Elissa Abi-Habib defended her thesis on Wednesday 2 December (ED GAIA). Carried out as part of the Interfaces flagship project, and financed by Agropolis Fondation, her subject was "Anthocyanins and tannins extraction in red winemaking : study of certain mechanisms and impact of the grape composition". Elissa was supervised by Céline Legrand-Poncet and Aude Verhnet and hosted at the UMR SPO (INRAE-Institut Agro) for three years. She is the first doctoral student of the Interfaces project to defend her thesis.
Grape polyphenols, mainly located in skins and seeds, play a key role in the taste and color of red wines. Their extraction occurs by diffusion during maceration. It is partial and modulated by several factors that can favour it or on the contrary limit it. After diffusion, they undergo other modifications linked to their chemical reactivity. It is therefore difficult to predict the final composition of a wine from that of grapes. The objectives of this work were to identify the factors impacting polyphenol extraction. To this end, two contrasted varieties were studied: Grenache and Carignan. The polyphenol compositions of the different compartments were determined and the analysis of the cell walls of the fleshes and skins was carried out.Studies in model solutions were performed: diffusion of polyphenols from skins and seeds, taken separately or together and in the presence or not of flesh water-insoluble materials (FWIM); adsorption of anthocyanins and tannins on FWIM. They were compared to microvinifications.
Results showed that:
I) the extraction of anthocyanins is dependent on the proportion of p-coumaroylated anthocyanins;
II) the diffusion of tannins from the skins and their adsorption on FWIM are influenced by the composition of the cell walls, in particular in terms of extensins and AGP ;
III) this adsorption is essentially irreversible, selective (tannins of higher DPs and galloylated) and little influenced by the presence of anthocyanins ;
IV) the tannin diffusions from the seeds alone are important but their concentration drops sharply as soon as they are in the presence of skins/fleshes cell walls and/or anthocyanins, in relation to adsorption and/or precipitation and/or chemical reactions; if the pulp insolubles can adsorb large amounts of tannins, this adsorption shifts the solid/liquid equilibrium in favour of diffusion. These results made it possible to account for the differences observed in microvinification between the two grape varieties studied and to progress in the comprehension of the impact of grape composition.