Postharvest physiology of wine grape dehydration

In this chapter an overview of the wines derived from dehydrated grapes is described. Historical origin of those wines is associated to the same original aim of drying grapes: reduce the grape berry water content for improving aptitude of harvesting, as well as enrich the internal content of quality attributes, mainly sugars. For this reason, the main enological destination of dehydrated wine grapes has been, traditionally, for producing sweet or “passito” wines, having high residual sugar content. A short description of the principal method and techniques for grape drying (in and off-plant, pre- or postharvest), and a distinction of the process character (short- or long-term, low or intense, different stages of grape water loss) is reported. In relation to the postharvest grape dehydration, managed in closed environments under the control of physical and technological parameters (mainly temperature, relative humidity, and ventilation), the induction, modulation, and regulation of the water stress in charge to the grape berry tissue and cells is clarified. This point of view is proposed as strictly related to the opportunity of controlling the possible evolution of the internal quality parameters and of metabolite profiles. Throughout the explanation of the principal biological, biochemical, and chemical pathways, the potential biosynthesis, accumulation, and/or degradation of primary (e.g. sugars and organic acids) and secondary (e.g. volatile organic compounds and polyphenols) metabolites is described. Most significant and problematic characters related to the vinification of dehydrated grapes are reported within protocols proposed as representative of white and red, sweet and dry wines. Finally, a specific section is dedicated to the Amarone wine and to the more modern tendency of producing red dry, structured, wines coming from withered grapes. The description of main qualitative, organoleptic, and sensorial attributes related to this specific wine is also included.2n