Selection and improvement of wine yeasts
The selection of wine yeasts is usually carried out within the species Saccharomyces cerevisiae. It aims at identifying the yeast strains that, besides fermenting grape juice vigorously and producing high ethanol yield, can also positively influence the composition and the sensorial characteristics of wine. The natural availability of yeast strains possessing an ideal combination of oenological characteristics is highly improbable. Moreover, selected S. cerevisiae wine strains usually produce wines with a plain aromatic profile. The extension of the selection of wine yeasts to S. cerevisiae not growing in oenological environments or to non-Saccharomyces yeasts has provided strains possessing novel and interesting oenological characteristics. Nevertheless, these strains cannot be directly used as starter cultures in wine fermentations, mainly because they are not vigorous or competitive in oenological conditions. Wine strains possessing innovative oenological traits that can influence the sensorial characteristics of wine can be constructed using genetic or molecular methods. Intraspecific S. cerevisiae hybridisation has provided useful oenological strains. Nevertheless, the traits of oenological interest that can be exchanged or introduced using this technique are only those commonly found in the species S. cerevisiae. Innovative oenological traits can be introduced or exchanged by hybridising strains belonging to different species but with a sufficient genetic affinity for them to mate. Interspecific Saccharomyces hybrids were found to be stable, vigorous and possessing the parental oenological traits in novel and interesting combinations. Nevertheless, they are sterile; the genetic improvement cannot therefore be taken further than the first generation. Moreover, the combination of the parental traits cannot be specifically programmed and a combination of positive traits is often the result of chance. The recent development of recombinant DNA technology has overcome the limitations of traditional genetic techniques as well as broadening the potential of wine yeast improvement.