Increase Bioethanol Production Research Articles

The Use of Molasses in Producing Bioethanol Catalyzed by Candida tropicalis (Isolated from Cocos nucifera. L) Immobilized MnFe2O4 Coated-Chitosan

Bioethanol is a sustainable fuel product to be an alternative energy source. Therefore, the study aims to observe and analyze the effect of MnFe2O4 coated-chitosan in increasing bioethanol production. This bioethanol was produced from molasses with a high total sugar content of up to 50% with Candida tropicalis as the microorganism. Fermentation is divided into two parts, namely using free C. tropicalis and C. tropicalis immobilized MnFe2O4 coated-chitosan. There was electrostatic interaction between MnFe2O4 and chitosan in 578 cm-1 and 659 cm-1 of FTIR, cubic spinel pattern in XRD, and SEM surface image of C. tropicalis immobilized MnFe2O4 coated-chitosan interaction. These characterization results show very good properties as a biocatalyst. The highest concentration was produced by fermentation using C. tropicalis immobilized MnFe2O4 coated-chitosan about 4.35% with residual sugar of 8.46 g/L. In summary C. tropicalis immobilized MnFe2O4 coated-chitosan have the potential to improve bioethanol products.
 Keywords: Candida tropicalis, fermentation, MnFe2O4 coated-chitosan, molasses

BIOETHANOL PRODUCTION FROM AGRICULTURAL WASTE: A REVIEW

The importance of this research lies in keeping pace with the global trend to diversify energy sources and reduce climate change and air pollution by shedding light on how to benefit from agricultural waste by using it as a raw material for the production of bioethanol, which is one of the most important types of renewable fuels. This review highlighted the; ways of converting agricultural and fruit waste into bioethanol and its environmental and economic benefits-including adding it to gasoline used as car fuel in different proportions, as it works to raise the octane number of gasoline and improve its quality, thus reducing its production costs and reducing gas emissions from vehicle exhaust. In addition, the global production of bio-ethanol was reviewed and is expected to reach it by the end of 2023. It also highlighted the results of several studies that dealt with the properties and uses of ethanol and the best agricultural wastes used for this purpose. It is suggested to focus on the agricultural waste in Iraq, such as rice husks, in terms of cost and conditions for improvement to increase bio-ethanol production using fermentation processes.

Integrated laccase delignification with improved lignocellulose recalcitrance for enhancing enzymatic saccharification of ensiled rice straw

The recalcitrant of straw biomass due to its severe lignification hinders its efficient use as a biofuel of feedstock. Therefore, this study was aimed to develop an integrated approach for treatment of rice straw to improve enzymatic saccharification and increase bioethanol production. We evaluated the effect of Lactobacillus plantarum (LP) and cellulases (CL), along with the ligninolytic enzyme laccase (Lac), on the quality of silage, enzymatic hydrolysis, and ethanol production from rice straw ensiled for 60 days. The rice straw was either ensiled without additive (control) or treated with five combinations of the additives, i.e., LP, CL, LP+CL, LP+Lac, or LP+CL+Lac (LPCL+Lac). The results indicated that adding laccase improved the fermentation and silage preservation quality of rice straw biomass by increasing lactic acid production and decreasing pH (<4) (P < 0.05). It also maintained high water-soluble carbohydrate and mono- and disaccharides. The LPCL+Lac treatment managed to reduce the lignin content of rice straw up to 3.82%. Fourier transform infrared spectroscopic analysis and scanning electron microscopic images confirmed that laccase degraded lignin and altered the physico-chemical structure of rice straw. The delignification and reducing the recalcitrance of lignocellulose of rice straw biomass with integrated laccase pretreatment (LPCL+Lac) resulted in release of high content of reducing sugar (52.42 mg/mL), and produced the highest quantity of ethanol (24.45 mg/mL). These results demonstrate that laccase effectively degraded lignin content of rice straw, and its integrated use with LP and CL provides a feasible approach for improving the preservation and bioconversion efficiency of highly lignified and recalcitrant straw biomass.

