The Dramatic Effects of Yeast in Winemaking

Yeast may not be the sexiest part of winemaking (surprising, right?), but it’s arguably the most important. Soil, oak, and blending all matter, but without yeast, you don’t get wine.

What Is Yeast?

Yeast is everywhere — on our skin, in our guts, on leaves, in the soil. The strain winemakers care about is called Saccharomyces, which literally means “sugar fungus.” Its job is simple: it eats grape sugar and produces alcohol and carbon dioxide when oxygen is scarce.

That carbon dioxide is why winemakers have to be very careful when handling tanks of fermenting juice. Overexposure to CO₂ has, more than once, led to unfortunate accidents while crushing grapes, stirring lees, or doing pumpovers.

Here’s what these microscopic little buggers look like under a microscope.

Wild versus Controlled Fermentation

Winemakers generally take one of two approaches: use wild, ambient yeasts — whatever is on the grapes or floating in the air — or lab-cultured strains. Is one better? That’s the million-dollar question.

Wild Fermentation

Historically, wild (or spontaneous) fermentation was the only option. Today, it’s still favored by many small or natural producers. Wild yeast clings to grapes, winery equipment, basically anything it can find. After crushing the grapes, winemakers just let nature take its course — hence the “spontaneous” label.

Fans of this approach argue it adds complexity, roundness, and a truer expression of terroir. But it comes with risk. Wild fermentation takes longer, requires more attention, and more things can go wrong. And, contrary to popular belief, it may not be as wild as people think.

Punchdown in progress on a red wine fermentation. 

With so much commercial yeast available, cross pollination of yeast is pretty much a given. Even if a winery has never used commercial yeast, chances are that some dominant strain of lab-cultured yeast finds its way into the tank where it proliferates and eventually takes over... maybe bees dropped it off from another winery or it landed on the grapes from a strong wind.

“Regardless of which yeast started the fermentation—indigenous or otherwise—a dominant commercial strain took over during the process, essentially wiping out any other forms of yeast that might have been present.” (source)

Controlled Fermentation

Controlled fermentation is the more common approach, and really the only practical option for larger wineries that need consistency across thousands of bottles. These producers use cultured, lab-grown yeast strains.

These strains are fast, reliable, and easier to manage. They tolerate temperature swings, won’t stall mid-ferment, and can consume nearly all the sugar, even at high alcohol levels. Winemakers can even choose specific strains to highlight certain flavors or textures. One might bring out the floral aromatics in Riesling or Gewürztraminer, while another enhances red berry and graham cracker notes in some reds.

Wild fermentation may be trendy and “natural,” but controlled fermentation is predictable. Wines made this way are consistent and dependable, though some argue they can be slightly less complex.

Next time you’re cruising the wine aisle, try a wild-fermented wine and see how it compares. Sometimes the little surprises from spontaneous yeast are exactly what make wine so magical.