Rising to the occasion: Friendly yeast plays key role in winemaking

For most wine drinkers, the role of yeast in winemaking can be whittled down to the basics of fermentation: Yeast eats the sugars in the grape juice and gives off ethanol, an alcohol that in limited quantities is safe to consume.

‘Nough said, open that bottle.

Last week, when discussing what are called “natural” wines, it was mentioned that some winemakers, in their desire not to add anything to the finished product, depend on naturally occurring yeasts on the grapes or in the winery to ferment their grapes.

Ripe grapes and other fruit have natural yeasts on their skin and as long as the skin isn’t broken, fermentation doesn’t happen.

Crushing the grapes at the winery releases the juice, which the yeast eats and fermentation begins. Some anti-additive winemakers seek spontaneous fermentation by simply piling whole berries in the fermentation tank, where the weight of the upper layers squeezes the lower ones.

They are betting there are enough yeasts either on the grapes or in the winery to take the juice all the way to the preferred level of dryness (a wine is completely dry when all the sugar has been converted to alcohol).

There are problems with that, said Stephen Menke, state enologist from Colorado State University and the Orchard Mesa Research Center, because not all yeasts are equal in their ability to start and complete a fermentation.

Menke said many native (or indigenous) yeasts are affected even by small amounts of the alcohol they produce and die before fermentation is finished.

If the yeasts die before all the sugar is consumed to the desired level of dryness, you get what’s called a “stuck” fermentation.

“What we call indigenous yeast bacteria are not very alcohol tolerant and may be involved in the earlier part of the fermentation process but not finish it,” Menke said.

As Menke noted, there are reasons other than alcohol levels for a stuck fermentation, including a lack of nitrogen or powdery mildew or even a killer yeast that takes over from the natural bugs but then topples.

If there is enough of the right yeast in the vats or winery to take over, you’re in luck. Many older wineries in Europe, where equipment has been used for generations, can be havens of yeast bugs, and have so much yeast on the walls and in the tanks there rarely are problems with stuck fermentations (that also contributes to a consistency in flavors but that’s for next time).

Not so in the Grand Valley and elsewhere where generally low humidity and relatively new (and clean) equipment don’t foster microbial growth.

But when fermentations stick, winemakers have an ace in the hole.

That ace is our (mostly) alcohol-tolerant and fermentation resistant yeast, Saccharomyces cerevisiae.

Archeologists tell us winemakers in Armenia and China may have relied on naturally occurring Saccharomyces cerevisiae at least 6,000 years ago because it has the key attributes needed for fermentation — a tolerance to high sugar, elevated acidity and high alcohol levels.

Writer, wine blogger (Wine-O-Scope) and scientist Erika Syzmanski, in a March 10, 2013, post on the online wine magazine Palate Press, said, “This is part of why ‘natural wine’ and ‘wild fermentation’ are slippery terms.”

“(W)inemaking practices have been influencing yeast survival for millennia,” Syzmanski writes. “No yeast that finds its way into a winemaker’s vat, in this age, has been untouched by human domestication.”

Syzmanksi also points out Saccharomyces cerevisiae is the only yeast to be named a state microbe (Oregon, 2013).

Winemakers today have hundreds of commercial Saccharomyces cerevisiae yeast options, most of them developed by a lab or a company to achieve specific results in the finished wine.

Finding the right yeast is a step toward finding that elusive balance in flavor and texture, Menke said.

“That dynamic balance is very hard to control, it’s always changing,” he said. “That’s why people play around with blending different grapes or experiment in using different types of oak or how much oak, and the kind of yeast they use.”


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