Kei Asada,* Atsushi Tanigawa, Koji Saito, Ayako Sanekata, Takahito Takase, and Koichi Toyoshima  
*Sapporo Breweries, Ltd., 10 Okatome, Yaizu, Shizuoka, 425-0013, Japan (kei.asada@sapporobeer.co.jp)

Diacetyl enhances the flavor of red wine and contributes to its fullness and depth. The diacetyl concentration in red wine is higher than that in white wine or other alcoholic beverages. Diacetyl is mainly produced by lactic acid bacteria during malolactic fermentation. However, some white wine varieties prepared with malolactic fermentation have low diacetyl concentrations. Yeast also produces diacetyl, which is a byproduct of branched chain amino acid (BCAA) biosynthesis. An intermediate metabolite of BCAA biosynthesis, 2-acetolactate, is oxidized non-enzymatically to diacetyl. We noted higher diacetyl concentrations in red wine than in white wine and examined the relationship between grape juice color and diacetyl concentration. These observations led to the hypothesis that some ingredients in red grape juice may influence yeast gene expression and contribute to higher diacetyl concentrations. We conducted a test fermentation by adding various fractions of red grape juice and measured the diacetyl concentration in the resulting wine. Malvidin-
3-glucoside, an anthocyanin, was the main ingredient of a grape juice fraction that showed the greatest diacetyl production. Subsequently, we performed another test fermentation after adding standard anthocyanin reagents (malvidin-3-glucoside, cyanidin-3-glucoside, delphinidin-3-glucoside, and petunidin-3-glucoside). Although all the anthocyanin reagents promoted greater diacetyl production, the diacetyl concentration depended on the anthocyanin type. These results indicate that anthocyanins contribute to greater diacetyl concentrations in red wine. Next, we investigated the influence of anthocyanins on yeast gene expression. BDH1 expression, important for the conversion from diacetyl to acetoin, was significantly down regulated by anthocyanin. These results support our hypothesis and suggest that a higher diacetyl concentration results from suppression of diacetyl metabolism by anthocyanins. To our knowledge, this is the first study showing that anthocyanins influence diacetyl production in wine by modulating yeast gene expression.

Funding Support: Sapporo Breweries, Ltd.