Alcoholic fermentation in winemaking
After the harvest, the grapes are transported from the vineyard to the cellar for crushing. The must is obtained from the pressing and the marc from the waste. The alcoholic fermentation process transforms sugar into alcohol and carbon dioxide. During the fermentation process, a whole series of by-products are created which will make up what are defined as the secondary aromas that make up the olfactory base of each wine.
The alcoholic fermentation takes place thanks to the yeasts are found naturally in the air and are transported by the wind and insects, depositing on the surface of the plants and on the grape skins, from which they will come into contact with the juice after pressing, causing spontaneous fermentation . These yeasts are generally defined as indigenous, indigenous or wild. In each vineyard there may be different species of autochthonous yeasts, some useful, others less or even harmful, being able to create unwanted substances such as acetic acid and other elements that compromise the stability of the wine. The solution was found by creating selected yeast cultures in the laboratory with the aim of obtaining better control over fermentation and finer quality wines. The most used selected yeasts belong to the Saccharomyces Cerevisiae family for the fermentation of normal musts and the Saccharomyces Bayanus for the fermentation of musts with high sugar content or for the production of classic method sparkling wines.
The role of yeasts in fermentation
In the initial phase of the transformation of sugars, the yeasts perform aerobic respiration (using oxygen from the air), which transforms sugars into CO2 and H2O. This metabolic phase allows the yeasts to obtain the amount of energy necessary for their rapid growth. The actual fermentation takes place inside the mass when the yeasts, due to lack of oxygen, pass from an aerobic to an anaerobic metabolism.
The products obtained from alcoholic fermentation
During alcoholic fermentation the sugars contained in the must are converted by yeasts into ethyl alcohol and carbon dioxide. Yeasts cause the oxidation of sugars and their transformation. Depending on the yeast used, approximately 50% of the sugar is transformed into alcohol, 45% into carbon dioxide, 3% into glycerol and 2% into other substances of different nature which play an essential role in determining the qualities as already mentioned. aromatic and taste of wine.
The quality and quantity of the fermentation products obviously depend on how the process is conducted. It is of fundamental importance that the must has not undergone oxidative phenomena before the start of fermentation, so it is good that it is started as soon as possible, after the normal operations of stabilization, decanting and clarification of the must. Since the must is highly sugary, it is in fact particularly sensitive to attacks by bacteria but also to microbial and oxidative alterations. The prefermentative phase must be as short as possible and the must can be further protected by adding sulfur dioxide, which allows to carry out the settling and clarification operations without risking the alteration of the must. The selected yeasts must be added to the must before the start of fermentation.
Duration of alcoholic fermentation
The duration of fermentation, depending on the type of must and how it has been treated, can vary between 5 and 15 days. It is important that fermentation does not take place too quickly, because the formation of carbon dioxide would be violent and would lead to the dispersion of aromatic substances. However, fermentation must not be too slow, because it would risk generating unwanted substances, including an excess of volatile acidity. The temperature must also be kept in the right range. If the temperature is too low (<15 ° C) the process may not start at all. Fermentation is an exothermic process, i.e. it generates heat, which in turn increases the speed of reactions. If the temperature becomes too high, the fermentation speed goes out of control, and if excessive, it leads to the death of the yeasts and the arrest of the process.
Fermentation for white wines
The white vinification involves the use of a must immediately after pressing the grapes. The must is filtered from the residual solid parts by the decantation process and clarifies. Thethe main objective of white winemaking is to preserve the aromatic complexity of the wine produced and to do this, rigorous temperature control, which must not rise above 20 ° C, is indispensable.
Fermentation for red wines
In red winemaking, must is used in which the skins are left to macerate for the extraction of the color. The extraction of color and polyphenolic substances is in fact one of the primary objectives of fermentation in red. The temperature favors the extraction of these substances and it can be roughly said that fermentation at higher temperatures produces wines with greater body and structure. However, the temperature must not in any case exceed 30 ° C, for the reasons explained above. If the temperature drops below 25 ° C, the extraction of substances from the skins would be difficult, especially that of the tannins, and the wine would be too light and "lackluster". Light red wines not destined for long aging can be obtained by fermentation in a relatively low temperature range, while more structured wines intended for aging in wood require higher temperatures to avoid the development of excessively "herbaceous" aromas.
The ideal moment for racking is chosen according to the type of wine to be produced. Young red wines are left to macerate for four or five days with the skins, and then they are drawn off before the end of the fermentation; the quality red wines and those produced with overripe grapes at the end of the alcoholic fermentation, while the robust and structured wines or those destined for long aging go off a few days later, allowing the red wines to macerate further with the skins and the white wines with the lees, thus giving a greater structure.
The racking is carried out in contact with the air, favoring a strong oxygenation, which has two important effects: first of all it leads to the oxidation of reductive compounds with an unpleasant smell such as hydrogen sulphide (which smells like rotten eggs) and also causes the reactivation of the lees left for the so-called slow fermentation of the residual sugars in the days following racking. The racking off causes the concentration and the decrease in the concentration of sulfur dioxide added as a precaution after the pressing, so it will be advisable to partially replenish it before the subsequent decanting, to avoid the risk of degradation and oxidation.