Higher Ethanol Production Rate Research Articles

Tungsten oxide nanosheets photoanode for highly efficient methane conversion

Photoelectrochemical (PEC) conversion of methane (CH4) to high-value-added liquid products has been regarded as an eco-friendly and sustainable strategy for the utilization of natural gas but remains challenging in terms of high yield and selectivity for diatomic carbon (C2) products. Herein, we report a facile oxalic acid reduction strategy to synthesize ultrathin tungsten oxide (WO3) nanosheets (NSs) with controllable oxygen vacancies (Ov) as photoanodes for efficient PEC CH4 conversion to ethanol. As a result, the ultrathin WO3 NSs photoanode shows a superior PEC CH4 conversion efficiency with a high ethanol production rate and Faraday efficiency of 98.9 mmol/g/h and 63.6% under a low applied positive potential of 0.9 V vs. reversible hydrogen electrode (RHE) during 4 h. Incident photon-to-current efficiency of 23.2% is achieved at 365 nm. The reaction path and catalytic mechanism of CH4→CH3→CH3OH→CH2OH→CH3CH2OH are studied by in-situ Fourier-transform infrared spectroscopy (FTIR), electron spin resonance spectroscopy (ESR), and density functional theory (DFT) simulation. The results indicate that in-situ-formed OOH is the main reactive oxygen species during CH4 conversion, and Ov significantly lowers the reaction energy of the rate-determinate elemental step.

NiO/ZnO heterojunction nanorod catalyst for high-efficiency electrochemical conversion of methane

Electrochemical conversion is promising for selective partial oxidation of methane. We present a NiO/ZnO shell/core nanorod catalyst used to enhance electrochemical conversion. The built-in potential at the NiO/ZnO interface and the 1D morphology of the nanorods allow for fast charge transfer, thereby enhancing electrochemical methane conversion. We achieved a high ethanol production rate of 1084.2 μmol/gNiO/hr with a selectivity of 81 % for a 600-nm long NiO/ZnO nanorod catalyst. Mechanistic analyses with isotopic labeling reactions suggested ethanol production pathways involving generation of active oxygen by dissociative adsorption of the anionic CO32- oxidant, formation of methanol by methane activation, and coupling of deprotonated methanol and methane. Furthermore, we improved the ethanol yield by improving methane solubility in sulfolane cosolvent mixtures. We also demonstrated conversion with in situ generation of CO32- from a CH4/CO2 mixture.

Optimization of Yeast, Sugar and Nutrient Concentrations for High Ethanol Production Rate Using Industrial Sugar Beet Molasses and Response Surface Methodology

Among the various agro-industrial by-products, sugar beet molasses produced by sugar refineries appear as a potential feedstock for ethanol production through yeast fermentation. A response surface methodology (RSM) was developed to better understand the effect of three process parameters (concentration of nutrient, yeast and initial sugar) on the ethanol productivity using diluted sugar beet molasses and Saccharomyces cerevisiae yeast. The first set of experiments performed at lab-scale indicated that the addition of 4 g/L of nutrient combined with a minimum of 0.2 g/L of yeast as well as a sugar concentration lower than 225 g/L was required to achieve high ethanol productivities (>15 g/L/d). The optimization allowed to considerably reduce the amount of yeast initially introduced in the fermentation substrate while still maximizing both ethanol productivity and yield process responses. Finally, scale-up assays were carried out in 7.5 and 100 L bioreactors using the optimal conditions: 150 g/L of initial sugar concentration, 0.27 g/L of yeast and 4 g/L of nutrient. Within 48 h of incubation, up to 65 g/L of ethanol were produced for both scales, corresponding to an average ethanol yield and sugar utilization rate of 82% and 85%, respectively. The results obtained in this study highlight the use of sugar beet molasses as a low-cost food residue for the sustainable production of bioethanol.