Enhancement of bioethanol production from tofu waste by engineered simultaneous saccharification and fermentation (SSF) using co-culture of mold and yeast

Tofu waste is a potential raw material for producing bioethanol since it has 52,82 % carbohydrate content, among which 35,12 % is starch. Indonesia has approximately 315 tofu business units that produce 2,6 million tons of tofu per year, so the availability of tofu waste is abundant, and its current use is only for feed. The use of starch as the raw material of bioethanol needs two steps, namely saccharification and fermentation. Application of Simultaneous Saccharification and Fermentation (SSF) technique may reduce the processing time of bioethanol production from starchy materials. The SSF involves co-culture, namely aerobic molds (Aspergillus niger) that hydrolyze starch into sugar and facultative anaerobic yeasts (Saccharomyces cerevisiae) that ferment sugar into ethanol. The objectives of the study were to obtain a production technique of bioethanol from the starch of tofu waste, which has more productive and higher yield through Engineered SSF/ESSF (stop aeration at a particular time) technique using co-culture of mold and yeast and to compare it with Conventional SSF/CSSF (complete aeration) technique. ESSF produced bioethanol at 11.39 g/L with the product yield per substrate used (Yp/s) of 0.36 g/g. In comparison, CSSF produced bioethanol at 7.69 g/L with the Yp/s of 0.23 g/g. ESSF technology has been proved to be more productive and efficient than CSSF since it can increase bioethanol production and Yp/s of 1.5 and 1.6 times, respectively. The study results also prove that the starch from tofu waste can be utilized as media for bioethanol production.

The Effect of Hydrolysis Time with Trichoderma Reesei on Soluble Sugar Concentration in Sweet Sorghum Stalk Pulp and Alcohol Concentration of Its Fermentation Results

It has been done a study on the use of Trichoderma reesei in the hydrolysis of sweet sorghum stalk pulp continued to fermentation by Saccharomyces cerevisiae to produce bioethanol. The purpose of this study was to examine the effect of hydrolysis time by T. reesei on the sugar content of sweet sorghum stalk pulp and the ethanol concentration obtained after its fermentation using Saccharomyces cerevisiae. The sweet sorghum stalks were cut into small pieces, blended until crushed, incubated with T. reesei at various times. and then fermented using S. cerevisiae for 24 hours after the incubation period was complete. The results showed that the soluble sugar concentration in the sweet sorghum stalk pulp increased after the incubation, which occurred from the 8th to the 24th hour. Furthermore, it continued to decrease until the longest incubation time. The alcohol concentration produced after the fermentation period changed according to the pattern of changes in sugar concentration in the substrate. These results suggest that to increase bioethanol production from sweet sorghum stalk, it will be better to incubate the pulp of stalk first with T. reesei for approximately 24 hours before fermented with S. cerevisiae.

Lessons from Brazil: Opportunities of Bioethanol Biofuel in Indonesia

Brazil has a long tradition of bioethanol production from sugarcane. Following the Middle East crisis in the 1970s, the oil price increased twofold, doubling Brazil’s spending on oil imports. As a result, the country had to find an alternative type of fuel to reduce its expenditure on conventional petrol fuels. With its vast tropical land conditions, the Brazilian government then put massive effort to establish the infrastructure and research of bioethanol from 1975 and 1989. Research plays a pivotal role in the success of Brazil’s ethanol industry. The country was heavily criticised at the beginning of the programme, but today, Brazil is the second-largest ethanol producer after the USA. Brazil produces the cheapest ethanol in the world and does not depend on fossil fuel to supply its energy demand. Indonesia and Brazil share the same landscape and geographical advantages. This paper aims to discuss the success of the Brazilian bioethanol industry and draw important lessons from it. In a nutshell, Indonesia should increase bioethanol production from renewable sources. Also, the modernization of distilleries facilities, the establishment of new production plants, the introduction of subsidies and the reduction of taxes for ethanol producers are needed. In summary, apart from massive research and development, we concluded that effective and strong long-term government commitment play important role in shaping the future of the Indonesian bioethanol industry.

Implementation of graphitic carbon nitride nanomaterials and laser irradiation for increasing bioethanol production from potato processing wastes.

