The harvested leaves undergo hours of withering, twisting and rolling to develop high levels of oxidation. This creates black tea’s familiar dark and rich nature. Lesser grades may be chopped. Long, aggressive rolling makes for a robust tea, while lighter treatment leads to a more delicate profile.
Black tea is rich in antioxidant theaflavins and thearubigins which combine with caffeine to create the astringent taste. You don’t want it too astringent (bitter) especially as it’s usually drunk in the Far East without milk or sugar. Actually the flavour should be mild, nutty, peppery, sometimes smoky, and a little sweet perhaps with a suggestion of malt, caramel, orange or berry.
Polyphenol oxydase and the polyphenols themselves are stored in the plant’s cells, but when the cells are damaged, say by slicing an apple or dropping and bruising it, the cells are ruptured and the enzyme comes into contact with air. With the help of oxygen in the air, the polyphenol oxidase initiates a series of chemical reactions, transforming the polyphenols and eventually producing melanins (brown pigments).
The general name for this process is “enzymatic browning,” and the fabulous thing (for tea at least) is that it doesn’t just change the appearance of produce: it also alters flavor, scent, and nutritional value.