The fermentation process doesn’t begin until the grapes are crushed, because the grape skin keeps the yeast and sugar apart.
This is (in very simple form) the ‘Great Wine Equation’ SUGAR + YEAST = ALOCOHOL + CARBON DIOXIDE (+heat).
Fermentation is a gradual process, and the winemaker can decide to stop the reaction at any stage, to make a semi-sweet wine, an off-dry one or a completely dry wine. The carbon dioxide gas is allowed to escape from the fermenting tank when natural, or ‘still’ wines are being produced. When sparkling wine is made, the carbon dioxide is trapped and stored in the wine under pressure, to be released later in the form of tiny bubbles, or mousse, which gives the sparkling wine its seductive twinkle.
In South Africa the sugar content of a wine determines what the wine may be called. Sugar content is expressed in grams per litre, and, in order to get an idea of the actual quantity involved, remember that a teaspoon holds about 2,5 grams of sugar. One litre is the capacity of a large cold drink bottle (the standard wine bottle size, 750 ml, is of course three quarters that volume). The official sugar classification in South Africa is as follows:
Extra Dry: 2,5 gm/l or less
Dry: 4,0 gm/l or less
Off –Dry: 4 to 12 gm/l
Semi-sweet: 4 to 30 gm/l
Special Late Harvest: 20 – 50 gm/l
Noble Late Harvest: 50-plus gm/l (there are other requirements to be met with a Noble Late Harvest as well).