Production of ethanol from carbohydrates from loblolly pine: a technical and economic assessment

Ethanol from lignocellulosic biomass has the potential to contribute substantially to bioethanol for transportation. We have evaluated the technical and economic feasibility of producing ethanol from the carbohydrates in loblolly pine. In the process evaluated, prehydrolysis with dilute sulfuric acid was employed to hydrolyze hemicellulose and make the cellulose more accessible to hydrolysis by enzymes. Residual biomass from hydrolysis and extraction of carbohydrates was burned in a CHP plant to generate power and process steam. Our analysis indicates that ethanol can be produced at a cost of dollars 1.53/gal, based on a delivered wood cost of $63.80/dry metric ton and 75% conversion of the carbohydrates in wood to sugars for ethanol production. Improving the conversion of wood carbohydrates to sugars to 95% would reduce the production cost to dollars 1.29/gal. These values are for a plant producing 74 million gal/yr and 93 million gal/yr, respectively. At current feedstock prices, ethanol produced from loblolly pine would be competitive with ethanol produced from corn or other lignocellulosic biomass. Based on our analysis, discounted cash flow rates of return would be 18% and 25%, respectively for plants of this capacity.     

The challenge of enzyme cost in the production of lignocellulosic biofuels

With the aim of understanding the contribution of enzymes to the cost of lignocellulosic biofuels, we constructed a techno-economic model for the production of fungal cellulases. We found that the cost of producing enzymes was much higher than that commonly assumed in the literature. For example, the cost contribution of enzymes to ethanol produced by the conversion of corn stover was found to be $0.68/gal if the sugars in the biomass could be converted at maximum theoretical yields, and $1.47/gal if the yields were based on saccharification and fermentation yields that have been previously reported in the scientific literature. We performed a sensitivity analysis to study the effect of feedstock prices and fermentation times on the cost contribution of enzymes to ethanol price. We conclude that a significant effort is still required to lower the contribution of enzymes to biofuel production costs.     

Flexible biorefinery for producing fermentation sugars, lignin and pulp from corn stover

A new biorefining process is presented that embodies green processing and sustainable development. In the spirit of a true biorefinery, the objective is to convert agricultural residues and other biomass feedstocks into value-added products such as fuel ethanol, dissolving pulp, and lignin for resin production. The continuous biomass fractionation process yields a liquid stream rich in hemicellulosic sugars, a lignin-rich liquid stream, and a solid cellulose stream. This paper generally discusses potential applications of the three streams and specifically provides results on the evaluation of the cellulose stream from corn stover as a source of fermentation sugars and specialty pulp. Enzymatic hydrolysis of this relatively pure cellulose stream requires significantly lower enzyme loadings because of minimal enzyme deactivation from nonspecific binding to lignin. A correlation was shown to exist between lignin removal efficiency and enzymatic digestibility. The cellulose produced was also demonstrated to be a suitable replacement for hardwood pulp, especially in the top ply of a linerboard. Also, the relatively pure nature of the cellulose renders it suitable as raw material for making dissolving pulp. This pulping approach has significantly smaller environmental footprint compared to the industry-standard kraft process because no sulfur- or chlorine-containing compounds are used. Although this option needs some minimal post-processing, it produces a higher value commodity than ethanol and, unlike ethanol, does not need extensive processing such as hydrolysis or fermentation. Potential use of low-molecular weight lignin as a raw material for wood adhesive production is discussed as well as its use as cement and feed binder. As a baseline application the hemicellulosic sugars captured in the hydrolyzate liquor can be used to produce ethanol, but potential utilization of xylose for xylitol fermentation is also feasible. Markets and values of these applications are juxtaposed with market penetration and saturation.     

Recent process improvements for the ammonia fiber expansion (AFEX) process and resulting reductions in minimum ethanol selling price

The ammonia fiber expansion (AFEX) process has been shown to be an effective pretreatment for lignocellulosic biomass. Technological advances in AFEX have been made since previous cost estimates were developed for this process. Recent research has enabled lower overall ammonia requirements, reduced ammonia concentrations, and reduced enzyme loadings while still maintaining high conversions of glucan and xylan to monomeric sugars. A new ammonia recovery approach has also been developed. Capital and operating costs for the AFEX process, as part of an overall biorefining system producing fuel ethanol from biomass have been developed based on these new research results. These new cost estimates are presented and compared to previous estimates. Two biological processing options within the overall biorefinery are also compared, namely consolidated bioprocessing (CBP) and enzymatic hydrolysis followed by fermentation. Using updated parameters and ammonia recovery configurations, the cost of ethanol production utilizing AFEX is calculated. These calculations indicate that the minimum ethanol selling price (MESP) has been reduced from $1.41/gal to $0.81/gal.     

