Dandan Wei (1), Misa Tanaka (1), Saori Takahashi (1), Kentaro Iwasaki (1), Hironori Inadome (1), Satoshi Yoshida (2), Hiroyuki Yoshimoto (1); (1) Research Laboratories for Alcoholic Beverage Technologies, Kirin Company, Limited, Namamugi, Tsurumi-ku, Yokohama, Japan; (2) Central Laboratories for Key Technologies, Kirin Company, Limited, Japan

Yeast, Fermentation, and Microbiology

In beer, the composition of terpene alcohol is strongly influenced by the type of hops used in the brewing process. Yeast cells are able to convert the monoterpenoid geraniol to beta-citronellol during the fermentation process. We have approximately 1,100 yeast strains in our yeast bank, which includes beer yeasts, sake yeasts, wine yeasts and whiskey yeasts. After collecting the various yeast strains in our fermentation experiments, we compiled a database of our yeast bank to collate the flavor and the brewing characteristics of the yeasts in the database. In the present study, we attempted to identify yeast strains with the ability to efficiently convert geraniol to beta-citronellol by monitoring for beta-citronellol production. Furthermore, to increase the sweetness of the finished product, the brewing conditions that increased residual maltose levels were investigated. The production of beta-citronellol by a total of 114 yeast strains (shochu, wine, and top-fermenting yeast strains) was evaluated by performing fermentation experiments in 10 mL test tubes with the bottom-fermenting yeast strain KBY011 as a control. Yeast strains identified as having high beta-citronellol production ability were further evaluated in larger scale fermentations of 500 mL, 20 L, and 200 L. Two strains, wine yeast strain WIY40 and top-fermenting yeast strain TFY192, were found to produce beta-citronellol levels that were threefold higher than that of the control yeast strain KBY011. As strain WIY40 was determined to produce beer with the best balance of flavors, we used this strain to investigate the optimal brewing conditions for increasing residual maltose levels as an approach for producing beer with higher sweetness. By adjusting the sugar composition enzymatically in wort, the optimal ratio of monosaccharide to maltose that allowed maltose consumption to be delayed was determined. In addition, rapid cooling of the optimized wort resulted in sufficiently high residual maltose concentrations. Thus, by combining a yeast strain with a high beta-citronellol–converting activity, wort with an optimal sugar composition, and rapid cooling during the fermentation process, beer with an enhanced citrus flavor, natural sweetness and reduced bitterness could be produced. These findings demonstrate that the combined use of wine yeast and optimal brewing conditions is capable of adding a citrus flavor and sweetness to beer, without the requirement for additional hops or sugars.

Dandan Wei received a master’s degree in agriculture from Kyoto University, Japan, in March 2011. She was employed in April 2011 by the Brewing Technology Development Center, Kirin Brewery Company Ltd., to conduct research on yeast. She also studied brewing technology at the University of California, Davis, from January to April 2015. The organization of Kirin Brewery Company changed in January 2013, leading to the current Kirin Company Ltd.