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Micro-oxygenation in Contemporary WinemakingCHAPTER 6: UNDERLYING CHEMISTRY OF MICRO-OXYGENATIONPart III of this report seeks to review the available scientific evidence related to the topic ofmicro-oxygenation. Chapter 7 will summarise the findings of those studies that haveevaluated the impact of micro-oxygenation directly. However, in the absence of conclusiveevidence from such studies, much of the justification and support for micro-oxygenation isderived from the more general research on phenolic reactions during winemaking andageing. That area of research is the topic of Chapter 6.The first section of this Chapter introduces the important phenolic constituents of winewhile the second section will describe some of the phenolic reactions that have beenshown to be important during winemaking and ageing.6.1 IMPORTANT PHENOLIC COMPOUNDS IN GRAPES AND WINE6.1.1 Importance of Phenolic Compounds in WineThe phenolic compounds are one of the most important constituents of wine, both in termsof their high concentration in wine, and also because of the key role that they play indetermining the organoleptic properties of wine (Ribéreau-Gayon and Glories, 1987;Cheynier et al., 1997; Kennedy et al., 2006; Ribéreau-Gayon et al., 2006c). Wine phenolsinclude the anthocyanins and tannins and it is these compounds that are responsible forthe colour, structure and mouthfeel of wines (Jones et al., 1999; Monagas et al., 2005;Cheynier et al., 2006; Parker et al., 2007).It is also the reactions of phenolic compounds that determine the development of wineduring maturation and ageing (Cheynier, 2002; Fulcrand et al., 2006; Ribéreau-Gayon etal., 2006a). Polymerisation of monomers as well as condensation reactions betweendifferent phenolic compounds produce the characteristic changes associated with ageing:young bright red wines turn red-orange, and harsh young tannins become softer and lessastringent.More recently, it has been identified that it is the anti-oxidant properties of phenoliccompounds that are responsible for most of the beneficial health effects of wine (Manzoccoet al., 1999; de Beer et al., 2002). By removing toxic Reactive Oxygen Species (oxygen34
Micro-oxygenation in Contemporary Winemakingfree radicals) wine phenolic compounds provide important protection against the tissuedamage that causes cardio-vascular disease and cancer (Heim et al., 2002).6.1.2 Basic Structure of PhenolsBenzene (C 6 H 6 ) is a cyclic hydrocarbon with a highly polyunsaturated structure. Theelectrons in the cyclic structure are actually shared by all 6 carbon atoms, a characteristicknown as aromaticity. The aromatic hydrocarbon structure is usually represented asalternating double carbon bonds or a circle inside the 6 carbon ring (Figure 4).HHHHCCCC56HCCHHCCOH41OHOHCCCC32HHHHBenzenePhenolFigure 4: Representation of Benzene and PhenolPhenol has the chemical formula C 6 H 5 OH. It consists of an aromatic benzene ring with anhydroxyl (–OH) group attached (Figure 4). So essentially phenol is a simple aromaticalcohol (Bowyer, 2002; Howell, 2005).All important phenolic compounds in wine are based on this simple structure. The basicphenol ring can be substituted by various other functional groups at carbon positions 2-6.Most wine phenolics are polyphenols which means that they have multiple hydroxyl groupsattached to the benzene ring.HO56565641OH41OH41OH323232HOOHHOOH1,3-diphenolVicinal diphenolPolyphenolFigure 5: Diphenols and Polyphenols35
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