Availability of corn stover as a sustainable feedstock for bioethanol production

The amount of corn stover that can be sustainably collected is estimated to be 80-100 million dry tonnes/yr (t/yr), a majority of which would be available to ethanol plants in the near term as only a small portion is currently used for other applications. Potential long-term demand for corn stover by non-fermentative applications in the United States is estimated to be about 20 million dry t/yr, assuming that corn stover-based products replace 50% of both hardwood pulp and wood-based particleboard, and that 50% of all furfural production is from corncobs. Hence, 60-80 million dry t/yr of corn stover should be available to fermentative routes. To achieve an ethanol production potential of 11 billion L (3 billion gal) per year (a target level for a non-niche feedstock), about 40% of the harvestable corn stover is needed. This amount should be available as long as the diversion of corn stover to non-ethanol fermentative products remains limited.     

Novel two‐stage fermentation process for bioethanol production using Saccharomyces pastorianus

Bioethanol produced from lignocellulosic materials has the potential to be economically feasible, if both glucose and xylose released from cellulose and hemicellulose can be efficiently converted to ethanol. Saccharomyces spp. can efficiently convert glucose to ethanol; however, xylose conversion to ethanol is a major hurdle due to lack of xylose‐metabolizing pathways. In this study, a novel two‐stage fermentation process was investigated to improve bioethanol productivity. In this process, xylose is converted into biomass via non‐Saccharomyces microorganism and coupled to a glucose‐utilizing Saccharomyces fermentation. Escherichia coli was determined to efficiently convert xylose to biomass, which was then killed to produce E. coli extract. Since earlier studies with Saccharomyces pastorianus demonstrated that xylose isomerase increased ethanol productivities on pure sugars, the addition of both E. coli extract and xylose isomerase to S. pastorianus fermentations on pure sugars and corn stover hydrolysates were investigated. It was determined that the xylose isomerase addition increased ethanol productivities on pure sugars but was not as effective alone on the corn stover hydrolysates. It was observed that the E. coli extract addition increased ethanol productivities on both corn stover hydrolysates and pure sugars. The ethanol productivities observed on the corn stover hydrolysates with the E. coli extract addition was the same as observed on pure sugars with both E. coli extract and xylose isomerase additions. These results indicate that the two‐stage fermentation process has the capability to be a competitive alternative to recombinant Saccharomyces cerevisiae‐based fermentations. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 30:300–310, 2014     

Technology and economics of fermentation alcohol — An update

Fermentation alcohol is being widely studied as an alternative fuel, and production is increasing, especially in Brazil, where the goal is more than 10 billion litres per year by 1985.Fuel markets are hundreds of times greater than the traditional ethanol markets which the existing industry supplies. To make a material contribution to fuel supply, fermentation ethanol must be treated as a major chemical and produced in large-volume, highly efficient plants. Such plants must be assured of a continuous supply of low-cost raw materials for which suitable processes have been developed and commercially proven. Sugar cane in the tropics and grains in some temperate countries meet these requirements; cellulosics do not qualify at present, nor will they in the foreseeable future, without major breakthroughs.Using techniques borrowed from the starch sweetener industry, starchy materials may be economically hydrolysed to fermentable sugars; rapid acid hydrolysis may prove superior to enzymatic processes. Major projects are under way to replace traditional batch or cascade fermentations with rapid, single-vessel continuous units, but these have not yet been fully proven. Where suitable, yeast recycle is being used as a means of increasing alcohol yields, and energy-efficient distillation methods of the petrochemical industry are being adopted. The consequent large reduction in steam consumption greatly reduces the appeal of other methods which have been proposed to remove water.Opportunities for process improvements abound, especially in developing (1) the means to provide cellulosic raw materials in large quantities at acceptable costs, (2) economically effective methods of pretreating and hydrolysing cellulosics, (3) practical organisms for converting five-carbon sugars to ethanol and (4) higher fermentation yields and efficiencies using bacteria or immobilized yeast.     

