All that is needed to turn grape juice into wine is the simple, all-natural process of fermentation. Fermentation is described as “the chemical change of sugar into alcohol and carbon dioxide gas brought about by yeasts” (micro-organisms which live (among other places) on grape skins). These micro-organisms need all tear in the grape skin to enter and to go to work on the sugar, which comprises about 30% of the pulp. And in an instant, there is wine.
Under normal conditions the yeast will go on working until all the sugar in the grapes is converted into alcohol, or until the alcohol level in the wine reaches about 15% of the volume – on the rare occasions when the grape are so sweet that this happens naturally the yeast is overcome, and fermentation stops. Left to nature, therefore, almost all wine would be dry.
But it is possible to curb the fermentation before all the sugar is used up; either by adding alcohol to raise the level up to 15% or by adding sulphur – both these anaesthetize the yeast, or by filtering the wine through a very fine filter to take the yeast out. These are the methods which are used to make sweet wine.