Yeast is single cell organism and belongs to the fungus family. The scientific name for wine yeast is saccharomyces cerevisiae. The way the yeast produces alcohol is quite interesting.
The whole process is accomplished with the aid of biological catalysts called enzymes. To simplify the process we will break it down in two stages.
In the primary fermentation, being aerobic process, the must needs to be stirred once or twice a day for approximately one week.
The purpose for stirring is to equilibrate all of the ingredients during active yeast reproduction and to introduce oxygen to the process. At this stage majority of the sugar is broken down to compounds called acetylaldehydes.
A good indicator for this reaction is the excessive foaming when you stir the must. Once acetylaldehydes are produced, about a week later, transfer the juice to the carboy.
It is crucial that the air lock is fitted. If oxygen is allowed to be present during the acetylaldehydes transformation, it will oxidize and produce acetic acid, A.K.A. vinegar.
During the secondary fermentation (anaerobic fermentation) the yeast, with the help of enzyme called alcohol dehydrogenase, without the presence of oxygen is allowed to do the job, to produce alcohol.
Yeasts require certain conditions for optimal growth (yeast nutrients, acidity...).
Ideal surrounding is 70ºF.
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