catechin, and its isomer epicatechin are phenolic compounds (see also flavonoids) found mainly in grape seeds, but also in stems and berry skins and, in lower amounts, in flesh. They contribute to bitterness in wines and are the constitutive units of tannins (see also proanthocyanidins), which are responsible for astringency and increasing the stability of anthocyanins (see co-pigmentation and pigmented tannins), leading to longer-lived colour in wines. Because of the increased skin contact involved during red winemaking, catechin concentrations are usually higher in red wines than in whites, but they are involved in browning reactions in both red and white wine. Catechin may play a role in protecting vine parts from microbial attack as a pre-existing chemical barrier, but it is also produced by vines in response to downy mildew infection.
