What is Oxidation?

Oxidation refers to a series of chemical reactions that result in the browning of tea leaves and the production of flavor and aroma compounds in finished teas. Depending on the type of tea being made, oxidation is prevented altogether, or deliberately initiated, controlled then stopped.

Much of the oxidation process revolves around polyphenols and the enzymes polyphenol oxidase and peroxidase. When the cells inside tea leaves are damaged and the components inside are exposed to oxygen and mix, specifically when polyphenols in the cell’s vacuoles and the peroxidase in the cell’s peroxisomes mix with polyphenol oxidase in the cell’s cytoplasm1 a chemical reaction begins. This reaction converts the polyphenols known as catechins into flavanoids called theaflavins and thearubigins (which are also polyphenols). Theaflavins provide tea with its briskness and bright taste as well as its yellow color, and thearubigins provide tea with depth, body and its reddish color. Also, during oxidation chlorophylls are converted to pheophytins and pheophorbides (pigments that lend to the black/brown color of dry oxidized tea leaves); and lipids, amino acids and carotenoids degrade to produce some of tea’s flavor and aroma compounds. Tea producers use special methods to initiate, fix, or even prevent oxidation in order to produce different flavors in a finished tea and inherently, different types of tea.

Oxidation begins when the cell walls within tea leaves are damaged. To achieve cell damage, tea producers macerate, roll or tumble tea leaves to intentionally initiate oxidation. Maceration is the quickest path to full oxidation because the leaves are, well… macerated which exposes much more of the insides of the leaves to oxygen and results in a greater mixture of the chemicals within. Maceration is typically used in mass production methods to create CTC (cut tear curl) tea or other broken-leaf teas and is achieved using a rotorvane or a CTC machine. Rolling results in a much slower and gentle oxidation and is usually done using a rolling table or by hand. Tumbling is an even gentler way to initiate oxidation and is achieved using large cylinders wherein the leaves are tumbled or by hand by shaking the leaves on top of a shallow bamboo basket. Regardless of the method of initiation, great care must be taken up to this point as any damage to the leaves before processing will cause premature oxidation and result in an unevenly processed finished tea.

Control over oxidation is maintained by introducing warm, moist, oxygen-rich air over time. The extent to which oxidation is allowed to occur has an astounding effect on the finished tea. Oxidation occurs best between 80-85F and is slowed, nearly to a halt at 140-150F2. Thus, when the tea producer wishes to halt oxidation, they heat the leaves. This heating process, known as fixing, denatures the enzymes responsible for the reaction. Oxidation is further slowed by drying the leaves. But it never completely stops, it just slows way down.

Fixing is a process in tea manufacture used to stop oxidation once it has started or to prevent it altogether. Fixing works by denaturing polyphenol oxidase and peroxidase – the enzymes primarily responsible for oxidation. Fixing is also commonly referred to as de-enzyming, denaturing or kill-green. The term kill-green is derived from the Chinese term shaqing 杀青, which translates to killing the green. In tea, the leaves must be heated to approximately 150 degrees Fahrenheit to halt oxidation. The process requires precise control of the temperature and length of heating, each has to be adjusted depending on the size and thickness of the leaves and the amount being processed.

Most common fixing methods:

  • Pan Firing: where tea leaves are heated in a large metal pan or wok that is heated by gas or wood fire.
  • Steaming: where steam is forced through the mass of tea leaves.
  • Tumblers: where a heated tumbler is used to heat the leaves.
  • Baking: where an oven type machine is used to heat the leaves.

Less common fixing methods:

  • Sun drying: where the heat of the sun denatures the enzymes in the leaf by dehydration.
  • Microwaving: where electromagnetic waves are used to quickly heat the leaves, seen more in commercial applications.
  • Plunging in boiling water: where tea leaves are literally plunged into boiling water.

When oxidation is prevented altogether, the tea leaves will keep their green color and vegetal characteristics in the cup as the catechins will be left largely intact. Think of an apple, once it is sliced open, it quickly turns brown; but yet, the apples in apple pie are not brown because the heat used to bake the pie denatured the polyphenol oxidase and peroxidase in the apples and prevented enzymatic browning (same goes for potatoes, avocados, bananas, etc).

When a semi-oxidized tea is being produced, some catechins will have converted to theaflavins and thearubigins, resulting in a slight browning in the leaves and yellower liquor. Lipids, amino acids, and carotenoids will have also begun to break down into flavor and aroma compounds.

When oxidation is allowed to run its course, the leaves will have undergone a full transformation and exhibit an aroma and taste profile completely unrecognizable from a finished tea that was exempted from oxidation. Theaflavins and thearubigins will now outnumber catechins resulting in a brisk tasting tea with a reddish color in the cup, the cholorophylls will have been converted to pheophytins and pheophorbides, turning the leaves a coppery brown, and a myriad of new volatiles will have developed. In this case, the leaves are often just dried to halt any still-occuring reactions and to bring the tea to a shelf-stable moisture level. This is a bit of a grey area in tea processing because here, drying can be considered a form of fixing. Heat is being used and oxidation is being halted. This is a great example of why it is sometimes important to view tea processing as more of a continuum rather than as a distinct set of steps.

