Identification and characterization of polyphenols in fining precipitate

Abstract1

Polyphenols are secondary metabolite widely distributed in plant kingdom such as in fruits, in grapes and in wine. During the winemaking process, polyphenols are extract from the skin and seed of the berries. Fining is an important winemaking step just before bottling which has an impact on wine stabilization and clarification. Most the time, fining agent are animal or vegetal protein while some of them can be synthetic polymer like PVPP (polyvinyl polypyrrolidone) or natural origin like bentonite.
The aim of this study was better understand colloidals phenomenons involved in fining process and determine how much polyphenols content and composition is impact in the finning. The wine used for fining experimentations were young (few mouth after the end of malolactic fermentation) from Merlot and Cabernet Sauvignon grape varieties. Different type and composition of fining agent were used during the investigation on each wine. Some of them were based on animal proteins, vegetal proteins, PVPP or bentonite alone while some were a mix of different type for example a mix with PVPP and vegetal proteins, PVPP and animal protein, and a mix with PVPP, vegetal proteins and bentonite. On the wine before and after fining, monomeric and total anthocyanins, monomeric, dimeric and total tannins, mDP (mean degree of polymerization), flavonol and phenolic acids content were measured. However in order to determine more precisely the content and composition of polyphenol lost during fining, a new strategy have been develop based on the re-solubilisation of the fining precipitate using an organic solvent. 

For these experiments, this new method highlight some drastic differences between fining agents. Indeed, depending of the fining agent the amount and the composition of the polyphenols present in the fining precipitate change. For example, some fining agent do not remove anthocyanins while some other precipitate mainly the p-coumarolylated anthocyanins. Moreover, important differences are also observed for condensed tannins according to the nature of the fining agent. Indeed, fining agent without PVPP do not have the ability to precipitate monomeric or dimeric condensed tannins. Similarly, according to the used fining agent the mDP of the precipitated condensed tannin change. Some fining agent are more selective of oligomeric tannins while some has tendency to precipitate tannins with higher mDP.

This new methodology allow a more precise and clear identification of the polyphenol precipitate by fining agent and will allow a better understanding of the impact of fining onorganoleptic properties of wine. Similarly, a better characterization of the fining precipitate will also help a better understanding of the colloidal structure of wine.