Kluyveromyces marxianus: Current State of Omics Studies, Strain Improvement Strategy and Potential Industrial Implementation

Bioethanol is considered an excellent alternative to fossil fuels, since it importantly contributes to the reduced consumption of crude oil, and to the alleviation of environmental pollution. Up to now, the baker yeast Saccharomyces cerevisiae is the most common eukaryotic microorganism used in ethanol production. The inability of S. cerevisiae to grow on pentoses, however, hinders its effective growth on plant biomass hydrolysates, which contain large amounts of C5 and C12 sugars. The industrial-scale bioprocessing requires high temperature bioreactors, diverse carbon sources, and the high titer production of volatile compounds. These criteria indicate that the search for alternative microbes possessing useful traits that meet the required standards of bioethanol production is necessary. Compared to other yeasts, Kluyveromyces marxianus has several advantages over others, e.g., it could grow on a broad spectrum of substrates (C5, C6 and C12 sugars); tolerate high temperature, toxins, and a wide range of pH values; and produce volatile short-chain ester. K. marxianus also shows a high ethanol production rate at high temperature and is a Crabtree-negative species. These attributes make K. marxianus promising as an industrial host for the biosynthesis of biofuels and other valuable chemicals.

The optimization of the Ambonese arrack fermentation using co-culture Pichia polymorpha and Kloeckera javanica

Abstract. Mahulette F, Astuti DI. 2020. The optimization of the Ambonese arrack fermentation using co-culture Pichia Polymorpha and Kloeckera javanica. Biodiversitas 21: xxxx. The fermentation process of the Ambonese arrack is still uncontrollable. As various microbes are involved in the fermentation, which causes inconsistent production of ethanol content. This research aimed to optimize the production of the Ambonese arrack using Pichia polymorpha and Kloeckera javanica in inoculum ratios of 1:1, 1:2, and 1:3. The number of yeast cells and chemical characteristics was observed every three hrs up to 24 hrs. The ethanol content of the fermented coconut sap and the arrack was measured using the titration and Gas Chromatography-Mass Spectrophotometry (GC-MS) methods, respectively. The numbers of P. polymorpha and K. javanica cells at 15 hrs of fermentation were 9.9 log CFU/mL and 10.7 log CFU/mL, respectively. The reducing sugar content decreased from 525 mg/L to 296 mg/L, while the pH from 6.46 to 4.82. The highest ethanol production rate was 1.4 mg/L.3hrs. (ratio 1:1), observed at 12 hrs of the fermentation, while the highest ethanol contents in the fermented coconut sap and the arracks were 11.3 mg/L and 300.400 mg/L, respectively. Different inoculum ratios affect the sensory characteristics of Ambonese arrack. The highest average value of sensory testing was 4.6 (ratio 1:2). Thus, the inoculum ratios of P. polymorpha and K. javanica 1:2 and 1:3 has the best quality in controlled Ambonese arrack processing.

Techno-economic analysis for incorporating a liquid-liquid extraction system to remove acetic acid into a proposed commercial scale biorefinery.

Mitigating the effect of fermentation inhibitors in bioethanol plants can have a great positive impact on the economy of this industry. Liquid-liquid extraction (LLE) using ethyl acetate is able to remove fermentation inhibitors-chiefly, acetic acid-from an aqueous solution used to produce bioethanol. The fermentation broth resulting from LLE has higher performance for ethanol yield and its production rate. Previous techno-economic analyses focused on second-generation biofuel production did not address the impact of removing the fermentation inhibitors on the economic performance of the biorefinery. A comprehensive analysis of applying a separation system to mitigate the fermentation inhibition effect and to provide an analysis on the economic impact of removal of acetic acid from corn stover hydrolysate on the overall revenue of the biorefinery is necessary. This study examines the pros and cons associated with implementing LLE column along with the solvent recovery system into a commercial scale bioethanol plant. Using details from the NREL-developed model of corn stover biorefinery, the capital costs associated with the equipment and the operating cost for the use of solvent were estimated and the results were compared with the profit gain due to higher ethanol production. Results indicate that the additional capital will add 1% to the total capital and manufacturing cost will increase by 5.9%. The benefit arises from the higher ethanol production rate and yield as a consequence of inhibitor extraction and results in a $0.35 per gallon reduction in the minimum ethanol selling price (MESP). © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:971-977, 2016.

Xylose fermentation as a challenge for commercialization of lignocellulosic fuels and chemicals.

