Yeast is probably the most important ingredient to a winemaker – it is a vegetable matter, a living cell that will multiply in the correct environment. Its needs are simple, just sugar and acid in water at a temperature of 70–75°F/21– 24°C. Yeast will break down the sugar, and use it, firstly to multiply itself, and then, when the colony is active and healthy, to turn the sugar into alcohol and carbon dioxide.
Do not listen to anyone who knows of a ‘no yeast’ wine because there is no such thing. While no yeast may be added, the wild yeasts in the atmosphere are allowed to take over. The result is likely to be a weak, oversweet wine, probably with off-flavours.
Nowadays a wide range of yeasts is available from ‘general purpose’ to yeasts for many of the commercial varieties. For makers of country wines, the general purpose yeasts will be adequate, but wine yeasts tend to leave a firmer deposit and are less wasteful when racking.