HPLC and SEC analysis on the flavonoids and the skin cell wall material of Merlot berries reveals new insights into the study of the phenolic maturity

Abstract

Anthocyanins and tannins contribute to important sensorial traits of red wines, such as color and mouthfeel attributes. Despite the evolution of flavonoids during berry ripening has been extensively studied and the properties of skin and flesh cell wall material (CWM) to bind tannins were described, the mechanism determining the reduction of unpleasant astringency in the last phases of ripening remained uncertain. In this regard, the present research was aimed to better understand the factors involved in the phenolic maturity by a detailed evaluation of the flavonoid characteristics and the CWM properties, in the last weeks before harvest and at harvest. The study was conducted in 2014 and 2015 in a Merlot vineyard located in the hills near Bologna (north of Italy). The analysis of flavonoids and skin CWM was performed on berries sampled 20 and 10 days before harvest and at harvest. Exhaustive extractions were conducted to analyze total anthocyanins and tannins (skin and seed separately), while a model hydroalcoholic solution was used for their extractable portion. Moreover, binding reactions between the CWM and an enological seed tannin were performed to evaluate the magnitude of tannin precipitation and the composition of the  tannins remained in solution.HPLC analysis showed the increase of total and extractable anthocyanins during ripening, while no change was found in the concentration, composition and mean degree of polymerization (mDP) of skin and seed tannins. Also, the composition of CWM did not change significantly, but the tendency of proteins to increase until the harvest was noticed in both years. Moreover, CWM bound increasing quantity of the enological seed tannin during ripening, favoring, in particular, the precipitation of the tannins with higher mDP and of the galloilated forms, which are reported to be the compounds more involved in the perception of unpleasant astringency. The analysis performed by SEC confirmed that CWM bound preferentially the tannins of high molecular mass (MM), while lower amounts of medium MM tannins and negligible quantities of low MM tannins interacted with CWM. Our results confirmed that more ripen berries may release higher amounts of anthocyanins and allowed us to hypothesize that CWM may play a role in the decrease of astringency which is associated with the progression of ripening.