Electronic nose to study postharvest dehydration of wine grapes

Montepulciano grapes were dehydrated at 10 and 20 degrees C, 45% RH and 1-1.5 m/s of air flow. Samplings were performed at 10%, 20%, 30%, and 40% of weight loss (wl). TSS (total solids content) increased up to 43 degrees and 36 degrees Brix at 20 and 10 degrees C, respectively, in 27 and 48 days. A straight regression line between M (kg water/kg dry matter) loss and days of dehydration with R(2) values, equal to 0.97, was found but an angular coefficient of -0.048 and -0.038, respectively at 20 and 10 degrees C indicated a different rate of weight loss. Volatiles compounds profile changed significantly with the temperature during the dehydration process. Anaerobic metabolites such as ethanol, acetaldehyde, and ethylacetate were much higher at 20 degrees C than at 10 degrees C and rose progressively, while at 10 degrees C, the significant rise was observed between 30% and 40% weight loss. Alcohols and esters prevailed at 20 degrees C while at 10 degrees C larger abundances of aldehydes and terpene alcohols are observed. PCA analysis of GC/MS confirmed the volatiles compounds separation between the samples held at 20 and 10 degrees C. A progressive evolution of the profile is observed for the samples held at 20 degrees C while those held at 10 degrees C show a change of volatiles compounds profile only at 40% wl. Loadings analysis show that aldehydes and terpenols are oriented towards the direction of 10 degrees C samples while alcohols and esters in that of 20 degrees C samples. The behaviours of GC/MS data were partially corroborated by the data of electronic nose. Electronic nose was able to follow the progressive change of aroma profile at 20 degrees C while at 10 degrees C grouped the responses obtained at 10%, 20%, 30% wl all together but at 40% wl, it discriminated the aroma response. This result opens a window on the potential use to monitor commercially the grape dehydration.