## 3 Results and Discussion
### Free Volatile Aroma Compounds in Fresh and Dehydrated Grapes
Among the volatiles, 1-hexanol, 2-hexen-1-ol, and 3-methyl-1-butanol were the major compounds in greenhouse-dried grapes. The opposite effect was found for chamber-dried grapes, whereby the concentration of C6 alcohols was significantly lower compared to fresh grapes. This differential effect on the concentration of C6 compounds could have been due to different metabolic changes occurring in grapes subjected to a fast dehydration rate at higher temperatures that cause a stronger concentrating effect and a faster water loss. A recent study by [PERSON] et al. [28] found that slow dehydration is necessary to induce gene expression and metabolite accumulation associated with the final quality traits of dehydrated berries. The accumulation of desirable key metabolites during postharvest dehydration is inhibited by rapid dehydration conditions that shorten the berry lifetime. Metabolomics and transcriptomics have demonstrated that the dehydration process results in modulation of the expression of various genes implicated in grape metabolism, such as secondary metabolism of phenols, terpen, and lipids [10]. The water loss is reflected in the cell metabolism that transitions from aerobic to anaerobic [18; 29]. This shift is associated with alcohol dehydrogenase (ADH) activity and may influence the metabolic changes in volatile compounds depending on the temperature, relative humidity, and level of dehydration [10; 28]. The activity of oxidative enzymes, such as lipoxygenase (LOX), leads to the formation of C6 alcohols and aldehydes [6; 9]. Among esters, ethyl acetate was found in the highest concentration, comprising more than 90% of the total ester concentration in both types of dried grapes. Although significant differences were found for ethyl lactate, ethyl hexanoate, ethyl octanoate, and hexyl acetate between the types of dried grapes, their concentrations were low and varied between 0.05 and 0.32 ug/L. Terpenes are of great interest to the aroma of wines, and it was important to examine the dehydration effect on their quantitative and qualitative composition in our study. The concentration of monoterenes significantly increased in dried grapes compare to fresh grapes, although no significant difference in the total terpen level was found between the two types of dried grapes. Terpinen-4-ol (7.67 ug/L) and linalool oxide (4.77 ug/L) were the prevalent monoterenes in greenhouse-dried grapes, whereas trans-limonene epoxide (6.38 ug/L) was the major one in chamber-dried grapes. The monoterpene profile is related to the unique sensory aroma of varietal white wine [30]. However, monoterenes in 'Marastina' dried grapes were found in low concentrations bellow their sensory thresholds, but they can still act as the significant odorants because of their synergy with other compounds [31; 32]. The most recent study showed that the composition of non-volatiles strongly increases the volatility of monoterene isomers in 'Pinot Gris' wine through a combination of enhancing and suppression effects, with the aroma perception changed when more components were present [33]. Monoterpenes were found to influence fruity aromas (orange flowers) in wine, suggesting that linalool (odor threshold of 6 \(\mu\)g/L) [34] and linalool oxide may interact with non-volatiles, resulting in change in their aroma perception [33; 35]. The concentration of free \(\beta\)-damascenone in both types of dried grapes significantly decreased in comparison to fresh grapes; their concentration was lowest in chamber-dried grapes. A decrease in the concentration of \(\beta\)-damascenone was also observed with the dehydration of grapes, in agreement with the observations of [PERSON] et al. [36].