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There's been an increasing number of well-designed studies looking at the question of 'wild' ferments, and it seems that there are really two questions that we need to ask: First, whether the predominant yeast in a spontaneous fermentation comes from the winery or the vineyard, and second, whether the yeast in the vineyard is itself native to the region. I think most researchers agree on the answer to the first question; depending on fruit handling and quality, the bulk of the alcoholic conversion will be performed by some strain or strains of Saccharomyces, either that resident in the winery from previous use of commercial yeast, or a blend of any resident yeast and yeasts present in the vineyard. It seems that the biggest question, however, is where the yeast in the vineyard originates. Genotyping has demonstrated that, as in the wineries themselves, vineyards adjoining facilities that use commercial yeast show high populations of strains identical to or closely related to those used in the winery. In theory, this means that fruit harvested in such a vineyard could be taken to a brand new facility, allowed to ferment without inoculation, and STILL produce a fermentation dominated by commercial yeast strains or their close relatives. It's important to note that yeast has relatively unstable genetics and reproduces rapidly, so the genetics (and subsequent fermentation performance) of yeast colonizing a winery may change over time. The strain that starts a fermentation 5 years after a winery stops using commercial yeast may subsequently be very different from the original commercial strain. The same would hold true with yeast migrating from a winery out into the vineyard, and may include genetic transfer from the dominant native Saccharomyces in the region. The fact that yeast are so genetically mutable leads to an even bigger question, though. The high tolerance to alcohol that we prize in wine yeast is only an evolutionary advantage in environments with high alcohol. Such environments are rare in nature but relatively frequent around humans; the apocryphal 'discovery of wine' story involving stored grapes leaking juice turning into the 'drink of the gods' is a classic example. It seems probable, then, that the yeast strains prized for wine (and beer, and bread, and other human activities) didn't exist on their own in the wild, but evolved overtime in the human-created environments where they out-competed their wimpier cousins. Over time, descendents of these strains were isolated and commercialized, and became the 'unnatural' yeasts used today. This theory is supported by the fact that, outside of human activities, most Saccharomyces species in the wild are found on oak or beech trees, and when isolated don't produce very tasty wine. A recent study (by Hyma and Fay, Molecular Ecology, 22(11)2917–2930) showed that there is some genetic transfer between the oak tree and vineyard strains in North America, and even went so far as to isolate several oak tree strains and use them for wine fermentations... with decided unsatisfactory sensory results. So- if wine yeast co-evolved with the winemaking industry, and are a sort of mutant Frankenyeast of our own making... how does this impact the argument about 'natural' fermentation and microbial terroir? It will be interesting to follow this discussion as yeast genotyping progresses.
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Aug 22, 2013