Definitive evidence for the actual contribution of yeast in the transformation of neutral precursors of grape aromas

Experiments were designed to demonstrate the actual contribution of ye I as t in the formation of the primary aroma during the vinification of neutral grapes. Ruche was chosen as the model wine to study because of its unique f ragrance. A yeast strain specific for Ruche was selected using a new and ra pid isolation method for red wines. The results of this study can be summar ized as follows: Skins from nonaromatic white or red grapes apparently cont ain most of the primary aroma compounds that are revealed in the must only after contact with yeast cells under defined conditions. Similar results we re obtained with the pulp and seeds fractions; however, the olfactory notes , although well characterized, differed from those obtained with skins alon e. Clarification, filtration, and centrifugation of the pulp and seed fract ions or sonification of the skins produce different and well-characterized olfaction notes during the contact with yeast. The primary aroma of nonarom atic white and red grapes contained in the skins can be revealed within 24- 48 h of yeast contact in a synthetic nutrient medium (SNM). The primary aro ma precursors extracted from the skins with methanol, water-saturated butan ol, or aqueous buffer at pH 3.2, concentrated and eluted from a C18 resin c olumn, can be transformed to the free form wine aroma markers within 6 h of contact with yeast cells in SNM. By contrast, prolonged maceration of the skins in aqueous alcoholic buffer at pH 3.2 or 1.1, at 50 or 70 degreesC di d not release primary odors typical of wine. The individual primary aroma c ompounds, identified by GC-MS analysis in Ruch6 wine samples or in Ruche sk in-yeast-SNM samples, could not explain the complexity of the typical Ruch6 wine odor. Only odors common to many wine varieties were identified by GC- olfactometry analysis.