Getting to Know Your Tea Terms: Oxidation

Tea terms are a bit of a mystery for many new to tea. Thank goodness for tea folks like James Norwood Pratt who take time to define them. (Mr. Pratt is author of Tea Dictionary, a great resource for anyone wanting to learn more about tea.)  For those of you, though, who don’t have time to read through that 370-page master work, we present our first in a series on some of the more oft used tea terms, starting with one that tends to cause quite a bit of puzzlement: oxidation.

First, let’s clarify that oxidation is not the same as fermentation (which will be covered in a later article), even though they are often used as being the same when writing about tea.

Next, be careful what your source for information on this and other tea terms is. There are lots of “forums” online now where people with no scientific background are posting answers to questions like this one: What exactly does “oxidation” mean in the process of making tea? (The “best answer” to the question that was chosen was by someone who says in her bio “I am a writer, blogger, app reviewer, voracious reader, aspiring photographer, cupcake eater, and movie lover.” Nothing wrong with that, but hardly the same as getting a true scientific answer.)

So, to give you a scientific answer, here is what I found on, a site that actually makes sure its “experts” are experts (I condensed several paragraphs into one here to save space):

Definition: Oxidation is the loss of electrons during a reaction by a molecule, atom or ion. Oxidation occurs when the oxidation state of a molecule, atom or ion is increased. An older meaning of oxidation was when oxygen was added to a compound. Example: Iron combines with oxygen to form iron oxide or rust. The iron is said to have oxidized into rust. (Anne Marie Helmenstine, Ph.D., Chemistry Guide)

How this relates to tea leaves: the leaves take in oxygen, which causes a chemical reaction in them which converts tea catechins into theaflavins and thearubigins. These change the leaf color from green to shades of brown, depending on the amount of oxidation.

Controlling the amount of oxidation is said to be the most important step in processing tea leaves. White and green teas are not allowed to oxidize at all (well, actually, tea leaves begin oxidizing as soon as they are picked, but they are rushed to the processor who halts that oxidation). Yellow teas are oxidized slightly. Oolongs are oxidized to varying percents. Black teas are oxidized fully.

Unoxidized green tea:

Partially oxidized oolong:

Fully oxidized black tea:

Now you know! Hope this helps you in selecting your next tea to enjoy.

About Janice and Stephen Shelton

Purveyors of Premium Teas
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