Oxidation

Definition

In tea making, oxidation is the chemical reaction that turns a green leaf brown, much like a cut apple. Fresh tea leaves hold catechins in one part of the cell and the enzymes polyphenol oxidase and peroxidase in another. When the leaf is bruised, rolled or cut, cell walls break, the two meet in the presence of oxygen, and the enzymes convert catechins into larger compounds: orange-yellow theaflavins and red-brown thearubigins. The maker controls the degree of oxidation by choosing when, and how hard, to damage the leaf and when to stop the reaction with heat. Green tea is heated soon after picking, so it stays essentially unoxidized. White tea oxidizes slowly and lightly during its long wither. Oolong is partly oxidized, from roughly 10 percent in green, floral Taiwanese high mountain teas to 60 or 80 percent in Oriental Beauty and some Dancong. Black tea is rolled hard and oxidized close to completion before drying. Oxidation changes color, aroma and taste. Lightly oxidized teas smell grassy, floral or vegetal; heavily oxidized teas move toward fruit, honey, malt and cocoa, with a darker, fuller liquor and less of the sharp bitterness of fresh catechins.