# Profiling Taste and Aroma Compound Metabolism during Apricot Fruit Development and Ripening
## 3 Discussion
The decrease in MS and increase in NADP-ME and NAD-ME were together responsible for the malate decrease throughout the development and ripening process, which is further confirmed by the correlation analysis between these enzyme activities and malate accumulation in this process (Figure 8b). CS was identified as the key enzyme for citrate synthesis, while citrate also can be metabolized in the GABA shunt pathway by GAD. The increase in CS enzyme activity mainly accounts for the increase of citrate while no increase of GAD enzyme activity was observed during the process, which indicates that the GABA shunt is not important for citrate accumulation in apricot fruit. Similarly, no significant content differences of organic acids were found between peels and pulps, but significantly higher citrate was found in pulps than that in peels. Even though citrate is not the souest of the organic acids, citrate gives a stronger taste than quinate and malate, which may strengthen the integration of sourness taste of apricot pulps. However, as no CS enzyme activity difference was found between them, GABA shunt may have some contribution to the difference. During the ripening period, all cultivars shared a similar pattern of organic acids accumulation, however, the ratio of quinate, malate and citrate depended on cultivars. The ratio of malate, quinate, and citrate in DX and HY was 1:1:0.7, in HY, LT and BX was 2:1:1, which led to the acidity difference between cultivars. Odor activity values (OAV) are usually used to identify the key aroma compounds in food, and in the present study a total of 18 aroma compounds were key components of apricot fruit by this method, which largely consistent with previous studies [29; 30]. Among these compounds, their content and compositions presented different patterns during apricot fruit development and ripening. Aldehydes and terpen decreased significantly during the whole development of apricot fruit, whereas lactones steeply increased with fruit ripening; these results are consistent with previous studies [18]. Seven key enzymes involved in aroma metabolism were analyzed in apricot during fruit development and ripening. In the fatty acid pathway, unsaturated fatty acids linoleic acid (18:2) and linolenic acid (18:3) can be cleaved into hydroperoxides by LOX, which are subsequently cleaved by HPL to form hexanal or nanonal. Aldehydes can then be reduced to the corresponding alcohols by ADH. AAT catalyzes the final linkage of an acyl moiety and an alcohol to form esters [31]. In the present study, all enzyme activities in the LOX pathway increased rapidly throughout fruit development and ripening process, which is consistent with the corresponding aroma products of the metabolic flux. The \(\beta\)-oxidation of fatty acid is considered as another pathway for lactones formation, the first enzyme acyl-CoA oxidase of this process is involved in lactone formation [32]. The accumulation of lactones, especially for \(\gamma\)-decalactone, kept consistent with ACX enzyme activities, which provide the further evidence for \(\beta\)-oxidation involved in lactones formation in fruit. Carotenoids can be cleaved into volatile apocarotonoids in fruit by CCD [33; 34]. During apricot development and ripening, a significant increase in CCD enzyme activity was observed in apricot fruit and it was in agreement with the correlation analysis between apocarotonoid accumulation and CCD enzyme activity. In addition, light-colored cultivars contained abundant \(\beta\)-ionone and dihydro-\(\beta\)-ionone. For example, LT and BX had higher CCD enzyme activity than other cultivars, which indicates that the carotenoids in these cultivars may be cleaved into volatile apocarotonoids and suggests that this enzyme is important for conformation of color and aroma quality, and future studies will be required to further elucidate this contribution.