Fuel ethanol production from lignocellulosic materials is at a level where commercial biofuel production is becoming a reality. The solubilization of the hemicellulose fraction in lignocellulosic-based feedstocks results in a large variety of sugar mixtures including xylose. However, allowing xylose fermentation in yeast that normally is used for fuel ethanol production requires genetic engineering. Moreover, the efficiency of lignocellulosic pretreatment, together with the release and generation of inhibitory compounds in this step, are some of the new challenges faced during second generation ethanol production. Successful advances in all these aspects will improve ethanol yield, productivity and titer, which will reduce the impact on capital and operating costs, leading to the consolidation of the fermentation of lignocellulosic biomass as an economically feasible option for the production of renewable fuels. Therefore the development of yeast strains capable of fermenting a wide variety of sugars in a highly inhibitory environment, while maintaining a high ethanol yield and production rate, is required. This review provides an overview of the current status in the use of xylose-engineered yeast strains and describes the remaining challenges to achieve an efficient deployment of lignocellulosic-based ethanol production.

Enhanced hexose fermentation by Saccharomyces cerevisiae through integration of stoichiometric modeling and genetic screening

In order to determine beneficial gene deletions for ethanol production by the yeast Saccharomyces cerevisiae, we performed an in silico gene deletion experiment based on a genome-scale metabolic model. Genes coding for two oxidative phosphorylation reactions (cytochrome c oxidase and ubiquinol cytochrome c reductase) were identified by the model-based simulation as potential deletion targets for enhancing ethanol production and maintaining acceptable overall growth rate in oxygen-limited conditions. Since the two target enzymes are composed of multiple subunits, we conducted a genetic screening study to evaluate the in silico results and compare the effect of deleting various portions of the respiratory enzyme complexes. Over two-thirds of the knockout mutants identified by the in silico study did exhibit experimental behavior in qualitative agreement with model predictions, but the exceptions illustrate the limitation of using a purely stoichiometric model-based approach. Furthermore, there was a substantial quantitative variation in phenotype among the various respiration-deficient mutants that were screened in this study, and three genes encoding respiratory enzyme subunits were identified as the best knockout targets for improving hexose fermentation in microaerobic conditions. Specifically, deletion of either COX9 or QCR9 resulted in higher ethanol production rates than the parental strain by 37% and 27%, respectively, with slight growth disadvantages. Also, deletion of QCR6 led to improved ethanol production rate by 24% with no growth disadvantage. The beneficial effects of these gene deletions were consistently demonstrated in different strain backgrounds and with four common hexoses. The combination of stoichiometric modeling and genetic screening using a systematic knockout collection was useful for narrowing a large set of gene targets and identifying targets of interest.

Maltose and glucose utilization during fermentation of barley and sorghum lager beers as affected by β-amylase or amyloglucosidase addition

The effect of supplementation of amyloglucosidase or β-amylase during mashing of sorghum and barley beers was studied. The research focused in the increments in maltose and glucose concentrations, and their consumption during 144 h fermentation of lager beers. Barley, red sorghum and white sorghum malt beers, produced with regular or waxy sorghum adjuncts, and supplemented with these exogenous enzymes, were produced. The addition of exogenous enzymes increased up to 20% the total sugar content. Regression lines to describe sugar consumption were obtained (R2 > 0.951). Amyloglucosidase treatments had higher glucose contents and up to 4 times higher glucose consumption rates compared with unsupplemented or with β-amylase treatments. The latter treatments had maltose as predominant sugar. Amyloglucosidase treatments had higher ethanol production rates, however the maximum production rate was delayed 24 h, compared with maltose-rich treatments. These worts observed incomplete fermentations and contained higher residual sugars. β-amylase treatments were not significantly different compared with their regular counterparts in terms of sugar and ethanol production. The selection of adequate sorghum cultivars for malt and adjuncts as well as the use of an osmotolerant yeast should be considered when exogenous enzymes are supplemented in brewing.

A thermo- and toxin-tolerant kefir yeast for biorefinery and biofuel production

Although biorefinery has become a common concept to convert biomass into biofuels and value-added chemicals for better cost-performance, good microbial hosts that can be used to implement the concept are still wanting. In this study, a Kluyveromyces marxianus yeast, named KY3, was isolated from a Taiwanese kefir microbial consortium. We showed that KY3 could grow on a broad spectrum of substrates, including hexose and pentose sugars. It is heat and toxin tolerant, can grow under a wide range of pH values (pH 2.5–9), and shows a high ethanol production rate at elevated temperatures. It also can produce value-added aromatic chemicals, such as 2-phenylethylethanol and 2-phenylethyl acetate, during the fermentative process. A genetic transformation was achieved in KY3 to express a rumen fungal β-glucosidase gene, and the transgenic host (KY3–NpaBGS) could efficiently convert cellobiose to ethanol. Furthermore, it was shown that a novel dual-microbe co-culture system of Bacillus subtilis and KY3–NpaBGS can be employed for bioethanol production from cellulosic material. Thus, KY3 has a high potential to be a good host for biorefinery.

