Tuesday, April 21, 2009

H2S in Beer

Genome-Wide Analysis of Gene Expression for Hydrogen Sulfide Production in the Bottom-Fermenting Yeast Saccharomyces pastorianus

TOSHIKO MINATO (1), Tetsuji Yasui (2), Tomohiro Tayama (3),
Hiroyuki Yoshimoto (1), Satoshi Yoshida (1), Kosuke Tashiro (4), Satoru
Kuhara (4), and Osamu Kobayashi (1)
(1) Central Laboratories for Key Technology, Kirin Brewery Co., Ltd.; (2)
Research Laboratories for Brewing, Kirin Brewery Co., Ltd.; (3)
Yokohama Plant, Kirin Brewery Co., Ltd.; (4) Graduate School, Gen. Res.
Tech., Kyushu University, Japan

The amount of hydrogen sulfide (H2S) produced by yeast during fermentation has a significant effect on beer flavor. Control of H2S production is important in brewing due to its very low threshold for detection. H2S is formed as an intermediate during reductive sulfate assimilation, which leads to formation of the sulfur-containing amino acids methionine and cysteine. Typically, a cropped yeast from one fermentation is continuously reused to pitch subsequent lager fermentations. The physiologic state of the cropped yeast is thought to influence the quality of the subsequent fermentations. DNA microarray analysis has recently been used to compare genome-wide patterns of gene expression in the bottom-fermenting yeast Saccharomyces pastorianus.

Here, we investigated the relationship between the physiological state of the cropped yeast and its ability to produce H2S. A strain of S. pastorianus was used to ferment wort at a normal and at an elevated temperature and was then cropped and subjected to microarray analysis. The ability of the cropped yeast to produce H2S during the subsequent fermentation was also analyzed. H2S production by the yeast cropped from the fermentation conducted at the higher temperature was greater than that at the normal temperature. Expression of genes involved in reductive sulfate assimilation was found to have increased in the yeast cropped from the fermentation conducted at the higher temperature, suggesting elevated consumption of methionine. Therefore, we examined the effect of adding methionine to the wort. H2S production by the yeast cropped from the fermentation supplemented with methionine was found to be lower. DNA microarray analysis indicated that the addition of methionine resulted in a decrease in the expression of genes involved in the sulfate assimilation pathway. These data suggest that H2S production can be controlled by alteration of fermentation conditions and wort composition.

Toshiko Minato graduated from Tokyo University of Agriculture and Technology in 1992. She works for Central Laboratories for Key Technology, Kirin Brewery Co., Ltd.

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