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Xylose Consumption (xylose + consumption)
Selected AbstractsEffect of acetic acid and pH on the cofermentation of glucose and xylose to ethanol by a genetically engineered strain of Saccharomyces cerevisiaeFEMS YEAST RESEARCH, Issue 4 2010Elizabeth Casey Abstract A current challenge of the cellulosic ethanol industry is the effect of inhibitors present in biomass hydrolysates. Acetic acid is an example of one such inhibitor that is released during the pretreatment of hemicellulose. This study examined the effect of acetic acid on the cofermentation of glucose and xylose under controlled pH conditions by Saccharomyces cerevisiae 424A(LNH-ST), a genetically engineered industrial yeast strain. Acetic acid concentrations of 7.5 and 15 g L,1, representing the range of concentrations expected in actual biomass hydrolysates, were tested under controlled pH conditions of 5, 5.5, and 6. The presence of acetic acid in the fermentation media led to a significant decrease in the observed maximum cell biomass concentration. Glucose- and xylose-specific consumption rates decreased as the acetic acid concentration increased, with the inhibitory effect being more severe for xylose consumption. The ethanol production rates also decreased when acetic acid was present, but ethanol metabolic yields increased under the same conditions. The results also revealed that the inhibitory effect of acetic acid could be reduced by increasing media pH, thus confirming that the undissociated form of acetic acid is the inhibitory form of the molecule. [source] Xylitol bioproduction from wheat straw: hemicellulose hydrolysis and hydrolyzate fermentationJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 9 2006Larissa Canilha Abstract A 22 central composite design with five center points was performed to estimate the effects of temperature (120, 130 and 140 °C) and acid loading (100, 150 and 200 mg g,1) on the yield of monomeric xylose recovery from wheat straw hemicellulose (YS/RM). Under the best hydrolysis condition (140 °C and 200 mg g,1), a YS/RM of 0.26 g g,1 was achieved. After vacuum concentration and detoxification by pH alteration and active charcoal adsorption, the hydrolyzate was used as source of xylose for xylitol bioproduction in a stirred tank reactor. A xylitol production of 30.8 g L,1 was achieved after 54 h,1 of fermentation, resulting in a productivity (QP) of 0.57 g L,1 h,1 and bioconversion yield (YP/S) of 0.88 g g,1. The maximum specific rates of xylose consumption and xylitol production were 0.19 and 0.15 g g,1 h,1, respectively. Copyright © 2006 Society of Chemical Industry [source] Effect of the reversal of coenzyme specificity by expression of mutated Pichia stipitis xylitol dehydrogenase in recombinant Saccharomyces cerevisiaeLETTERS IN APPLIED MICROBIOLOGY, Issue 2 2007J. Hou Abstract Aims:, To determine the effects on xylitol accumulation and ethanol yield of expression of mutated Pichia stipitis xylitol dehydrogenase (XDH) with reversal of coenzyme specificity in recombinant Saccharomyces cerevisiae. Methods and Results:, The genes XYL2 (D207A/I208R/F209S) and XYL2 (S96C/S99C/Y102C/D207A/I208R/F209S) were introduced into S. cerevisiae, which already contained the P. stipitis XYL1 gene (encoding xylose reductase, XR) and the endogenously overexpressed XKS1 gene (encoding xylulokinase, XK). The specific activities of mutated XDH in both strains showed a distinct increase in NADP+ -dependent activity in both strains with mutated XDH, reaching 0·782 and 0·698 U mg,1. In xylose fermentation, the strain with XDH (D207A/I208R/F209S) had a large decrease in xylitol and glycerol yield, while the xylose consumption and ethanol yield were decreased. In the strain with XDH (S96C/S99C/Y102C/D207A/I208R/F209S), the xylose consumption and ethanol yield were also decreased, and the xylitol yield was increased, because of low XDH activity. Conclusions:, Changing XDH coenzyme specificity was a sufficient method for reducing the production of xylitol, but high activity of XDH was also required for improved ethanol formation. Significance and Impact of the Study:, The difference in coenzyme specificity was a vital parameter controlling ethanolic xylose fermentation but the XDH/XR ratio was also important. [source] Comparison of glucose/xylose cofermentation of poplar hydrolysates processed by different pretreatment technologiesBIOTECHNOLOGY PROGRESS, Issue 2 2009Yulin Lu Abstract The inhibitory effects of furfural and acetic acid on the fermentation of xylose and glucose to ethanol in YEPDX medium by a recombinant Saccharomyces cerevisiae strain (LNH-ST 424A) were investigated. Initial furfural concentrations below 5 g/L caused negligible inhibition to glucose and xylose consumption rates in batch fermentations with high inoculum (4.5,6.0 g/L). At higher initial furfural concentrations (10,15 g/L) the inhibition became significant with xylose consumption rates especially affected. Interactive inhibition between acetic acid and pH were observed and quantified, and the results suggested the importance of conditioning the pH of hydrolysates for optimal fermentation performance. Poplar biomass pretreated by various CAFI processes (dilute acid, AFEX, ARP, SO2 -catalyzed steam explosion, and controlled-pH) under respective optimal conditions was enzymatically hydrolyzed, and the mixed sugar streams in the hydrolysates were fermented. The 5-hydroxymethyl furfural (HMF) and furfural concentrations were low in all hydrolysates and did not pose negative effects on fermentation. Maximum ethanol productivity showed that 0,6.2 g/L initial acetic acid does not substantially affect the ethanol fermentation with proper pH adjustment, confirming the results from rich media fermentations with reagent grade sugars. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 [source] Xylitol Production from Sugarcane Bagasse Hydrolyzate in Fluidized Bed Reactor.BIOTECHNOLOGY PROGRESS, Issue 4 2003Effect of Air Flowrate Cells of Candida guilliermondiiimmobilized onto porous glass spheres were cultured batchwise in a fluidized bed bioreactor for xylitol production from sugarcane bagasse hemicellulose hydrolyzate. An aeration rate of only 25 mL/min ensured minimum yields of xylose consumption (0.60) and biomass production (0.14 gDM/gXyl), as well as maximum xylitol yield (0.54 gXyt/gXyl) and ratio of immobilized to total cells (0.83). These results suggest that cell metabolism, although slow because of oxygen limitation, was mainly addressed to xylitol production. A progressive increase in the aeration rate up to 140 mL/min accelerated both xylose consumption (from 0.36 to 0.78 gXyl/L·h) and xylitol formation (from 0.19 to 0.28 gXyt/L·h) but caused the fraction of immobilized to total cells and the xylitol yield to decrease up to 0.22 and 0.36 gXyt/gXyl, respectively. The highest xylitol concentration (17.0 gXyt/L) was obtained at 70 mL/min, but the specific xylitol productivity and the xylitol yield were 43% and 22% lower than the corresponding values obtained at the lowest air flowrate, respectively. The concentrations of consumed substrates and formed products were used in material balances to evaluate the xylose fractions consumed by C. guilliermondii for xylitol production, complete oxidation through the hexose monophosphate shunt, and cell growth. The experimental data collected at variable oxygen level allowed estimating a P/O ratio of 1.35 molATP/molO and overall ATP requirements for biomass growth and maintenance of 3.4 molATP/C-molDM. [source] | ||||||||||