Tag Archives: science

Some Notes on Oxidation

Back in the old days I wrote a post on Dobra Tea’s blog about tea and oxidation. This is an updated version of that post.

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 or banana is cut open, its inside surface is generally white. Once exposed to air, however, the interior begins to change to brown and eventually black. The flavor of the fruit changes as well. This process is an enzymatic reaction called Oxidation. Many plants experience this darkening when parts of their cells are exposed to oxygen, including tea leaves.

Photo by Midori.

Photo by Midori.

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, as the Chinese call it, Red) tea.

Gently rolling oolong tea to encourage oxidation.

Gently rolling oolong tea to encourage oxidation. San Lin Shi, Taiwan.

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” (Shāqīng, 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 is still be quoted by many people in the tea industry, particularly in China.

Stopping the oxidation in Liu'An, Anhui.

Stopping the oxidation in Liu’An, Anhui.

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. In addition, oxidation has basically no effect on the caffeine molecule, so green tea is likely to have roughly the same amount of caffeine per cup as a similarly infused black tea (but that is also a different article).