micro-oxygenation in contemporary winemaking - Cape Wine ...

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Micro-oxygenation in Contemporary WinemakingTannin (Carbocation)OHOHOHHOδ -8 O7 12δ 6- H 3 +5 C4OHOHHOOOHOHR 1OHOH+R 1HOOOHR 2HOHOδ -8 O7 12635 4OOHR 2OHHydrationOGlucoseT-A (Carbinol)[Colourless]OHGlucoseAnthocyanin (Carbinol)OHOHHOOOHR 1OHOHT-A + (Flavylium)[Red]HOO +R 2OGlucoseFigure 17: Direct T-A Condensation ReactionOHDrawn from (Monagas et al., 2005; Ribéreau-Gayon et al., 2006c)6.2.4 Aldehyde-Mediated Condensation Reactions between Anthocyanins and TanninsIndirect reactions between anthocyanins and tannins can also occur. These reactions aremediated by acetaldehyde and other similar molecules (Figure 18). Acetaldehyde is aproduct of alcoholic fermentation. But as discussed in Section 6.2.2, acetaldehyde is alsoan important product of the oxidation of ethanol in the presence of phenols (Figure 15).Therefore, this is the most important mechanism for anthocyanin-tannin condensationunder oxidative conditions (Saucier et al., 1997). It is also the reaction that would be mostcritical in producing the beneficial effects of micro-oxygenation treatment.50
Micro-oxygenation in Contemporary WinemakingAs shown in Figure 18 the acetaldehyde produced through coupled phenolic oxidationreactions is activated to the carbocation form and then reacts with tannin at C6 or C8. Thenew carbocation attacks the anthocyanin at C8 resulting in an anthocyanin-tannin complexjoined by an ethyl bridge (Wildenradt and Singleton, 1974; Timberlake and Bridle, 1976).The resulting compound is stabilised by deprotonation to form a violet coloured quinoidalbase.HOTannin8 O7 12OHOH+C H 3COHAcetaldehydeHOHOHC CH 3OOHOH6543OHOHOHOHR 1OHHC + CH 3OHOHHO8 O +712R 2+HOO6543OOHOHGlucoseOHAnthocyaninTannin-AcetaldehydeCarbocationGlucoseOHR 2HOR 1O324O + 1H5678 OHC CH 3OHOHHOOEthyl-linked AT[Red]OHOHFigure 18: Acetaldehyde-induced Anthocyanin-Tannin CondensationDrawn from (Monagas et al., 2005; Ribéreau-Gayon et al., 2006c)51
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ACKNOWLEDGEMENTSI must acknowledge
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