Fermentation caused spontaneously by natural yeasts (also natural yeasts, environmental yeasts, wild yeasts, grape yeasts, vineyard yeasts) during winemaking. Until the middle of the 19th century, yeasts were 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 usually the product of spontaneous fermentation. The natural yeasts are found in larger quantities in the air in the vineyard and are spread by insects such as vinegar flies (fruit flies). They then enter the cellar with the grapes. However, they do not only come from the vineyard, but also from the cellar from the surrounding area of the farm. The quantity is sufficient to trigger spontaneous fermentation.