A Complex Chemical Composition
Producers of Aromas
The production of wine consists of countless choices and chances. Steps in the process range widely, from berry composition to filtration options. From year to year, each passing season brings a new berry composition. With these grapes comes something that carries the influence of soil type, vine growth and biological substrates: a growth medium for yeasts.
Yeast is, by nature, unpredictable. Winemakers have discovered a strain of this microorganism that--thankfully--has robust, vigorous fermentation capabilities and a high tolerance for alcohol. That species is known as Saccharomyces cerevisiae. Believed to have been originally isolated from the skin of grapes, S. cerevisiae is also linked to brewing, baking and astrobiology (its hardiness has been the subject of survival throughout deep space missions).
During the course of fermentation, wine gains the chemical and sensorial properties necessary to evolve into a bouquet. Bacteria and yeasts are responsible for synthesizing an abundance of many aroma compounds: fruity scents from esters; spicy leather notes from volatile phenols; and cooked vegetable characteristics from sulfides. Different species of yeast can vary the dimensionality and complexity of wine by the way in which they enhance these different notes.
Impact on Wine Quality
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.
From Fermentation to Stabilization
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.
Options and Perceptions
One conclusion to draw from the varying preferences and proclivities of yeast is that experience isn't everything. A vintage might be driven by native yeasts at a certain temperature one year, but could require different nutrients and organic growth additives for successful fermentation in the following year. While it may never be possible to conquer the problem of producing the perfect culture, winemakers are nearly limitless in their ability to create well-balanced wines from a thriving selection of biodiverse organisms.
Sources and Further Reading:
Carpena, M. et al. Foods, 2021. "Secondary Aroma: Influence of Wine Microorganisms in Their Aroma Profile." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7824511/pdf/foods-10-00051.pdf
Rieger, Ted. Wine Business Monthly, 2019. "Wine Management Strategies, from Fermentation to Stabilization."
Wikipedia. "Saccharomyces cerevisiae." https://en.wikipedia.org/wiki/Saccharomyces_cerevisiae