WINETECH Technical Yearbook 2020

cultivars, like Sauvignon blanc, irreversibly. Such bound thiols are consequently no longer available as aromas. In the absence of glutathione, the (+)-catechin and (-)-epi- catechin will be bound and the aromatic thiols will still be available. Hydroxycinnamic acid is not only the most important form of non-flavonoids in grapes, but also comprises 80% of the phenolic compounds in white wines, without skin contact. It occurs in the berry pulp as esters of tartaric acid and is important in white wines seeing that it is the first to be oxidised. Hydroxycinnamates play a key role in the browning of juice seeing that enzymes are released from the juice, which will lead to

• Finings during fermentation can be utilised beneficially to remove surplus phenols. • Although the enzymatic oxidation of grape juice occurs rapidly, the chemical oxidation can occur at a much slower rate over time. (Seabrook, 2019.) REFERENCE Seabrook, A., 2019. White wine phenolics: What compounds are there and which ones cause problems? Australian and New Zealand Grapegrower and Winemaker , January 2019: 53-54.

its browning. In the presence of glutathione the oxidised hydroxycinnamate will form a colourless grape reaction product and decrease the browning. Sulphur dioxide will limit the enzymatic oxidation, but chemical oxidation will over time not be prohibited if hydroxycinnamates are still available. In oxidised form it can bind thiols as flavan-3- ols, but it is not very effective in doing so. In combination with sulphur dioxide the presence of caffeic acid can however slow down the removal of the volatile aroma compounds. The most important aspects, which can be summarised from the above-mentioned are the following:

• A combination of fining agents is better to remove a broad spectrum of oxidisable or oxidised phenols. • Grape juice or wine will be oxidised if oxidisable compounds occur in it, if it is not protected against such compounds. • Sulphur dioxide additions are not sufficient to prevent the oxidation of key phenolic compounds. • The volatile compounds of certain cultivars like Sauvignon blanc, which require protection, must be understood. • The influence of different press pro- grammes on the phenolic concentra- tions of juice must be understood.

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