This is generally understood to mean alcoholic fermentation (derived from the Middle High German jären = to boil, to foam), in which sugar is unconverted into ethanol and carbon dioxide. In this process, the microbial decomposition of organic substances takes place without the inclusion of oxygen(anaerobic). However, the degradation of organic substances can also take place with the inclusion of oxygen (aerobic). Acetic acid fermentation consumes oxygen, which is why, strictly speaking, it is not a fermentation in the scientific sense.
In German, however, the term fermentation is an umbrella term for aerobic and anaerobic processes. In other languages, however, "fermentation" is used for alcoholic fermentation or does not have its own term.
Cause of fermentation
The process was long thought to be a natural decomposition. The influence of air (although oxygen was already known) was not taken into account. For this reason, the processes involved in converting must into wine, wort into beer and wine into vinegar were referred to as fermentation. The involvement of previously unknown microorganisms was recognised by the French chemist Louis Pasteur (1822-1895) in 1857.
Fermentation occurs spontaneously without human intervention wherever yeast fungi come into contact with aqueous sugar solutions at an optimum temperature of between 18 and 27 °C. Such fungi exist in the vineyard and in the wine cellar. Such fungi exist in the vineyard and also in the wine cellar, which is why we speak of vineyard and cellar yeasts. Many producers deliberately allow this without the addition of pure yeasts. As a rule, fermentation starts automatically with grape must or other fruit juices, but it can also be other substances dissolved in water, such as cereals (barley, wheat, maize, rice) in the production of beer or other sugary substances such as potatoes or bread.
The picture on the left shows open stainless steel red wine tanks for mash fermentation, the picture in the centre shows wooden fermentation racks for mash fermentation and the picture on the right shows stainless steel white wine tanks ( Gerhard Markowitsch winery, Carnuntum Lower Austria).
Yeast types
Yeasts play a decisive role in fermentation. After pressing, they multiply rapidly in the grape must by sprouting and dividing. Towards the end, there are 50 to 200 billion yeast cells per litre of wine. The biochemical processes are controlled by enzymes contained in the yeasts; the zymase complex is responsible for fermentation. Natural or wild yeasts are present in large quantities in the vineyard and elsewhere (air) and enter the cellar with the grapes. They can also trigger fermentation on their own, which was common practice in the past. This spontaneous fermentation is more difficult to control, but is favoured for organic wines and natural wines, especially in connection with organic viticulture.
Pure culture fermentation is initiated using pure culture yeasts (cultured yeasts). These guarantee a fast fermentation start. If necessary, this is supported by starter cultures (also fermentation starters). Saccharomyces cerevisiae is the most important yeast species involved in the start worldwide. The alcohol-sensitive yeast species die quickly during fermentation, in the case of natural yeasts from around 14% vol. Only the alcohol-resistant (cultured yeasts) survive until they too stop working at 16 to 18% vol. alcohol content. In the production of spirits, "turbo yeasts" are used which are even viable up to 20% vol. Sugar conversion is a multi-stage process caused by the yeast complex zymase. A special type of yeast is flor, which forms during the production of certain types of sherry.
Fermentation process
As already mentioned, fermentation takes place in an anaerobic environment in the absence of oxygen. However, if there is a higher concentration of glucose (grape sugar) in the grape must, alcohol can also be formed in the presence of oxygen, i.e. in an aerobic environment....
