How do yeasts influence wine?

Yeast are the unsung heroes of winemaking — single-celled fungi that accomplish one of nature's most delicious chemical transformations. But they do far more than just make alcohol.

During fermentation, yeast converts sugar (glucose and fructose) into ethanol and CO₂. That's the main reaction. But simultaneously, yeast produces over 400 secondary compounds — esters, higher alcohols, aldehydes, glycerol, organic acids — that collectively create the aromatic profile of the wine. Change the yeast, change the wine.

**Wild vs. commercial yeast** is the big debate. Wild (indigenous) yeast — the community of microorganisms living on grape skins and in the winery — includes dozens of species that work in sequence. Non-Saccharomyces yeasts like *Hanseniaspora* and *Torulaspora* kick off fermentation, producing fruity esters and floral compounds. As alcohol rises above 4-6%, they die off and *Saccharomyces cerevisiae* takes over to finish the job.

Winemakers who use wild yeast argue it adds complexity and a sense of place — 'microbial terroir.' Research from New Zealand and Burgundy has shown that indigenous yeast populations differ measurably between vineyards, potentially contributing to site-specific character. The downside: wild fermentations are slower, less predictable, and occasionally go wrong (stuck fermentations, off-flavours).

Commercial yeast strains — freeze-dried cultures of selected *S. cerevisiae* — offer reliability and targeted flavour profiles. Strain EC1118 is a workhorse for sparkling wine. D47 enhances tropical fruit in Chardonnay. BM45 is prized for its ability to extract colour in red wine. They're like choosing a specific paintbrush for a specific effect.

One special case: *Brettanomyces* (Brett) is a spoilage yeast that produces barnyard, leather, and medicinal aromas. In small amounts, some people find it adds complexity (especially in traditional Rhône and Barossa wines). In large amounts, it's a fault. It's the most controversial microbe in the wine world.