Alcoholic Fermentation of Wine
For this juice to finally become wine it has to undergo alcoholic fermentation. This happens almost automatically due to the naturally occurring yeast. During the fermentation process, the sugar the grape juice contains turns into alcohol. With dry wines, this happens to almost all of the sugar, whilst a significantly larger amount of residual sugar can be found in medium dry or sweet wines.
Natural or Industrial Yeast
The yeast bacteria without which a wine could not be made, exists almost everywhere. South Africa’s winemakers see the great advantage of natural yeast in the fact that because they are a part of the terroir, they create a unique taste in the wines; this is why many winemakers swear on them. They also have disadvantages, though. On the one hand, there is the possibility that the yeast does not exist of the required concentration or that it is simply not strong enough to finish the fermentation process. This can result in the fermentation coming to an end too early, and it is very difficult and laborious to restart this process. Furthermore, you never know exactly what taste you will get when using natural yeast, so there is always a moment of uncertainty.
Industrially produced yeast - so-called selected pure yeast - has the advantage that the winemakers exactly know its characteristics and what to expect from the final result. Furthermore, the vintners can precisely determine the amount they need of industrial yeast and therefore better control the fermentation process. But a few winemakers see the risk of alignment of the wineries and even the individual wines. If everyone uses the same yeast, the wines will increasingly taste more alike.
The Right Temperature Determines the Success
Despite their wide and natural occurrence, yeasts are very sensitive when it comes to temperature fluctuations. When the temperature is too cold, they cannot reproduce sufficiently and the fermentation process is delayed. This can lead to the mash or juice standing for too long which is unwanted for a premium wine. This is why in some regions winemakers have to heat up the mash in order to initiate the fermentation process.
At the same time, temperatures too high are also counterproductive as the yeast bacteria stop being active then - the fermentation comes to a halt too early, and once stopped it is difficult to get it going again. Alcoholic fermentation constantly causes heat; this is why the winemakers have to continuously check the temperatures and cool the juice or mash if required. Most South African winemakers prefer to not let things come so far and try to keep the degrees in the tanks at a constant level.