Tea Oxidation/Fermentation - what is it?

Tea Oxidation/Fermentation - what is it? I used to be quite ignorant of this word, and preferred to just drink and enjoy my tea without questioning the degree of its oxidation/ fermentation. Who cares that green tea is unfermented and oolongs are partly fermented, when you can simply enjoy the drink?

But then it became obvious that it's not possible to explain the difference between tea types without understanding what oxidation is.

Oxidation (sometimes called Fermentation) is the process, which happens inside a tea leave under the influence of oxygen, temperature, water and mechanical manipulations of a tea farmer. This process give results in a tea's distinct aroma, taste and colour.

The same thing happens with an apple if you cut it in pieces and leave them for some time - they get dark, or oxidised.

And so is with tea. The process is such that tea leaves are crushed, left to rest, then watered, then heated up, then crushed again, and so on and on, in different variations. The more manipulations are done with tea, the more it is "tortured" - the higher the degree of oxidation.

So you can often hear that "this tea is lightly fermented", or "that tea is heavily oxidised", or "that type is 40% oxidised" - basically all of these meaning one thing - the amount of manipulations done on tea leaves before the final baking stage.

So green and white teas are the least fermented. Red and Puerh - the most fermented. And Oolongs are in between with oxidation being in the range of 20-70%.

Oxidation is an infinite process and continues every single second but it is dramatically stopped with the fixing (heating stage) of the process.

tea processing