Optimization of simultaneous saccharification and fermentation for the production of ethanol from sweet sorghum (Sorghum bicolor) bagasse using response surface methodology

The response surface method was used to investigate the effects of process parameters, including temperature from 35 to 45°C, enzyme loading from 10 to 30 filter paper units (FPU)/g-glucan, yeast concentration from 0.5 to 1.5g/L, and bagasse solid concentration from 4 to 10% on ethanol yield, concentration and production rate during simultaneous saccharification and fermentation (SSF) of sweet sorghum bagasse. The results showed that the maximum ethanol yield, concentration and production rate were obtained at different SSF conditions. Under the recommended SSF condition of the temperature at 37°C, yeast concentration at 1.4g/L, enzyme loading at 25 FPU/g-glucan and bagasse solid concentration at 10%, the ethanol yield, concentration and production rate were 89.4%, 38g/L and 1.28g/L/h, respectively. The bagasse solid concentration had significant effects on the ethanol concentration and ethanol production rate. The optimum bagasse solid concentration for the ethanol yield was 7%. The increase of the bagasse solid concentration significantly increased the final ethanol concentration but decreased the ethanol production rate. Fed batch operation could be used to maintain the bagasse solid concentration at a low value (e.g., 7%) to achieve high ethanol yield and production rate while increasing the final ethanol concentration.

Immobilization of Saccharomyces cerevisiae to modified bagasse for ethanol production

Ethanol production was carried out by growing yeast cells immobilized on bagasse carriers. The effects of cellulase hydrolysis of carriers on cell immobilization and ethanol production were investigated. The electrical conductivity of carriers after three days cellulase hydrolysis was 3.7 times higher than that of basic bagasse carrier and the immobilized cell concentration of modified carriers was 1.46 times higher than that of unmodified ones. The scanning electron microscopy (SEM) of the carriers indicated that cellulase hydrolysis disrupted the sleek surface into rough and porous, which promoted the mass transfer and resulted in high specific ethanol production rate. Cellulase hydrolysis of carriers has positive effects on both of cell immobilization and ethanol productivity.

Continuous Fermentation of Undetoxified Dilute Acid Lignocellulose Hydrolysate by Saccharomycescerevisiae ATCC 96581 Using Cell Recirculation

Saccharomyces cerevisiae ATCC 96581 was cultivated in a chemostat reactor with undetoxified dilute acid softwood hydrolysate as the only carbon and energy source. The effects of nutrient addition, dilution rate, cell recirculation, and microaerobicity were investigated. Fermentation of unsupplemented dilute acid lignocellulose hydrolysate at D = 0.10 h(-1) in an anaerobic continuous reactor led to washout. Addition of ammonium sulfate or yeast extract was insufficient for obtaining steady state. In contrast, dilute acid lignocellulose hydrolysate supplemented with complete mineral medium, except for the carbon and energy source, was fermentable under anaerobic steady-state conditions at dilution rates up to 0.14 h(-1). Under these conditions, washout occurred at D = 0.15 h(-1). This was preceded by a drop in fermentative capacity and a very high specific ethanol production rate. Growth at all different dilution rates tested resulted in residual sugar in the chemostat. Cell recirculation (90%), achieved by cross-flow filtration, increased the sugar conversion rate from 92% to 99% at D = 0.10 h(-1). Nutrient addition clearly improved the long-term ethanol productivity in the recirculation cultures. Application of microaerobic conditions on the nutrient-supplemented recirculation cultures resulted in a higher production of biomass, a higher cellular protein content, and improved fermentative capacity, which further improves the robustness of fermentation of undetoxified lignocellulose hydrolysate.

