Taming a Wild Wine Fermentation

Experts at UC Davis discuss wild or native fermentation, other yeast issues

by Andrew Adams

Kloeckera, seen here, is a wild yeast that can initiate a fermentation at colder temperatures.

Davis, Calif.—You can definitely fill a tank with must or juice, cross your fingers and hope the “native” yeast produce a successful fermentation that expresses true vineyard terroir and varietal characteristics.

But if you’re not careful—and don’t know what’s really going on in the vineyard—your fermentation could instead by marred by Pediococcus and acetic acid bacteria, and the Saccharomyces cerevisiae could stall out without finishing the job.

 At a winemaking session held recently at the University of California, Davis, yeast expert Dr. Linda Bisson provided an outline of the key factors that ensure a successful native fermentation. She said such fermentations are defined by relying on the microbial flora of grapes and wineries to conduct fermentation without any deliberate inoculation of commercial strains.

The day also included a few sessions and tastings exploring the relatively new process of flash détente, in which grapes are heated and run through a vacuum chamber to remove unwanted flavor compounds and achieve specific stylistic goals. (For more on those sessions visit the MOG blog.)

Many winemakers allow “native” microbes to initiate a fermentation and then finish with an inoculated strain or inoculate with a malolactic bacteria at the onset to reduce the chance a spoilage bacteria could get established, and let ambient yeast finish the primary fermentation at their own pace.

Such fermentations can add complexity to the finished wines while also resulting in slower primary fermentations with less volatilization of aromas. On the other hand, a native fermentation that goes wrong can result in a stuck fermentation, sulfur taints, ethyl acetate and a loss of varietal character.

Factors to consider
Some of the most important variables in a native fermentation are the condition of the grapes, organisms on the fruit, time of harvest, processing conditions, juice/must adjustments, pH, sulfite additions and temperature.

Bisson said grape rot “amplifies” acetic acid bacteria and can be a host for mold. Material other than grapes can provide means for unwanted microbes to enter the fermentation, and clusters that are not evenly ripe or contain raisins will contain a mix of flora. Additional hang time also increases the amount of lactobacilli.

The organisms brought in on grapes may not always be those found in the vineyard. Bisson said wineries with chronic issues from bad dairy lactic bacterium are often located near dairies. Insects carry the bacteria from the dairy into the vineyard, where it gets on the grapes and eventually ends up in the winery.

Cooler temperatures can also help initiate a fermentation with native yeast, or at least give non-Saccharomyces yeast a chance to conduct some of the fermentation. “You can bias toward wild yeasts if you’re lower in temperature,” Bisson said. “Sometimes in a cold soak or cool fermentation it’s not just Saccharomcyes present; you’re getting the other organisms to grow.”

When adding nutrients, Bisson reminded the audience that all microorganisms could feast upon the added food source, not just the desired yeast. She said she’s been surprised by inquiries from winemakers who didn’t seem to realize the nutrients they added for yeast enabled unwanted bacteria to bloom. She quipped that it’s like laying out a tin of food for your cat and then being surprised when your dog gobbles the food up.

Bisson said it’s also crucial to get a microscope and a thorough understanding of what microorganisms you’re working with in the winery and vineyard. “I would never advise doing native without knowing what’s in there,” she said.

Winemakers’ perspectives
The session also included a panel in which winemakers discussed how they work with yeast. Neil Bernardi, vice president of winemaking for Duckhorn Wine Co., said he works with six winemakers while overseeing six brands and eight facilities. Bernardi expects to crush 10,000 tons in 2015.

Bernardi recalled that when he was the associate winemaker at Littorai Wines in Sebastopol, Calif., he was able to babysit long and slow native fermentations—but at Duckhorn that’s not an option. He said he wants to work with yeasts with predictable fermentation curves that produce little to no off-characteristics.

He said he uses different strains at different points during the harvest, and for different reasons. Lalvin RC-212 is a good strain for a quick Pinot Noir fermentation, while Assmanhausen has a long lag phase, making it better suited for one of the last tanks filled for the year.

Lalvin CY3079 is good for barrel-fermenting Chardonnay, while Enoferm M2 is good for pure aromatics and minerality. Using different strains with specific goals in mind is a good way to add complexity while still ensuring efficiency, simplifying logistics and maintaining consistency, he said.

Tom Stutz, winemaker at La Rochelle Winery in Kenwood, Calif. and the Livermore Valley, said he doesn’t use sulfur dioxide at the crush pad because the grapes he works with arrive in pristine condition, and he wants the native yeasts to start fermentation and provide some complexity. After cold soak, he said he adds a small amount of yeast at around 1 pound per 1,000 gallons, and he just sprinkles it over the top of fermentation bins without mixing. If any wine starts to smell a little funky, he will add some nutrients—but that’s it.

Once malolactic fermentation is complete, Stutz will make a 50 ppm of sulfur dioxide addition and just top regularly, eschewing any further sulfur additions. He joked that he tells other winemakers, “Don’t try this at home,” but his style has worked so far. Stutz said he recently checked a two-year-old bottle of wine, and found its free sulfur dioxide was at 9 or 10 ppm, and the total SO2 was at 40.

He said in his opinion, one of the biggest advances in ensuring wine quality during élevage is the humble silicone bung. The old redwood bungs could never achieve a proper seal, allowing for greater evaporation and possible oxidation.

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