## 3 Results and Discussion
### Evolution of the Concentration of Volatile Compounds during Grape Ripening
#### 3.2.1 Free Volatile Compounds  
The accumulation pattern of free volatile compounds during the ripening process in the four varieties of wine grapes is shown in Figure 1, which are presented by hierarchical cluster analysis. The free volatiles can be divided into two categories according to different trends, with the first category showing a decrease in concentration during ripening, the second category showing an increase in concentration during ripening. The aromas of wine grapes in the D0-D7 period were dominated by carbonyl compounds and showed a decreasing trend during grape ripening. Among the first category compounds, C6 aldehydes were predominant, including 1-hexenal, 3-hexenal, 2-hexenal, (E)-2-hexenal and (E,E)-2.4-hexadien. These compounds all had the maximum content in the D0-D7 period, which is just the source of green aromas in the unripe berry. During grape maturation, C6 aldehydes are formed from fatty acids. Fatty acids such as linoleic acid and linolenic acid are oxidized by lipoxygenase (LOX) and lipid hydroperoxide lyase to produce C6 aldehydes. Furthermore, C6 aldehydes are reduced to C6 alcohols by alcohol dehydrogenase (ADH), resulting in a reduction of C6 aldehydes during grape berry ripening [29]. It was suggested that the ratio of C6 aldehydes to C6 alcohols could be used to determine the ripeness of grapes from an aromatic point of view and that this ratio continued to decrease during the ripening process [30; 31]. The sensory threshold of C6 alcohols is higher than that of C6 aldehydes and has less effect on the aroma. This conversion reduces the green flavor during grape berry ripening. In contrast, C6 alcohols were mostly classified as the second category of compounds, including n-hexanol, (E)-3-hexenol, (Z)-3-hexenol and (Z)-2-hexenol, all of which tended to increase during ripening, with some decreasing during the over-ripening process. Differences in C6 alcohols in different varieties of wine grapes occurred mainly during over-ripening, for example, n-hexanol and (Z)-3-hexenol peaked at D42 in Cabernet Sauvignon and Cabernet Gernischet, while n-hexanol in Cabernet Franc reached a maximum at D21, after which the levels decreased. It was found that C6 alcohols increased and then decreased rapidly during water loss from grapes at over-ripening, the content of C6 compounds was proportional to the activities of ADH and LOX [32], while LOX and ADH activity levels varied with the rate and amount of water loss [33]. In addition, the degree of water loss in the early stages of over-ripening in different varieties of grapevines is susceptible to soil conditions, climate and genetic factors, leading to different levels of water loss in different varieties at the same time and thus having different levels of LOX and ADH activity, while varieties with less water loss continue to synthesize C6 alcohols at the early stages of over-ripening, leading to differences in the levels of C6 alcohols at the early stages of water loss in different varieties. Figure 1: Clustering heat map of free volatile compounds during grape ripening. ((a), Cabernet Sauvignon; (b), Cabernet Germischet; (c), Cabernet Franc; (d), Merlot). As can be seen from Figure 1, wine grape aroma was dominated by alcohols, esters and terpenes in the D28-D42 period. Most of the alcohols, esters and terpen were clustered together and showed an overall upward trend during berry ripening, which belonged to the second category. Among them, the contents of benzyl alcohol, phenylethyl alcohol, theaspirane and butyl acetate showed an upward trend during the ripening of berries from all four cultivars and were able to enhance the floral and fruity notes in ripened berries. However, not all alcohols, esters and terpenes changed equally during ripening in different cultivars. For example, 2-heptanol and methyl heptenone increased gradually during ripening in Cabernet Sauvignon, Cabernet Franc and Merlot berries, whereas they decreased in Cabernet Gernischet.