DOES LIGNIN AN ACCEPTABLE MARKER OF GRAPESEED MATURATION AND QUALITY?

Abstract

Usually the winemaker consider polyphenols from the grape berry as an actor of the wine quality. There are frequently consider as a marker of grape maturity. It is commonly known that winemaker consider tannins and anthocyanins as main polyphenol actors for winemaking practices and wine quality. Here we will focus on the characterisation of lignins in grape seeds. Previous studies suggest that the seed is lignified [1], which could explain the change in colour of the seed when it reaches maturity and thus provide a reliable indicator for describing the maturity stage in the seed. Furthermore, lignin is contained in the outer envelope of the seeds [2] and would constitute a mesh that affects the extractability of tannins and therefore the quality of the wine obtained following the winemaking process [3], since these are mainly responsible for the astringency and bitterness in the wine. In order to provide initial answers on the establishment of lignin in the seed, as well as on the quantity and type of lignin found in the seed, a preliminary two-stage study was conducted.

In a first stage, the lignin biosynthesis pathway was studied using qPCR approach with a focus on key genes of the lignin pathway (PAL, 2 isoforms of COMT, CCOAMT, F5H and 2 isoforms of CAD). The analysis of level of transcripts show a differential regulation and timing of transcripts accumulation depending of the stage of maturity and the vintage studied.

In a second step, it was undertaken to identify and quantify the different lignin monomers present in the grapeseed. For this purpose, an extraction of lignins was carried out with an ethanol:toluene, ethanol, water sequence on seed powder. In order to determine the lignin content after extraction, an acetyl bromide procedure was performed as well as a thioacydolysis protocol to cleave the β-O-4 bonds of the lignin polymer and release the different lignin monomers G from guaiacyl, S from syringyl and H from p-hydroxyphenyl. Their identification and quantification was undertaken by HPLC-MS.

This first work on lignin determination in grapeseed give a solid baseline to go further in the comprehensive way to transfer of oenological molecules from grapeseed to must and wine almost in the context of climate change.