Single-celled microorganisms belonging to the fungus family (Thallophyde = plants without roots and leaves), in spherical, oval, elongated to cylindrical or pointed form. The size is between 5 and 14 thousandths of a millimetre (but considerably larger than bacteria). Most of them reproduce rapidly by cell sprouting, which is why they are also called "shoot fungi". This process can occur up to 35 times. The yeasts mainly need sugar as an energy source, as well as some nutrients and trace elements, most of which are present in grape must. The yeasts play a decisive role in winemaking. During fermentation, the sugars glucose (grape sugar) and fructose (fruit sugar) are converted into ethanol (alcohol) and carbon dioxide. The French scientist Louis Pasteur (1822-1895) made a special contribution to the study of this complex process.