Varietal relationship between skin break force and off-vine withering process for winegrapes.

This paper presents a detailed study on the influence of the winegrape variety, skin hardness of fresh berries and withering thermo-hygrometric conditions on the dehydration kinetics of the grape. Although some works already published are focused on the changes in the weight loss rate during grape dehydration under different environment conditions for several varieties, the effect combined of the skin mechanical properties and the thermo-hygrometric conditions has not been previously considered. The skin break force was confirmed as varietal marker. Furthermore, its role in the selection of the withering conditions was also assessed. The fresh grape berries were classified in two groups (soft and hard) according to the skin break force, and four off-vine withering conditions were evaluated. The softer skins facilitated the dehydration process increasing the weight loss rate but significantly only at low temperatures. The increase in temperature and/or the decrease in relative humidity favored the grape dehydration process. A weight loss of about 50% after 12 withering days was achieved at air temperature of 28º C and relative humidity of 40%. At the same dryer environment conditions, certain varietal effect in the dehydration kinetics was observed because a faster weight loss occurred for those grapes characterized by lower values of the skin break force. These results suggest that the grape dehydration rate, influencing the quality of dessert wines, should be planned considering the skin hardness of fresh berries and the withering conditions. The skin break force represents a new variable that should be considered in modelling the withering process and in selecting, prior to harvest, which is the most appropriate vineyard to elaborate the dessert wines desired.