玉米秸秆分批补料获得高还原糖浓度酶解液的条件优化

木质纤维素高浓度还原糖水解液的获得是纤维乙醇产业化发展的方向。在发酵工业领域,分批补料法是实现这一目标的重要研究途径。本研究采用分批补料法对获得高浓度玉米秸秆酶解还原糖的条件进行了优化。以稀硫酸预处理的玉米秸秆为原料,考察了液固比、补加量与补加时间对分批补料糖化的影响。结果表明,秸秆高浓度酶解液条件的初始物料为20％ (重量/体积),木聚糖酶220 U/g (底物),纤维素酶6 FPU/g (底物),果胶酶50 U/g (底物),在24 h、48 h后分批补加8％预处理后的物料,同时添加与补料量相应的木聚糖酶20 U/g (底物),纤维素酶2 FPU/g (底物),72 h后,最终糖化结果与非补料法相比,还原糖浓度从48.5 g/L提高到138.5 g/L,原料的酶解率最终达到理论值的62.5％。试验结果表明补料法可以显著提高秸秆水解液还原糖浓度。     

Chemical and enzymic pretreatment of corn stover to produce soluble fermentation substrates

Corn stover was pretreated with various chemical agents, including sodium hydroxide, sulfuric acid, ethylenediamine, n-butylamine (either alone or in solution with methanol), and acetonitrile or ethanol containing hydrochloric acid. Of these chemicals, n-butylamine was the best reagent for pretreatment of corn stover, considering the degree of loss of total carbohydrate, delignification, cumulative weight loss, cumulative yield of reducing sugars per original total carbohydrate, and the potential ease of recovery and reuse of reagent. In comparison to the other reagents tested, n-butylamine (n-BA) selectively delignified corn stover. The best conditions were as follows: a 12-h presoak of about a 155 g dry wt/L slurry (1 mm average particle size) in 100% n-BA at room temperature, followed by 30 min of refluxing (86.5 degrees C) with 40% (w/w) n-BA-distilled water solution. The cumulative yield of reducing sugars after enzymic hydrolysis was 44.5% of the original total carbohydrate and the cumulative total weight loss (dry basis) was 59%. Degradative loss of total carbohydrate during pretreatment was not detected.     

Biorefinery for combined production of jet fuel and ethanol from lipid‐producing sugarcane: a techno‐economic evaluation

Replacing fossil fuels with an economically viable green alternative at scale has proved most challenging in the aviation sector. Recently sugarcane, the most productive crop on the planet, has been engineered to accumulate lipids. This opens the way for production of far more industrial vegetable oil per acre than previously possible. This study performs techno‐economic feasibility analysis of jet fuel production from this new cost efficient and high yield feedstock. A comprehensive process model for biorefinery producing hydrotreated jet fuel (from lipids) and ethanol (from sugars), with 1 600 000 MT yr^?1 lipid‐cane processing capacity, was developed in SuperPro Designer. Considering lipid‐cane development is continuing for higher oil concentrations, analysis was performed with lipid‐cane containing 5%, 10%, 15%, and 20% lipids. Capital investments for the biorefinery ranged from 238.1 to 351.2 million USD, with jet fuel capacities of 12.6–50.5 million liters (correspondingly ethanol production of nil to 102.6 million liters). The production cost of jet fuel for different scenarios was estimated Replacing fossil fuels with an economically viable green alternative at scale has proved most challenging in the aviation sector. Recently sugarcane, the most productive crop on the planet, has been engineered to accumulate lipids. This opens the way for production of far more industrial vegetable oil per acre than previously possible. This study performs techno‐economic feasibility analysis of jet fuel production from this new cost efficient and high yield feedstock. A comprehensive process model for biorefinery producing hydrotreated jet fuel (from lipids) and ethanol (from sugars), with 1 600 000 MT yr^?1 lipid‐cane processing capacity, was developed in SuperPro Designer. Considering lipid‐cane development is continuing for higher oil concentrations, analysis was performed with lipid‐cane containing 5%, 10%, 15%, and 20% lipids. Capital investments for the biorefinery ranged from 238.1 to 351.2 million USD, with jet fuel capacities of 12.6–50.5 million liters (correspondingly ethanol production of nil to 102.6 million liters). The production cost of jet fuel for different scenarios was estimated $0.73 to $1.79 per liter ($2.74 to $6.76 per gal) of jet fuel. In all cases, the cost of raw materials accounted for more than 70% of total operational cost. Biorefinery was observed self‐sustainable for steam and electricity requirement, because of in‐house steam and electricity generation from burning of bagasse. Minimum fuel selling prices with a 10% discount rate for 20% lipid case was estimated $1.40/L ($5.31/gal), which was lower than most of the reported prices of renewable jet fuel produced from other oil crops and algae. Along with lower production costs, lipid‐cane could produce as high as 16 times the jet fuel (6307 L ha^?1) per unit land than that of other oil crops and do so using low‐value land unsuited to most other crops, while being highly water and nitrogen use efficient.     

