Growing & Winemaking

 

Role of Yeast in Winemaking

January 2015
 
by Paul Franson
 
 
Sampling yeast strains
 
Attendees sampled wines inoculated with both wild and cultivated yeast strains.

In an effort to find what microbes might be affecting commercial wineries, professor David Mills from the University of California, Davis, swabbed locations all over the UC Davis research winery before and after harvest. His findings included a large number of microorganisms—some unfamiliar to most winemakers, and others that are identified with toxic microbes (though not the strains found).

Mills revealed his discovery during a session about “wild” yeast strains at Rootstock, a Napa Valley Grapegrowers conference and exhibition held in Napa, Calif. (See “Napa Grapegrowers Consolidate Events” on page 162.)

The session also examined the role of what could be called feral yeasts (cultured yeasts that have gone wild).Researchers at the UC Davis research winery found Saccharomyces yeast around the garage-sized door used to bring in grapes—even before there were grapes in the winery.

Mills also found Brettanomyces in a stainless tank. It turns out the tank had been used for work involving that yeast a year before, and it had been thoroughly cleaned after use.

The researchers are trying to determine how the microbes got into and moved around the winery. Some possibilities include humans, equipment and insects, notably the fruit fly Drosophilia. The latter turned out to mostly spread Saccharomyces and other yeasts, not acetic acid and lactic acid bacteria as commonly thought.

    KEY POINTS
     

     
  • UC Davis researchers discovered microbes including Brettanomyces in locations presumed clean.
     
  • The process for identifying strains of yeast has become quicker and cheaper in recent years.
     
  • Controlling pH, temperature and oxygen level can help steer which yeast strain takes over fermentation.
     

Quick and cheap identification
Mills noted that traditional methods of identifying yeasts and other microbes are tedious and time consuming, but recent developments using new technology including parallel computing have made enormous progress in the past decade. He said that it’s even outpacing computer storage, a notoriously fast-moving field. “We’re having trouble storing the results,” he complained.

Mills added that in his early days, it took $1.6 million to sequence lactic acid bacteria. Now he can map 50 microbes for less than $1,000, and much more quickly. This allows researchers to take more samples, and he noted that adequate work could require more than 300 samples.

The research also led them to various regions of the world, where they found different yeast and bacteria profiles. Even different regions of the relatively small Napa Valley exhibit diversity, leading many researchers to speculate whether this might have something to do with the realities of terroir, the concept that every winegrowing region produces a distinct wine.

Saccharomyces domestication
Lucy Joseph, who manages the yeast collection for the UC Davis Department of Viticulture & Enology, discussed how Saccharomyces comes to dominate wine production.

Saccharomyces is fairly rare in sound berries in the vineyard, and it is found in about one of 1,000 berries tested. This is partly because the yeast is very sensitive to ultraviolet light. However, the concentration is much higher in damaged berries (about one in four berries tested).

The number of Saccharomyces cells on damaged berries is about 104 to 105 cfu/ml (colony forming units), but the total microbial counts in damaged berries is much higher: 106 to 107 cfu/ml.

The yeasts in vineyards seem to come primarily from trees that were previously in the vineyard or surrounding it, but Joseph says that the wild yeasts apparently haven’t been studied extensively.

Those yeasts found in wineries can get there from many sources including the grapes, surrounding environment, equipment (especially barrels) and, of course, introduced cultured yeasts. Wine yeasts originate primarily from Europe with other yeasts found throughout the world (like sake yeasts in Asia).

Only the strong survive
So why do Saccharomyces yeasts come to dominate wine fermentation?

It occurs partly due to natural selection favoring Saccharomyces like low pH, high sugar concentration, some nutrients like nitrogen, high phenolic content, low oxygen and alcohol concentration.

In addition, winemakers encourage Saccharomyces through raising temperature, adding potassium metabisulfite, lowering pH with tartaric acid and adding lysozyme and nutrients.

One result is that non-Saccharomyces yeasts start out strong but are discouraged by rising temperatures and alcohol levels (ideal conditions for Saccharomyces).

In general, high acid favors growth of yeasts early in fermentation, but most microbes (especially bacteria) are not tolerant of acid. Juice pH often increases during fermentation, which favors malolactic (ML) fermentation, but pH greater than 3.6 encourages spoilage by lactic acid bacteria.

Brettanomyces is more tolerant of low pH. It can tolerate a pH of 2, making it one of the few yeasts of concern to makers of Coca-Cola, which has a pH of about 2.8.

Adding SO2 inhibits the growth of spoilage bacteria and growth of wild yeasts including Brettanomyces. It also destroys thiamin and inhibits oxidation.

Of course, adding cultured yeast encourages it to take over, but Joseph said that doesn’t ensure the added yeast will finish the fermentation. Other strains may prevail.

Winemakers can take many steps to adjust temperature (including the use of cold soaks, tank temperature controls and pump overs) and adjust oxygen (with pump overs and rack and return, punch down, stirring or aeration and micro-oxygenation).

Joseph said that cold soaks (15°-20° C) encourage growth of non-Saccharomyces yeast early in fermentation, while cool temperatures during fermentation inhibit growth of spoilage bacteria and some yeasts. Warm temperatures can favor ML bacteria, and cool temperatures discourage spoilage organisms.

She added that strict aerobic organisms like filamentous fungi and acetic acid bacteria cannot compete in low-oxygen conditions, but anaerobic and facultative anaerobes like Saccharomyces and ML bacteria do grow.

Wild and domestic yeasts
Finally, Michael Silacci of Opus One described trials he conducted with yeasts isolated from various parts of the vineyard away from its winery by Anaïs Houlette-Cassou. He compared them to wolves (purely wild), coyotes (wild but venturing into domestic territory) and dogs (purely domestic).

Attendees were offered samples of Cabernet Sauvignon inoculated with these yeasts plus a control (prise de mousse yeast) for 2013 and 2014. Subtle differences were noted, less so in the older wines. Silacci suggested that the differences tend to diminish over time.

 
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