Unicellular microorganisms belonging to the fungi (formerly categorised as thallophytes, now classified as fungi) in spherical, oval, elongated to cylindrical or pointed form. The size is between 5 and 14 thousandths of a millimetre (but significantly larger than bacteria). Yeast cells are colourless to slightly cream-coloured and mostly odourless, but can develop a slightly yeasty, bread-like odour in larger accumulations. Most of them multiply rapidly by cell sprouting (sprout fungi); this process can take place up to 35 times.

Picture left: a "Saccharomyces cerevisiae" (1 graduation mark is 1 µm). Picture right: Yeast cells with sprouting scars.

Sugar as an energy source

Yeasts primarily need sugar as an energy source as well as some nutrients and trace elements, most of which are present in grape must. During fermentation, the sugars glucose (grape sugar) and fructose (fruit sugar) are converted into ethanol (alcohol) and carbon dioxide. Louis Pasteur (1822-1895) recognised that alcoholic fermentation is a biological process controlled by microorganisms.

Process

The process usually takes place under anaerobic conditions (absence of oxygen). However, aerobic conditions (presence of oxygen) are important before or at the start of fermentation, as the yeasts can only multiply in an oxygen-rich environment. Glucose is processed much faster, which is why fructose is mainly present in the residual sugar. This explains why fructose often remains as the main sugar at the end of fermentation.

However, alcohol can also be formed under aerobic conditions with higher amounts of glucose in the grape must from around 100 g/litre. This is known as the crabtree effect (Pasteur effect). In the Crabtree effect, a high sugar content inhibits the aerobic metabolism so that yeasts also produce alcohol under oxygen supply.

Yeast genera and species

There are over 30 scientifically described species of the yeast genus "Saccharomyces" (sugar fungi); a total of more than 1000 yeast species are known worldwide. The species most frequently involved in the fermentation of wine, beer and sourdough is Saccharomyces cerevisiae (see all species there), which aptly means "brewer's yeast" or "baker's yeast". An older name is "Saccharomyces ellipsoideus" due to the mostly elliptical shape of these yeasts.

Natural yeasts (wild yeasts)

Natural yeasts are sufficient for triggering spontaneous fermentation without intervention, which used to be the only possible and common practice. This form has become popular again in connection with organic viticulture. However, most wild species are sensitive to sulphur dioxide and alcohol and their activity is at least inhibited or even halted at 3 to 4%...