anaerobic and aerobic 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.
Yeast genera and species
The most important yeast genus is "Saccharomyces" (sugar fungus), of which there are over a hundred 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. Due to these new findings, the taxonomic order will certainly change in the future in one point or another.
Natural yeast (wild yeast)
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. On a single (unharmed) berry there are about eight million cells, with cracked ones 40 times as many. These overwinter in spore form in the soil and are also present in the air during the grape harvest. Together with the grapes, they reach the cellar, where they multiply quickly during pressing and are released into the must. Without the intervention of the winemaker, the natural yeasts are sufficient to trigger spontaneous fermentation, which used to be the only 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% vol. alcohol content. They play a role especially at the beginning of fermentation, regardless of whether other yeasts are also used. However, the fermentation process is difficult to control, as it is subject to chance which of the strains prevails. That is why today mostly laboratory-produced cultured yeasts (pure cultured yeasts, culture yeasts, vaccination yeasts) are added to the must. Special merits for the development of pure breeding yeasts have been acquired by Dr. Julius Wortmann (1856-1925), who founded the first pure yeast breeding station in Geisenheim (Germany) in 1894, and Dr. Wenzel Seifert (1862-1942) at the Klosterneuburg Wine Institute (Austria).
Breeding yeasts
Due to the cooled fermentation that is common today, the yeast genus Saccharomyces is mainly used. The most important species is Saccharomyces cerevisiae, other species are S. bayanus, S. paradoxus and S. uvarum. The cultured yeasts are specifically selected and cultivated in the laboratory as liquid cultures or in the form of dry preparations. 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 in which even vineyard-specific yeasts are cultivated, is called Individual Yeast Designing. The different types are characterized by purity of taste and certain characteristics. Normally, the yeasts are sensitive to low or high temperatures, alcohol and carbon dioxide, cease activity when certain limits are reached (16 to 18% vol. alcohol content) or even die off.
Depending on the desired fermentation or the type of wine, different types are therefore cultivated. Desired characteristics of yeasts are, among others, rapid fermentation (possibly supported by starter cultures or fermentation starters), problem-free fermentation (no sticking), low foam formation, few by-products, good alcohol yield, no sulphur dioxide formation, sugar and alcohol compatibility and gentle colouring of red wine. The cold fermentation yeasts are also still active at low temperatures of 6 to 9 °Celsius. The sparkling yeasts are resistant to high alcohol and carbon dioxide, the sulphite yeasts to high sulphur content. In order to promote yeast formation, yeast nutrient salts may be added during fermentation.
Special yeasts
Turbo yeasts have the highest alcohol tolerance and under optimal conditions they can produce up to 20%. However, pure fruit juice or grape juice is not sufficient as a raw material for this; sugar must be added. They are used for the production of tasteless alcohol, which is then concentrated by distillation. As a last development, there are genetically modified yeasts permitted in the USA which can convert sugar into alcohol and at the same time malic acid into lactic acid. This saves the need for malolactic fermentation. In Geisenheim, there are attempts to convert glucose into gluconic acid using the enzyme glucose oxidase. Gluconic acid cannot be converted into alcohol by the yeasts. Experiments are also being carried out with the selection of ineffective yeasts. In this way, wines with a balanced alcohol content are specifically produced for the purpose of improving the taste (see under alcohol reduction).
At the Swiss research station Agroscope Changins-Wädenswil (ACW), experiments have been conducted with pure cultured yeasts since the 1970s. The wine yeasts Cerevisiae Lalvin W27 (1980) and Lalvin W15 (1997) selected in Wädenswil are particularly successful, with special properties which are already being used worldwide. These inhibit unwanted microorganisms and increase the content of high-quality ingredients. They produce only little acetic acid and more glycerine than other yeasts, which makes the wine smoother and more harmonious. The concentration of succinic acid is increased, lowering the pH of the wine below 3.5, which prevents the growth of undesirable acetobacter (lactic acid bacteria). Other advantages are low susceptibility to buckser, fast settling of the yeast and good alcohol tolerance, which allows wines up to 16% vol alcohol (almost) without residual sugar. 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 aromas produced during fermentation are specifically promoted. For example, they strengthen fruity notes. However, under certain circumstances these may mask varietal characteristics of the grape variety and are therefore not undisputed. At the beginning of fermentation, different species of natural yeasts are active, but this changes as the fermentation progresses. A total of at least ten different subspecies of wild and cultured yeasts are involved. Many farms carry out the entire fermentation with natural yeasts as so-called spontaneous fermentation, which is particularly common in the production of organic wines. In order to take specific care of the natural yeasts, the soil is fertilised with the marc or the tank bottoms (pressing and fermentation residues), so that over time an ideal mixture of the "vineyard's own" yeasts is produced in the vineyard. One type of yeast that is undesirable in vinification is Brettanomyces, which causes the wine defect horse sweat. See also under pile, yeast storage and cream.
Picture left: By Bob Blaylock - Own work, CC BY-SA 3.0, Link
Picture right: From Mogana Das Murtey and Patchamuthu Ramasamy, CC BY-SA 3.0, Link