Yeast is an unsung hero. It hangs around, invisible to the eye (40,000 of them can fit on the head of a pin!) and then after we crush grapes, it goes to work turning their sugars into alcohol. There would be no wine without yeasts. In fact, there would be no alcohol at all.
Winemakers use either the ambient (naturally present) or cultivated yeasts when making wine. The yeasts take all the natural sugars in the grapes, convert them to alcohol, and in the process give off CO2 and a lot of heat. The yeasts continue to work until they are either stopped by the winemaker (usually by shocking the wine with a dose of sulphur dioxide or by cooling the wine down to a temperature where the yeasts cannot survive, thus stopping the conversion process and stabilizing the wine) or they actually kill themselves off: yeasts cannot live in an environment of over 16% alcohol, so once the wine reaches this level, the alcohol actually kills the yeast which created it. I think any parent would tell you that this is simply a metaphor for having children…
Assuming that the yeasts were not killed off by high alcohol levels or stabilization, yeasts continue their job even after they have converted a wine’s sugars into alcohol. In the barrel, they interact with the oak itself, sometimes even absorbing some of the harsher tannin flavors that a new oak barrel can impart on wine. Their interactions add even more complexity to the wine.
After the yeasts have done their job, they still remain in the wine as dead particles. They must be removed in order for the winemaker to create the final, particle-free product; however, some winemakers actually leave these dead yeasts (called the lees) in the barrels for four months to a year because it gives the wine a nice full mouth feel. On some bottles of wine you will see the phrase “sur lie”, which translate from French as “on the lees”, meaning that the winemaker chose to leave these yeasts in with the wine after they expired to impart that fleshy flavor into the wine.
Thank you, little yeasts, for your undying love of sugar and for the end result of your sweet tooth!