New Research on Role of Yeast in Winemaking

Wild, feral and cultured yeasts discussed at Rootstock event in Napa

by Paul Franson
Attendees listen during a trial and comparison tasting session at the Rootstock event Thursday in Napa. Source: CeliaCarey.com Film & Photography
Napa, Calif.—The Napa Valley Grapegrowers on Thursday inaugurated the first Rootstock, an annual conference and exhibit in Napa that replaced two biennial events.

It was well attended, with a sell-out group of trade exhibitors as well as attendees, and both the exhibit halls and seminars were crowded with growers, winemakers and staff seeking to learn the latest information about making quality wine.

Among the most interesting sessions was one about “wild” yeast strains that also examined the role of what could be called feral yeasts, cultured yeasts that have gone wild.

It featured David Mills and Lucy Joseph from the University of California, Davis, plus winemaker Michael Silacci of Opus One, who offered samples of 10 wines fermented with various yeasts.

Mills started the discussion with a look at how microbes impact wine production and quality.

He 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 can require more than 300 samples.

In his search to find what microbes might be affecting wine, and with new powers for identification, his researchers swabbed locations all over the UC Davis research winery before and after harvest.

They found a large number of microorganisms including some unfamiliar to most winemakers and even some that are identified with toxic microbes (though not the strains found).

Most interesting, they found Saccharomyces yeast around the garage-sized door used to bring in grapes even before there were grapes in the winery.

He also found Brettanomyces in a stainless tank; it turns out the tank had been used for work involving that yeast a year before and the tank had been thoroughly cleaned.

The researchers are trying to determine how the microbes got into and moved around the winery. Some possibilities were 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.

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.

They performed research to try to correlate the microbial mix with wine characteristics and found differences in the finished wines, but they haven’t reported definitive conclusions.

Saccharomyces domestication
In the second section of the seminar, Lucy Joseph discussed how Saccharomyces comes to dominate wine production.

Saccharomyces is fairly rare in sound berries in the vineyard, and it is found in about 1 of 1,000 tested. This is partly because the yeast is very sensitive to ultraviolet light. However, the concentration is much higher in damaged berries, about 1 in 4 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 and 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.

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 yeasts other than Saccharomyces start out strong, but are discouraged by rising temperatures and alcohol levels, which Saccharomyces encourages and its population expands.
During fermentation, non-Saccharomyces yeasts (thin black dashed lines decline as Saccharomyces cerevisiae (thin black solid lines) grows as ethanol level and temperature rise. Population size is shown in colony forming units, cfu, in four separate barrels over 20 days of ferment. Source: Matthew R. Goddard
In general, high acid favors growth of yeasts early in fermentation but most microbes, especially bacteria, are not tolerant of acid. pH often increases during fermentation, which favors malolactic fermentation but pH greater than 3.6 encourages spoilage 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 that yeast to take over,  but interestingly, Joseph said that that doesn’t ensure the added yeast will finish the fermentation. Other strains may prevail.

Winemakers can take many steps to adjust temperature including cold soaks, tank temperature 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 (150-200C) 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 during storage.

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

She concluded that the winemaker can take many steps to tailor the microbes in wine, and hence affect flavor.

Wild and domestic yeasts in practice
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), coyote (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 drop over time.

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