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82 WINES&VINES April 2016 WINEMAKING WINE EAST Spontaneous Fermentations A case study in the Finger Lakes explores where 'wild' yeast comes from By Anna Katharine Mansfield and Camila Tahim KEY POINTS Sources for the yeast and bacteria for spontaneous fermentations can come from the vineyard, from microflora resident in the winery, or from equipment, bins or insects. Because the mix of microorganisms in spontaneous fermentations is not known, it is difficult for winemakers to manage the nutritional requirements. The Cornell Enology Extension Lab tested the YAN requirements and microflora in five spontaneous fermentations of Riesling grapes at two Finger Lakes wineries. T he complex microbial reac- tions that transform grape juice into wine involve a se- quential evolution of different yeast species. Since the first (likely accidental) fermenta- tion of grape juice, winemakers have relied on whatever yeast happened along to do the work of sugar-to-alcohol conversion. Though Dr. Hermann Müller-Thurgau first used a pure yeast culture as an inoculate in the 1890s, it wasn't until the 1970s that commercial cultures were widely available. Winemakers turned to these products to increase predictability in fermentation speed and wine quality, especially in regions with a limited history of wine production. More recently, the notion that wine should be terroir-driven, and that commer- cial yeast somehow obscures terroir effects, has prompted a resurgence of spontaneous (or "wild") fermentations in New World wine regions. This trend seems to be driven by the perception that uninoculated fer- mentations produce wines of higher quality, or at least more sensory complexity. Grow- ing consumer interest also has prompted new research into spontaneous fermenta- tion; consequently, advances in microbial and sensory methods are shedding new light on an age-old processing method. Microbial ecology One question taunting winemakers and researchers alike relates to origin. Namely, where do the yeast and bacteria that bring about spontaneous fermentation come from? There are three potential sources: Organisms can originate in the vineyard, exist as resident winery microflora and be transferred through the movement of pick- ing bins and equipment, and even move about via insects like social wasps or fruit flies. The composition of the microbial community at each source is influenced by many factors. On grape berry surfaces, populations are impacted by physical dam- age to the fruit, disease pressure, rainfall and use of pesticides. In the winery, prac- tices like sulfur dioxide use, cleaning and sanitization programs, and degree of juice clarification further modify the active play- ers in spontaneous fermentations. Yeast in the vineyard Saccharomyces cerevisiae, the yeast that almost universally completes wine fermen- The authors sampled this New York Riesling for microflora.