Effect of different conditions of dehydration on the volatile composition of Nebbiolo grapes (Vitis vinifera L.)

In the wine sector when we speak of post-harvest, we mean of dehydration or withering technologies used for making not only sweet wines. This work is part of a research project aimed to optimize, in field and after harvest, the production of chemical compounds of oenological interest, that is polyphenols and volatile organic compounds. Until some years ago, the dehydration of grapes has been seen as a process of concentration of simple sugars without any important regard for other quality characteristics, in particular aroma. But, the water loss is responsible for large and significant changes in the metabolism of fruits and vegetables (e.g. respiration and cell wall enzyme activity). For this reason, the technology or practice of dehydrating grapes can play an important role in modulating the production and release of secondary metabolites like free and bound volatile compounds in grape and wine (1, 2). The biosynthesis of aroma compounds is a very complex process that involves various metabolisms. The activation or inhibition of these metabolisms is strongly dependent on the grape maintenance procedure (temperature, relative humidity, dehydration rate, light) before and after harvest. Moreover, the drying process affects the physical state of the berry (shape, tissue texture) depending on ambient conditions and this could change the extractability of aroma compounds from the different portions of the berry during winemaking. In this paper we present the results of an experimental study conducted using a dehydration tunnel for the dehydration of Nebbiolo grapes at different condition of temperature (10°C and 20°C; 60% relative humidity). Our attention was mainly focused on comparing the free volatiles present in skin and pulp of grapes dehydrated at 0%, 10% and 20% of weight loss.