Wine Fermentation Process
In the crushing, today's winemaker usually adds minute amounts of a
solution of sulphur dioxide to kill the natural yeasts, which occur as a
fine white bloom on the grape. Some of these are what the winemaker
calls "wild" yeasts, enzymes that will contribute to fermentation, but,
by their nature, will introduce unwanted reactions and flavors. In place
of the naturally occurring yeasts, the winemaker adds a cultured yeast,
grown in the laboratory and capable of withstanding small amounts of
sulphur and large amounts of alcohol which kill the natural wild yeasts.
He finds no need to do this with every vat of juice, for the yeasts will
travel over the short distances from one vat to another, inoculating
each vat full of juice on the way. Perhaps once a week throughout
vintage he will need to add a new inoculation of yeast culture. Cultured
wine yeasts today are available from the laboratories of the Australian
Wine Institute, at Adelaide, in South Australia, including the
remarkable flower or flor yeasts used in the making of good quality
sherries. These can be seen in many of the larger wineries floating on
top of a special glass ended barrel of maturing wine.
In the process of becoming wine, the juice of the grape ferments, that
is it undergoes a chemical change. Grape juice consists of water (the
greatest part), sugar and acids (mainly tartaric, acetic, malic and
lactic). Fundamentally the chemical change is simple, although indeed
miraculous, even to the chemist and micro-biologist. While the juice is
inside the unbroken skin of the grape it is protected from yeasts (but
not from all airborne bacteria), those tiny but so vitally important
enzymes that are one of the real mysteries of nature. But once the skin
is broken the yeasts are able to enter the juice and go to work on the
sugar; in fact, they feed on it and, in the process of feeding, convert
the sugar into carbon dioxide and alcohol. This is the process of
fermentation. Anyone who has watched a vat of grape juice fermenting in
a winery will have seen with fascination the vigorous bubbling liquid,
and will have been told that this is nothing more than escaping carbon
dioxide.
Fermentation usually takes about six days. When it is complete, that is
when the yeasts have used up all the sugar in the juice (or must as
fermenting grape juice is called), the new wine is taken to the
winepress to separate the liquid from the mart. Marc is the winemaker's
term for the skins and other solids that are left in the winepress after
pressing. Pressing is a very important stage in wine making. It is
important economically, for the wine-maker must recover the maximum
quantity of liquid from the fermented must. It is important
qualitatively, for he must exercise keen judgment on how he combines the
first or "free" run of the press with the second and third "pressings".
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