Ethylene treatments on harvested grape berries: physiological responses and effects on wine composition

In this study, we elucidated how ethylene post harvest treatments may affect grape metabolism and the resulting wine. key categories of secondary metabolites have been analyzed in berries after 24 and 36 h of treatments, and after two days from the end of treatment. In berries, ethylene was shown to increase anthocyanins already after 24h, whereas total phenols, flavonoids, condensed tannins, anthocyanins and tartaric esters increased at 36 h. Ethylene appeared to stimulate the degradation of carotenoids and a progressive increase of the content of norisoprenoids. Chemical analyses were integrated with target gene expression analyses performed at 24 h of treatment. Two key genes, encoding aphenyl ammonia lyase (PAL) and an expansinA were upregulated in ethylene-treated berries. Considering cell wall metabolism, the activity of both pectin-methyl esterase and beta-glucosidase were enhanced in ethylene-treated berry skins, thus eventually affecting the vinification process (extraction of metabolites from the skins) and the composition of the resulting wines. Indeed, in the wine obtained from ethylene-treated berries, higher concentrations of specific phenol compounds belonging to the classes of flavonols, anthocyanins, flavan-3-ols, and stilbenes, were detected. Ethylene induced a significant change in the wine aroma profile by increasing free volatile categories, such as phenols and fatty acids, and reducing the content of carbonyl compounds and, in particular, of esters. Less pronounced differences between control and ethylene-treated wines were observed in terms of glycoside-bound volatile compounds. Results indicate that postharvest treatments with ethylene on grapes are effective in modulating specific metabolic processes of the berries, resulting in wines with increased concentration of specific polyphenol classes and some changes in the aroma profile