Single-celled, to the mushrooms counting microorganisms (Thallophyde = plants without roots and leaves), in spherical, oval, oblong to cylindrical or pointed form. The size is between 5 and 14 thousandths of a millimeter (but much larger than bacteria ). Most of them multiply rapidly by cell sprouting, which is why they are also called "yeasts". This process can take up to 35 times. The yeasts need above all else sugar as an energy source, as well as some nutrient and trace elements, most of which are in the grape available. The yeasts play in the winemaking a crucial role. In the fermentation become the sugars glucose (Glucose) and fructose (Fructose) in ethanol (Alcohol) and carbon dioxide transformed. The French scientist Louis Pasteur (1822-1895) earned particular merit in exploring this complex process.