Agricultural and agro-industrial wastes (e.g., potato peel waste) are causing severe environmental problems. The processes of pretreatment, saccharification, and fermentation are the major obstacles in bioethanol production from wastes and must be overcome by efficient novel techniques. The effect of exposing the fungi (yeast) Saccharomyces cerevisiae to laser source with the addition of graphitic carbon nitride nanosheets (g-C3N4) with different concentrations on bioethanol production was investigated through the implementation of a batch anaerobic system and using potato peel waste (PPW). Dichromate test was implemented as quantitative analysis for quantification of the bioethanol yield. The benefits of this test were the appearance of green color indicating the identification of ethanol (C2H5OH) by bare eye and the ease to calculate the bioethanol yield through UV-visible spectrophotometry. The control sample (0.0ppm of g-C3N4) showed only a 4% yield of bioethanol; however, by adding 150ppm to PPW medium, 22.61% of ethanol was produced. Besides, laser irradiations (blue and red) as influencing parameters were studied with and without the addition of g-C3N4 nanomaterials aiming to increase the bioethanol. It was determined that the laser irradiation can trigger the bioethanol production (in case of red: 13.13% and in case of blue: 16.14% yields, respectively) compared to the control sample (in absence of g-C3N4). However, by adding different concentrations of g-C3N4 nanomaterials from 5 to 150ppm, the bioethanol yield was increased as follows: in case of red: 56.11% and, in case of blue: 56.77%, respectively. It was found that using fungi and exposing it to the blue laser diode source having a wavelength of 450nm and a power of 250 mW for a duration of 30min with the addition of 150mg L-1 of g-C3N4 nanomaterials delivered the highest bioethanol yield from PPW.

Effect of the Sugarcane Bagasse Deacetylation in the Pentoses Fermentation Process

Biomass alkaline deacetylation prior to acid pretreatment can be a promising alternative to reduce the toxicity of hemicellulosic hydrolysates and improve second-generation bioethanol production. In this paper, the effect of alkaline deacetylation of sugarcane bagasse on bioethanol production by Spathaspora passalidarum was evaluated. Sugarcane bagasse deacetylated hemicellulosic hydrolysate (DHH) was processed using the following sequence: (1) deacetylation (0.4%, w/v NaOH, 70 °C, 3 h) and (2) acid pretreatment (0.5% (v/v) of H2SO4, 140 °C, 15 min). Non-deacetylated, hemicellulosic hydrolysate (HH), was obtained applying only acid pretreatment (0.5% (v/v) of H2SO4, 140 °C, 15 min). Biomass deacetylation reduced the content of acetic acid and some phenolic compounds in the hydrolysate (DHH) compared to acid pretreatment (HH), which resulted in its low toxicity. Thus, the bioethanol production with DHH was of 16.92 g L−1, whereas only 1.3 g L−1 of bioethanol was obtained with HH fermentation. Deacetylation process provided a 13-fold increase in bioethanol production by S. passalidarum, showing that alkaline deacetylation followed by sulfuric acid pretreatment is a promising strategy to increase bioethanol production. This procedure provided a simple and practical alternative to the classic methods of detoxification of hemicellulosic hydrolysate from sugarcane bagasse.

Assessing the biofuel – transport nexus. The case of the sugar industry in Cuba

Cuba currently faces a limited availability of transportation to support the development needs of the country. Transport availability is mostly limited because of fuel shortage. Moreover, Cuba has an important production of sugarcane, with a significant potential to further increase its production. Using sugarcane-based bioethanol is a significant opportunity for sugarcane producer countries. There are different raw materials available in the sugar industry to produce bioethanol. Therefore, there are different scenarios to increase the production of sugarcane and energy cane, to increase bioethanol production. In this study, two scenarios of sugarcane and energy cane production were considered, from which there are eight possible scenarios of bioethanol production. These bioethanol production scenarios were matched with three transport scenarios, including a business-as-usual scenario, a scenario considering the use of bioethanol blends in standard gasoline and diesel engines, and the and introduction of vehicles running on high ethanol blends or pure bioethanol (i.e flexible fuel vehicles, and ethanol buses and trucks). In total, the production of sugarcane-based bioethanol might support from 4 to 58% of the yearly demand for transport energy in the transport scenarios. Additionally, the use of bioethanol as a transport fuel can potentially reduce transport-related greenhouse gas emissions by an estimated 3–30%.

