Biotransformation of hop-derived monoterpene alcohols by lager yeast and their contribution to the flavor of hopped beer.

It is well-known that various beers contain many flavor compounds derived from barley malts, hops, yeast fermentation, and other raw materials. Among these flavor compounds, terpenoids are mainly derived from hops. Linalool, one of the monoterpene alcohols, has been found in various beers and been regarded as an important factor for a hop-derived beer flavor. We focus on contributions of other monoterpene alcohols (geraniol, beta-citronellol, nerol, and alpha-terpineol) to hopped beer flavor. Several researchers have reported that monoterpene alcohols are biotransformed by yeast and that geraniol is mainly transformed to beta-citronellol during the first 2-4 days in model fermentation. In this study, we investigated the biotransformation of monoterpene alcohols during fermentation of hopped beer by using various hop cultivars. As a result, geraniol drastically decreased during the first 3 days. beta-Citronellol was almost absent in wort and gently increased during the total fermentation period. The concentrations of geraniol and beta-citronellol in finished beer increased, depending on the initial concentration of geraniol in the wort. The continuous increase of beta-citronellol did not correspond to the fast decrease of geraniol. This increase of beta-citronellol might be partly explained by an occurrence of glycosidically bound flavor precursor and a glucoside hydrolase activity secreted from lager yeast. In addition, we examined flavor characteristics of monoterpene alcohols and found that there was an additive effect among linalool, geraniol, and beta-citronellol and that only 5 microg/L of geraniol and beta-citronellol were enough for this effect. Therefore, it is suggested that not only linalool but also geraniol and beta-citronellol might contribute to hopped beer flavor at lower levels, at which OAVs of these compounds become below 1.0.