Study of yeast biocatalytic activity on grape aroma compounds

Abstract

Many volatile compounds of different chemical/biochemical origin contribute to wine aroma. Certain key ‘varietal’ aroma compounds such as methoxypyrazines are formed in the grape and appear to be only scarcely influenced by fermentation. Conversely, other grape-derived compounds undergo important transformation during fermentation, so that grape varietal volatile pattern is substantially different than the one observed in the corresponding wine. While this phenomenon is generally regarded from the point of view of the cleavage of glycosidic or amino acidic precursors, recent studies highlighted the existence of other bio-catalytic processes taking place during fermentation, which could be relevant to wine aroma generation. Accordingly, in addition to enzymatic activities such as glycosidase, b-lyase, and/or acetyl-transferase, other enzymes could play a role in the expression of wine varietal aroma typicality, although these have been poorly characterized so far. Certain key volatile such as norisoprenoids (fruity attributes), lactones, (dry fruit attributes), cyclic terpenoids (minty and balsamic attributes), sesquiterpenes and benzenoids (balsamic and spicy attributes) could be associated with such processes. Some of them could also arise from the combination of yeast enzymatic and acidic rearrangements taking place at wine pH. 

The aim of this work was to investigate the biotransformation of potentially relevant grape metabolites by Saccharomyces cerevisiae. Cyclic, oxydrylated, or ketonic terpenes, sesquiterpenes, aliphatic lactones and aldehydes, hydroxyl acids and benzenoids were all investigated, as well as precursors extracts from different grapes. Biotransformations were screened by placing target compounds under incubation (at 37 °C) with yeast resting cells for 72 hours under variable conditions. After incubation, the products of biotransformation were analyzed by SPME-GC-MS and their aroma evaluated by GC-O. 

The results highlighted the occurrence of several complex transformations involving, among others, reduction of allylic carbonyl and carbon-carbon double bond, stereospecific reduction of terpenic ketones, acetylation. These reactions occurring to grape metabolites produced odoriferous molecules considered to participate to the characteristic aroma of some wines. The methodology employed in our study turned out as an effective approach to study the process of aroma generation from neutral grape into wine. As first application, this study has allowed to elucidate some aspects concerning the balsamic notes appearing in wines made with Corvina grapes.