Process design and economic studies of alternative fermentation methods for the production of ethanol

Cell recycle and vacuum fermentation processes are described for the continuous production of ethanol. Preliminary process design studies are employed to make an economic comparison of these alternative fermentation schemes with continuous and batch fermentation technologies. Designs are based on a production capacity of 78,000 gal 95% ethanol (EtOH)/day employing molasses as the fermentation substrate. The studies indicate that a 57% reduction in fixed capital investment is realized by continuous rather than batch operation. Further decreases in required capital investment of 68 and 71% over batch fermentation were obtained for cell recycle and vacuum operation, respectively. However, ethanol production costs were dominated by the cost of molasses, representing over 75% of the total manufacturing cost. But, when a reasonable yeast by-product credit was assumed, the net production cost for 95% ethanol was estimated at 82.3 and 80.6 cent/gal, for the cell recycle and vacuum processes, respectively.     

Weedy lignocellulosic feedstock and microbial metabolic engineering: advancing the generation of ‘Biofuel’

Lignocellulosic materials are the most abundant renewable organic resources (~200 billion tons annually) on earth that are readily available for conversion to ethanol and other value-added products, but they have not yet been tapped for the commercial production of fuel ethanol. The lignocellulosic substrates include woody substrates such as hardwood (birch and aspen, etc.) and softwood (spruce and pine, etc.), agro residues (wheat straw, sugarcane bagasse, corn stover, etc.), dedicated energy crops (switch grass, and Miscanthus etc.), weedy materials (Eicchornia crassipes, Lantana camara etc.), and municipal solid waste (food and kitchen waste, etc.). Despite the success achieved in the laboratory, there are limitations to success with lignocellulosic substrates on a commercial scale. The future of lignocellulosics is expected to lie in improvements of plant biomass, metabolic engineering of ethanol, and cellulolytic enzyme-producing microorganisms, fullest exploitation of weed materials, and process integration of the individual steps involved in bioethanol production. Issues related to the chemical composition of various weedy raw substrates for bioethanol formation, including chemical composition-based structural hydrolysis of the substrate, need special attention. This area could be opened up further by exploring genetically modified metabolic engineering routes in weedy materials and in biocatalysts that would make the production of bioethanol more efficient.     

低水用量约束条件下的高固体含量纤维乙醇生物加工技术策略

木质纤维素原料生物转化生产纤维乙醇需要使用大量的水和蒸汽,从而使过程能耗和废水排放显著增加,大幅度增加了加工成本。最大限度地降低水和蒸汽用量对过程节能和废水减排并对最终成本控制极为重要。对极限低水用量约束条件下木质纤维素生物转化关键路径进行了实验研究和计算分析,确定了极低水和蒸汽用量的新型预处理技术,实现高效率预处理过程的废水零排放;采用独特的生物脱毒技术,用从自然界筛选的煤油霉菌Amorphotheca resinae ZN1对预处理原料中的抑制物进行了快速生物脱毒;对极限高固体含量下高粘度多相流物系在复杂抑制物胁迫下的酶水解与发酵行为以及放大准则进行了研究;建立了基于Aspen plus平台上的生物质加工物性数据库和严格热力学意义上的全过程流程模拟数学模型,实现了对过程的局部和全局设计与调优。这一综合技术在生物炼制微型工厂中进行了测试,并在纤维素乙醇工业示范装置中得到了应用。该研究结果将为构建具有工业实用价值的节能和清洁化木质纤维素生物转化技术提供依据。     

Subcritical and supercritical technology for the production of second generation bioethanol

