Microcalorimetric monitoring of grape withering

Highlights: • Microcalorimetry was applied to monitor grape withering. • Heat flow was used to estimate rate of respiration. • Calorimetry was coupled with analysis of volatile compounds. • Data analysis showed three steps of withering. - Abstract: This work aimed at monitoring the metabolic activity of grapes during withering by microcalorimetry. Samples of Corvina grapes, a cultivar used in the production of Amarone wine, were dehydrated for about 120 days at an industrial scale plants (fruttaia). Single berries, sampled in the course of the withering process, were closed in ampoules and maintained at constant temperature. As biochemical events (i.e. berry respiration, microbial growth, etc.) are always accompanied by the production of heat (q), the heat-flow (dq/dt) emitted by berries enclosed in the ampoules was used to monitor their metabolic activity during withering, i.e. respiration. For each sampling time, the heat rate production of the berries at 298 K was monitored till a steady state signal was achieved (within 60 h). Such heat flow value was used as marker during the entire withering process (120 days). Its trend allowed to characterize the changes in the metabolic activity of the grape berries along the withering process. To understand the origin of such changes, the emission of volatile organic compounds (VOCs) were also measured by proton transfer mass spectrometry (PTR-MS). The use of microcalorimetry associated with the analysis of specific VOCs fragments offered a valuable information to describe the withering process.