Why the Difference Between Oxidation and Fermentation Matters

Having recently seen yet another article on oxidation versus fermentation as those terms related to the processing of tea leaves, we thought that not only should we repeat here the essential parts of our previous articles about those terms but also why the difference even matters. A lot of hair-splitting goes on in the tea world, such as how to spell pu-erh (with some insisting we should use “puer” or “pu’er” despite the negative impact on SEO and the total ho-hum response from our customers), but in this case, a whole wig is being split, and with good reason.

First, the Difference in the Terms:


“Oxidation” and “fermentation” are often used as synonymous by many tea bloggers, but that’s not very accurate or scientific. Here is a scientific answer:

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.

Examples (click on each photo for tea name and details):


Now it’s time to address the term “fermentation” and see if it can shed some light on things even further. Actually, fermentation is commonly mis-applied to oolong teas and other non-fermented teas. While there may be a minor amount of fermentation occurring, oxidation is the primary process involved here. Part of the confusion is probably due to Westerners calling fully oxidized tea by the term “black tea” instead of what the Asians call it (“red tea”). What we call “fermented teas” are the true black teas.

So, what is “fermentation”?

In fermentation of tea leaves, they are exposed to microflora, humidity, and oxygen in the air which may also produce some reactivated oxidative enzymes in the leaves. This process alters the aroma and flavor of the tea, mellowing it and turning them from being astringent or bitter into ones with pleasant mouthfeels and aftertastes.

A properly fermented tea has been stored under the proper conditions (right humidity level, clean, good air flow, etc.) and has thus matured in a consistent manner without fear of mold developing. It infuses a darker transparent liquid with wonderful aroma, a flavor with little or no bitterness and astringency, and a texture that ranges from silky to velvety. Repeated infusions will be increasingly better.

Pu-erh teas are the best known of this class of teas. They are fermented either naturally or using the Do Wui process developed by the Menghai and Xiaguan tea factories a few decades ago. Westerners call fully-oxidized teas by the term “black tea” but they are not actually true “black teas” (aka, “dark teas” or heichas). Pu-erh is a true heicha.

Some heichas have several sub-versions and come from Anhui, Guangdong, Guangxi, Hubei, Hunan, Sichuan, and Yunnan provinces in China. Examples are Anhui Liu’an Lancha, Guangxi Liubao Cha, Hubei Laobian Cha, Hunan Heicha, and Sichuan Biancha. There is also Ddok Cha from South Korea and Japanese Awabancha and Goishicha.

The best known fermented teas, according to Wikipedia:

English Chinese Pinyin
Dark tea 黑茶 hēi chá
Pu-erh tea 雲南普洱茶 yúnnán pǔ’ěr chá
Liu’an tea 安徽六安籃茶 ānhùi lìu’ān lán chá
Liubao tea 廣西六堡茶 guǎnxī lìubǎo chá
Hunan dark tea 湖南黑茶 húnán hēi chá
Laobian tea 湖北佬扁茶 húběi lǎobiǎn chá
Kangzhuan tea 四川康砖茶 sìchuān kangzhuan chá
Bian tea 四川邊茶 sìchuān biān chá
Pha tea 发茶 fā chá

Example (click on photo for tea name and details):


This is a term used by many tea folks, even those who have been dealing with tea on a professional basis for many years, who are using “fermentation” instead of “oxidation.” The term “post-fermented” is supposed to indicate a tea that undergoes further processing after the normal steps are done: withering, oxidizing (which they call “fermenting” out of a sense of tradition instead of clarity), fixing, shaping, sorting, drying. Some sites, including Wikipedia, have switched already from using “post-fermented” to using “fermented” and sticking with “oxidized” for the processing step after withering. It’s more accurate and simpler. Oxidation is not fermentation. And these teas are not undergoing additional processing after the true fermentation process. So “post-“ is not needed.

Example (click on photo for tea name and details):

So, Why Does All This Matter?

In a word: clarity.

It’s tough enough for newbies to get started enjoying tea in a serious way without running into a lot of tea term confusion. They can also end up buying the wrong tea and get a bit of a shock that will lead them off of that particular tea for years to come, or even for the rest of their tea drinking lives.

A good example is pu-erh. First, some people bring up the whole name spelling thing and get in a big huff about it with anyone who doesn’t want to change away from the spelling that has been used for about a century. Then, some vendors start calling it “heicha,” “dark tea,” “post-fermented” and other terms. So someone innocently orders one of these thinking he or she is buying true black tea. The earthy character smacks them in the face when they first open the package, and things go downhill from there. Expectation is a large part of the tea experience.

Proper labeling is what we strive for as a tea vendor and hope others will, also.


  • Oxidized: teas where the leaves have been allowed to absorb some oxygen.
  • Fermented: teas where the leaves have been fully processed and then undergone the fermentation process.
  • Post-fermented: used by those who confuse the above terms and is meant to mean “fermentation.”

Hope this all helps!

© 2017-2021 World Is a Tea Party photos and text


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Hi, humans, this site is under my editorial excellence. I, your lovable and sassy Little Yellow Teapot, authors articles on tea, etc., and edit the occasional guest article. All in the interest of helping you humans have a better tea experience. TOOOT!


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