Anaerobic and aerobic process
The process usually takes place for the most part under anaerobic conditions (meaning absence of oxygen). Areobic conditions in the presence of oxygen are, however, important before or at the beginning of fermentation, as the yeasts can only multiply in an oxygen-rich environment. Glucose is processed much faster, which is why the residual sugar is mainly fructose. Louis Pasteur reported as early as 1861 that yeasts consume much less sugar in an aerobic environment. However, at higher glucose levels in the grape must from about 100 mg/l, alcohol can also be formed under aerobic conditions. This is called the Crabtree effect (or Pasteur effect). At low glucose levels, yeasts directly metabolise the sugar in the presence of oxygen, so it is not converted into alcohol.
Yeast genera and species
The most important yeast genus is "Saccharomyces" (sugar fungi), of which there are over 100 different species. The species most commonly involved in the fermentation of wine, beer and sourdough is "cerevisiae" (grain), aptly named "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 are extremely complicated. DNA analyses have shown that many of the previously assumed relationships, and thus also the names, are incorrect. The taxonomic order will certainly change quite a bit in the future due to these new findings.
Natural yeasts (wild yeasts)
More than a dozen different yeasts are present in the vineyard. They are called natural yeasts, wild yeasts, environmental yeasts or indigenous yeasts. However, only some of them can ferment the must completely. The most important are Candida, Hansenula, Kloeckera, Pichia and Torulopsis. There are about eight million cells on a single (uninjured) berry, 40 times as many on cracked ones. These overwinter in spore form in the soil and are also present in the air during the grape harvest. They reach the cellar with the grapes, where they multiply rapidly during pressing and enter the must. The natural yeasts are sufficient to trigger spontaneous fermentation without intervention by the winemaker, 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 at 3 to 4% alcohol by volume, or they stop at all. 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, however, as it is subject to chance which of the strains will prevail.
Cultured yeasts
For this reason, cultivated yeasts (pure-breeding, culture, inoculation yeasts) are usually added to the must in order to avoid possible fermentation faults. Dr. Julius Wortmann (1856-1925), who founded the first pure-breeding yeast station in Geisenheim (Germany) in 1894, and Dr. Wenzel Seifert (1862-1942) at the Klosterneuburg Institute of Viticulture (Austria) were particularly responsible for the development of pure-breeding yeasts.
At the Swiss institute Agroscope Changins-Wädenswil (ACW), experiments with pure-breeding yeasts have been going on since the 1970s. The wine yeasts Cerevisiae Lalvin W27 (1980) and Lalvin W15 (1997) selected in Wädenswil with special properties have been particularly successful and are used worldwide. These inhibit undesirable microorganisms and increase the content of high-quality ingredients. They form only a little acetic acid and more glycerine than other yeasts, which makes the wine smoother and more harmonious. The concentration of succinic acid is increased, which lowers the pH of the wine below 3.5, which prevents the growth of undesirable Acetobacter (lactic acid bacteria). Other advantages include low susceptibility to boil, rapid settling of the yeast and good alcohol tolerance, which allows wines up to about 16% alcohol by volume (almost) without residual sugar. Malolactic fermentation is up to three times faster than with conventional products.
Due to the cooled fermentation common today, mainly the yeast genus Saccharomyces is used. The most important species is Saccharomyces cerevisiae, other species are S. bayanus, S. paradoxus and S. uvarum. The cultivated yeasts are specifically selected and bred as liquid cultures or in the form of dry preparations in the laboratory. They are inoculated from the spontaneously fermenting must, after which the desired strain is propagated from individual cells. This process, which is individually adapted to a particular farm and even involves the cultivation of vineyard-specific yeasts, is called Individual Yeast Designing. The different types are characterised by purity of taste and certain properties. Natural yeasts are sensitive to low or high temperatures, alcohol and carbon dioxide, and stop working or even die when certain limits are reached (for example, from 14% alcohol by volume).
Special cultivated yeasts
Depending on the desired fermentation or wine type, different types are therefore bred. Desired characteristics of yeasts are rapid fermentation (if necessary supported by starter cultures or fermentation starters), problem-free to ferment fully (no sticking), low foaming, few by-products, good alcohol yield, no formation of sulphur dioxide, sugar and alcohol tolerance and colour preservation in red wine. The cold fermentation yeasts are still active at low temperatures of 6 to 9 °Celsius. The sparkling wine yeasts are resistant to high alcohol and carbon dioxide content, the sulphite ye asts to high sulphur content. To promote yeast formation, yeast nutrient salts or yeast rind may be added during fermentation.
The so-called turbo ye asts have the highest alcohol tolerance and create up to 20% alcohol by volume under optimal conditions. However, pure fruit or grape juice is not sufficient as a starting material for this; sugar must be added. They are used to produce tasteless alcohol, which is then concentrated by distillation. As the latest development, there are genetically engineered yeasts permitted in the USA that can convert sugar into alcohol and malic acid into lactic acid at the same time. This saves the malolactic fermentation. In Geisenheim, there are attempts to convert glucose into gluconic acid by means of the enzyme glucose oxidase. Gluconic acid cannot be converted into alcohol by the yeasts. Similarly, experiments are also being carried out with the selection of ineffective yeasts. This is used to specifically produce wines with a balanced alcohol content for the purpose of taste amelioration (see under alcohol reduction).
With aromatic yeasts certain secondary aromas that develop during fermentation can be specifically forced. They enhance fruity notes, for example. However, they may mask varietal characteristics of grape varieties and are therefore not uncontroversial. At the beginning of fermentation, different species of natural yeasts are active, and this changes as fermentation progresses. In total, at least ten different subspecies of wild and cultivated yeasts are involved. Many wineries carry out the entire fermentation process with natural yeasts as spontaneous fermentation, which is particularly common in the production of organic wines or natural wines. In order to specifically nurture the natural yeasts, the soil is fertilised with the marc or the lees (press and fermentation residues), so that over time an ideal mixture of the "vineyard's own" yeasts is created. A yeast genus undesirable in winemaking is Brettanomyces, which causes the wine defect horse sweat. On the subject of yeasts, see also the keywords Flor, yeast sediment storage and Kahm.
Picture left: By Bob Blaylock - Own work, CC BY-SA 3.0, Link
Picture right: By Mogana Das Murtey and Patchamuthu Ramasamy, CC BY-SA 3.0, Link
