Juan J. Moreno

Analytical differentiation of wine fermentations using pure and mixed yeast cultures

SummaryThirty-three fermentations of Pedro Ximénez grapes, collected in three degrees of ripeness, were carried out by inoculation with three types of inoculum: pure cultures ofSaccharomyces cerevisiae races and ofTorulaspora delbrueckii, indigenous yeasts, and mixed cultures of indigenous yeasts enriched with the pure cultures. By means of variance analysis 21 compounds were determined whose final concentrations in the wines significantly depended on the musts, the inocula or both. Eleven products that depended significantly on the inocula were subjected to a discriminant analysis in which most of the pure cultures gathered in a discriminant space area different from that occupied by the indigenous yeasts. The centroids corresponding to most of the mixed cultures were shifted to the central area of the discriminant space, moved away from their corresponding pure cultures and approached the indigenous yeasts. The results show a high similarity between the fermentations carried out with mixed cultures with the addedS. cerevisiae races and those fermentations carried out with the indigenous yeasts, with regard to those compounds which were significantly dependent on the inocula.

Discrimination of sweet wines partially fermented by two osmo-ethanol-tolerant yeasts by gas chromatographic analysis and electronic nose.

Some special sweet wines are obtained by partial fermentation of musts from off-vine dried grapes containing large amounts of sugars. This process is very slow and subject to serious stop problems that can be avoided by using osmo-ethanol-tolerant yeasts. Musts containing 371g/l of sugars were partially fermented with selected Saccharomyces cerevisiae strains, X4 and X5, to 12% (v/v) and the wines obtained with X5 exhibited a higher volatile acidity but lower concentrations of higher alcohols, carbonyl compounds and polyols than those obtained with X4. A principal component analysis (PCA) of the data provided by an electronic nose (E-nose) afforded discrimination between fermented and unfermented musts, but not between wines obtained with X4 or X5. The PCA applied to the major volatile compounds and polyols shows similar results, but a clear discrimination between wines is obtained by removing the polyols glycerol and 2,3-butanediol from the PCA.