Fermentation does occur naturally. But natural yeasts cause unreliable and unsteady fermentation that is disturbed by any and all changes in temperature. Once these yeasts have died, acetic bacteria invades and turns the wine into vinegar.
Cultivated yeasts are used because they will tolerate higher alcohol levels and continue to work. Although the final character of the wine is affected by the fruit or other flavorings added at the beginning, there is no wine without the yeast.

Yeast can be very fickle, and the preparation of the starter mix is important in getting off to a good start with the brew. Before adding yeast to the must, it is best to get the fermentation going and the yeast active. Adding a small amount of sugar and a warm fluid will do this.

  • Baker’s yeast, such as you find in cubes or sachets in the supermarket, takes 10 to 20 minutes to be activated. A small, narrow-necked bottle is best to use. Add the yeast, 2 tablespoons of white sugar and enough warm water to fill the bottle half full. Shake the mixture well and use a cotton wool ball to lightly plug the neck. Stand the yeast starter on a surface that can be readily cleaned. Within about 10 minutes, a froth will form.

Use this starter as soon as possible; it will climb out of the bottle if you don’t!

  • Brewer’s yeasts and wine yeasts come in sachets or inactive mixes. Follow the instructions on the label or prepare as for the Baker’s yeast, but leave 2 to 3 days to activate. Plug the bottle neck loosely with a wad of cotton wool to stop the mix from becoming contaminated while it works.
see also Environmental Yeasts question ~ Aug 2009

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