Fermentation caused spontaneously by natural yeasts (also natural yeasts, environmental yeasts, wild yeasts, grape yeasts, vineyard and cellar yeasts). This was the common practice until the late Middle Ages. The clergyman Johann Rasch (1540-1612) writes about this in his famous work "Weinbuch: Von Baw, Pfleg und Brauch des Weins" as follows: "Take the soil where the wine grew and throw it into the barrel, and it will ferment." This referred to possible fermentation problems. At the time, this was very aptly called "wild fermentation". Until the middle of the 19th century, however, yeasts were completely unknown as a causative agent and were only proven by the French chemist Louis Pasteur (1822-1895) through analyses of the fermentation process. Until the 1970s, wines were still generally the product of such spontaneous fermentation. The natural yeasts are found in large quantities in the air and also in the soil of the vineyard and are spread by insects such as vinegar flies (fruit flies). These then enter the cellar with the grapes. They come not only from the vineyard but also from the cellar of the farm. The quantity is sufficient to trigger spontaneous fermentation.
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