Oxidation
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Summary
Often incorrectly referred to as "Fermentation." Oxidation is a chemical reaction that occurs in the presence of an oxidizing agent such as the oxygen in our atmosphere. (Fermentation, on the other hand, is an anaerobic metabolic process which results in the production of ethanol. This process DOES NOT occur in the production of tea.)
Other common examples of oxidation reactions are the rusting of iron and the browning of various fruits, vegetables, and meats when exposed to oxygen for long periods of time (Note: this is not the same as the browning reaction caused by cooking, which is the Maillard reaction).
Oxidation in Processing
Technically, oxidation occurs continuously from the time a tea is picked until it is drunk. However, it typically refers to the period of rapid oxidation between the time a tea is picked and when the rapid oxidation is effectively stopped via firing or rapid heating (this is also known as the "kill-green" step). Even green teas are fired or heated to stop oxidation.
Note that the initial firing step (the "kill-green" step) is a separate process from the roasting step later performed on many teas. To illustrate: Sencha is a green tea that is heated during the kill-green step, but not roasted later. Houjicha, on the other hand, is a green tea that undergoes the kill-green step and then roasting, at a later step in processing.
Oxidation and Tea Classification
The earlier a tea is fired/heated, the greener it remains–. Thus, green teas are the least oxidized, while black teas (by the western definition) are the most oxidized.
As oolongs fall between these two extremes, some vendors will note an estimated oxidation percentage of their oolongs, using green teas at 0% and black teas at 100% for reference.
Post-production Oxidation
After the firing, a tiny amount of oxidation (called post-production oxidation) still occurs, but most packaging and storage schemes seek to minimize this as it changes the characteristics of the tea, usually for the worse (notable exceptions are Puerh and other Hei Cha). Greener teas, generally meaning green tea and lighter oolongs, are usually more sensitive to post-production oxidation, though again, there are exceptions.
