Wine Yeast

“This narrative of terroir yeast associated with a specific vineyard or estate has always existed,” says Isabelle Masneuf-Pomarede, professor of viticulture and enology at Bordeaux Sciences Agros's Institute of Agricultural Sciences. Masneuf-Pomarede has been involved in yeast research for about 20 years and argues that both winemakers and drinkers tend to glorify the use of indigenous cultures as opposed to commercially available alternatives for the former’s ability to express terroir and bring unique characteristics to wine. Indeed, while a diverse array of yeast strains and bacteria are commonly found in nature, beliefs such as the contribution of indigenous cultures to a wine’s organoleptic complexity and their use as a means to express a vineyard’s unique character encounter considerable academic skepticism. Other wine professionals maintain that indigenous yeasts bring unique characteristics to wine, even if those strains are difficult to identify. Commercial yeasts are unique strains that have been isolated from wild and indigenous cultures because of specific phenotypic traits and properties of interest, then propagated on a large scale. Their effect on the vinification process and a wine’s final flavor profile may be somewhat predictable, but they ensure a hiccup-free fermentation and a final product free from undesirable taints. Indigenous or “wild” yeasts, on the other hand, might cause stuck or sluggish fermentations, off-aromas or, in worst-case scenarios, spoilage. However, their supporters believe that, when handled with care, they translate into greater complexity and in a truer expression of each vineyard’s unique attributes. “The expression of terroir and the connection between the vineyard and the winery are strengthened by utilizing native ferments,” says Rebekah Wineburg, enologist at Napa Valley’s Quintessa. “And the fermentation microbiology is more complex with native ferments; different strains will dominate the fermentation at various stages, resulting in a more complex wine.” Masneuf-Pomarede is skeptical that such yeasts are necessarily unique to a given place. “It is not actually demonstrated that winemakers can ever claim that any given yeast is associated with their winery,” she says, highlighting that, at any given time, the same strain or selection may be found across a number of adjacent wineries. A large-scale survey of vineyard-associated saccharomyces cerevisiae strains published in FEMS Yeast Research found that no strains could be associated with a given ecosystem, while a more recent paper wonders: “What would be a terroir yeast? Does this suppose that a yeast would be dominant in a wine area and present only in this area, and that this same yeast would be responsible for spontaneous alcoholic fermentation during each vintage?” The article points out a number of elements that contradict the concept of terroir yeast, concluding that “there is no strong evidence that microbes contribute to the so-called ‘terroir effect’.” Masneuf-Pomarede explains that yeast strains do not permanently reside on a single plot of land. Instead they regularly travel long distances. “A yeast or group of yeast strains in any given vineyard can easily be the same found in your neighbor’s plot,” she says. “Also, yeast strains cannot represent the terroir of your vineyard because they tend to vary from one year to another. They can’t be part of the terroir because they are not a permanent environmental factor. It’s not like soil or microclimate.” The complexity debate By inoculating commercially available yeast, winemakers ensure that a high population level of one individual strain will dominate the vinification process, while a number of strains might carry out an uninoculated fermentation. “The yeasts that are present on the skins when the fruit is harvested [starts] the fermentation,” says Ames Morison, cofounder of Medlock Ames winery in California’s Alexander Valley. “As these yeasts begin their work, they change the environment of the juice to a slightly less sweet and slightly alcoholic medium. These changed conditions are no longer optimal for the initial yeast population, and other types of yeast floating around the cellar begin to take over the fermentation. As more sugar is converted to alcohol other strains begin to thrive. Each does part of the work and passes the baton to the next one.” A multiple-strain fermentation, however, is never guaranteed, nor will an indigenous selection necessarily consist of different yeasts in the first place. “You may have a dominant genome that will conduct the entire alcoholic fermentation, in which case that would be the same as what a commercial yeast does,” says Masneuf-Pomarede, pointing out that wild cultures can never guarantee great complexity, regardless of their diversity. “The complexity of a wine is actually quite difficult to quantify to begin with, but there is no evidence that if there are distinct strains of saccharomyces conducting the fermentation, those will lead to a more complex result. Different cultures simply make different wines.” Morison believes that winemakers can better achieve greater organoleptic complexity by blending different wines. He ferments musts from the same lot of grapes in multiple vessels, each inoculated with a distinct commercial or native yeast selection. “This gives us multiple expressions of the same wine and adds more nuances to the blend,” he says. Whether looking for uniqueness or for enhanced flavor, winemakers must ensure an unproblematic fermentation, free from contamination of Brettanomyces genomes. “That’s what can really lead to the standardization of flavor, by producing high levels of volatile phenols,” says Masneuf-Pomarede. “For instance, if you get Brett [Brettanomyces] in a Cabernet Sauvignon, you won’t be able to tell whether it comes from Saint-Emilion, Pessac-Léognan, Australia or California. There are many strains of Brett, but high phenols really are a signature of this species and can have a masking effect on the wine’s fruitiness, the typicity of the grapes, and finally, the terroir.”