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Fermentation

fermentazione (I)
fermentation (GB)
fermentação (PO)
fermentación (ES)
fermentation, cuvaison (F)

The term for alcoholic fermentation used to refer to the apparent "boiling" or "foaming" in grape must. Fermentation is understood to be the microbial degradation of organic substances without the inclusion of external electron acceptors such as nitrate or oxygen (i.e. anaerobic) for the purpose of energy production. However, the degradation of organic substances can also take place with the inclusion of oxygen (aerobic). For example, acetic acid fermentation consumes oxygen, which is why it is not a fermentation in the scientific sense as described above. The term fermentation, on the other hand, 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.

Cause of fermentation

The process was long thought to be a natural decomposition. The influence of air (even when oxygen had already been discovered) was ignored. This is why even today breweries still use the advertising slogan "just malt, hops and water", which is actually incomplete. For this reason, the processes involved in converting must into wine, wort into beer and wine into vinegar were referred to as fermentation. The involvement of previously unknown microorganisms was recognised by the French chemist Louis Pasteur (1822-1895) in 1857.

Fermentation occurs spontaneously without human intervention wherever yeast fungi come into contact with aqueous sugar solutions at an optimum temperature of between 18 and 27 °C. Such fungi exist in the vineyard and in the wine cellar. Such fungi exist in the vineyard and also in the wine cellar, which is why we speak of vineyard and cellar yeasts. Many producers deliberately allow this instead of adding pure yeasts. As a rule, fermentation starts automatically with grape must or other fruit juices, but it can also be other substances dissolved or added in water, such as cereals (barley, wheat, maize, rice) in the production of beer or other sugary substances such as potatoes or bread.

Gärung - Gärungsbehälter

The picture on the left shows open stainless steel red wine tanks for mash fermentation, the picture in the centre shows wooden fermentation racks for mash fermentation and the picture on the right shows stainless steel white wine tanks ( Gerhard Markowitsch winery, Carnuntum Lower Austria).

Yeast types

Yeasts play a decisive role in fermentation. After pressing, they multiply rapidly in the grape must by sprouting and dividing. 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. They can also trigger fermentation on their own, which used to be common practice. In this case we speak of spontaneous fermentation. However, this is much more difficult to control, but is favoured especially in connection with organic viticulture for organic wines and natural wines.

Pure yeasts (cultured yeasts) are used to initiate the mainly used pure yeast fermentation. These guarantee a fast, complication-free start to fermentation. If necessary, this is supported by starter cultures (also fermentation starters). Saccharomyces cerevisiae is the most important type of yeast involved at the start worldwide. The alcohol-sensitive yeast species die quickly during fermentation, in the case of natural yeasts from around 14% vol. Only the alcohol-resistant (cultured yeasts) survive until they too stop working at 16 to 18% vol. alcohol content. In the production of spirits, "turbo yeasts" are used 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 flor, which forms during the production of certain types of sherry.

Fermentation process

Fermentation is the enzymatic decomposition of organic substances in an anaerobic environment, i.e. in the absence of oxygen. However, if there is a high concentration of glucose (grape sugar) in the grape must, alcohol can also be formed in the presence of oxygen, i.e. in an aerobic environment. This is also known as 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 Madeira, port and sherry.

Today, fermentation of white wine and rosé is usually temperature-controlled in stainless steel or ceramic tanks, although this is also done by computer in larger wineries. However, barrel fermentation is also used by many producers. Mash fermentation, which is mainly obligatory for red wines but also common for white wines, especially in the production of natural wines, is also used. mash fermentation often takes place in open vats. In larger wineries, however, computer-controlled, coolable and heatable fermentation tanks are also used. Higher fermentation temperatures are necessary here. If the temperature is too high, above 35 °Celsius, boiling can occur, which can lead to acetic souring and spoilage of the wine. Cooling is required to maintain the desired temperatures. Fermentation activators are also added to optimise the fermentation process.

Fermentation time

Fermentation takes place after the pressing process and subsequent must treatment with, for example, removal of lees and sulphurisation. Depending on the type of wine and fermentation method, the duration of alcoholic fermentation can range from a few days to several months in extreme cases for special types of wine (e.g. the Kachet process). On average, this is around 6 to 8 days, often two to three weeks. It takes place in three phases.

In the fermentation phase, the yeast sprouts and multiplies and carbon dioxide is already formed. This is followed by the stormy fermentation (bouillage), which is characterised by strong yeast propagation, carbon dioxide formation and heat development; the must can literally bubble. Malolactic fermentation (BSA), which is triggered by special bacteria, can then begin in parallel, unless this is deliberately prevented. In the silent secondary fermentation phase, the last sugar residues are broken down and valuable flavours and aromas are formed. Depending on the cellar temperature, must temperature and sugar concentration, the yeasts can stop working at an early stage.

