I briefly mentioned oxidation in my getting sorted post when I was talking about the different categories of tea and thought it might be worth talking about a little more.
Oxidation is basically what causes the leaves of the tea plant (that are green when they grow) to turn brown. It is a biochemical reaction which involves the absorption of oxygen (like when an apple is cut). Black tea is generally close to fully oxidised, Green teas are usually non-oxidised and Oolongs tend to be partially oxidised to varying degrees. Oxidation is pivotal to the processing of tea and will change its colour, smell and flavour.
Oxidised teas, are bruised (from lightly to extensively) to break the cell walls and allow the enzymes in the leaves to cause natural oxidation reactions. Heating the leaves stops oxidation by deactivating the enzymes. In this way the tea producer can decide on the extent of oxidation by introducing heat. Green teas are non-oxidised and so are heated early in the production process so that the oxidation process is skipped.
Sometimes tea-oxidation is called fermentation but no microorganisms are used so this is a misnomer. Generally when people talk about tea-fermentation they are talking about oxidation.
However, just to complicate matters, the proper micororganism fermentation does take place with pu-erhs.
There are exceptions to this, but here are the usual oxidation and fermentation levels:
Black tea – almost fully oxidised
Oolong tea – partialy oxidised (ranging from 12% – 80%)
White tea – minimal oxidation
Green tea – No oxidation
Yellow tea – No oxidation
Pu-erh – fermented (sheng pu-erh is not oxidised but shou pu-erh is)