Sustainability analysis of bioethanol promotion in Thailand using a cost-benefit approach

This study aims at analyzing the sustainability of bioethanol promotion in Thailand using cost-benefit analysis. The analysis is carried out based on the bioethanol production targets in Thailand’s Alternative Energy Development Plan 2015, covering cassava and molasses ethanol. The role of taxes and production efficiency improvement on the entire economy is presented via bioethanol promotion options. The socio-economic and environmental effects of bioethanol promotion are also considered. The effects of bioethanol promotion are derived via a hybrid approach including computable general equilibrium modeling and life cycle impact assessment. Environmental effects are converted into monetary units using the conversion factors for Thailand. The results show that increasing bioethanol production and use can bring about a net benefit to society of around 62 billion Thai Baht over 2016–2026. Improving the production efficiency of feedstock along with increasing bioethanol production can increase the net benefit to society. Furthermore, not only does bioethanol promotion enhance real Gross Domestic Product, but also creates more jobs in the society and leads to less environmental cost per unit of domestic production when air emissions and irrigation water consumption are considered. The study can thus support decision-making towards sustainable biofuel development in Thailand.

Exogenous application of rosmarinic acid improves saccharification without affecting growth and lignification of maize

Biomimetically incorporated into the lignin structure, rosmarinic acid improves in vitro maize cell wall saccharification; however, no in planta studies have been performed. We hypothesized that rosmarinic acid, itself, could inducer saccharification without disturbing plant growth. Its effects on growth, enzymes of the phenylpropanoid pathway, lignin, monomeric composition, and saccharification of maize were evaluated. In a short-term (24 h) exposure, rosmarinic acid caused deleterious effects on maize roots, inhibiting the first enzymes of the phenylpropanoid pathway, phenylalanine ammonia-lyase and tyrosine ammonia-lyase, altering lignin composition and slightly increasing saccharification. In a long-term (14 d) exposure, rosmarinic acid increased saccharification of maize stems by about 50% without any deleterious effects on plant growth, the phenylpropanoid pathway and lignin formation. This demonstrated that exogenous application of rosmarinic acid on maize plants improved saccharification, and represented an interesting approach in facilitating enzymatic hydrolysis of biomass polysaccharides and increasing bioethanol production.

Optimization of Bioethanol Production from Durian Skin by Encapsulation of Saccharomyces cerevisiae

The optimum condition of durian skin using SSF process with encapsulation of Saccharomyces cerevisiae takes place at pH 5.0 at 37 °C. S. cerevisiae encapsulation can increase bioethanol production activity. Bioethanol production is produced at pH 4.5; 5.0 and 5.5 were 35.85, 41.25 and 39.89 g/L medium, respectively. While using free cells S. cerevisiae, bioethanol produced was 21.42, 34.94 and 28.15 g/L medium. The percentage of bioethanol produced by S. cerevisiae was 15.3 % at pH 5.0; 40.25 % at pH 4.5 and 29.43 % at pH 5.5. Encapsulation of S. cerevisiae cells can increase the resistance to process temperature by differences in bioethanol production between encapsulation with cells free of S. cerevisiae by 19.34 % at 37 °C, 24.02 % at 40 °C and 49.01 % at 45 °C. These results are higher compared to cells free of S. cerevisiae.

Selenium supplementation during fermentation with sugar beet molasses and Saccharomyces cerevisiae to increase bioethanol production

AbstractA bench scale submerged fermentation process was used to bioethanol produce using sugar beet molasses andSaccharomyces cerevisiae, as substrate and microbial strain, respectively. Effects of selenium amount on growth ofS. cerevisiaeand bioethanol production were evaluated. The obtained results indicated that growth ofS. cerevisiae(manifested as turbidity intensity) in the samples containing 0, 5, 10, 15, 20 and 25 μg sodium selenite, during aerobic process, was 0.1707, 0.1678, 0.1679, 0.1664, 0.1627 and 0.160% a.u./h (after 14 h incubation), respectively. Statistical analysis based on compression test indicated that there were insignificant (p &gt; 0.05) differences between growth rate of the yeast in the fermented samples containingS. cerevisiaeand 5 to 25 μg selenium salt. Response surface methodology was utilized to evaluate effects of two fermentation parameters namely, amount of selenium (5-25 μg) and substrate brix (10-25°Bx) on the concentration (g/L) of produced bioethanol. Obtained results revealed that maximum bioethanol concentration (55 g/L) was achieved using 15 μg selenium and molasses with 25°Bx. Furthermore, results have also indicated that, without using selenium and using molasses with 25°Bx, bioethanol with concentration of 29 g/L was produced.

