These compounds occur in great profusion in grapes. They are particularly rich in stems, seeds, and skins but also occur in juice and pulp. The concentration of phenolics in grape skins increases if the berries are exposed to sunlight, in particular to ultraviolet radiation, because the phenolic compounds act as a natural sunscreen. Phenolic compounds strongly absorb ultraviolet light, a fact used in their laboratory analysis. Manipulating fruit exposure in the vineyard is therefore a way to affect the phenolic content of berries and wine made from them. See canopy management.
