After having pressed the grapes and once obtained the must, it is time to start those chemical processes that will turn the juice into wine.

Alcoholic fermentation – called primary fermentation as well - is a fundamental process for the production of wine. Its role is not limited just to the conversion of sugar into alcohol and carbon dioxide - as well as other products - but it is what defines, or rather reveals, the organoleptic qualities of the grape. In fact, all the potential qualities of a wine are hidden in the grapes. The role of the alcoholic fermentation is essential for the formation of aromatic and gustatory qualities of the wine; it is said that "the primary aromas are hidden in grapes, but it is the fermentation that reveals them."

Fermentation is a natural process that allows the transformation of grape juice - the juice - into wine. Alcoholic fermentation, or primary, can be considered as a chain reaction of chemical phenomena. During these processes, the sugar content in grape juice is converted by enzymes of yeast in ethanol and carbon dioxide. Depending on the type of yeast, from the conversion of sugar, we obtained about 60% alcohol, 35% of carbon dioxide, 3% glycerol and 2% of other by-products. This complex process, misunderstood for centuries, also produces other substances, in fact regarded as "sub", and which play an essential role in taste and aromatic qualities of wine. Among these, the most important are acetaldehyde, acetic acid, ethyl acetate, glycerol and other alcohol. These "secondary" products are vital to the development of the aromatic qualities of each wine and, depending on how they were turning the alcoholic fermentation, the aromas may deteriorate or refine.

Fermentation is an extremely complex process that develops in more than thirty subsequent reactions caused by enzymes of yeasts, unicellular microorganisms that exploit the sugars present in the must to ensure their growth and multiplication. The type of yeast most commonly used in wine making is Saccharomyces Cerevisiae, also used for beer fermentation and leavening of bread. The entire process is done under anaerobic conditions, ie in the absence of oxygen, so the yeast produce energy by converting sugars. In the early stages of fermentation, yeasts play an aerobiotica respiration, use oxygen that is present in wine, turning the sugar into water and carbon dioxide. When the must run out of oxygen, begins the fermentation process itself - a process carried out under anaerobic conditions - during which the yeast cells produce energy through the oxidation of sugar, turning them into ethanol, carbon dioxide and other secondary products.

The quality and quantity of primary and secondary substances produced during fermentation, depend largely on the conditions that occur during this process. It is essential that the juice is in the best possible conditions before the start of fermentation.

For this reason, it is very important that the fermentation of must be initiated as soon as possible, immediately after the normal stabilization operations. At this stage, it is particularly sensitive to attack by bacteria, deterioration caused by microbial activity and oxidation.
It is also of main importance that the fermentation is carried out according to the typical timing of this process, a time which may vary between 5 and 15 days. A fermentation conducted too slowly can produce substances responsible for secondary flavours and aromas "ordinary" and an excessive amount of volatile acidity. On the contrary, if the fermentation is carried out too quickly, this will cause an excessive rise in temperature causing the loss of aroma - which is dispersed by the huge amounts of carbon dioxide - with the result of obtaining a regular and free wine quality Organoleptic pleasing. The temperature plays a key role for the smooth conduct of the fermentation. Depending on the type of wine that is produced, it is essential that the temperature is kept constant within a specified range, as we shall see later.

A too low  temperature - usually less than 15 ° C - to prevent the initiation and progress of fermentation, increasing the risk of oxidation or in the case of red wines, an insufficient extraction of colour. Temperature too high cause the performance of a too rapid fermentation with the risk of obtaining a wine rather coarse and ordinary, devoid of every quality of finesse. An excessively high temperature also causes the fermentation lock, and even the death of yeast. Temperature is also important during the addition of selected yeasts in the must. It is important that the temperature of selected yeasts is as equal as possible to that of which must be added, because excessive differences in temperature may cause the death of most of yeast, thus making it useless so. Finally, it should be remembered that the selected yeasts are added to the must before fermentation.