The character of a wine is very much determined by the type of yeast used as well as the speed of fermentation. Without yeast there would be no ferment, and although most wines are made by using an added dried yeast, all fruits have their own natural yeast. But why is yeast so important in wine making?
Yeasts are mostly single celled microorganisms, of which the species Saccharomyces cerevisiae has been used in the fermenting of alcoholic drinks for millennia. During fermentation, these fungii are responsible for converting natural sugar in the fruit into carbon dioxide and alcohol. The art of winemaking really comes down to the art of managing and controlling the yeast.
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 off the natural yeast with a controlled amount of sulfur dioxide (Campden tablets in home winemaking). Next, the yeast is 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 consumes more and more of the sugar, the temperature will start to rise sharply. It is now that the skills of the winemaker come in to play as he or she must intervene and try to control the speed of the ferment, especially for the production of delicate white wines. The slower the ferment goes on, the longer it lasts, which usually results in more complex flavors and characters showing through. Should a wine ferment quickly at higher temperatures, much of these flavors will be removed via the carbon dioxide, and the yeast will burn itself out sooner.
For the fermentation of many white wines, the ideal is to cool the vessel at intervals, keeping the temperature stable and controlled enabling a prolonged ferment. This is done either by cooling the outside surface of the vessel or by passing coolant through internal pipes built inside the vessel.
It is important to taste the wine regularly during fermentation to ensure that no nasty off-flavors are being produced. Once the yeast has completed its task, it dies and sinks to the bottom of the vessel and forms the 'lees' (dead yeast cells). By leaving wine on the lees for some time can enhance flavors dramatically, while at the same time it can also impart hydrogen sulphide or bad eggs flavors into the wine. This is a winemakers balancing act, and negligence at this stage is the cause of a many a poor wine.
There are several types of yeast, 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 yet more are specifically for restarting stuck or difficult fermentations.
Yeast is so important in wine making that large choices of many strains are now available to winemakers, and great care is taken to select the right one for a particular style of wine. Next time you drink a glass of wine, remember it was the yeast that helped to give the wine it's defined character.