Biotransformation of monoterpene alcohols by lager yeast and their contribution to the citrus flavor of beer

KIYOSHI TAKOI (1), Yutaka Itoga (2), Koichiro Koie (2), Yuuta Katayama (1), Masayuki Shimase (1), Takayuki Kosugi (1), Yasuyuki Nakayama (1), Junji Watari (1)
(1) Sapporo Breweries Ltd., Yaizu, Shizuoka, Japan; (2) Sapporo Breweries Ltd., Sorachi-gun, Hokkaido, Japan

Various beers contain many flavor compounds derived from barley malts, hops, and yeast fermentation. Among these flavor compounds, terpenoids are mainly derived from hops. Especially, linalool, one of the monoterpene alcohols, has been regarded as an important factor for a hop-derived beer flavor. In this report, we focus on contributions of other monoterpene alcohols (geraniol, β-citronellol, nerol, and α-terpineol) to hopped beer flavor. Several researchers have reported that monoterpene alcohols are biotransformed by brewing yeasts during beer fermentation, and that geraniol is mainly transformed to β-citronellol. First, we investigated biotransformation of monoterpene alcohols during fermentation of hopped beer by using various hop varieties. As a result, geraniol drastically decreased during the first 3 days. β-Citronellol was almost absent in wort and gently increased during total fermentation period. The concentrations of geraniol and β-citronellol in finished beer increased depending on the initial concentration of geraniol in wort. We next examined flavor characteristics of monoterpene alcohols and found that there was an additive effect among linalool, geraniol, and β-citronellol. The test-brewed beer containing these three monoterpene alcohols had a citrus flavor. Therefore, we carried out the screening of various hop varieties and selected Citra as a geraniol-rich hop. The use of Citra was effective for enriching the concentration of geraniol and β-citronellol in the finished beers and this Citra beer had a strong citrus flavor. We evaluated the synergy of geraniol and β-citronellol under coexistence with excess linalool, simulating the composition of the three monoterpene alcohols in the Citra beer, and found that the flavor impression became lime-like citrus by coexistence of all three monoterpene alcohols. From these results, it was suggested that geraniol metabolism by brewing yeasts contribute to the citrus flavor of beer. In addition, the variation on geraniol contents among the U.S. hop varieties is also discussed.

Kiyoshi Takoi graduated from Tohoku University with an M.S. degree in agricultural chemistry in 1989 and joined the Brewing Research Laboratories of Sapporo Breweries, Ltd. as a biochemist. From 1989 to 2002, he worked on brewing chemistry and mainly investigated beer foam stability. During 2002–2005, he evaluated the brewing properties of malts and hops using the pilot malting and brewing plants in the Production & Technology Development Center. In 2006, he managed product development in the New Product Development Center. During 2007–2008, he worked in the Frontier Laboratories of Value Creation as a lead research brewer and mainly investigated hop-derived flavor compounds. At present, he is working in the Value Creation Department of Sapporo and developing new products. He received a Ph.D. in agricultural chemistry from Tohoku University in 2011.