Yeasts Associated With Biological Ageing of Fortified Wines

Fortified wines are obtained by adding ethanol at some point during grape fermentation, in order to stabilize them and avoid wine spoilage. Fortification results in wines with high alcohol contents, from 15 to 23%. Exquisite, careful grape selection, harvesting, fermentation and above all, the ageing procedure and blending system, have given rise to the production of excellent fortified wines, such as Sherries. Sherries are subjected to biological and/or physico-chemical ageing in a dynamic process of coninuous blending called soleras and criaderas. In the process of biological ageing, Saccharmyces cerevisiae flor strains, genetically different from fermenting S. cerevisiae yeasts, form a velum on the wine surface through respiratory metabolism, thus protecting wine from oxidation, which is responsible for its pale color and many of its delicate organoleptic properties. Yeasts involved in the maturation of these types of wines show a high degree of variability in their physiological and genetic features. Genome analysis indicated reorganizations reflecting adaptation to specific environments and some of their enological features. A flocculin-like protein encoded by the FLO11 gene is responsible for velum formation, although transcriptional regulators and other genes involved in velum formation and stress resitance have been identified. Moreover, new technologies in Sherry production regarding to increase flor yeast viability and velum stability during Sherry ageing are beeing attempted.