Single-celled microorganisms belonging to the fungi (thallophytes = plants without roots and leaves), in spherical, oval, elongated to cylindrical or pointed shape. They range in size from 5 to 14 thousandths of a millimetre (but are significantly larger than bacteria). Most of them reproduce furiously by cell sprouting, which is why they are also called "sprouting fungi". This process can take place up to 35 times. In the process, the yeasts mainly need sugar as a source of energy, as well as some nutrients and trace elements, most of which are present in grape must. The yeasts play a crucial 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) earned special merit for his research into this complex process.