Single-celled, to the mushrooms counting microorganisms (Thallophyde = plants without roots and leaves), in spherical, oval, oblong to cylindrical or pointed form. The size is between 5 and 14 thousandths of a millimeter (but much larger than bacteria ). Most of them multiply rapidly by cell sprouting, which is why they are also called "yeasts". This process can take up to 35 times. The yeasts need above all else sugar as an energy source, as well as some nutrient and trace elements most of which are in grape available. The yeasts play in the winemaking a crucial role. In the fermentation become the sugars glucose (Glucose) and fructose (Fructose) in ethanol (Alcohol) and carbon dioxide transformed. The French scientist Louis Pasteur (1822-1895) earned particular merit in exploring this complex process.
The process is usually undercut for the most part anaerobic Conditions (that means absence of oxygen). Areobe conditions in the presence of oxygen are important before or at the beginning of fermentation, as the yeasts can only multiply in an oxygen-rich environment. The glucose is processed much faster, that is why in the residual sugar especially fructose present. As early as 1861, Louis Pasteur reported that yeasts consume much less sugar in aerobic environments. At higher glucose levels in grape must from about 100 mg / l but can also under aerobic Conditions alcohol are formed. This is called Crabtree effect (or Pasteur effect). At low glucose levels, yeast in the presence of oxygen causes the sugar to breathe directly, so it is not converted into alcohol.
The most important yeast genus is "Saccharomyces" (sugar mushrooms), of which there are over a hundred different species. The most common in the fermentation of Wine. beer and sourdough involved type is "cerevisiae" (grain), in German aptly "brewer's yeast" or also "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 previously assumed kinship relationships and thus the designations have turned out to be wrong. The taxonomic Because of this new knowledge, order will surely change in one way or another in the future.
There are over a dozen different yeasts in the vineyard. They are referred to as natural yeasts , wild yeasts , environmental yeast or indigenous yeasts . Of these, only some can completely ferment the must. The most important are Candida, Hansenula, Kloeckera, Pichia and Torulopsis. On a single (uninjured) berry there are about eight million cells, with a cracked 40 times as much. These overwinter in spore form in the ground and are also contained in the air during the grape harvest. With the grapes they reach the cellar, where they multiply quickly during pressing and get into the must. The natural yeasts range without intervention of the winemaker for the release of the spontaneous fermentation from what used to be the only and common practice. This form is related to Organic viticulture become 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 activity. They play a role especially at the beginning of the fermentation, regardless of whether it is also working with other yeasts. However, the fermentation process is difficult to control, since it is subject to chance, which of the tribes prevail. Therefore, today mostly produced in the laboratory yeasts (purebred, cultural, Impfhefen) are added to the must. Special services to the development of pure culture yeasts have come to Dr. med. Julius Wortmann (1856-1925), who died in Geisenheim (Germany) founded the first yeast pure breeding station in 1894, and Wenzel Seifert (1862-1942) on Klosterneuburger Weinbauinstitut (Austria).
By today's conventional refrigerated fermentation mainly the yeast genus Saccharomyces is used. The most important species is Saccharomyces cerevisiae , other species are S. bayanus, S. paradoxus and S. uvarum. Breeding yeasts are selectively selected and bred as liquid cultures or in the form of dry preparations in the laboratory. They are picked from spontaneously fermenting must, then the desired strain is multiplied from individual cells. This process, which is individually tailored to a particular farm, where even wine-garden-specific yeasts are cultivated, is called Individual Yeast Designing, The different species are characterized by purity in taste and by certain properties. Normally, the yeasts are sensitive to low or high temperatures, alcohol and carbon dioxide, cease or even die when certain limits (16 to 18% by volume of alcohol) are reached.
Depending on the desired fermentation or the type of wine is therefore grown different species. Desirable characteristics of yeasts include rapid broaching (possibly assisted by starter cultures or starter), trouble-free fermentation (no sticking), low foaming, little by-products, good alcohol yield, no sulfur dioxide, sugar and alcohol compatibility and color preservation in red wine. The cold fermented yeasts are still active at low temperatures of 6 to 9 ° Celsius. The champagne yeasts are resistant to high levels of alcohol and carbon dioxide, while sulphite yeasts are resistant to high levels of sulfur. In order to promote yeast production, fermentation may occur mineral yeast nutriments added.
The turbo yeasts have the highest alcohol tolerance and create under optimal conditions up to 20%. However, pure fruit or grape juice as starting material is not enough, but sugar must be added. They are used for the production of tasteless alcohol, which then passes through distillation is concentrated. The latest development in the US is allowing GM yeasts that contain sugar in alcohol and at the same time malic acid in lactic acid can convert. One saves thus the malolactic fermentation, In Geisenheim there are experiments, by means of the enzyme glucose oxidase, the glucose in gluconic convert. Gluconic acid can not be converted into alcohol by the yeasts. Likewise experimenting with the selection of ineffective yeasts . This targeted wines with balanced Alcohol content produced for the purpose of flavor enhancement (see alcohol reduction ).
At the Swiss Research Institute Agroscope Changins-Wädenswil (ACW) has been experimenting with pure-yeasts since the 1970s. Particularly successful are the in 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 are very little acetic acid and more glycerin as other yeasts, which makes the wine smoother and more harmonious. The concentration of Succinic acid is increased, causing the PH value the wine sinks below 3.5, causing unwanted growth Acetobacter (Lactic acid bacteria) prevented. Other advantages are low susceptibility to sulfurous off, rapid settling of the yeast and good alcohol tolerance, the wines to 16% vol alcohol (almost) without residual sugar allows. The malolactic fermentation is up to three times faster than traditional products.
In recent years, so-called aromatic yeasts have become popular, with which certain secondary in the fermentation flavors be forcibly promoted. They reinforce for example fruity Grades. However, these may cover varietal characteristics of the vine and are therefore not undisputed. At the beginning of the fermentation, different species of natural yeasts are active, later on this changes. In total, at least ten different subspecies of wild and farmed furs are involved. Of many farms, the entire fermentation with natural yeasts is called as spontaneous fermentation This is especially true in the generation of organic wines common. To cultivate the natural yeasts, the soil with the rape or the lees Fertilized (pressing and fermentation residues), so that over time results in an ideal blend of "vineyard-owned" yeasts in the vineyard. An undesirable in winemaking Hefegattung is Brettanomyces that the wine error horse sweat caused. See also below pile. lees and And arrived,