Characterization of various groups of pyranoanthocyanins in Merlot red wine

Abstract

In red wines, anthocyanins evolve during the wine-making process and ageing. They react with other compounds (such as vinylphenols, acetaldehyde, pyruvic acid…) to form a stable family of compounds called pyranoanthocyanins. Furthermore, the oxidation process can modify the anthocyanic profile of a red wine. It is also interesting to evaluate the occurrence of the different subclasses of pyranoanthocyanins and to characterize their chemical properties. The first objective of this study is to evaluate the occurrence of the different groups of pyranoanthocyanins in an oxidised Merlot wine by a centrifugal partition chromatography strategy. The second goal is to evaluate their relative impact in red wines from Bordeaux region by measuring their concentrations. Centrifugal partition chromatography (CPC), as the key purification technique, is commonly used in phytochemistry to separate natural substances. Based on the partition of compounds in a non-miscible liquid-liquid system, it provides many advantages. On one hand, the fractionation is applied on a significant quantity of product, in a short period of time, and thus leads to high purification yield. On the other hand, the selectivity of the solvent system provides efficiency for separating molecules from each other. The red wine used in this study was an oxidized sample from Merlot. It was also fractionated with a gradient elution solvent system. Each obtained fraction from CPC was submitted to HPLC-ESI in order to group the same UV and visible profiles. The pigments were also distributed in 8 blocks and the wash fraction, which were finally analysed with a UHPLC-ESI/Q-ToF strategy. Attention was first focused on blocks 1 and 2. The study of their anthocyanic profile by UHPLC-ESI/Q-ToF revealed the occurrence of various adducts depending on the polarity. Some pyranomalvidin-3-O-coumaroylglucoside adducts, and pyranomalvidin3-O-glucoside with a procyanidin dimer were detected in block 1, and pyranomalvidin-3-O-glucoside-4-vinyl(epi)catechin or acetylglucoside-4-vinyl(epi)catechin were found in block 2. HPLC at the preparative scale allowed separating and collecting each pigment in order to determine and validate their molecular structure by nuclear magnetic resonance (NMR). With the aim to complete this study, further investigation will determine the chemical properties of these molecules. Finally, a first evaluation of their concentrations in a few red wines from Bordeaux region
(oxidized or not) will determine a kinetic pattern of the pigments and their relative importance as markers of wine ageing. Furthermore, CPC which is used in this study is an appropriate anthocyanin fractionation and purification technique at the preparative scale towards the complexity of the red wine sample.