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

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Micro-oxygenation in Contemporary WinemakingHowever, the main focus of this section is on the reactions responsible for the ageing of redwine which are also the reactions that appear to be important in micro-oxygenation. Thefollowing key phenolic reactions will be briefly discussed (Monagas et al., 2005):1. Enzymatic + chemical oxidation;2. Direct condensation reactions between anthocyanins and tannins;3. Aldehyde-mediated condensation reactions between anthocyanins and tannins; and4. Cyclo-addition reactions leading to the formation of pyranoanthocyanins.6.2.2 Phenolic Oxidation ReactionsOxidation reactions involve the exchange of electrons.Oxidation - e -X O+ Y RX R + Y OReduction + e -Figure 13: Basic Redox ReactionPhenolic compounds are the most important oxidation substrates in wine. Phenols areinvolved in both enzymatic and chemical oxidation reactions which results in the phenolsbeing converted to quinones.EnzymaticOxidationoxygenOOPPOwaterH 2 OOHOR 1OHR 1OphenolquinoneChemicalOxidationOOoxygenHOOHhydrogen peroxideFigure 14: Phenolic Oxidation Reactions46
Micro-oxygenation in Contemporary WinemakingEnzymatic oxidation is produced by oxidases (primarily polyphenoloxidase, PPO, andlaccase) derived from grapes. Because oxidases are rapidly degraded and inactivated inmust, enzymatic oxidation only occurs early in the vinification process. However, it is animportant cause of browning in white wine making. The main phenolic substrates forenzymatic oxidation are the hydroxycinnamic acids and hydroxycinnamic acid esters suchas caftaric and coutaric acid.Chemical oxidation is much slower but plays an important role in wine ageing. The majorchanges that occur during barrel ageing are primarily due to the slow diffusion of oxygenthrough the barrel walls and the slow oxidation reactions that occur as a result. It is thesebeneficial oxidation reactions that micro-oxygenation treatment aims to produce byreplicating a similar slow exposure to oxygen.The main phenolic substrates for chemical oxidation during ageing are the anthocyaninsand tannins. They become oxidised through complex coupled reactions which convert thephenols to semi-quinones and quinones (Figure 15). This conversion is coupled to theformation of hydrogen peroxide (H 2 O 2 ) which is an even stronger anti-oxidant and whichthen, in turn, oxidises ethanol to acetaldehyde (ethanal).R 1phenolOHOHOOoxygenO OHOOHhydrogen peroxideR 1quinoneC H 2ethanolOHCH 3ODirect Linkingreactions2H 2 OwaterHCCH 3acetaldehydeethanalicLinkingreactionsFigure 15: Coupled Chemical Oxidation of Phenolic CompoundsDrawn from : (Kennedy et al., 2006; du Toit et al., 2006b)47
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