WINETECH Technical Yearbook 2021

after rearrangement and oxidation, to the formation of yellow to brown pigments. This is particularly the case with polymers formed through glyoxylic acid or oak- derived compounds, such as furfural, hydroxymethylfurfural and vanillin. In red wine, at the start of ageing and for as long as the quantity of anthocyanosides is sufficiently high, vinylflavanols and vinyl- tannins, resulting from cleavage of the ethyl bridge, have the capacity to react relatively quickly with anthocyanins to form flavanyl-pyranoanthocyanins; these pigments are orange in colour, like vitisins and pinotins. These conversion reactions probably explain the change from the ruby red colour of young red wines towards the brick red colour of older wines. In old wines with reduced anthocyanoside content, vinylflavanols and vinyl-tannins can react preferentially with vitisin A, the predominant pyranoanthocyanin at this stage, to form A-type portisins blue-

coloured pigments. They are present in very low concentrations, but their contribution to the colour of old wines with high pH should not necessarily be neglected. CONCLUSIONS • Today, we have the analytical means to study phenolic compounds in detail. • The structure of each of the polyphe- nolic constituents is characteristic of each grape variety and there are family resemblances within groups of grape varieties. • Within the grape bunch, the phenolic compounds are compartmentalised both at the level of the different tissues (skin, pulp, seed and stalk) and at the sub-cellular level. • These different locations influence their extractability, which nevertheless remains strongly dependent on the

technology used during the preparation of the grapes for wine production. • Their fate in wine depends on a large number of parameters. • Once in solution, the phenolic com- pounds that come into contact react by various reaction pathways: chemical, biochemical and physico-chemical. • The kinetics of the chemical reactions in particular are very slow and affect only a fraction of the constituents extracted, so the impact on the wine composition depends on time and temperature. • Dihydroxylated phenolic compounds are the structures that preferentially participate in oxidation phenomena. Those catalysed by the enzymatic pathway quickly cause changes in the must composition. The strong potential for chemical and physicochemical reactivity of flavonoids

gives rise to a diverse range of new polyphenolic constituents in wine, leading to changes in colour and taste. Knowledge of the chemical structures and the conditions for their formation, as well as those for the formation of supramolecular structures, are important to choose the appropriate winemaking practices, supported by the availability of effective oenological products like enzymes and tannins.

For more information, contact Elda Binneman at Anchor Oenology, ebinneman@anchor.co.za.

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