The Fermentation Yeasts

Appears in
On Food and Cooking

By Harold McGee

Published 2004

  • About

Fermentation can begin with or without the addition of a starter culture of yeast. The winemaker can choose among many different strains of Saccharomyces, or allow the fermentation to begin spontaneously with “wild” yeasts from the grape skins (species of Kloeckera, Candida, Pichia, Hansenula, and others). These are always eventually displaced by Saccharomyces cerevisiae, which has a greater tolerance for alcohol, but they do contribute flavor compounds to the finished wine.

The primary job of the yeast is to convert sugar to alcohol, but it also produces various volatile, aromatic molecules that the grape itself cannot supply. Prominent among them are the longer-chain alcohols, and esters, a class of compounds that combine an acid with an alcohol or phenol. Both yeast and grape enzymes and the acid conditions also liberate aromatic molecules from the nonvolatile sugar complexes in which some are stored in the grape, so fermentation also brings out the grape’s own flavor potential.