In commercial winemaking this is seldom allowed to happen, as the ‘wild’ yeasts on the grape skins are sometimes unpredictable, so commercial yeasts of known characteristics are used instead.
To make white wines, the grapes are picked at their optimal ripeness, when the sweetness and acidity are perfectly balanced, and are taken to the cellar where they are crushed and the stalks removed.
Sulphur dioxide gas is pumped through the juice to kill the wild yeasts and selected yeasts are added to begin the fermentation.
As this is a slow process, lasting several days, the winemaker can elect to stop the fermentation at any time, leaving part of the grape sugar unfermented.
In this way the same grapes can be used to make semi-sweet, off-dry or very dry white wines. It is simply a matter of deciding when to stop the fermentation process.
Red wine is made in a similar way, except that the colour of the wine is obtained by leaving the juice to soak on the grape skins, which contain the red pigmentation.
If the juice is drawn off the skins immediately after crushing, and the grapes are cold enough, there will be no colour at all. So you can, in fact, make a white wine from red grapes. If you leave the juice in contact with the skins for a short period of time so that it has a coppery-pinkish colour then you have a blanc de noir (say blanh den nwah) -- white from black.
Rose (say ro-zay) wines are made in the same way, either from red grapes only or from mixing both red and white.
Most reds are allowed to remain on the skins until fermentation is complete; sometimes as long as three weeks. Red wines are generally made completely dry.
What is the difference between a Cabernet Sauvignon and a Sauvignon Blanc?
You know there are different types of apples: Granny Smith, Starking, Golden Delicious. There are also different types of grapes. So when you see a name like Cabernet Sauvignon or Sauvignon Blanc on a label it means the wine is made from a grape of that name. If there is no name the wine might be a blend, a mixture of different varieties.
White wine grape varieties include the popular Chenin Blanc (say shen-in blanh). Chardonnay, noted for its nutty, toffee character with hints of fresh lime juice. Sauvignon Blanc (say so-veen-yon blanh), with its fresh, herbal character and undertones of grassiness, green pepper and asparagus. Semillon (say semi-yon) with its pine, almonds and lanolin flavours. And the honey-flavoured muscat grape varieties, like our famous hanepoot or muscat d’Alexandrie.
The best known red varieties include Cabernet Sauvignon (say ca-ber-nay so-veen-yon) , the basis of all good French clarets. Merlot (say mer-lo), used in the claret blend as a softening influence. Shiraz, rich and smoky. Earthy Pinot Noir (say pee-no nwah), which made the wines of Burgundy famous. And our own Pinotage, developed as a cross between Pinot Noir and Hermitage, or Cinsaut (say sin-so).Which grapes make the best wines? Not a fair question. You should rather ask, ‘This is a Chenin Blanc – is it a good Chenin Blanc?’ Each wine has characteristics of its own. Some are meant for easy drinking and early enjoyment. Others are serious wines, with depth and character, and need to be laid down before they are at their best. When you feel like one or other type of wine the important thing is not to be disappointed. The best wine is a well made wine, whatever the variety.