If you have learned a little about tea processing, you have doubtless come across the saying that Black tea is more “fermented” than Green tea. This is more than a little inaccurate, as you’ll see below.
When an apple is cut open, its inside surface is generally white. Once exposed to air, however, the apple’s interior begins to change to brown and eventually black. The flavor of the apple changes as well. This process is an enzymatic reaction called Oxidation. Many plants experience this darkening with exposure to oxygen, including tea leaves.
When a tea leaf is bruised during the rolling process, the contact of the polyphenol oxidase in the cells of the leaf with other parts of the leaf’s chemistry begins a browning process. This process changes the leaf from what we know as Green Tea to what we call Oolong Tea, and eventually to Black (or Red) tea.
When an apple is cut, there are several options we can use to stop its oxidation and retain its white color and fresh flavor. Applesauce, for example, does not turn black. This is because when heat is applied, it deactivates the enzymes necessary for the reaction to continue. This principle was discovered by early tea producers and used to stabilize or “fix” the tea leaf at a desired stage of oxidation. For green tea, the leaf is heated almost immediately and no oxidation occurs.
The Chinese call this heating process “kill green” (殺青). When this technique was developed, tea producers didn’t understand the mechanism involved and it was assumed that the browning process was a form of fermentation. Even though scientific understanding has progressed in the thousands of years since, this term has remained in use and will still be quoted by many people in the tea industry, particularly in China.
Fermentation in Chinese is Fāxiào (發酵). Oxidation in Chinese is Yǎnghuà (氧化). Because it has become an industry term in its own right, Chinese articles discussing tea will use the term fāxiào, causing confusion for many Westerners. Making this more complicated is the actual bacterial fermentation involved in Puer tea, but that is another topic entirely.
So, to summarize, Green tea is less oxidized than Black tea, and neither are fermented. (And, as an aside, oxidation has no effect on the caffeine.)