Irrigation, a way to affect the phenolic and anthocyanic composition of grapes
Hydric stress in certain epochs, such as before and after veraison, slows down the development of the fruit and increases concentration of certain compounds, thus affecting significantly the composition of wine
To irrigate or not to irrigate the grapevine? This is one of the most common questions for wine grower, especially in function of the production they wish to obtain.
Notoriously, hydric stress can limit the development of fruits, because of a limited flexibility of cell walls, especially after veraison. In such instances, a higher accumulation of anthocyanic and phenolic compounds in response to the hydric deficit, with consequences on the final phenolic composition, and on the sensorial profile, of the wine.
The research team of Prof. Bindon, from University of Stellenbosch, South Africa, carried out a research to keep track of phenolic alterations due to different irrigation modes during two grape harvests on a Merlot wine. The concentration of anthocyanins, tannins and flavonoids in grapes were measured by means of liquid chromatography (RHPLC) following extraction with ethanol. The concentration of the same compounds in wines, following fermentation, was monitored for one season only, with a specific focus on the influence of fermentation on the formation of pigments, in order to achieve a detailed statistical analysis of color and phenolic composition in 6 month old and 18 month old wines.
The first interesting element is that, if anthocyanin content increased with hydric stress while fruit weight decreased, no effect was measured on tannins, which remained unaltered in the different irrigation modalities. More interesting results, though, were found during vinification, where non irrigated grapes presented a higher content of anthocyanins and flavonoids with respected to irrigated ones. In particular, for reduced irrigation, bisulphite resistant pigments were increased, with a consequent increase in color stability and some aromatic sensation of wines.
The difference in anthocyanin and flavonoid concentration was found also in 6 and 18 month old wines, with a remarkable difference on wine color especially, due to the higher presence of bisulphite resistant pigments, which stabilized color.