed for 12 h. Wheat straw regenerated were hydrolyzed more easily than that treated with water. The fermentability of the hydrolyzates, obtained after enzymatic saccharification of the regenerated wheat straw, was evaluated using Saccharomyces cerevisiae. This microbe could ferment glucose efficiently, and the ethanol production was 0.43 g/g glucose within 26 h. In conclusion, the IL [EMIM]DEP shows promise as pretreatment solvent for wheat straw, although its cost should be reduced and in-depth exploration of this subject is needed.     

乙醇预处理麦秆的酶解性能研究简

采用H2 SO4催化和自催化乙醇法对麦秆进行预处理,比较预处理后麦秆的主要化学组成、纤维素酶解性能和半同步糖化发酵生产乙醇特性,并进行物料衡算。结果表明：H2 SO4催化和自催化乙醇预处理过程中纤维素固体回收率大于90％。添加非离子表面活性剂吐温20和吐温80没有显著提高H2 SO4催化乙醇预处理后纤维素的酶解葡萄糖得率及半同步糖化发酵过程中乙醇的产量,而对自催化乙醇处理后麦秆的酶解和半同步糖化发酵过程有一定程度的促进作用,相应的酶解葡聚糖转化率由72.7％提高到85.0％,而半同步糖化发酵过程中乙醇质量浓度提高了11.4％。物料衡算结果表明：酸催化和自催化乙醇预处理后葡聚糖回收率分别为91.0％和95.4％;半同步糖化发酵生产乙醇的得率分别为10.4和11.6 g（按100 g原料计）。     

白腐菌协同热水预处理对毛白杨化学组分和酶水解效果的影响

【目的】增强真菌预处理的效率和降低热水预处理对反应条件的要求。【方法】综合利用白腐菌和热水预处理毛白杨,分析此方法对毛白杨化学组分和酶水解效果的影响。【结果】白腐菌Lenzites betulinus C5617协同热水处理,损失率最高达70.70%。纤维素在2个预处理阶段都有损失,其中L.betulinus C5617达到29.62%。木质素的降解主要集中在白腐菌预处理阶段,其中L.betulinus C5617降解的酸不溶木素较多,达到了16.98%。综合预处理显著改善了毛白杨的酶水解效果。与只经热水预处理的样品相比较,L.betulinus C5617和P.sanguineus D9497协同热水处理分别引起还原糖得率上升了20.60%和12.23%。【结论】综合预处理降低了热水解对反应条件的要求,节约了预处理成本。     

Production and characterization of biopolyols and polyurethane foams from crude glycerol based liquefaction of soybean straw

The feasibility of using crude glycerol to liquefy soybean straw for the production of biopolyols and polyurethane (PU) foams was investigated in this study. Liquefaction conditions of 240 °C, >180 min, 3% sulfuric acid loading, and 10-15% biomass loading were preferred for the production of biopolyols with promising material properties. Biopolyols produced under preferential conditions showed hydroxyl numbers from 440 to 540 mg KOH/g, acid numbers below 5 mg KOH/g, and viscosities from 16 to 45 Pa.s. PU foams produced under preferential conditions showed densities from 0.033 to 0.037 g/cm³ and compressive strength from 148 to 227 kPa. These results suggest that crude glycerol can be used as an alternative solvent for the liquefaction of lignocellulosic biomass such as soybean straw for the production of biopolyols and PU foams. The produced biopolyols and PU foams showed material properties comparable to their analogs from petroleum solvent based liquefaction processes.     

Fungal pretreatment: An alternative in second-generation ethanol from wheat straw

The potential of a fungal pretreatment combined with a mild alkali treatment to replace or complement current physico-chemical methods for ethanol production from wheat straw has been investigated. Changes in substrate composition, secretion of ligninolytic enzymes, enzymatic hydrolysis efficiency and ethanol yield after 7, 14 and 21 days of solid-state fermentation were evaluated. Most fungi degraded lignin with variable selectivity degrees, although only eight of them improved sugar recovery compared to untreated samples. Glucose yield after 21 days of pretreatment with Poria subvermispora and Irpex lacteus reached 69% and 66% of cellulose available in the wheat straw, respectively, with an ethanol yield of 62% in both cases. Conversions from glucose to ethanol reached around 90%, showing that no inhibitors were generated during this pretreatment. No close correlations were found between ligninolytic enzymes production and sugar yields.     

Enhanced enzymatic hydrolysis and structural features of corn stover by FeCl3 pretreatment

Corn stover was pretreated with FeCl₃ to remove almost all of the hemicellulose present and then hydrolyzed with cellulase and β-glucosidase to produce glucose. Enzymatic hydrolysis of corn stover that had been pretreated with FeCl₃ at 160 °C for 20 min resulted in an optimum yield of 98.0%. This yield was significantly higher than that of untreated corn stover (22.8%). FeCl₃ pretreatment apparently damaged the surface of corn stover and significantly increased the enzymatic digestibility, as evidenced by SEM and XRD analysis data. FTIR analysis indicated that FeCl₃ pretreatment could disrupt almost all the ether linkages and some ester linkages between lignin and carbohydrates but had no effect on delignification. The FeCl₃ pretreatment technique, as a novel pretreatment method, enhances enzymatic hydrolysis of lignocellulosic biomass by destructing chemical composition and altering structural features.     