Metabolic regulation analysis of an ethanologenic Escherichia coli strain based on RT-PCR and enzymatic activities

BackgroundA metabolic regulation study was performed, based upon measurements of enzymatic activities, fermentation performance, and RT-PCR analysis of pathways related to central carbon metabolism, in an ethanologenic Escherichia coli strain (CCE14) derived from lineage C. In comparison with previous engineered strains, this E coli derivative has a higher ethanol production rate in mineral medium, as a result of the elevated heterologous expression of the chromosomally integrated genes encoding PDCZm and ADHZm (pyruvate decarboxylase and alcohol dehydrogenase from Zymomonas mobilis). It is suggested that this behavior might be due to lineage differences between E. coli W and C.ResultsThis study demonstrated that the glycolytic flux is controlled, in this case, by reactions outside glycolysis, i.e., the fermentative pathways. Changes in ethanol production rate in this ethanologenic strain result in low organic acid production rates, and high glycolytic and ethanologenic fluxes, that correlate with enhanced transcription and enzymatic activity levels of PDCZm and ADHZm. Furthermore, a higher ethanol yield (90% of the theoretical) in glucose-mineral media was obtained with CCE14 in comparison with previous engineered E. coli strains, such as KO11, that produces a 70% yield under the same conditions.ConclusionResults suggest that a higher ethanol formation rate, caused by ahigher PDCZm and ADHZm activities induces a metabolic state that cells compensate through enhanced glucose transport, ATP synthesis, and NAD-NADH+H turnover rates. These results show that glycolytic enzymatic activities, present in E. coli W and C under fermentative conditions, are sufficient to contend with increases in glucose consumption and product formation rates.

Optimization of fermentation conditions for the production of the mezcal from Agave salmiana using response surface methodology

Response surface methodology was applied to optimize the fermentative phase for the mezcal production from Agave salmiana. A 3 k factorial design was used to obtain models describing the relationship between the ethanol production, process productivity, and product yield with respect to the fermentation temperature and the initial sugar concentration. The results showed that the fermentative conditions affected the composition of higher alcohols (referred as a quality indicator) in the mezcal as well as the amount of ethanol. The highest ethanol production was attained by employing the following predicted optimum operational conditions: temperature of 28 °C and an initial sugar concentration of 105 g/l. However, the maximum productivity process was attained with 34.6 °C and 90 g/l, whereas the maximum product yield and the best quality mezcal at 28 °C and 77 g/l. Results show that the simultaneous optimization for high ethanol production and fast production rate are not compatible, since high ethanol production requires a high substrate concentration, which in turn inhibit the growth rate.

Life cycle assessment of various cropping systems utilized for producing biofuels: Bioethanol and biodiesel

A life cycle assessment of different cropping systems emphasizing corn and soybean production was performed, assuming that biomass from the cropping systems is utilized for producing biofuels (i.e., ethanol and biodiesel). The functional unit is defined as 1 ha of arable land producing biomass for biofuels to compare the environmental performance of the different cropping systems. The external functions are allocated by introducing alternative product systems (the system expansion allocation approach). Nonrenewable energy consumption, global warming impact, acidification and eutrophication are considered as potential environmental impacts and estimated by characterization factors given by the United States Environmental Protection Agency (EPA-TRACI). The benefits of corn stover removal are (1) lower nitrogen related environmental burdens from the soil, (2) higher ethanol production rate per unit arable land, and (3) energy recovery from lignin-rich fermentation residues, while the disadvantages of corn stover removal are a lower accumulation rate of soil organic carbon and higher fuel consumption in harvesting corn stover. Planting winter cover crops can compensate for some disadvantages (i.e., soil organic carbon levels and soil erosion) of removing corn stover. Cover crops also permit more corn stover to be harvested. Thus, utilization of corn stover and winter cover crops can improve the eco-efficiency of the cropping systems. When biomass from the cropping systems is utilized for biofuel production, all the cropping systems studied here offer environmental benefits in terms of nonrenewable energy consumption and global warming impact. Therefore utilizing biomass for biofuels would save nonrenewable energy, and reduce greenhouse gases. However, unless additional measures such as planting cover crops were taken, utilization of biomass for biofuels would also tend to increase acidification and eutrophication, primarily because large nitrogen (and phosphorus)-related environmental burdens are released from the soil during cultivation.

Factors influencing ethanol production rates at high-gravity brewing

The aim of this work was to investigate the influences of different fermentation factors on ethanol production rates by Saccharomyces cerevisiae (lager strain), at high-gravity brewing using response surface methodology. An empirical linear polynomial model was developed to describe the behaviour of the dependent variable as a function of significant factors. The resultant functional relationship in terms of coded values for predicting ethanol production rates was: Y=0.421+0.155 X 2+0.0575 X 2 X 3, where Y represents the ethanol production rate (g/lh), and X 2 and X 3 are coded levels for fermentation temperature and nutrient supplementation, respectively. Patterns of yeast growth, decrease in wort gravity and ethanol production were also evaluated at the maximum ethanol production rate (0.694 g/lh). It was concluded that higher ethanol production rates could be achieved by increasing fermentation temperature and supplementing high-gravity worts with yeast extract, ergosterol and Tween 80.

