The character of a wine is largely determined by the type of yeast used, as well as the rate of fermentation. Without yeast there would be no ferment, and although most wines are made using an added dry yeast, all fruits have their own natural yeast. But why is yeast so important in winemaking?
Yeasts are mostly single-celled microorganisms, of which the species Saccharomyces cerevisiae has been used in the fermentation of alcoholic beverages for millennia. During fermentation, these fungi are responsible for converting the natural sugar in the fruit into carbon dioxide and alcohol. The art of winemaking really comes down to the art of yeast management and control.
Most winemakers choose to introduce a cultured yeast with a known specific character that they want in their wine. Before introducing the desired yeast, they have to kill the natural yeast with a controlled amount of sulfur dioxide (Campden tablets in home winemaking). The yeast is then brought to life by dissolving it in warm water, between 40-43°C, and allowing it to multiply for 15 minutes, before introducing it into the must (grape juice).
After inoculation, a fermentation will begin within a few days, and as the yeast consume more and more sugar, the temperature will begin to rise sharply. It is now that the skills of the winemaker come into play, who must intervene and try to control the speed of the ferment, especially for the production of delicate white wines. The slower the ferment progresses, the harder, which generally results in more complex flavors and characters. If a wine ferments quickly at higher temperatures, much of these flavors will be washed out through carbon dioxide and the yeast will burn off sooner.
For the fermentation of many white wines, the ideal is to cool the vessel at intervals, maintaining a stable and controlled temperature that allows a prolonged fermentation. This is done by cooling the outer surface of the vessel or by passing coolant through internal pipes built into the vessel.
It is important to taste the wine regularly during fermentation to ensure that off-flavours are not produced. Once the yeast has completed its task, it dies and sinks to the bottom of the container and forms the ‘lees’ (dead yeast cells). Leaving the wine on its lees for some time can enhance the flavors dramatically, while at the same time it can also impart hydrogen sulfide or rotten egg flavors to the wine. This is a balancing act for winemakers, and neglect at this stage is the cause of many poor wines.
There are several types of yeasts, some can tolerate high alcohol levels and are used in red wines, others are low foaming and ideal for barrel or low temperature fermentations. While even more are specifically for restarting stuck or difficult fermentations.
Yeast is so important in winemaking that winemakers now have wide choices of many strains at their disposal, and great care is taken to select the right one for a particular style of wine. The next time you drink a glass of wine, remember that it was the yeast that helped give the wine its distinct character.