Oxidation is a chemical process that results in the browning of tea leaves and the production of flavor and aroma compounds in finished teas, oxidation is also responsible for the browning of some fruits and vegetables when they are cut open including potatoes, apples, and avocados. Controlled oxidation usually begins after tea leaves are rolled or macerated, two processes that break down the cell walls in tea leaves. Chemically speaking, oxidation occurs when the polyphenols in the cell’s vacuoles and the peroxidase in the cell’s peroxisomes come in contact with the polyphenol oxidase in the cell’s cytoplasm. The resulting reaction converts tea catechins into theaflavins and thearubigins. Theaflavins provide tea with its briskness and bright taste as well as its yellow color, and thearubigins provide tea with depth and body and its orange-brown color. This conversion of catechins to theaflavins and thearubigins means that the longer the oxidation, the lower the amount of catechins in the finished tea. Also, during oxidation chlorophylls are converted to pheophytin, a pigment that lends to the dark color of oxidized teas. Lipids, amino acids and carotenoids also degrade during oxidation to produce some of tea’s flavor and aroma volatile compounds. The oxidation of tea leaves is controlled by the tea master and requires moist, oxygen-rich air over time. Controlled oxidation is halted by drying the leaves, which renders the enzymes responsible for the reaction inoperable.
Oxidation is often incorrectly referred to as fermentation in the tea world. This is entirely false as fermentation involves the breakdown of chemicals by microorganisms. Until enough research was done, it was actually believed that bacterial fermentation was responsible for the browning of tea leaves and thus, the word “fermentation” was used, however, it is now a widely accepted fact that enzymatic oxidation is responsible for this browning. It is important to note that true bacterial fermentation does occur in the production of post-fermented teas, namely heicha and pu-erh. It is also important to note that there is no such thing as a “fully oxidized” tea; black tea is commonly referred to as a “fully oxidized” tea, but it is impossible to “fully oxidize” tea leaves unless they are first ground into a powder.