There is increased interest in reducing our reliance on fossil fuels and increasing the share of renewable raw materials in our energy supply chain due to environmental and economic concerns. Ethanol is emerging as a potential alternative to liquid fuels due to its eco-friendly characteristics and relatively low production costs. As ethanol is currently produced from commodities also used for human and animal consumption, there is an urgent need of identifying renewable raw materials that do not pose a competitive problem. Lignocellulosic agricultural residues are an ideal choice since they can be effectively hydrolyzed to fermentable sugars and integrated in the context of a biorefinery without competing with the food supply chain. However, the conventional hydrolysis methods still have major issues that need to be addressed. These issues are related to the processing rate and generation of fermentation inhibitors, which can compromise the quality of the product and the cost of the process. As the knowledge of the processes taking place during hydrolysis of agricultural residues is increasing, new techniques are being exploited to overcome these drawbacks. This review gives an overview of the state-of-the-art of hydrolysis with subcritical and supercritical water in the context of reusing agricultural residues for the production of suitable substrates to be processed during the fermentative production of bioethanol. Presently, subcritical and/or supercritical water hydrolysis has been found to yield low sugar contents mainly due to concurrent competing degradation of sugars during the hydrothermal processes. In this line of thinking, the present review also revisits the recent applications and advances to provide an insight of future research trends to optimize on the subcritical and supercritical process kinetics.     

Ethanolic fermentation of hydrolysates from ammonia fiber expansion (AFEX) treated corn stover and distillers grain without detoxification and external nutrient supplementation

External nutrient supplementation and detoxification of hydrolysate significantly increase the production cost of cellulosic ethanol. In this study, we investigated the feasibility of fermenting cellulosic hydrolysates without washing, detoxification or external nutrient supplementation using ethanologens Escherichia coli KO11 and the adapted strain ML01 at low initial cell density (16 mg dry weight/L). The cellulosic hydrolysates were derived from enzymatically digested ammonia fiber expansion (AFEX)-treated corn stover and dry distiller's grain and solubles (DDGS) at high solids loading (18% by weight). The adaptation was achieved through selective evolution of KO11 on hydrolysate from AFEX-treated corn stover. All cellulosic hydrolysates tested (36-52 g/L glucose) were fermentable. Regardless of strains, metabolic ethanol yields were near the theoretical limit (0.51 g ethanol/g consumed sugar). Volumetric ethanol productivity of 1.2 g/h/L was achieved in fermentation on DDGS hydrolysate and DDGS improved the fermentability of hydrolysate from corn stover. However, enzymatic hydrolysis and xylose utilization during fermentation were the bottlenecks for ethanol production from corn stover at these experimental conditions. In conclusion, fermentation under the baseline conditions was feasible. Utilization of nutrient-rich feedstocks such as DDGS in fermentation can replace expensive media supplementation.     

Evaluation of pelleting as a pre-processing step for effective biomass deconstruction and fermentation

Densification of bulky forages by pelleting reduces their transportation, handling, and storage costs. Because of high shearing force and frictional heating during the pelleting process, it is hypothesized that pelleting of lignocellulosic biomass could also partially deconstruct its complex structure and facilitate bioethanol production. In this study, pelleted wheat straw, corn stover, big bluestem, and sorghum stalk were evaluated for sugars and ethanol production, and compared with those of unpelleted biomasses. Mass recovery after alkali pretreatment increased by 14%, 11%, 2%, and 5%, respectively, in unpelleted biomasses. Lignin content reduced significantly more in pelleted samples for all types of biomass, except sorghum stalk. Volumetric productivity of enzymatic hydrolysis was 23%, 21%, 20% and 12% higher, respectively, in pelleted samples; ethanol yield on the basis of released sugars did not differ significantly between pelleted and unpelleted samples. These results indicate that the pelleting process led to better enzymatic hydrolysis of pretreated biomasses without affecting the quality of sugars for fermentation. However, overall yield of ethanol from the raw biomass was not significantly higher in pelleted biomasses because of higher mass loss during pretreatment process. In our study, we propose a schematic for complete utilization of various byproducts for enhanced economic viability.     

Conversion of biomass hydrolysates and other substrates to ethanol and other chemicals by Lactobacillus buchneri*

Aims: A Lactobacillus buchneri strain NRRL B‐30929 can convert xylose and glucose into ethanol and chemicals. The aims of the study were to survey three strains (NRRL B‐30929, NRRL 1837 and DSM 5987) for fermenting 17 single substrates and to exam NRRL B‐30929 for fermenting mixed substrates from biomass hydrolysates. Methods and Results: Mixed acid fermentation was observed for all three L. buchneri strains using various carbohydrates; the only exception was uridine which yielded lactate, acetate and uracil. Only B‐30929 is capable of utilizing cellobiose, a desired trait in a potential biocatalyst for biomass conversion. Flask fermentation indicated that the B‐30929 strain can use all the sugars released from pretreated hydrolysates, and producing 1·98–2·35 g l^?1 ethanol from corn stover hydrolysates and 2·92–3·01 g l^?1 ethanol from wheat straw hydrolysates when supplemented with either 0·25× MRS plus 1% corn steep liquor or 0·5× MRS. Conclusions: The L. buchneri NRRL B‐30929 can utilize mixed sugars in corn stover and wheat straw hydrolysates for ethanol and other chemical production. Significance and Impact of the Study: These results are valuable for future research in engineering L. buchneri NRRL B‐30929 for fermentative production of ethanol and chemicals from biomass.     

