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.
The process usually takes place for the most part under anaerobic conditions (i.e. absence of oxygen). However, areobic conditions in the presence of oxygen are important before or at the beginning of the fermentation, as the yeasts can only multiply in an oxygen-rich environment. The glucose is processed much faster, therefore the residual sugar contains mainly fructose. Louis Pasteur reported already in 1861 that yeasts consume much less sugar in an aerobic environment. However, higher amounts of glucose in grape must from about 100 mg/l can also produce alcohol under aerobic conditions. This is known as the Crabtree effect (or Pasteur effect). At low glucose levels, yeasts in the presence of oxygen breathe the sugar directly, so it is not converted into alcohol.
The most important yeast genus is "Saccharomyces" (sugar fungus), of which there are over 100 different species. The species most frequently involved in the fermentation of wine, beer and sourdough is "cerevisiae" (cereals), aptly named "brewer's yeast" or "baker's yeast". An older name is "Saccharomyces ellipsoideus" due to the mostly elliptical shape of these yeasts. The naming and classification of the different yeasts is extremely complicated. DNA analyses have shown that many of the previously assumed relationships and thus also the names have turned out to be wrong. The taxonomic order will certainly change in the future due to these new findings.
There are over a dozen different yeasts in the vineyard. They are called natural...