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Micro-oxygenation in Contemporary Winemaking6.2 IMPORTANT PHENOLIC REACTIONS OCCURING DURING WINEMAKING ANDAGEING6.2.1 OverviewThe developments that occur in red wine during maturation and ageing have beendescribed for centuries: There is a decrease in colour intensity over time; The colour hue changes from bright red through brick-red to red-orange; The astringency decreases and the mouthfeel softens; and Tertiary aromas and flavours develop.However, there has been significant progress in recent years in understanding thechemistry which underlies these observed changes (Monagas et al., 2005; Ribéreau-Gayon et al., 2006a). The changes in colour and mouthfeel, in particular, are due toreactions involving the anthocyanins and tannins in wine (Fulcrand et al., 2006).Phenolic compounds are relatively reactive compounds due to acid-base reactions of thehydroxyl function and the nucleophilic character of the benzene nucleus.R 1phenolOHOHpKaO -+H +R 1 OHphenolateFigure 11: Phenol Acid-Base ReactionsThe main reactions of anthocyanins and tannins in wine, and their impact on wine colourand astringency, are summarised in Figure 12. The changes in anthocyanin form andcolour with pH, and the reactions between anthocyanins and SO 2 were discussed inSection 6.1.3.2. Tannin-protein interactions and tannin polymerisation reaction were alsobriefly mentioned in Section 6.1.3.2. Co-pigmentation is a phenomenon observed in winewhere the anthocyanins form complexes with other non-coloured organic compoundsresulting in colour enhancement or colour shift (Boulton, 2001). Anthocyanins and tanninsalso undergo degradation reactions, particularly when exposed to high doses of oxygen,resulting in the formation of brown/yellow pigments in wine.44
Micro-oxygenation in Contemporary WinemakingFigure 12: Summary of Key Phenolic ReactionsR 1OHOHOHHOO +R 2HOOOGlucoseOHOHOHAnthocyanins[A]Tannins[T]Co-PigmentationpHEquilibriumReactionwith SO2DegradationOxidationCyclo-addition AT condensationReactions Indirect DirectPolymerisationReaction withproteins &polysaccharidesOxidationA-C A + AO AOH AHSO3 Ad AOHRedRed, violet,colourlessPyranoanthocyaninsColourless Yellow Brick red Orange VioletAeT T-A + T-AO T-AOH Tc TP TdRed, colourless,orangeYellow red Colourless Yellow↑ Colour ↑ or ↓ Colour ↓ Colour Colour becomes more orange Modified Astringency ↓ AstringencyStabilised ColourAC: Co-pigmented complex; A + : Flavylium; AO: Quinoidal; AOH: Carbinol; AHSO3: Anthocyanin-SO2 complex; Ad: Degraded Anthocyanin; T-A + / T-AO / T-AOH: Polymeric pigments (flavylium / quinoidal / carbinol)AeT: Ethyl-linked Anthocyanin-Tannin complex: T-A: Tannin-Anthocyanin complex; Tc: Condensed Tannin; TP: Tannin-polysaccharide; Td: Degraded TanninDrawn from (Ribéreau-Gayon and Glories, 1987; Ribéreau-Gayon et al., 2006a)45
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