Dispersion of alkali on the surface of Co-MoS 2/clay catalyst: a comparison of K and Cs as a promoter for synthesis of alcohol

The surface of alkali (K, Cs) promoted Co-MoS 2/clay catalysts were investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The surface enrichment of K and Cs on the surface of catalysts increased with syngas exposure. XPS suggests that syngas treatment slightly shifted the K 2p lines of K-promoted catalyst (K 2p 3/2=292.9 and 2p 1/2=295.7 eV) to a higher binding energy (BE) (K 2p 3/2=293.2 and 2p 1/2=296 eV), and for Cs-promoted catalyst, the Cs 3d 5/2 (724.6 eV) and Cs 3d 3/2 (738.7 eV) were shifted slightly to a higher binding energy (0.2 eV). Co 9S 8 was formed during syngas exposure of Cs-promoted catalyst, but it was not formed during syngas exposure of the K-promoted catalyst. XPS, SEM and EDS analysis indicated that potassium and cesium distributions on the fresh catalysts were not uniform when a mortar and pestle was used to mix the active chemicals. The formation of Co 9S 8 on the surface of Cs-promoted Co-MoS 2/clay catalyst may result in a reduction of Co-Mo-S surface area and its activity toward alcohol production. The highest ethanol production rate (129 g/kg cat h −1) was achieved for K-promoted Co-MoS 2/clay catalyst at 300 °C and 2000 psig for gas hour space velocity (GHSV=2000 h −1). The total alcohol synthesis (AS) production rate increased upon increasing the reaction temperature, however, the total alcohol selectivity was decreased.

Breeding and Cultivation of Glucoamylase-Producing Yeast with Inactivation of MAT Locus.

The breeding of a recombinant yeast having glucoamylase activity, Saccharomyces cerevisiae SR93, has already been reported by the authors, and further improvement of this yeast is attempted to significantly increase the conversion rate of starch into ethanol. It is generally known that the disruption of the MAT locus, which produces a repressor protein, enhances the expression of glucoamylase gene. A glucoamylase-producing yeast with the disrupted MAT locus, Saccharomyces cerevisiae SR96, is bred to convert starch into ethanol rapidly. Disruption of the MAT locus is performed by inserting the LEU2 gene into the MAT locus of S. cerevisiae SR93 and was confirmed by Southern blot analysis. The induction effect of starch and the repression effect of glucose on the glucoamylase synthesis are examined experimentally. The glucoamylase activity per unit cell concentration increases about 1.6-fold due to the disruption of the MAT locus. The specific growth rate, the glucoamylase synthesis rate, and the ethanol production rate of S. cerevisiae SR96 are much higher than S. cerevisiae SR93, and the direct alcohol fermentation of starch using S. cerevisiae SR96 gave the highest ethanol production rate in the various incubation systems.

Controlled limited aeration and metabolic regulation during the production of ethanol from D-xylose by Pichia stipitis

Under aerobic conditions Pichia stipitis CBS 5773 can grow on D-xylose with reduced specific growth rate, compared to D-glucose, but with comparable yield. In the absence of oxygen xylose is converted to ethanol and xylitol. Under conditions of controlled limited respiration ethanol production is increased whereas no xylitol is formed. With the aim of maximizing ethanol formation batch processes were performed with constant oxygen transfer rates by means of computer controlled stirrer speed. At a volumetric oxygen uptake rate (OUR) of 3.75 mmol l −1 h −1 the highest ethanol production rate was found to be 0.12 g ethanol per g cell mass and hour at a product yield, Y of 0.39. From our enzyme kinetic studies we calculate that, under aerobiosis, xylose in the first step is reduced to xylitol nearly exclusively with NADPH as the coenzyme, and that in the absence of oxygen NADH is the preferred electron donor. The activities of the two key enzymes, xylose reductase and xylitol dehydrogenase, seem to be under the control of reciprocal competitive and non-competitive inhibition by their substrates/products and the cofactors. These regulations can explain the kinetic behaviour of this yeast.