Phenolic Content in Wine - Origin of The Phenolic Compounds

Origin of The Phenolic Compounds

The natural phenols are not evenly distributed within the fruit. Phenolic acids are largely present in the pulp, anthocyanins and stilbenoids in the skin, and other phenols (catechins, proanthocyanidins and flavonols) in the skin and the seeds. During the growth cycle of the grapevine, sunlight will increase the concentration of phenolics in the grape berries, their development being an important component of canopy management. The proportion of the different phenols in any one wine will therefore vary according to the type of vinification. Red wine will be richer in phenols abundant in the skin and seeds, such as anthocyanin, proanthocyanidins and flavonols, whereas the phenols in white wine will essentially originate from the pulp, and these will be the phenolic acids together with lower amounts of catechins and stilbenes. Red wines will also have the phenols found in white wines.

Wine simple phenols are further transformed during wine aging into complex molecules formed notably by the condensation of proanthocyanidins and anthocyanins, which explains the modification in the color. Anthocyanins react with catechins, proanthocyanidins and other wine components during wine aging to form new polymeric pigments resulting in a modification of the wine color and a lower astringency. Average total polyphenol content measured by the Folin method is 216 mg/100 ml for red wine and 32 mg/100 ml for white wine. The content of phenols in rosé wine (82 mg/100 ml) is intermediate between that in red and white wines.

In winemaking, the process of maceration or "skin contact" is used to increase the concentration of phenols in wine. Phenolic acids are found in the pulp or juice of the wine and can be commonly found in white wines which usually do not go through a maceration period. The process of oak aging can also introduce phenolic compounds into wine, most notably vanillin which adds vanilla aroma to wines.

Most wine phenols are classified as secondary metabolites and were not thought to be active in the primary metabolism and function of the grapevine. However, there is evidence that in some plants flavonoids play a role as endogenous regulators of auxin transport. They are water soluble and are usually secreted into the vacuole of the grapevine as glycosides.