Tips for a proper withering of grapes
The Passito wines are conquering bigger and bigger quotas of the market; their success depends on their peculiar aromas that need to be preserved during the withering stage. In order to do that it is necessary to know where the aromatic components are at the end of the dehydration process
Aromatic grapes, typically employed for the production of Passito wines, differ from normal grapes because of the preponderance of aromatic components in the free form over the glycosylised form, which is not immediately perceivable.
This is clearly an oversimplification if we consider that some aromatic varieties, such as the Sauvignon, derive their aromatic components from biochemical processes which take place during the pre-fermentative and the fermentative stages.
Beside this, it is clear that in order to get high quality Passito wines, it is pivotal to preserve as much as possible the aromatic component, also in its free form, contained in the Moscato, Aleatico, Brachetti and Malvasia grapes.
Hence, it is important to understand both the qualitative/quantitative dynamics of aromas during withering and the localization of them in the grape.
Withering needs to be done very carefully, avoiding any bacterial or fungal contamination, preserving the grape health and providing a uniform dehydration of grapes. As a matter of fact, a too fast withering induces an uneven dehydration of grapes, with negative effects on the quality of the Passito.
Optimal conditions for the withering are the following: 10°C constant temperature, 45% humidity and an air flux of 1.2 m/s. These are the parameters employed by Domenico Tiberi (CRA) during his researches on the chemistry of grapes during withering.
In these conditions, after six weeks, the grapes show a 40% reduction in weight, with a 70% increase in sugar content. The polyphenol and anthocyanin concentrations are increased, as well. The pH tends to rise, given a total titrable acidity, for the buffering effect of the juice that salifies the weak acids.
The aromatic component is basically preserved in respect to the concentration in the fresh fruit. In particular, for two classes of molecules (linalool and norisoprenoids), the concentration has even been shown to increase until a weight loss of 30%, and to slowly decrease after that.
The most interesting thing to say is that, while pulp loses its aromatic components, skin increases it; this keeps on average the total aromatic component constant during the process. The most reasonable explanation of this phenomenon is of course a translocation of the aromatic component from the pulp to the skin.
However, it is also possible that the increment in the skin could derive from an actual increased synthesis of the compounds there, since skin has the biological function to protect pulp and seeds from stress. As a matter of fact, it is known that some secondary compounds have a role as biologic messenger molecules in the fruit.