Gene expression measurement by real time PCR, relevant for the synthesis and the degradation of acetate esters and 4-vinylguaiacol, in top fermenting yeast

Microbiology Session
Hubertus Schneiderbanger, Research Center Weihenstphan for Brewing and Food Quality, Freising, Germany
Co-author(s): Mathias Hutzler and Fritz Jacob, Research Center Weihenstephan for Brewing and Food Quality, Freising, Germany

ABSTRACT: Gene expression measurement by PCR is a well-known instrument to get a view into yeast and microorganism cells. So far research has been focused on bottom fermenting yeast. In this study the gene expression responsible for aroma compound synthesis of top fermenting wheat beer yeast (yeast strain TUM 68) was analyzed. The most important aroma compounds, especially for wheat beer (isoamyl acetate, ethyl acetate, 4-vinylguaiacol, and higher alcohols) were synthesized and degraded by different enzymes, which are encoded in the genetic structure of the yeast cells. It is therefore possible to measure messenger-RNA, which encodes the buildup plan for enzymes, using real-time PCR like it has been done for bottom fermenting yeasts in the past. In this study a method was introduced for measuring gene expression levels in top fermenting wheat beer yeasts of the genes ATF1, ATF2, IAH1, and PAD1. The genes ATF1 and ATF2 encode alcoholtransferases of Saccharomyces cerevisiae, which are responsible for the synthesis of ethyl acetate and isoamyl acetate. These esters are degraded by an IAH1-encoded esterase. The PAD1 gene encodes the yeast enzyme phenylacrylic acid decarboxylase, which has previously been associated with the synthesis of 4-vinylguaiacol. The aim of this work was to further the monitoring of the enzymatic synthesis and degradation including the flavor production during main fermentation. Therefore it is possible to gain knowledge of the preferential synthesis and degradation times of these important aroma compounds under realistic conditions. The fermentation trials, which have been done, have been varied using harvested yeast, propagation yeast, and yeast under pressure conditions. It was shown that top fermenting yeast mainly synthesizes enzymes for acetate-ester production between 12 and 24 hr after the beginning of fermentation (depending on the type of yeast used). The expression of IAH1 genes increased mainly at the end of fermentation. PAD1 genes were present at relatively low levels in all trials, which were done with yeast strain TUM 68. This method enables measurement of genes in order to predict the behavior of yeast cells. For the future it could therefore be possible to predict yeast behavior in a technological way in order to achieve the best aroma for this type of beer.

Hubertus Schneiderbanger was born in 1982 in Scheßlitz, Germany. In 2003 he started studying at the TU Munich and graduated in 2008 as an engineer (Dipl.-Ing.) for brewing science and beverage technology. After graduation he worked at the Lehrstuhl fuer Technologie der Brauerei II (2008–2009). Since 2009 he has been working as a consulting engineer and a Ph.D. student at the Research Center Weihenstephan for Brewing and Food Quality. His research focuses on the aroma profile of wheat beer.