Valorization of Sugar-Ethanol Industry Waste Vinasse for Increased Second-Generation Ethanol Production Using Spathaspora passalidarum Yeast Strains

In this study, sugarcane bagasse hydrolysate supplemented with different proportions of sugar-ethanol industrial waste (vinasse) was used to optimize fermentative ethanol production using two Spathaspora passalidarum yeast strains isolated from decaying wood in Amazonian biome. The bagasse samples were pretreated chemically and enzymatically before using a binary factorial experimental design. The factorial design was carried out in two stages; in the first, a 24 factorial design was done to assess the effects of pretreated sugarcane bagasse hydrolysate and vinasse, ammonium sulfate concentration, yeast biomass concentration, and two different strains of yeast, S. passalidarum; in the second stage, a 23 factorial design was done to provide a more detailed analysis of the effects of vinasse supplementation on the fermentation process—this time, by modifying the ratio of sugarcane bagasse hydrolysate and vinasse and, more significantly, the same factor—biomass concentration. The results showed that the most significant factors for increasing bioethanol production in the 24 factorial design proved to be the following: inoculum and hydrolyzed bagasse supplementation with vinasse (the optimal being 75:25), followed by yeast concentration (optimal at 14% w/v cell wet weight) and then the type of strain used (S. passalidarum UFMG-HMD-14.1). The use of only hydrolyzed bagasse without vinasse (23 factorial design) had no significant influence on the results, nor did varying the concentration of nitrogen. This increase in ethanol production, which allows the same amount of ethanol productivity through a reduction in the carbon source, suggests it can lead to an improved final use for vinasse.

Pressurized pretreatment and simultaneous saccharification and fermentation with in situ detoxification to increase bioethanol production from green coconut fibers

In this study, we investigated four conventional pretreatments under pressurized conditions, as well as in situ detoxification using polyethylene glycol (PEG 400), for the simultaneous saccharification and fermentation (SSF) of green coconut fibers. The highest fermentable sugar production (22.05% g/g untreated fiber) was reached with pressurized dilute alkaline pretreatment. In the SSF, the addition of PEG favored ethanol production compared to that in experiments without the polymer, reaching a maximum value of 7.87 g/L at a liquor content of 50% v/v for the pressurized water pretreatment. After lignin recovery by acid precipitation, the liquors had interesting antioxidant properties, similar to those of fruit extracts. The proposed pressurized water pretreatment could be an advantageous choice because it can be performed at mild temperatures (70 °C) and high solid loadings (26.67% w/v) and achieves a high mass yield (72.84% w/w) without the addition of a chemical agent. Furthermore, the liquor can be used in the SSF, reducing the water consumption and number of steps in the cellulosic ethanol process.

Macroeconomic modeling for assessing sustainability of bioethanol production in Thailand

The development of bioethanol can bring about socio-economic and environmental impacts. To support the sustainability analysis of bioethanol development and the decision-making on bioethanol promotion in Thailand, this study aims at assessing the socio-economic and environmental effects of bioethanol production using recursive dynamic computable general equilibrium (CGE) modeling. The assessment is carried out based on the bioethanol production targets in Thailand’s Alternative Energy Development Plan (AEDP) 2015, capturing cassava ethanol and molasses ethanol. Simulation scenarios are set up to incorporate the role of bioethanol support policies, such as tax reduction, production efficiency improvement, and widening the price gap between gasoline and gasohol, into the analysis. The results show that increasing bioethanol production together with improving the efficiency of feedstock cultivation is the most efficient way to enhance economic growth. However, the higher production and consumption would also lead to more irrigation water demand and air emissions. The presentation of bioethanol support policies’ role in promoting bioethanol along with widespread socio-economic and environmental effects focus are novel. Not only are the results useful for the decision-making towards sustainable bioethanol development, the methods presented in this study should also be a guideline for other socio-economic and environmental impact assessment schemes.

