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Appl. Environ. Microbiol. doi:10.1128/AEM.02109-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Isolation and characterization of brewer's yeast variants with improved fermentation performance under high-gravity conditions

Laboratory of Molecular Cell Biology, Institute of Botany and Microbiology, Katholieke Universiteit Leuven and Department of Molecular Microbiology, Flanders Interuniversity Institute for Biotechnology (VIB), Kasteelpark Arenberg 31, B-3001 Leuven- Heverlee, Flanders, Belgium; Centre for Malting and Brewing Science, Department of Food and Microbial Technology, Katholieke Universiteit Leuven, Kasteelpark Arenberg 22, B-3001 Leuven- Heverlee, Flanders, Belgium

^* To whom correspondence should be addressed. Email: Patrick.Vandijck{at}bio.kuleuven.be.

	Abstract

To save energy, space and time, today's breweries make use of high-gravity brewing in which concentrated medium (wort) is fermented, resulting in a product with higher ethanol content. After fermentation, the product is diluted to obtain beer with the desired alcohol content. While economically desirable, the use of wort with an even higher sugar concentration is limited by the inability of brewer's yeast to efficiently ferment such concentrated medium. Here, we describe a successful strategy to obtain yeast variants with significantly improved fermentation capacity under high-gravity conditions. We isolated better performing variants of the industrial lager strain CMBS33 by subjecting a pool of UV-induced variants to consecutive rounds of fermentation in very high-gravity wort (>22 °Plato). Two variants (GT336 and GT344) showing faster fermentation rates and/or more complete attenuation as well as improved viability under high ethanol conditions were identified. The variants displayed the same advantages in a pilot-scale stirred fermenter under high-gravity conditions at 11°C. Microarray analysis identified several genes whose altered expression may be responsible for the superior performance of the variants. The role of some of these candidate genes was confirmed by genetic transformation. Our study shows that proper selection conditions allow the isolation of variants of commercial brewer's yeast with superior fermentation characteristics. Moreover, it is the first study to identify genes that affect fermentation performance under high-gravity conditions. The results are of interest to the beer and bioethanol industries, where the use of more concentrated medium is economically advantageous.

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