This technique consists in the dehydration of the berries, so that, while most of the water evaporates, the sugars and other organic compounds concentrate more. In this important winemaking technique is the type of grapes that you decide to use: the choice will fall on the aromatic type, different from the others because in it the greater is the presence of free aromatic compounds, which are therefore immediately identifiable. In any case, the main vines used are Moscato, Aleatico, Malvasie and Brachetto. This process also requires particular attention in preserving the grape from bacterial and fungal contamination, as well as in ensuring a dehydration that is homogeneous over the total of the grapes, so as to preserve the quality of the finished product.
Homogeneity of dehydration and duration of the procedure
The grape drying represents a very delicate phase in the production of dessert wines, therefore requiring ideal environmental conditions, as well as a long period of time for its optimal completion. In fact, a hasty and disrespectful process of the physiological timing could negatively influence the quality of the wine. Furthermore, according to some researches, the optimal environmental conditions for a good outcome of the procedure are represented by a temperature around 10 ° C, humidity rate of 45% and air flow of 1.2 m / s. In compliance with these conditions, after a period of about six weeks, on one hand the body weight of the berries will decrease by about 40%, while on the other hand the concentration of sugars will increase by 70%; at the same time, an increase in pH due to the process chemistry will be appreciated.
On-vine withering technique
The different techniques used can be divided into two main categories: on-vine and off-vine withering. In the first case, the grapes are left for long periods on the plant, so that, during the process of over-ripening, the berries continue to be fed by the plant, which opposes the continuous dehydration with the supply of nutrients, including of course the sugars. This category includes techniques such as withering in the plant with the development of Botrytis Cinere, a "noble rot" capable of making the berry softer and increase the internal glycerin rate; in other cases the bunches are left on the plant until after the winter season: the grapes dehydrate and concentrate the sugars, which will prevent them from freezing, according to Raoult's law.
With the off-vine process we mean all those techniques that are put in place when the bunches of grapes have already been separated from the plant. In this case, the phenomenon of concentration of sugars is not due to the fact that it increases their absolute quantity, but rather due to the fact that the water, which leaves the berries by evaporation, decreases: in essence, the sugars are more concentrated in relation to the quantity of liquids left in the berries. The sugar concentration is however lower than that observed in dried on-vine grapes, with the result that the wines obtained with this latter technique will be sweeter. Immediately after the harvest, the process can take place in widely ventilated rooms, called fruit cellars, or in rooms where environmental control is electromechanical.