Modulation of geraniol metabolism during alcohol fermentation

Geraniol is the main monoterpenol present in Gewurtztraminer white grapes and in some red aromatic grapes from northern Italy. During fermentation, the geraniol concentration showed a dramatic drop and a small amount was transformed into linalool and α-terpineol by an acid catalysed chemical reaction. Yeast are responsible for changing most of the geraniol to citronellol through enzymatic reactions. The final aroma of wine is due mainly to both the untransformed geraniol and the formed citronellol. In this study, competition between the geraniol-citronellol transformation and another metabolic process, in which according to our hypothesis sterol biosynthesis is involved, was investigated. Geraniol, in the active form of geranyl pyrophosphate, is one of intermediates in that metabolic pathway. The addition of ergosterol, one of final products of sterol biosynthesis, to the fermentation medium, appeared to reduce the amount of geraniol involved in this metabolic pathway; as a result geraniol-citronellol transformation was favoured. The result was a higher production of citronellol with the same geraniol consumption. The relationships between oxygen and regulation of the metabolic pathways involving geraniol were also studied. Results showed that aerobic conditions could improve citronellol production. Finally, it was observed that metabolism of geraniol also depended on the physiological state of the yeast during fermentation.