The term for alcoholic fermentation during winemaking used to refer to the apparent "boiling" or "foaming" in the grape must. Fermentation is the microbial degradation of organic matter without the inclusion of external electron acceptors such as oxygen(anaerobic) for the purpose of energy production. In principle, however, organic substances can also be degraded with the inclusion of oxygen(aerobic). For example, acetic acid fermentation consumes oxygen, so it is not a fermentation in the scientific sense as described above. The term fermentation, however, is an umbrella term for aerobic and anaerobic processes. This often leads to confusion, because in other languages "fermentation" is used for alcoholic fermentation or there is no separate term for it.
The process was long thought to be a natural decay. The influence of air (even when oxygen had already been discovered) was ignored. Therefore, the processes involved in the transformation of must into wine, of wort into beer and of wine into vinegar were called fermentation. The involvement of microorganisms was recognized by Louis Pasteur (1822-1895) in 1857. Fermentation occurs spontaneously wherever yeast fungi come into contact with aqueous sugar solutions at an optimum temperature between 18 and 27 °C. Usually these are fruit juices, but they can also be other substances dissolved or added to water, such as cereals (barley, wheat, maize, rice) in the production of beer or other sugar-containing substances such as potatoes.
Today, fermentation of wine is mostly temperature-controlled in stainless steel tanks, although in larger companies this is also done by computer. But also barrel fermentation is used by many producers. For an optimal fermentation process, an appropriate must treatment is required beforehand. Fermentation is the enzymatic decomposition of organic substances in an anaerobic environment, i.e. in the absence of oxygen. However, if a higher concentration of glucose (glucose) is present in the grape must, alcohol can also be produced in the presence of oxygen, i.e. in an aerobic environment. This is also called the Crabtree Effect (or Pasteur Effect) after the English biochemist Herbert Grace Crabtree. An aerobic environment with oxidative processes is also common in the production of certain wines such as port and sherry.
The yeasts play a decisive role in fermentation. After pressing, they multiply rapidly in the grape must through sprouting and division. Towards the end, there are 50 to 200 billion yeast cells per litre of wine. The biochemical processes are controlled by enzymes contained in the yeasts; the zymase complex is responsible for fermentation. Natural or wild yeasts are present in large quantities in the vineyard and elsewhere (air) and enter the cellar with the grapes. This allows the fermentation to be triggered completely independently, which used to be the usual custom. In this case one speaks of Spontaneous fermentation. However, this is much more difficult to control, but is increasingly preferred, especially in connection with organic viticulture, for the production of organic wine.
However, the main method still used is pure-bred fermentation, which is initiated by means of pure-bred yeasts (culture yeasts). These guarantee a fast, complication-free fermentation start. If necessary, this is supported by starter cultures (also fermentation starters). The world's most important yeast species involved at the beginning is Saccharomyces cerevisiae. The alcohol-sensitive yeast varieties die off quickly during fermentation. Only the alcohol-resistant yeasts survive until they also die at 16 to 18% vol. Alcohol content. Today, newly developed "turbo yeasts" are used in the production of spirits, which are even viable up to 20% vol. Sugar conversion is a multi-stage process caused by the yeast complex zymase. A special type of yeast is the pile which is effective in the production of certain types of sherry.
At the beginning of fermentation, the grapes or grape must contain approximately equal amounts of the simple sugars glucose and fructose. They are converted to 90% ethanol and carbon dioxide in a ratio of 51:49, whereby this occurs more quickly with glucose. Two molecules of ethanol and two molecules of carbon dioxide are formed from each molecule of fructose or glucose. Aldehydes, esters, glycerol, methanol, succinic acid, acetic acid, lactic acid, sulphurous acid, citric acid and higher alcohols or fusel oils are formed as by-products, as well as energy in the form of heat. Therefore the actual alcohol yield (ethanol) is not 51%, but only 47 to 48%. As a rule of thumb, 10 g of sugar per 1,000 g of must yields 0.66% by volume of alcohol.