Fermentation temperature

An important criterion is the fermentation temperature. The "right" fermentation temperature depends on the desired wine type. As a rule, white wines and rosé wines are fermented at 15 to 18° Celsius and red wines at 25° to 30° Celsius. This is also the optimum temperature for the yeasts to multiply. At higher temperatures, fermentation is vigorous, at lower temperatures it is slow. The warmer the fermentation, the more flavours and alcohol are lost, but more glycerine is produced, among other things. This also ensures a safe to ferment fully with the result of a dry wine.

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 around 15 to 18 °Celsius. Cold fermentation, which is particularly common for white wine cold fermentation using special cold-fermenting yeasts at temperatures of 10 to 15° Celsius. If the fermentation temperature is too high, wine faults such as brown break, acetic souring and carbonisation can occur.

Sugar & by-products

At the beginning of the fermentation process, the two simple sugars glucose and fructose are present in the grapes or grape must in roughly equal proportions. They are converted to 90% ethanol and carbon dioxide in a ratio of 51:49, with glucose being converted faster. Two molecules each of ethanol and carbon dioxide are formed from one molecule of fructose or glucose. Aldehydes, esters, glycerine, methanol, various acids such as succinic acid, acetic acid, lactic acid, sulphurous acid and citric acid, higher alcohols or fusel oils and energy in the form of heat are formed as by-products. This is why 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% alcohol by volume.

Carbon dioxide

The carbon dioxide makes up about fifty times the volume of the must and collects at the bottom as it is heavier than air (relative density 1.52). There is therefore a risk of asphyxiation in the fermentation cellar and despite the knowledge of this fact, a few deaths occur every year due to carelessness. The candle test, which was often used in the past to check the oxygen content of the air, is unsuitable and life-threatening. Discharge via fermentation grilles, which used to be standard practice, can also be dangerous if the fermentation room is lower than the cellar entrance. Today, the carbon dioxide is extracted via exhausters (ventilators) and led outside or, in large cellars, collected and utilised. A small proportion of the carbon dioxide is bound in the wine, which also produces real carbon dioxide. In the production of sparkling wine, the primary aim and actual purpose is to produce the finest possible bubbles and a high degree of effervescence.

offene Tanks für Maischegärung / Maische

End of fermentation

Fermentation ends when the sugar has been almost completely converted into alcohol and carbon dioxide, at which point the yeasts no longer have any food. The second main reason is the alcohol content, which is toxic to the yeasts above a certain level. However, it is not possible to ferment the sugar completely by natural means; a certain amount of residual sugar remains in the wine, even in dry wines. The amount of residual sugar remaining in the wine results in the flavours defined by wine law, ranging from dry to sweet (see sugar content).

If a certain proportion of residual sugar is to be retained, fermentation must be stopped at the relevant time. This is done either by cooling down to minus 3 to 2 °C, by adding sulphur, by heating the mash to 75 °Celsius or, for certain wines, by spriting (addition of pure alcohol). The dead yeasts settle together with other substances as lees at the bottom of the fermentation tank. An unintentional end to fermentation(stuck fermentation) is a common fermentation error that can have a negative impact in the form of flavour loss and acetic souring.

Malolactic fermentation

It is now decided whether malolactic fermentation (BSA = biological acid degradation) should take place or be authorised. This process is mainly used in red wine production, but is also used for white wines. It is a natural process without any necessary intervention. However, it may also be deliberately initiated.

Acid treatment and lees storage

If acidity is to be retained in the wine (which can be useful in warmer regions), this is prevented by removing the proteins required for this. The question of whether acid should be added or removed varies according to the wine-growing zone or country (see Acidification and deacidification). After fermentation is complete, certain types of wine are left on the lees for a longer period of time to give them more flavour and fullness as well as carbon dioxide. This is known as lees ageing (French: sur lie), whereby a distinction is made between ageing on the full lees (immediately after fermentation) and fine lees freed of sediment and sediment.

Techniques & processes

Various forms of fermentation have been developed in recent decades. The main aim is intensive extraction, i.e. to extract as much colour and flavour as possible from the grapes. There are a myriad of techniques and processes, many of which are alternative:

Autovinification to bottle fermentation

Whole-cluster fermentation to cold fermentation

Macération carbonique to must fermentation

Ripasso to over-pumping

Further information

For information on the production of alcoholic beverages, see Champagne (sparkling wines), Distillation (distillates), Speciality wines, Spirits (types), Winemaking (wines and wine types) and Wine law (wine law issues).

Cellar with fermentation tanks: Gerhard Markowitsch winery
red fermentation tanks: by Cjp24 - Own work, CC BY-SA 4.0, Link

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