Obtaining the highest concentration of polyphenols in grapes is one of the main agronomic objectives that has a positive influence on the entire course of red wine making, giving the winemaker the possibility to obtain a wine of certainly higher quality. For this reason, it is important to identify the moment when the concentration of phenolic compounds in grapes is at its highest to start the harvest.
During the ripening period of grapes the concentrations of anthocyanins and tannins in the skins of the berries are subject to variations.
From veraison to so-called technological maturity (maximum sugar/total acidity ratio) the grape skins are enriched with phenolic compounds. In particular, anthocyanins appear at veraison and continue to increase throughout the course of ripening, reaching their maximum concentration near technological maturity, and then decreasing during the over-ripening phase of the grapes. The tannins present in the skins show the same behaviour, with the difference that at the time of veraison they are already present in significant quantities. The concentration of tannins present in the grape seeds decreases from the moment of veraison and during the entire ripening period of the grapes. Depending on the environment, the maximum concentration of anthocyanins and tannins present in the grapes can coincide with the maximum ratio of sugar to total acidity, but it can also be prior or subsequent. It is for this reason that it is deemed important to identify the phenolic maturity of the grapes, as well as the technological maturity, in order to establish the optimal moment for the harvest.
The method considered to be the reference method for the determination of phenolic maturity is the Glories method based on the extraction of anthocyanins from a sample of must, partly under conditions simulating the wine-making process (extraction by means of a buffered solution at pH 3.2), and partly under extreme conditions, capable of completely eliminating diffusion barriers (acid solution at pH 1).
This method is very time-consuming because it requires a long extraction and sample preparation phase. In addition, the analyses must be carried out by an operator specialised in laboratory techniques.
Are there methods to quickly and easily determine when grapes should be harvested in order to maximise the extraction of phenolic compounds?