Oxidation describes the process of enzymes in the tea leaves reacting with oxygen. During oxidation, leaves undergo changes in their chemical composition turning them brown and changing the flavor and aroma profile.
Oxidation is an essential step in the processing of tea. Depending on the type of tea which is made, oxidation is either fully prevented or intentionally caused and stopped at a certain level. Whereas green, white and yellow teas are unoxidized or lightly oxidized, black teas are highly or fully oxidized. In between are oolong teas, which cover a wide range of oxidation levels from lightly oxidized to highly oxidized.
The process of oxidation is intentionally caused by crushing, rolling or tumbling the leaves. Thereby the walls of the cells are damaged, exposing the inside of the leaves with oxygen, resulting in a series of chemical reactions. In order to stop the oxidation at a certain level, leaves have to be heated or dried. Heat denatures the enzymes responsible for oxidation.