A Win for Microorganisms

A specific set of grapes will be accompanied by non-Saccharomyces yeasts, which exist on the fruit and begin a process of spontaneous fermentation when the skins are crushed. Winemakers can then induce a guided fermentation with winery yeasts--strains selected for their behavior, uniformity and ethanol resistance--to improve the aromatic profile. It's only logical that other species besides S. cerevisiae have been found to impact a wine's overall sensory quality. Some yeasts, such as Torulaspora delbrueckii and its pastry-like scents in sparkling wine, are added intentionally as part of a sequential or concurrent inoculation with Saccharomyces yeasts; however, a majority of non-Saccharomyces yeasts come from the vineyard itself.

Microbial populations tend to be quite diverse. Ambient, native yeasts from the growing environment can change significantly each year based on seasonal conditions. The rise of mixed fermentation, wherein more than one strain of a wine yeast is added, has added the intrigue of synergistic effects between different cultures. It is in the addition itself that winemakers can gain adept microbiological control. For example, adding T. delbrueckii simultaneously with S. cerevisiae showed an increase in fruity aromas when compared to a sequential addition.

It's possible to manage fermentation so that hundreds of aroma compounds are balanced within the resultant wine. Winemakers monitor fermentation every day by checking temperature, analyzing phenolics and examining other chemical parameters. After yeasts from either a commercial starter culture or the grapes are combined, their performance depends on finding an optimal temperature for yeast activity and the concentration of added nitrogen for the production of metabolites. A mismanaged attempt to promote the growth of non-Saccharomyces yeasts might lead to lower ethanol yields, spoiled microbes, off-flavors, or overproduction of sweet glycerins.