Comparison of some new pretreatment methods for second generation bioethanol production from wheat straw and water hyacinth

Pretreatment of lignocellulosic residues like water hyacinth (WH) and wheat straw (WS) using crude glycerol (CG) and ionic liquids (IL) pretreatment was evaluated and compared with conventional dilute acid pretreatment (DAT) in terms of enzymatic hydrolysis yield and fermentation yield of pretreated samples. In the case of WS, 1-butyl-3-methylimidazolium acetate pretreatment was found to be the best method. The hydrolysis yields of glucose and total reducing sugars were 2.1 and 3.3 times respectively higher by IL pretreatment than DAT, while it was 1.4 and 1.9 times respectively higher with CG pretreatment. For WH sample, CG pretreatment was as effective as DAT and more effective than IL pretreatment regarding hydrolysis yield. The fermentation inhibition was not noticeable with both types of pretreatment methods and feedstocks. Besides, CG pretreatment was found as effective as pure glycerol pretreatment for both feedstocks. This opens up an attractive economic route for the utilization of CG.     

Effect of compounds released during pretreatment of wheat straw on microbial growth and enzymatic hydrolysis rates

In the process of producing ethanol from lignocellulosic materials such as wheat straw, compounds that can act inhibitory to enzymatic hydrolysis and to cellular growth may be generated during the pretreatment. Ethanol production was evaluated on pretreated wheat straw hydrolysate using four different recombinant Saccharomyces cerevisiae strains, CPB.CR4, CPB.CB4, F12, and FLX. The fermentation performance of the four S. cerevisiae strains was tested in hydrolysate of wheat straw that has been pretreated at high dry matter content (220 g/L dry matter). The results clearly showed that F12 was the most robust strain, whereas the other three strains were strongly inhibited when the fraction of hydrolysate in the fermentation medium was higher than 60% (v/v). Furthermore, the impact of different lignin derivatives commonly found in the hydrolysate of pretreated wheat straw, was tested on two different enzyme mixtures, a mixture of Celluclast 1.5 L FG and Novozym 188 (3:1) and one crude enzyme preparation produced from Penicillium brasilianum IBT 20888. From all the potential inhibiting compounds that were tested, formic acid had the most severe influence on the hydrolysis rate resulting in a complete inactivation of the two enzyme mixtures.     

Optimization of processing conditions for the pretreatment of wheat straw using aqueous ionic liquid

Pretreatment of wheat straw with the aqueous ionic liquid, 1-ethyl-3-methylimidazolium acetate, was optimized to maximize fermentable sugars recovery. The optimization process employed a central composite design, where the investigated variables were temperature (130-170 °C), time (0.5-5.5 h) and ionic liquid concentration (0-100%). All the tested variables were identified to have significant effects (p < 0.05) on fermentable sugars recovery. The optimum pretreatment conditions were 158 °C, an ionic liquid concentration of 49.5% (w/w), and a duration of 3.6 h. Cellulose and xylan digestibility generally increased with increasing temperature, time and ionic liquid concentration; but, the carbohydrates recovered in the washed solids following pretreatment decreased. Thus, the final optimum conditions for maximizing fermentable sugars from the starting biomass were a compromise between greater digestibility and minimal carbohydrates loss during pretreatment.     

Fractionation of wheat and barley straw to access high-molecular-mass hemicelluloses prior to ethanol production

The cost efficiency of the biorefining process can be improved by extracting high-molecular-mass hemicelluloses from lignocellulosic biomass prior to ethanol production. These hemicelluloses can be used in several high-value-added applications and are likely to be important raw materials in the future. In this study, steam pretreatment in an alkaline environment was used to pretreat the lignocellulosic biomass for ethanol production and, at the same time, extract arabinoxylan with a high-molecular-mass. It was shown that 30% of the arabinoxylan in barley straw could be extracted with high-molecular-mass, without dissolving the cellulose. The cellulose in the solid fraction could then be hydrolysed with cellulase enzymes giving a cellulose conversion of about 80–90% after 72 h. For wheat straw, more than 40% of the arabinoxylan could be extracted with high-molecular-mass and the cellulose conversion of the solid residue after 72 h was about 70–85%. The high cellulose conversion of the pretreated wheat and barley straw shows that they can be used for ethanol production without further treatment. It is therefore concluded that it is possible to extract high-molecular-mass arabinoxylan simultaneously with the pretreatment of biomass for ethanol production in a single steam pretreatment step.     

Hydrothermal processing and enzymatic hydrolysis of sorghum bagasse for fermentable carbohydrates production

Untreated and hydrothermally treated sorghum bagasse (SB) was hydrolyzed to simple sugars by the synergistic action of cellulases and hemicellulases produced by the fungi Fusarium oxysporum and Neurospora crassa. Synergism between the two lignocellulolytic systems was maximized with the application of higher fraction of N. crassa enzymes.Hydrothermolysis of SB was studied at a wide range of treatment times and temperatures. At intense pretreatment conditions (210 °C for 20 min; logR₀ = 4.54), the residual hemicellulose percentage was 17.45%, while formation of inhibitory products, 5-hydromethyl-furfural (HMF), furfural, acetic and formic acid, (0.21, 0.51, 3.36 and 1.80 g/l, respectively) remained in acceptable levels.Maximum conversion of cellulose and total polysaccharides of the untreated SB were 23.18% and 18.79%, respectively. Combining hydrothermal treatment and enzymatic hydrolysis of released oligosaccharides and insoluble solids resulted in improvement of cellulose (approximately 15% increase) and total polysaccharides (two fold) hydrolysis compared to that of untreated SB.