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

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Micro-oxygenation in Contemporary WinemakingPolymeric pigments are produced by reactions between anthocyanins and tannins, andthese forms of anthocyanins are much more resistant to SO 2 bleaching and pH changes(Jones et al., 1999). For example, at a pH of 3.4, 53% of polymeric pigments are in the redform (Table 13) which is significantly higher than 18% for the free form.b) Flavanols and TanninsGrape tannins are found in the skin, seeds and stems and are important determinants ofthe astringency and mouthfeel of wine (Kennedy et al., 2006). Astringency is due to tanninsforming complexes with salivary proteins (Monteleone et al., 2004). By definition, tanninsare water soluble molecules which are capable of reacting with proteins and polyamides.Protein binding requires a very specific molecular weight of between 1000 and 3000Daltons – smaller molecules aren’t able to produce cross-linkages between molecules andlarger molecules can’t penetrate the active sites of proteins (Margalit, 2004; Herderich andSmith, 2005).There are two main types of tannins in wine:1. Hydrolysable tannins:2. Condensed tannins:Hydrolysable tannins are gallotannins and ellagitannins which actually originate from woodrather than grapes. They are called gallotannins and ellagitannins because they releasegallic acid and ellagic acid on hydrolysis. The most important ellagitannins are vescalaginand castalagin (Ribéreau-Gayon et al., 2006a).The tannins derived from grapes are known as condensed tannins. The monomericbuilding blocks of condensed tannins are the flavanols (also known as flavan-3-ols orcatechols) (Table 12). The most important flavanol monomers are (−)-epicatechin, (+)-catechin, gallocatechin, epigallocatechin and epicatechin-3-O-gallate (Monagas et al.,2005; Herderich and Smith, 2005).The monomeric units, mostly (−)-epicatechin and (+)-catechin, form linkages in a variety ofways to produce dimers, trimers, oligomers and eventually condensed tannins (Figure 10).(In another confusing twist of nomenclature, condensed tannins are also calledproanthocyanadins or procyanidins in the literature because they release anthocyaninswhen heated under acidic conditions). The polymers are distinguished by the length of thechain and the type of inter-flavanic bond formed. For example, Type B dimers have C4-C6or C4-C8 linkages, while Type A dimers have an additional ether bond between the two42
Micro-oxygenation in Contemporary Winemakingmonomers. Oligomers have from 3-10 monomeric units whereas condensed tannins havemore than 10 units (Cheynier et al., 2006). Tannins from grape seeds and skins have anaverage chain length of 10 units whereas skin tannins are usually about 30 units long(Monagas et al., 2005).OHOHOHnOHHOOHOOOHOHOHOHOHOHOHOHHOOHOOOHOHOH4,8-linked Catechin DimerType-B ProcyanidinOHCatechin PolymerFigure 10: Grape Tanninsc) FlavonolsThe flavonols (Table 12) are yellow pigments which are produced in grapes in response toUV radiation (Monagas et al., 2005). Quercetin, myricetin and kaempferol are the mostcommon flavonols in grapes and wine, occurring mostly as 3-O-glycosides rather than freeaglycones.These compounds are important as co-pigments for anthocyanins and contribute to wine’santi-oxidant capacity.43
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