WINETECH Technical Yearbook 2020

The phenolic compounds of white wines

AUGUST 2020

CHARL THERON: Private consultant KEYWORDS: Phenolic compounds, white wines

wine making. The enzymatic oxidation of both can occur in fresh grape juice or chemical oxidation can occur over time. In the oxidised form it can cause sensory faults by the binding of thiol compounds or browning of the colour. Enzymatic oxidation can be caused rapidly by the grape enzyme, tyrosinase, but this can be controlled by sulphur dioxide and settling practices. Enzymatic oxidation can however be negative if it is caused by the laccase enzyme, which occurs in Botrytis infected grapes. This enzyme is more resistant against sulphur dioxide and better adjusted to the pH of grape juice. Flavan-3-ol (flavonoid) consists mainly of (+)-cat - echin and (-)-epicatechin, which are important in white wines, because they are very reactive towards oxygen. The concentration in grape juice is mainly determined by vineyard practices like the harvesting method, press cycles and press pressure, because it occurs mainly in the skin and seeds. In its oxidised form the flavan-3-ols are very reactive towards oxygen and it can bind the thiols, which are especially formed by the yeast in thiol

Phenolic compounds occur widely in nature. Due to the colour and astringency of red wines everybody is aware of the role of phenols. It can however also play an important role in the colour and quality of white wines. Phenolic compounds are characterised by a six carbon atom benzene ring. Its concentration in white juice and wine is influenced by different fac - tors ranging from the vineyard into the cellar. Oak wood and grapes are the two sources thereof in white wines and the phenols originating from the grapes can be divided in flavonoids and non-fla - vonoids. The latter occurs mainly in the berry pulp, while the first-mentioned occurs in the skin, seeds and stems. If the sources are borne in mind it is obvious that concentrations are influenced by viticultural and winemaking practices. Phenolic compounds develop and change over time and are also influenced by pH, ageing conditions, oxygen and temperature. Hydroxycinnamates (non-flavonoids) and flavan- 3-ol (flavonoid) contribute the most during white

Phenols can cause the browning of white wines.

WINETECH TECHNICAL YEARBOOK 2020 168