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Alcoholic Fermentation of Wine

In order for the grape juice to finally become wine, alcoholic fermentation must take place. Thanks to the naturally occurring yeast, this happens almost automatically. During fermentation, the sugar contained in the grape juice is transformed into alcohol. In dry wines, this is almost all of the sugar; in semi-dry or sweet wines, a significantly larger amount of residual sugar remains.

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Natural or industrial yeast

Actually, the important yeast bacteria exist everywhere, without which a wine cannot be created at all. In the case of natural yeasts, South African winemakers have the advantage that, as part of the terroir, they create a very individual taste in the wines, which is why many a wine lover swears by them. But they also have disadvantages. On the one hand, it can happen that the yeast is not present in sufficient concentration or is simply not strong enough to carry out the fermentation to the desired end. The fermentation can thus end prematurely and it is difficult and takes a lot of effort to get it going again. In addition, with natural yeast you never know exactly what flavour you will get in the end, so there is always a moment of uncertainty involved. Industrially produced yeast – so-called pure culture yeast – has the advantage that you know exactly what properties it has and what you can expect from the final result. In addition, winemakers can determine the exact amount of industrial yeast used and thus better control the fermentation process. Quite a few, however, see this as a danger of harmonisation between wineries and even between individual wines. If everyone used the same yeasts, the wines would almost inevitably have to converge in taste.

The right temperature determines success

Despite their wide and natural distribution, yeasts are extremely sensitive to temperature fluctuations. If the temperature is too cold, they cannot multiply sufficiently and the fermentation process is delayed. As a result, the mash or juice may stand for too long, which is not desirable for a premium wine. Therefore, in some areas, winemakers have to heat the mash to get the fermentation process going. At the same time, however, too high temperatures are counterproductive. Then the yeast bacteria stop their activity, fermentation stops too soon, and once stopped it is extremely difficult to restart it. Since heat is constantly generated during alcoholic fermentation, winemakers must continuously check the temperature and cool the juice or mash if necessary. Most winemakers in South Africa, however, prefer not to let it get that far in the first place and try to keep the degrees in the tanks at a constant level.