Yeast produce varietal character?

“It is the winemaking which reveals the aroma hidden in the fruit. The wine tastes more of fruits than the grape does.” This quote from Peynaud was used in a research article in the current edition of AJEV (57:1; 2006) that investigated the impact of yeast on varietal flavor of Sauvignon blanc. It is publications such as these that at the same time allow me to get my scientific fix (which more and more I realize I need) AND help me understand why we marvel over wine. I

think fondly of working in New Zealand with the seemingly innocuous SB fruit that when fermenting became a showy, sweet mixture of boxwood, passionfruit, and guava. The 4 sulfur containing compounds responsible for these aromas are locked away in the grape, odorless, waiting to be unveiled by the fermentation process. The data clearly demonstrate that the varietal character of SB that I have come to adore is almost exclusively released by the catabolic activity of yeast. (Incidentally, the odoriferous forms of some of these have been found in guava, passion fruit, and boxwood). Not only that, but the strain of yeast can make a significant difference in the amount of character that develops. This makes me think differently about the yeast catalogues we receive which always have a nice little blurb informing you that this yeast does that, and that yeast does this. Ok, but here we have well documented evidence not just the word of someone trying to push a product.

Interestingly, natural strains from Sancerre were isolated and some found to be Saccharomyces bayanus var uvarum, and not S. cerevisiea. Unfortunately S. bayanus often produces two other compounds that can nullify the sensory impact of the delicious SB character. What does this mean for terroir, wine quality? Aren’t we told that maintaining varietal character is a key component to a quality wine? At the same time I have read that true wines of terroir are made from indigenous yeast. What is a winemaker to do? Enhance the character that most people look for and associate with SB, or let his wines ferment with an unknown indigenous yeast that may or may not ‘reveal’ the true varietal character? If the soil and the climate are perfect for SB, but your cellar doesn’t contain an inoculum that produces significant SB character, are we to conclude you’re in the wrong terroir? All those compounds responsible for the aroma of passion fruit, boxwood, guava, grapefruit are just sitting in those grapes waiting to be revealed, but if you don’t have the right strain you don’t get the aromas.

I think this represents such a great example of how amazing wine is. We love SB for characters that the grape produces, but that we can only appreciate because it is a fermented beverage. One final note: although the yeast are required to release these compounds, the amount of precursor existing in the grape correlates with the amount revealed in the wine. Additionally of the 4 compounds explored in the study, presumably your climate, soil, and vineyard management could impact the concentration of each differently.