Ethanol, fermentation Saccharomyces yeasts

Further Improvement of Glucose-Xylose-Fermenting Saccharomyces Yeasts for Effective, Economical Conversion of Sugars from Cellulosic Biomass to Biofuel Ethanol. 

Saccharomyces yeasts are rapid fermentors. S. cerevisiae and S. bayanus produce up to 18—20% ethanol. The cells are ovoid to spherical, eUiptical, or elongated (especially under conditions of nitrogen starvation). Vegetative propagation is by multilateral budding. S. uvarum and S. rosei occur earher in the fermentation, when S. rosei may produce up to 6—8% ethanol before being overgrown by the other Saccharomyces yeasts. S. cerevisiae may produce up to 18-20% ethanol (28). 

Romani et al. (2011) also evaluated the yeast population dynamics and fermentation kinetics, and their influences on the analytical profiles of Vin Santo obtained at industrial scale utilizing in separate trials two non-Saccharomyces yeasts, T. delbrueckii and Z. bailii. These results were compared with those obtained both with spontaneous fermentation and with an inoculum of a S. cerevisiae yeast strain. The standard kinetics of fermentations were observed in all of the trials, also if a higher fermentation rate was observed in the trials inoculated with S. cerevisiae compared to those inoculated with the two non-Saccharomyces yeasts, and in the spontaneous one. A rapid decrease in non-Saccharomyces yeast was observed in the trials inoculated with S. cerevisiae. In these last ones, after 6 months, 18.4% ethanol was reached versus 16% of the trials inoculated with the non-Saccharomyces strains. No substantial differences were seen for the higher alcohols or other byproducts assayed. 

Kuyper, M., Harhangi, H. R., Stave, A. K., Winkler, A. A., Jetten, M. S., et al., High-level functional expression of a fungal xylose isomerase The key to efficient ethanolic fermentation of xylose by Saccharomyces cerevisiae FEMS Yeast Res 2003, 4(1), 69-78. 

Ethanol has traditionally been and is currently still produced from glucose-based food crops, such as cane sugar, corn starch, and other starch-rich grains, via fermentation of glucose present in these feedstocks by Saccharomyces yeasts. However, these agricultural crops are expensive and in limited supply. 

Yeasts and bacteria metabolize xylose by following sHghtly different pathways as showing in Fig. 1. Yeasts rely on xylose reductase and xyHtol dehydrogenase, but bacteria rely on xylose isomerase, to convert xylose to xylulose [2, 3]. Although the Saccharomyces yeasts as well as other fermentative yeasts are not able to ferment xylose, Saccharomyces yeasts are able to ferment xylulose to ethanol [4]. Furthermore, they are also able to ferment xylose when a bacterial xylose isomerase is present in the medium [5]. This indicates that Saccharomyces yeasts lack only the enzymes for the conversion of xylose to xylulose. 

Fig. 9 A, B. Comparison of fermentation of xylose under identical conditions by A genetically engineered Saccharomyces yeast strain 1400(pLNH32) which contains the cloned and genetically modified XR, XD, and XK genes and by B 1400 (pXR-XD) which contains only the same cloned XR and XD genes, but not the cloned XK gene. These results demonstrate the importance of cloning the XK gene to enable the Saccharomyces yeasts such as 1400 (pLNH32) to ferment xylose to ethanol...

Fig. 12. Fermentation of glucose and xylose present in the crude hydrolysate of corn fiber by genetically engineered Saccharomyces yeast strain 1400(pLNH32). The corn fiber hydrolysate was provided by Cargill, Inc. The major sugars present in the hydrolysate are glucose, Xylose, and-arabinose. Symbols solid square glucose solid circle xylose solid triangle ethanol open circle arabinose open triangle xylitol and arabitol open square glycerol...

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