Land Use Change under Biofuel Policies and a Tax on Meat and Dairy Products: Considering Complexity in Agricultural Production Chains Matters

Growing demand for meat and dairy products (MDP), biofuels, and scarcity of agricultural land are drivers of global land use competition. Impacts of policies targeting demand for MDP or biofuels have only been analysed separately. We use the computable general equilibrium model DART-BIO to investigate combined effects, since MDP and biofuel production are closely related via feestock use and co-production of animal feed. We implement four scenarios: (a) a baseline scenario; (b) halving MDP consumption in industrialised countries by a tax; (c) abolishing current biofuel policies; and (d) no exogenous land use change. We find that a MDP tax and exogenous land use change have larger effects on land use and food markets than biofuel policies. International trade is affected in all scenarios. With respect to combined effects of a MDP tax and biofuel policies, we find decreasing biodiesel but increasing bioethanol production. In addition, the MDP tax decreases the impact of biofuel policies on agricultural markets and land use. Our results highlight the importance of a detailed representation of different vegetable oils used in biodiesel production and related by-products. Finally, since the MDP tax increases the use of fossil fuels, the net climate mitigation potentials of such a tax should be investigated further.

Energetic shift of sugarcane bagasse using biogas produced from sugarcane vinasse in Brazilian ethanol plants

Worldwide environmental policies demand each time more biofuel consumption and less emission. In this context, this work presents a 2G ethanol study as a mean to increase bioethanol production and availability. Currently, technologies use sugarcane bagasse for lignocellulosic ethanol production, which may unbalance the ethanol and sugar mill energy matrix, since bagasse and straw are the main fuel for power and steam generation. A possible solution is using biogas produced from vinasse biodigestion as a fuel instead of using biomass, enabling to shift a fraction of the sugarcane bagasse to 2G ethanol production and, at the same time, keeping power and steam production constant. This paper assesses that energy shift by analyzing ten different scenarios for power generation, comparing the amount of bagasse shifted, the increase in straw consumption, the increase in ethanol production and the reduction of environmental emissions in each scenario. The results show that, at least from the technical and environmental perspective, a combined cycle operating at a high pressure is the best alternative. It is possible to shift from 56.5% to 100% of the available bagasse using the combined cycle technology, which is also followed by an increase in straw consumption. In addition to that, the ethanol availability increase ranges from 28.5 to 50.4%. Moreover, the organic load disposal to the ground also decreases more than 90% compared to the conventional process due to the introduction of vinasse biodigestion.

Magnesium sulphate and β-alanine enhanced the ability of Kluyveromyces marxianus producing bioethanol using oil palm trunk sap

ABSTRACTThe abundance of oil palm trunk waste generated each year has encouraged research in using its sap for fermentation to produce value-added products. One of these value-added products is bioethanol production using yeast strains. In this study, the ability of Kluyveromyces marxianus ATCC 46537 to produce bioethanol using oil palm trunk sap (OPTS) was examined. The nutrients (ammonium sulphate, di-ammonium hydrogen phosphate, magnesium sulphate, β-alanine, calcium chloride and potassium dihydrogen phosphate) required to enhance production were screened and optimised. The concentrations of bioethanol and sugars were monitored with high performance liquid chromatography. The results showed that K. marxianus could attain maximum bioethanol concentration at 16 h with a higher productivity as compared to S. cerevisiae. Magnesium sulphate and β-alanine were found to increase bioethanol production. When 7.93 g/L of magnesium sulphate and 0.90 g/L of β-alanine were supplemented to OPTS, bioethanol production increased 20% with a bioethanol yield of 0.47 g/g and a productivity of 2.22 g/L.h. Therefore, minimum supplementation of OPTS with inorganic nutrients could enhance the bioethanol production of Kluyveromyces marxianus.

The addition of hydrolyzed rice straw in xylose fermentation by Pichia stipitis to increase bioethanol production at the pilot-scale

The pretreatment of agricultural biomass by diluted acid is often employed to facilitate the release of monosaccharide for the subsequent enzyme hydrolysis for lignocellulosic ethanol production. However, furfural and hydroxymethylfurfural are usually generated and markedly decrease the yield of pentose fermentation during this pretreatment. In the present study, the enhancement of lignocellulosic ethanol production was successfully demonstrated at pilot scale with extra addition of hydrolyzed rice straw into pentose fermentation by Pichia stiptis. This way has resulted into the increase of P. stiptis cell mass was shown to play a positive role. The ethanol yield, 0.45 gp/gs, with the addition of hydrolyzed rice straw in hemicellulosic hydrolysate from plywood, bagasse and bamboo were increase 20–51% to demonstrate the applicability of this technology in a variety of lignocellulosic ethanol processes due to the efficient conversion of xylose.