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What is alcoholic fermentation and why is it so important?

During fermentation, yeast cells convert sugar into alcohol - this is basic knowledge from chemistry lessons and also one of the first subjects of every WSET course. But of course there is much more to it than that!

Historically, knowledge about fermentation is still quite young - for thousands of years, the reason for the exhilarating effect of wine or beer was unknown and seemed like magic to people. Until in the 19th century the "father of microbiology" Louis Pasteur in France (where else?!) was able to trace the formation of alcohol back to the metabolic process of yeasts.

Pasteur deciphered what we describe in words like this: During alcoholic fermentation, the yeast cells metabolise the sugar of the grape must, consisting of fructose and glucose, to ethanol, carbon dioxide and heat of reaction. This also explains why the cellar always gurgles in autumn: the carbon dioxide produced during fermentation escapes through the so-called fermentation bung.

In addition, the fermenting must becomes warm due to the heat of the reaction - we will come back to this point later.

First, let's take a closer look at the actual star of fermentation, the yeast: The single-celled fungus occurs in nature in many different forms. For winemakers, saccaromyces cerevisiae is usually of particular interest. It is used in the form of cultured yeast, which winemakers selectively add to a must to start fermentation. The advantage: cultured yeasts deliver reliable results - the wine can be fermented to dryness, i.e. all sugar is consumed, and no undesirable by-products are produced. Nevertheless, these yeasts are also criticised because a certain standardisation of all wines is suspected. The counter-concept to cultured yeast is spontaneous fermentation: here, the naturally occurring yeast strains in the vineyard and in the cellar are used and fermentation is allowed to start spontaneously. In the opinion of many, this produces more interesting wines. However, one can rely less on the "wild yeasts" and risks off-flavours.

Once fermentation is underway, there is not much to do other than to observe the process carefully - regular measurements are part of the process. If the fermentation should stagnate, you can help the yeasts with a little air supply, with special yeast nutrients or with a little heat.

Speaking of heat: Temperature is a crucial parameter for fermentation. On the one hand, the yeasts need some heat to be able to work at all. On the other hand, the fermentation temperature has a decisive influence on the later wine style. As a general rule, white wines are fermented at cooler temperatures (12 - 22 degrees Celsius) than red wines (20 - 32 degrees Celsius). Within this spectrum, the temperature determines, among other things, the later aroma world of a wine: the cooler the fermentation, the more the emergence of so-called cold fermentation aromas, reminiscent of pear, banana or iced candy, is favoured. And also the many other wine aromas that we appreciate so much are only released thanks to fermentation - but that is a topic for a new blog article!

Photo Credit: German Wine Institute

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