Unicellular, to the mushrooms counting microorganisms (Thallophyde = plants without roots and leaves), in spherical, oval, oblong to cylindrical or pointed shape. The size is between 5 and 14 thousandths of a millimeter (but significantly larger than bacteria ). Most of them multiply rapidly by cell sprouting, which is why they are also called "sprout mushrooms". This process can take place up to 35 times. The yeast need above all sugar as an energy source, as well as some nutrient and trace elements most of which are in grape available. The yeasts play at the winemaking a crucial role. In the fermentation become the types of sugar glucose (Dextrose) and fructose (Fructose) in ethanol (Alcohol) and carbon dioxide converted. French scientist Louis Pasteur (1822-1895) earned special merit in researching this complex process.
The process is usually largely undermined anaerobic Conditions (that is, the absence of oxygen). Areobic conditions in the presence of oxygen are important before or at the start of fermentation, since the yeasts can only multiply in an oxygen-rich environment. The glucose is processed much faster, so in the residual sugar especially fructose present. As early as 1861, Louis Pasteur reported that yeast consumed much less sugar in an aerobic environment. With higher amounts of glucose in the grape must from about 100 mg / l can also under aerobic Conditions alcohol are formed. That is called Crabtree effect (or Pasteur effect). With small amounts of glucose, yeast inhale the sugar directly in the presence of oxygen, so it is not converted into alcohol.
The most important genus of yeast is "saccharomyces" (sugar mushrooms), of which there are over a hundred different types. The most common in the fermentation of Wine. beer and the type of sourdough involved is "cerevisiae" (cereals), in German meaning "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. By DNA analysis many of the family relationships assumed up to now, and thus also the names, have turned out to be wrong. The taxonomic Due to these new findings, order will certainly change in one or the other point in the future.
There are over a dozen different yeasts in the vineyard. They are called natural yeasts , wild yeasts , environmental yeasts or indigenous yeasts . However, only a few of these can ferment the must completely. The most important are Candida, Hansenula, Kloeckera, Pichia and Torulopsis. There are around eight million cells on a single (uninjured) berry, and 40 times as many if they open. These hibernate in the soil in spore form and are also contained in the air during the harvest. With the grapes they reach the cellar, where they quickly multiply during the pressing and reach the must. The natural yeasts are sufficient to trigger the wine without the winemaker's intervention spontaneous fermentation from what used to be the only and common practice. This form is related to Organic viticulture became popular again.
Most wild species are sensitive to sulfur dioxide and alcohol and are already at 3 to 4% vol alcohol content at least inhibited in their work. They play a role especially at the beginning of fermentation, regardless of whether other yeasts are also used. The course of fermentation is difficult to control because it is subject to chance which of the strains prevails. Therefore today yeasts mostly produced in the laboratory (purebred, cultural, Impfhefen) are added to the must. Dr. has made special contributions to the development of pure breeding yeast. Julius Wortmann (1856-1925) who in Geisenheim (Germany) founded the first yeast pure breeding station in 1894, as well as Dr. Wenzel Seifert (1862-1942) on Klosterneuburg Wine Institute (Austria) acquired.
The yeast genus Saccharomyces is mainly used due to the currently cooled fermentation. The most important species is Saccharomyces cerevisiae , other species are S. bayanus, S. paradoxus and S. uvarum. The cultivated yeasts are specifically selected and grown as liquid cultures or in the form of dry preparations in the laboratory. They are vaccinated from the spontaneously fermenting must, after which the desired strain is propagated from individual cells. This process, which is individually tailored to a specific farm and in which even vineyard-specific yeasts are grown, is called Individual yeast designing, The different types are characterized by purity in taste and certain properties. Usually the yeasts are sensitive to low or high temperatures, alcohol and carbon dioxide, stop working when certain limits (16 to 18% vol. Alcohol content) are reached or even die.
Depending on the desired fermentation or the type of wine you therefore breed different types. Desired properties of yeast include rapid fermentation (possibly supported by starter cultures fermentation starter), easy fermentation (no getting stuck), low foaming, few by-products, good alcohol yield, no sulfur dioxide formation, sugar and alcohol tolerance and color protection for red wine. The cold proofing yeasts are also active at low temperatures of 6 to 9 ° Celsius. The champagne yeasts are resistant to high alcohol and carbon dioxide, the sulfite yeasts to high sulfur content. To promote yeast formation, fermentation may be necessary mineral yeast nutriments added.
The turbo yeasts have the highest alcohol tolerance and achieve up to 20% under optimal conditions. However, pure fruit or grape juice is not sufficient as a raw material, but sugar must be added. They are used for the production of tasteless alcohol, which then goes through distillation is concentrated. As a recent development, there are permitted genetic engineering yeasts in the USA that contain sugar in alcohol and at the same time malic acid in lactic acid can convert. You save yourself the malolactic fermentation, In Geisenheim there are attempts to use the enzyme glucose oxidase to reduce glucose in gluconic convert. The yeast cannot convert gluconic acid into alcohol. Experiments are also being carried out with the selection of ineffective yeasts . In this way, wines are targeted balanced Alcohol content generated to improve taste (see under alcohol reduction ).
At the Swiss research institute Agroscope Changins-Wädenswil (ACW) has been experimenting with pure breeding yeasts since the 1970s. The in. Are particularly successful Wädenswil selected wine yeasts Cerevisiae Lalvin W27 (1980) and Lalvin W15 (1997) with special properties that are already used worldwide. These inhibit unwanted microorganisms and increase the content of high-quality ingredients. They form little acetic acid and more glycerin than other yeasts, which makes the wine smoother and more harmonious. The concentration of Succinic acid is increased, causing the PH value the wine drops below 3.5, making the growth undesirable Acetobacter (Lactic acid bacteria) prevented. Other advantages are low vulnerability to sulfurous off, fast settling of the yeast and good alcohol tolerance, the wines up to 16% vol alcohol (almost) without residual sugar allows. The malolactic fermentation is up to three times faster than with conventional products.
In recent years, so-called aroma yeasts have become popular, with which certain secondary fermentation arises flavors be targeted. For example, they reinforce fruity Grades. Under certain circumstances, however, these cover the characteristics typical of the variety vine and are therefore not without controversy. At the start of fermentation, different species of natural yeast are active, and this changes later on. A total of at least ten different subspecies of wild and breeding yeast are involved. Many companies use all of the fermentation with natural yeasts spontaneous fermentation carried out, especially when generating organic wines common. In order to maintain the natural yeasts in a targeted manner, the floor is covered with the rape or that lees (Press and fermentation residues) fertilized, so that over time an ideal mixture of the "vineyard's" yeasts results in the vineyard. An undesirable type of yeast in winemaking is Brettanomyces that the wine bug horse sweat caused. See also under pile. lees and And arrived,