Wild and Cultured Yeasts: How Microbiology Shapes Wine Character

Alcoholic fermentation is the process by which yeasts consume natural sugars (glucose and fructose) in the grape must and break them down into ethyl alcohol and carbon dioxide. Along the way, heat is released and hundreds of secondary aromatic compounds are synthesized: esters, higher alcohols, and acids. It is these byproducts of yeast activity that form the bouquet we later evaluate in the glass.

A winemaker can entrust this process to two different groups of microorganisms. The first is wild yeasts (or spontaneous microflora) that enter the winery directly from the grape skins and cellar equipment. The second is cultured (selected) yeasts, bred in laboratories and preserved for precise control over fermentation. The choice between these approaches divides winemakers into proponents of classic non-intervention and advocates of technological precision.

Selected yeasts guarantee a stable result. The winemaker knows exactly at what temperature the strain will start working, how much sugar it will convert, and when the process will end. This minimizes the risk of stuck fermentation, where unfermented sugar remains in the must, leaving it vulnerable to bacteria.

Wild yeasts are unpredictable. They can stop working halfway through, leaving the wine semi-sweet and unstable. The winemaker must constantly monitor must density and temperature, ready to intervene at any moment to save the batch.

Modern science offers hundreds of cultured yeast strains capable of enhancing specific grape characteristics. For example, there are strains to unlock thiols in Sauvignon Blanc (notes of tropical fruit and freshly cut grass) or to emphasize berry tones in Pinot Noir.

Wild yeasts do not aim for standardized varietal purity. They yield more muted but complex aromas. Instead of a bright mono-aroma, a complex palette is achieved, where varietal characteristics intertwine with fermentation tones and terroir features.

Using a pure yeast culture virtually eliminates off-odors. Fermentation runs cleanly, and the wine is predictable and commercially reliable.

In spontaneous fermentation, undesirable yeast species (such as Brettanomyces) or bacteria can produce volatile acids and phenolic compounds. This leads to the appearance of barnyard, wet wool, or medicinal plaster tones in the wine. In small doses, these nuances add complexity, but if control is lost, they can completely ruin the wine.