The Use of Naturally Occurring (‘Wild’) Yeasts in Fermentations
Formerly, producers relied on making yeast starter cultures from juice (sulfited to eliminate wild yeast and promote S. cerevisiae), which were inoculated into fermentors at 2–5% after several days of spontaneous fer-mentation. Spontaneous fermentation can be regarded as a heterogeneous microbiological process involving the sequential development of various yeasts and other microbiological species, affected by the prevailing fermentation conditions in a particular vat or tank (Pretorius, 2000). When grape must is used as a culture medium, selective pressures favor the yeasts with the most efficient fermentative catabolism, particularly strains of S. cerevisiae and perhaps strains of closely related species such as S. bayanus. For this reason, S. cerevisiae is almost universally preferred for initiating alcoholic fermentation, and has earned itself the title of ‘the wine yeast.’ For winemaking purposes, yeasts are frequently categorized as ‘wild’ yeasts or ‘wine’ yeasts.
The Killer Factor