The carbon dioxide results in about fifty times the volume of the must and collects at the bottom because it is heavier than air (relative density 1.52). Therefore there is a danger of suffocation in the fermenting cellar and despite knowledge of this fact, several deaths occur every year. The candle sample, which was often used in the past to check the oxygen content of the air, is unsuitable and life-threatening. Also the discharge over formerly usual fermentation grids can be dangerous if the fermentation room is lower than the cellar entrance. Today the carbon dioxide is extracted via exhausters (deaerators) and led into the open air or is also used in large cellars. To a small extent, the carbon dioxide is bound in the wine, which also produces real carbon dioxide. In the production of sparkling wine, the primary goal is the formation of as fine bubbles as possible or a high degree of pearlability.
An important criterion is the fermentation temperature. At higher temperatures fermentation is stormy, at lower temperatures slow. The optimum temperature for the propagation of yeasts is 25 °Celsius. The "correct" fermentation temperature depends on the type of wine desired. The warmer the fermentation, the more aromas and alcohol are lost, but more glycerine is produced. In addition, a safe through fermentation with the result of a dry wine is achieved. It should be noted that the must heats up more in larger containers (600 litre containers up to 22 °C, 7,000 litre containers up to 33 °C). In preparation, the pre-clarified must is brought to a starting temperature of about 15 to 18 °Celsius. From a temperature which is above all usual for white wine Cold fermentation by means of special cold fermentation yeasts is spoken at temperatures of 10 to 15° Celsius.
With the usual red wine Mash fermentation higher temperatures are necessary, which are between 25 and 30 °Celsius. If the fermentation temperatures are too high, starting at 35 °Celsius, the wine may boil, which can lead to acetic acid and spoil the wine. Cooling is necessary to maintain the desired temperatures. Nowadays, this is often controlled automatically by computer with coolable and heatable fermentation tanks. These are usually made of stainless steel or ceramic. The fermentation time ranges from eight days to six weeks, depending on temperature, yeasts and wine type. For special wines, such as Vin Santo, this can also take months. Fermentation activators are also added for an optimal fermentation process.
Fermentation takes place in three phases. In the fermentation phase, the sprouting and propagation of the yeast takes place, whereby carbon dioxide is already formed. This is followed by the stormy fermentation (french bouillage), which is characterised by strong yeast propagation, carbon dioxide formation and heat development; the must can literally bubble during this process. In the still post-fermentation phase, the last sugar residues are broken down and valuable flavour and aroma substances are formed. If a certain amount of residual sugar is to be obtained, fermentation must be stopped at this point. This is done either by cooling down to minus 3 to 2 °C, by adding sulphur, by heating to 75 °C for a short time or, for certain wines, by spritification (addition of pure alcohol). Depending on cellar temperature, must temperature or sugar concentration, the yeasts can stop working early. An unintentional end of fermentation (sticking) is one of the most common Fermentation error.
The fermentation ends when the sugar is almost completely converted, then the yeasts have no more food. The second main reason is when the conditions have reached a status that is hostile to the life of the yeasts, for example the alcohol content. However, a complete fermentation of the sugar is not possible by natural means. Even with completely dry wines, a certain amount of residual sugar remains in the wine. The dead yeasts settle on the bottom of the fermentation tank together with other substances as trub. Now it has to be decided whether or not the malolactic fermentation (BSA = Biodegradation of Acid) is taking place or is to be approved.
If the acidity is to be retained in the wine (which can be useful in warmer climates), this is prevented by removing the necessary proteins. There are different legal regulations for each wine-growing zone or country-specific regulations on the question of whether acid should be added or removed (see Acidification and deacidification). After fermentation is complete, certain types of wine are left on the lees for a longer period of time in order to give them more flavour and body and also to make them more carbonated. This is known as yeast sediment storage (French sur lie = storage on fine or full yeast).
During the last decades, various forms of fermentation have been developed. The main goal is an intensive extraction, i.e. to extract as many colour and flavour substances as possible from the grapes. There is a myriad of techniques and procedures:
The amount of residual sugar remaining in the wine after fermentation gives rise to the tastes defined by wine law; see sugar content. Complete lists of the numerous cellar techniques, as well as the types of wine, sparkling wine and distillate regulated by wine law are included under the keyword vinification. Comprehensive information on wine law can be found under the keyword wine law.