Tea Science 6 min read

Tea Oxidation Explained: Green, Oolong and Black Tea

What oxidation does to tea leaf, why black tea is not fermented, how oxidation ranges differ by source, and what it means for caffeine and brewing.

By the CupCura editorial team. How we work

Published August 2026 · Reviewed August 2026

Tea oxidation is the browning of bruised tea leaf by the leaf's own enzymes reacting with oxygen.

The tea maker decides when to stop it with heat: almost at once for green tea, partway for oolong, and not until it has run its course for black tea. All of these come from the same species, Camellia sinensis, and it is this one step, more than the plant, that sets the colour, body and flavour of the cup.

Where each type sits on the oxidation range

The table gives the figures that appear most often in reference sources, with the range each source quotes. Percentages are not standardised, and they are the part of tea talk that varies most.

Type	Oxidation	What the maker does
White	Minimal	Withers the leaf for a day or more, then dries it gently, with no rolling
Green	Stopped almost immediately	Heats the fresh leaf (steaming in Japan, pan-firing in China) before the leaf can brown
Oolong	About 5 to 70% in one source, 8 to 85% in another	Bruises the withered leaf, lets it oxidise for a controlled time, then heats to halt it
Black	Fully oxidised	Rolls or cuts the withered leaf, oxidises it in a cool, humid room, then dries it
Dark (pu-erh and similar)	Not a simple percentage	Relies on microbial post-fermentation rather than on oxidation alone

Oolong is the problem category. The Food Technology magazine overview of tea processing puts oolong at roughly 5 to 70% oxidation. Wikipedia's tea processing article describes light oolongs at 5 to 40% and darker ones at 60 to 70%, while its oolong article gives 8 to 85%. These are descriptions of ranges, not measurements of a standard, and two oolongs both labelled "semi-oxidised" can taste nothing alike. A tea sold as 20% oxidised is a claim by the seller unless it comes with a method.

Oxidation is not fermentation

Black tea is often called "fermented", especially in older trade usage. The process is enzymatic: enzymes already in the leaf, set off when the leaf cells are broken, react with oxygen. Nothing microbial is needed. A 2020 review in Chemical and Pharmaceutical Bulletin describes black tea polyphenols as produced by enzymatic oxidation of four tea catechins, while still using the trade word "fermentation" for the step.

The word is correct for one group: post-fermented dark teas such as pu-erh, where microbes work on the leaf after it has been fixed. If you want that style, the pu-erh tea guide covers it separately.

What happens in the leaf, in order

Withering. The leaf loses up to about half of its moisture and becomes pliable enough not to crumble, and its proteins begin to break down into amino acids.
Bruising or rolling (oolong and black only). Breaking cell walls mixes the enzymes with their raw material. Orthodox rolling keeps the leaf mostly whole; the CTC method uses a machine to chop it into small pieces.
Oxidation. Chlorophyll breaks down and the catechins are converted into new pigments. In black tea these include theaflavins and the poorly understood thearubigins. The review above notes that almost 60% of the solids in a black tea infusion are still chemically uncharacterised, which is a good reminder of how much remains unmapped.
Fixation ("kill green"). Heat deactivates the enzymes and stops the clock. Green tea is fixed early. Wikipedia lists pan-firing and steaming as the traditional methods, and baking or panning in rotating drums as modern ones. Black tea is the exception: it has no separate fixation step, and the final drying ends the reaction.
Shaping and drying. Rolling, balling or twisting gives the leaf its shape, and drying leaves it shelf-stable.

Does more oxidation mean more caffeine?

Not in a simple way. The idea that black tea has more caffeine than green is common, but laboratory data do not show a clean ladder from green to black. A 2025 study in Foods brewed 68 commercial teas of six types at 2 g per 200 ml. At 100°C (212°F) for 10 minutes the mean caffeine was about 27 mg for green, 31 for white, 29 for oolong and 27 for black, so the types were close. A cooler, shorter brew looked different: at 80°C (176°F) for 2 minutes the means were about 14 mg for black, 11 for green, 6 for white and 4 for oolong. Within a type there was large variation, with coefficients of variation around 30 to 60% in many cells, so these are direction, not cup values.

What does change caffeine in the cup is how it is brewed. In the same study, after 10 minutes at 100°C, black tea in cup-size bags gave about 39 mg per 200 ml against 24 mg for loose leaf. A separate Brazilian survey of dry leaf found 14.3 to 34.8 mg of caffeine per gram across white, black and green teas, with 51.5 to 85.2% passing into the infusion (that range also covered maté, which is not tea). So grade, leaf form, dose and time matter more than the label. For cup figures with serving sizes, see the tea caffeine guide.

How to use this when buying and brewing

Place a tea before you brew it. As a rule of thumb (editorial, not a standard), a lightly oxidised oolong suits cooler water and a darker, roasted one tolerates hotter water. The Tea Association of the USA range for oolong is wide, about 79 to 91°C (175 to 195°F).
Compare, do not rank. Tasting a green tea, a light oolong, a dark oolong and a black tea from one region side by side tells you more than any percentage. The oolong oxidation article explains why oolongs resist simple labels.
Treat percentage claims with care. Oxidation level alone does not tell you roast, leaf grade or freshness.
Who can skip this. If you only drink bagged black tea with milk, oxidation is already decided for you. It matters mainly if you buy loose-leaf oolong or want to understand why green and black teas brew so differently.

Questions

Is oxidised tea bad?

No. Oxidation is the intended step that makes oolong and black tea. It is a different matter from tea that has gone stale or absorbed moisture in storage, which is a storage problem.

Is more oxidised tea better for you?

This page does not rank teas for health, and the sources cited here do not compare them in that way. Green, oolong and black teas all contain caffeine, so the usual cautions about caffeine apply to each. Anyone pregnant or on medication should check intake with a clinician or pharmacist.

Sources

Wikipedia, Tea processing (definitions, oolong ranges, kill-green methods, post-fermentation): https://en.wikipedia.org/wiki/Tea_processing
Wikipedia, Oolong (8 to 85% range): https://en.wikipedia.org/wiki/Oolong
Food Technology magazine (IFT), "Processing: how tea is processed": https://www.ift.org/news-and-publications/food-technology-magazine/issues/2018/january/columns/processing-how-tea-is-processed
Production mechanisms of black tea polyphenols. Chem Pharm Bull 2020. doi:10.1248/cpb.c20-00295
Determination of L-theanine and caffeine contents in tea infusions with different fermentation degrees and brewing conditions. Foods 2025. doi:10.3390/foods14132313
Tfouni SAV et al. Caffeine in teas: levels, transference to infusion and estimated intake. Food Sci Technol 2018. doi:10.1590/1678-457X.12217
Tea Association of the USA ranges as reproduced by Bunn: https://resource.bunn.com/library/eZine/tea_basics_brochure/files/basic-html/page3.html