Tannins themselves are found principally in the bark, leaves and immature fruit of a wide range of plants. They form complexes with proteins and other plant polymers such as polysaccharides. It is thought that the role of tannins in nature is one of plant defence: they have an astringent, aversive taste that is off-putting to wannabe herbivores. As an animal or insect begins to munch on plant tissue, the tannins are released from cellular compartments and bind with the proteins and other cell components, making them taste unpleasant and rather indigestible. Significantly for winemaking, the grape vine exploits tannins in a rather clever way in its fruit. Grapes start life small, green, mean, and extremely bitter and astringent, through a combination of searingly high acidity and green, aggressive tannins. The grapes are also camouflaged green, the same colour as the rest of the plant. This is because the grape berry’s function in life is to act as a carrier for seeds, and it doesn’t want birds to eat them all before they’re ready. The idea is that the palatability and attractiveness of the berry is timed to coincide with the ripeness of the seed: at the right time, the berry changes colour so it stands out, acidity diminishes, sugar increases and the bitter tannins soften, in order to make it attractive. The birds eat the berries and some time later, the seeds are deposited in a new location. The change in colour from green to red (or purple or black) is brought about through the anthocyanin pigments in the skins.