Tea oxidation

Tea oxidation (often loosely called fermentation) is the reaction in which the enzymes of the Camellia sinensis leaf itself turn catechins into colour compounds. How long it runs decides what you get: green tea, oolong or black. Since no microbes are involved, the process is enzymatic oxidation, not fermentation.

The key enzyme is polyphenol oxidase. It oxidises catechins into theaflavins, which are orange-red, and thearubigins, which are red-brown. In black tea production about 75 % of the leaf catechins change this way. In a typical factory the leaf spends 2–4 hours at 24–29 °C, in air of 95–98 % humidity with oxygen in good supply. Run it too long and the colour compounds fall again after their peak.

Green tea never gets the chance: the leaf is steamed at about 100 °C or pan-fired at 300–350 °C and the enzymes switch off. Oolong is oxidised partly, Black tea fully. White tea is not deliberately oxidised; it is only withered for 36–48 hours and dried. With the CTC method, where the leaf is crushed between rollers, oxidation in a drum takes 60–90 minutes; for Darjeeling tea it takes 2–4 hours.

Genuine microbial fermentation happens only in dark tea, ripe pu-erh included: the leaf is piled so that Aspergillus, yeasts and Eurotium can work on it. The Types of tea page keeps this group separate.

Calling black tea fermented is a historical slip. A 2020 review by Tanaka and Matsuo shows that enzymes only start the oxidation and most later reactions run without them. EU Regulation 2022/2340 describes green tea as made without fermentation, meaning without this oxidation.

Why this matters to you: once you can read the processing on the pack, you know what colour and taste to expect in the cup.