If you found this interesting, be sure to check out my posts some other individual processing steps: witheringkill green and drying. And finally, my tea processing chart.


  1. Harney, Michael. 2008. The Harney & Sons Guide to Tea. New York: The Penguin Press.
  2. Basu, R.P., and M.R. Ullah. 1978 “Notes on Tea Fermentation.” Two and A Bud 25(1): 7–11.

About the Author:

Tony has been studying tea for over ten years and has traveled to many tea producing regions throughout Asia. His book, "Tea: A User's Guide" is available now.


  1. Asha Thomas November 19, 2018 at 9:06 pm - Reply

    Tea from tender leaves oxidized, and heated to 97 c to the colour of sunshine makes the best brew. Could add mint and/or ice.

  2. David D December 5, 2017 at 1:36 am - Reply

    Handy, but I came here looking for times. How long do the oxidation processes take for different teas?

  3. David December 1, 2017 at 10:51 pm - Reply

    I am trying to oxidise tea leaves in a lab from store bought tea. I want to get high levels of theaflavins for experimenting it’s affect on dihydrotestosterone. Would anyone care to share some information on how I could proceed?

    • Tony Gebely December 2, 2017 at 6:36 pm - Reply

      Typically store bought leaves have already been oxidized. Can you tell me more about the quality / type of store bought leaves you are using?

  4. syazwani October 2, 2017 at 9:56 pm - Reply

    really helpful n satisfied article i hv ever read.. really informative..i’m just love it.. tq tony.. kipidup:)

  5. amy April 9, 2017 at 11:23 am - Reply

    Thanks, Tony. Nice article. However, and my question is related to Ryan’s, I’ve never read an article actually properly explained what “percentage of oxidation” mean, concretely. This one is no exception. For example, what is a 30% oxidized tea, and why it is classified as 30% oxidized? And what is a 70% oxidized tea, and why it is classified as such? I bought you book, “Tea, a User’s Guide,” hoping to find the answer, as you explained what oxidation is here pretty well. I was disappointed your book did’t shed more light on the specifics of tea oxidation. If you do know the answer, I would love to read it in a future article of yours. Thx.

  6. Jinny Ciesluk March 13, 2017 at 3:02 am - Reply

    Thanks, this website is really beneficial.|


  7. Mudassir January 27, 2016 at 8:42 pm - Reply

    I want to ask that in pyrolysis we observe that tea polyphenols undergo oxidation.
    What is the reson of this oxidation??

  8. SM February 21, 2015 at 4:17 pm - Reply

    Cool article! I want to pin your page on Pinterest but you don’t have the option. I suggest adding it!

  9. mohit November 11, 2014 at 4:35 am - Reply


  10. telsah July 1, 2014 at 11:56 pm - Reply

    Thanks for the nice insight. I would like to stop by the oxidation/fermentation proces name. Fermentation is an enzymatic degradation as well. Often carried out by bacterial enzymes but not only by them. As far as I know fermentation is carried out in muscles to produce energy. You can stop fermentation by deactivating the enzyme.
    To call a process oxidation usually a strong direct reaction with O2 is present. Finally I I always thought that enzymatic oxidation is called fermentation. But maybe it’s just because the black tea production and pu-erh production would be called the same it was changed.
    I was looking for some more information on this topic but it’s hard to find. Thanks for your help.

    • Eric Scott April 5, 2016 at 8:48 am - Reply

      Fermentation is a metabolic process carried out by a living organism for getting energy out of a molecule by breaking it down. That is NOT what is happening when apples brown or when black/red tea is produced. Lactic acid fermentation happens in animals and ethanol fermentation happens in plants when low on oxygen, so it isn’t technically restricted to microbes, but in both of these cases the purpose is to extract energy from food which is not what is happening in a browning tea leaf.

    • shiVksaria December 4, 2016 at 8:58 pm - Reply

      To call it fermentation is a misnomer. It is actually an oxithermic reaction where oxygen is consumed, heat water and CO2 is released in the various ways described by TG above.

  11. Steve May 2, 2014 at 6:10 am - Reply

    Thanks Tony – we’ve just moved from Chicago to China and found your blog very useful for understanding oxidation. I just wrote about Longjng harvesting and processing after a trip to Hangzhou: http://blancavalencia.com/2014/05/01/lost-in-longjing-part-i/ …. cheers, Steve

  12. Ryan March 6, 2012 at 9:35 pm - Reply

    Nice piece, Tony. Didn’t realize that “fully oxidized” tea actually isn’t.

    If we’re talking in terms of percentages, how oxidized is traditional black tea? 90%? 95%?

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