Uncertainty in techno-economic estimates of cellulosic ethanol production due to experimental measurement uncertainty

ABSTRACT: BACKGROUND: Cost-effective production of lignocellulosic biofuels remains a major financial and technical challenge at the industrial scale. A critical tool in biofuels process development is the techno-economic (TE) model, which calculates biofuel production costs using a process model and an economic model. The process model solves mass and energy balances for each unit, and the economic model estimates capital and operating costs from the process model based on economic assumptions. The process model inputs include experimental data on the feedstock composition and intermediate product yields for each unit. These experimental yield data are calculated from primary measurements. Uncertainty in these primary measurements is propagated to the calculated yields, to the process model, and ultimately to the economic model. Thus, outputs of the TE model have a minimum uncertainty associated with the uncertainty in the primary measurements. RESULTS: We calculate the uncertainty in the Minimum Ethanol Selling Price (MESP) estimate for lignocellulosic ethanol production via a biochemical conversion process: dilute sulfuric acid pretreatment of corn stover followed by enzymatic hydrolysis and co-fermentation of the resulting sugars to ethanol. We perform a sensitivity analysis on the TE model and identify the feedstock composition and conversion yields from three unit operations (xylose from pretreatment, glucose from enzymatic hydrolysis, and ethanol from fermentation) as the most important variables. The uncertainty in the pretreatment xylose yield arises from multiple measurements, whereas the glucose and ethanol yields from enzymatic hydrolysis and fermentation, respectively, are dominated by a single measurement: the fraction of insoluble solids (fIS) in the biomass slurries. CONCLUSIONS: We calculate a $0.15/gal uncertainty in MESP from the TE model due to uncertainties in primary measurements. This result sets a lower bound on the error bars of the TE model predictions. This analysis highlights the primary measurements that merit further development to reduce the uncertainty associated with their use in TE models. While we develop and apply this mathematical framework to a specific biorefinery scenario here, this analysis can be readily adapted to other types of biorefining processes and provides a general framework for propagating uncertainty due to analytical measurements through a TE model.     

Improving Enzymes for Biomass Conversion: A Basic Research Perspective

The cost of enzymes for converting plant biomass materials to fermentable sugars is a major impediment to the development of a practical lignocellulosic ethanol industry. Research on enzyme optimization with the goal of reducing the cost of converting biomass materials such as corn stover into glucose, xylose, and other sugars is being actively pursued in private industry, academia, and government laboratories. Under the auspices of the Department of Energy Great Lakes Bioenergy Research Center, we are taking several approaches to address this problem, including “bioprospecting” for superior key enzymes, protein engineering, and high-level expression in plants. A particular focus is the development of synthetic enzyme mixtures, in order to learn which of the hundreds of known enzymes are important and in what ratios. A core set comprises cellobiohydrolase, endoglucanase, β-glucosidase, endoxylanase, and β-glucosidase. Accessory enzymes include esterases, proteases, nonhydrolytic proteins, and glycosyl hydrolases that cleave the less frequent chemical linkages found in plant cell walls.     

Simultaneous saccharification and co-fermentation of glucose and xylose in steam-pretreated corn stover at high fiber content with Saccharomyces cerevisiae TMB3400

The two main sugars in the agricultural by-product corn stover are glucose and xylose. Co-fermentation of glucose and xylose at high content of water-insoluble solids (WIS) without detoxification is a prerequisite to obtain high ethanol concentration and to reduce production costs. A recombinant strain of Saccharomyces cerevisiae, TMB3400, was used in simultaneous saccharification and fermentation (SSF) of whole pretreated slurry of corn stover at high WIS. TMB3400 co-fermented glucose and xylose with relatively high ethanol yields giving high final ethanol concentration. The ethanol productivity increased with increasing concentration of pretreatment hydrolysate in the yeast production medium and when SSF was performed in a fed-batch mode.