Alcoholic Fermentation
Yeasts are naturally found in the air and are carried by wind and insects, depositing on the surface of plants and grape skins. After crushing, they come into contact with the juice, causing spontaneous fermentation.
Indigenous and Selected Yeasts
Yeasts are unicellular fungal microorganisms, spherical, oval, or elliptical in shape, about 5-30 μm in length and 1-5 μm in width. They are already present on grape skins, less so on immature berries, but with maturation and up to the harvest, sugars reach the surface of the berries, providing nourishment to the yeasts and increasing their population.
The so-called indigenous yeasts (naturally occurring) belong to different families than artificially selected ones (Saccharomyces cerevisiae). On immature grape clusters, genera such as Torulopsis, Cryptococcus, Rhodotorula, and Candida, as well as Aureobasidium, Sporobolomyces, Filobasidium, which are generally present in the vineyard (soil, leaves, bark), predominate. In mature grape clusters, apiculate yeasts with oxidative metabolism, such as Hanseniaspora and Metschnikowia, are mainly found, while the main fermentation agent, Saccharomyces cerevisiae, is only present in very small quantities on the clusters.
Yeast Respiration and Growth
In the initial phase of sugar transformation, yeasts perform aerobic respiration (using oxygen from the air), which converts sugars into CO2 and H2O. This metabolic phase allows yeasts to obtain the energy needed for their rapid growth. The actual fermentation occurs within the mass when yeasts, due to lack of oxygen, switch from aerobic to anaerobic metabolism.
Glycolysis and Alcoholic Fermentation
The set of reactions for the transformation of sugars (glucose and fructose), both anaerobically and aerobically, is called glycolysis. Glycolysis occurs within yeast cells, leading to the production of ATP (adenosine triphosphate), high-energy molecules used by cells, and by-products such as pyruvic acid, which in turn is a starting molecule for subsequent reactions and biosynthesis intermediates, including glycerol. Under anaerobic conditions, yeasts convert pyruvic acid into carbon dioxide and ethyl alcohol, which conclude alcoholic fermentation.
Products of Alcoholic Fermentation
During alcoholic fermentation, the sugars contained in the must are converted by yeasts into ethyl alcohol and carbon dioxide. Yeasts are unicellular microorganisms that use the sugars present in the must to grow and multiply. In the initial stages of this process, which develops in over thirty subsequent reactions, yeasts carry out aerobiotic respiration, meaning they use the oxygen present in the must to transform sugars into water and carbon dioxide. Soon the little oxygen present in the must is exhausted, and it is at this point, under anaerobic conditions, that the actual fermentation begins. In this phase, yeasts cause the oxidation of sugars and their transformation. Depending on the yeast used, about 50% of the sugar is transformed into alcohol, 45% into carbon dioxide, 3% into glycerol, and 2% into other substances of various natures which, as already mentioned, play an essential role in determining the aromatic and gustatory qualities of the wine. The most important by-products worth mentioning are acetaldehyde, acetic acid (responsible for "volatile" acidity), ethyl acetate, glycerol, and other types of polyvalent alcohols that, among other things, determine the softness of the wine's taste.
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 occur too quickly, as the formation of carbon dioxide would be violent and would lead to the dispersion of aromatic substances. However, fermentation must not be too slow either, as it would risk generating unwanted substances, including an excess of volatile acidity. The temperature must also be maintained within the correct range. If the temperature is too low (<15°C), the process might not start at all. Fermentation is an exothermic process, meaning it generates heat, which in turn increases the speed of the reactions. If the temperature becomes too high, the fermentation rate goes out of control, and if excessive, it leads to the death of the yeasts and the arrest of the process.
Alcoholic Fermentation: White Wines
White winemaking involves using a must from which the skins have been separated (draining) immediately after crushing the grapes. The must is prepared by separating the residual solid parts (decantation and clarification). The main objective of white winemaking is the preservation of the aromatic complexity in the produced wine, and to achieve this, strict temperature control is essential, which must not exceed 20°C. The fermentation tanks made of stainless steel are equipped with an external jacket through which cooling water flows; in its absence, cold water can be run along the tank walls. An alternative technique involves fermenting the musts in small wooden barrels (barriques) and subsequently aging the wine “sur lies“ (on the lees) for a more or less extended period. Periodically, in this case, "bâtonnage" is performed, which is the action of putting the wine lees back into suspension, carried out using a stick (hence the term) whose movement agitates the wine. In this way, the lees previously deposited at the bottom of the container rise back into suspension. The yeasts undergo partial decomposition (autolysis), releasing colloidal substances into the wine that have the ability to bind phenolic compounds, particularly more reactive tannins. The consequence is an increase in the wine's body and roundness sensation.
Alcoholic Fermentation: Red Wines
In red winemaking, must is used in which the skins are left to macerate for the extraction of color. The extraction of color and polyphenolic substances is, in fact, one of the primary objectives of red fermentation. Temperature favors the extraction of these substances, and it can be broadly 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 tannins, and the wine would be too light and "flat." Light red wines not intended for long aging can be obtained by fermentation within a relatively low temperature range, while more structured wines intended for aging in wood will require higher, but not excessive, temperatures, to avoid the development of excessively "herbaceous" aromas. High temperatures are sometimes necessary for musts from grapes poor in coloring substances, such as Pinot Noir.
Conclusion of Alcoholic Fermentation
The indicator of the fermentation process's progress is density. The presence of sugars makes the must denser than water (d>1), while the presence of alcohol and polyalcohols lowers the density below this value. A common densimeter, also called a mustimeter, is used to control density. When the must's density falls below unity, fermentation is practically complete, and the tumultuous bubbling of the wine ceases. At this point, with a concentration of 1-2% residual sugars, it is time to perform "racking," which is the separation of the wine from yeast residues and other solid substances that settle at the bottom at the end of fermentation, to prevent their degradation from potentially damaging the wine's stability and quality.
Racking
The ideal time for racking is chosen based on the type of wine to be produced. Young red wines are left to macerate for four or five days with the skins, and are then racked before the end of fermentation; quality red wines and those produced with overripe grapes are racked at the end of alcoholic fermentation, while robust and structured wines or those intended for long aging are racked a few days later, allowing red wines to macerate further with the skins and white wines with the lees, thus imparting greater structure.
Racking should be performed in contact with air, promoting significant oxygenation, which has two important effects: firstly, it leads to the oxidation of reductive compounds with unpleasant odors such as hydrogen sulfide (which smells of rotten eggs), and secondly, it causes the reactivation of the remaining yeasts for the so-called slow fermentation of residual sugars in the days following racking. Racking causes the removal and decrease in the concentration of sulfur dioxide added as a precaution after crushing, so a partial reintegration before subsequent transfers will be advisable, to avoid the risk of degradation and oxidation.
