Listen to the podcast episode here: https://podcasts.apple.com/us/podcast/after-wine-school/id1787586745

Open a bottle and the room changes. Pear. Brioche. Wild strawberry. Sometimes a hint of leather or something pleasantly earthy. Those aromas aren’t just “from the grape.” They’re built—quietly, relentlessly—by yeast.

Yeast are single-celled fungi that digest grape sugars and convert them into alcohol and carbon dioxide. That’s the job description. But the reason wine smells and feels the way it does has as much to do with what yeast also make along the way: esters, acids, polysaccharides, sulfur compounds, and a swirl of other molecules that shape fruit notes, texture, and finish.

The Workhorse (and Its Friends)

The main player in wine is Saccharomyces cerevisiae—“Sac” in winery shorthand. Think of Sac as the reliable pro: alcohol-tolerant, clean, and predictable when you want it to be. Commercial strains have been isolated for decades for specific jobs—fermenting cool whites without fuss, pushing through high-sugar musts, or coaxing out particular fruit notes in Grenache, Zinfandel, or Napa Cabernet.

But Sac never works alone. In the early hours and days, non-Saccharomyces yeasts—ambient species living on grape skins and around the cellar—often kick off fermentation. They bring wild edges: pineapple here, a touch of spice there, sometimes a left-field note you didn’t ask for. As alcohol climbs toward ~5%, most non-Sac yeasts tap out; Sac, which tolerates 15–16% alcohol, takes the baton and runs to dryness. The handoff matters. It can add complexity. It can also add chaos if you’re not steering.

Yeast Make Fruit—Not Just Alcohol

Those peach, apricot, pear, and banana notes? They largely come from esters—aromatic compounds created when yeast metabolize sugars and acids. Strain choice, nutrient availability, and temperature all change which esters form, how intense they are, and how long they stick around.

Yeast shape mouthfeel, too. During and after fermentation, the breakdown of yeast cells (autolysis) releases mannoproteins and polysaccharides that soften edges, build mid-palate weight, and lengthen the finish. That’s a big reason wines aged sur lie (on the lees) feel creamier and more complete.

Key terms

• Lees: Spent yeast and grape solids that settle after fermentation.
• Autolysis: Yeast self-decomposition; source of bready, nutty, creamy notes.
• Esters: Fruity aroma compounds (pear, peach, banana, etc.).

When Yeast Live On Top: Sherry and Vin Jaune

Yeast can also shape wine after fermentation by forming a veil on the surface. In biologically aged sherry and vin jaune, a living layer of yeast protects wine from oxidation while consuming oxygen and transforming flavor. The result: distinct notes of almond, walnut, dough, and saline complexity you will not find elsewhere. Different organism; different job. Same headline: yeast drive style.

Champagne’s Signature: Time on Lees

In traditional-method sparkling wines, the famous toast, brioche, and nuttiness are autolytic aromas from extended time on yeast after bottle fermentation. The longer the rest, the deeper the savory complexity and creamier the texture. Yeast give you bubbles and the bakery.

Temperature: The Hidden Helm

Fermentation is a living ocean—circulating, exothermic, and unruly. Temperature is the helm.

• Cool, long ferments (especially for whites) protect delicate esters and build nuance.
• Warmer peaks (often above ~80°F / 27°C in reds) can crack open glycosidically bound precursors, unlocking deeper fruit and spice—if controlled.
• Overheats kill yeast and strip aroma. Over-cooling stalls ferments. The art is anticipating the curve hours ahead and nudging gently, not slamming the brakes.

Good winemakers also strategically stress yeast—sparing nutrients, adjusting oxygen, or timing cap management—to encourage complexity. Go too far and you invite cabbage or rotten-egg sulfides. Done right, you get savory layers that play under ripe fruit.

Stuck Fermentations: Everyone’s Nightmare

A stuck fermentation stops short with sugar still in the tank. It’s almost always avoidable, and it’s always painful: loss of aromatic freshness, microbial risk, and a rescue plan that rarely equals the wine you set out to make. You can restart with a robust strain and a carefully built starter, but you’re fixing, not crafting. The better path is prevention—sound fruit, stable temps, appropriate nutrition, and patience.

Native, Ambient, “Wild”: What Those Labels Really Mean

Text sheets increasingly tout “native,” “ambient,” or “wild” yeast. Practically, that means no commercial inoculation: the fermentation is initiated by yeasts on the grapes and in the cellar’s microbiome (yes, wineries become their own biodomes over time). Two clarifications help:

1. Alcohol reality. If a wine finishes at 13–14% without inoculation, Saccharomyces still did the heavy lifting; non-Sac yeasts simply can’t survive that alcohol level.
2. Modern nuance. Many wineries quietly isolate and propagate their favorite ambient strains—then pitch them on future lots. It’s “wild,” but it’s also controlled. There are even commercial non-Sac preparations used at the start to gain aromatic benefits while reducing risk.

Brettanomyces: Complexity or Contaminant?

Brettanomyces (often shortened to “Brett”) is an unconventional yeast that produces aromas ranging from leather and clove to barnyard and, at high levels, bandage or ashtray. In tiny doses, some traditional regions accept it as part of style. In most cellars, it’s treated as a fault: difficult to eradicate, quick to spread, and capable of hijacking house character. The line between “intrigue” and “ruin” is thin.

How Yeast Read in the Glass: A Few Styles

• Muscadet sur lie — Lees aging softens a bright, briny white and adds subtle bread dough and nutty notes.
• Oregon Pinot Noir (native ferments) — Often shows wild strawberry, red cherry, and a gentle humus/forest-floor edge from early non-Sac contribution, layered under clean red fruit as Sac finishes the job.
• Traditional Rioja (long aging) — Savory leather and cured-meat tones can develop with time and oxygen exposure; oak regime and cellar culture play major roles.
• Chablis — Precision often comes from cool ferments with neutral/inoculated yeasts, stainless or old oak, and minimal interference to spotlight terroir’s mineral line.

Yeast don’t just make alcohol. They write the wine’s accent—fruit register, texture, finish—and they co-author terroir with site and cellar. Strain choice matters. So does temperature, nutrition, oxygen, and time on lees. “Native” can be thrilling, if managed. “Commercial” can be transparent, if chosen well. Either way, great wines come from a thoughtful relationship with the smallest workers in the room.