Varietal Relationship Between Skin Break Force and Off-Vine Withering Process for Winegrapes

This article presents a detailed study on the influence of winegrape variety, skin hardness of fresh berries, and withering thermo-hygrometric conditions on the dehydration kinetics of the grape. Although some works focused on the changes in the weight loss rate during grape dehydration under different environment conditions for several varieties, the combined effect of the skin mechanical properties and the thermo-hygrometric conditions has not been previously considered. The skin break force was confirmed as the varietal marker. Furthermore, its role in the selection of the withering conditions was assessed. The fresh grape berries were classified into 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 the increase was significant only at low temperatures. The increase in temperature and/or decrease in relative humidity was beneficial to the grape dehydration process. A weight loss of about 50% after 12 days of withering was achieved at an air temperature of 28°C and relative humidity of 40%. Under the same dryer environmental conditions, a certain varietal effect on the dehydration kinetics was observed because a faster weight loss occurred for those grapes characterized by lower values of 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 modeling the withering process and in selecting, prior to harvest, the most appropriate vineyard for the dessert wines desired.