What is happening chemically
When leaf cells rupture, polyphenol oxidase enzymes inside meet oxygen and begin converting catechins into larger compounds — theaflavins and thearubigins. Those are what turn the leaf coppery and the liquor red, and what replaces green tea’s fresh grassiness with malt, dried fruit and body. It runs over a few hours in a warm humid room, and it is stopped by heat, which denatures the enzymes.
Why it is not fermentation
Fermentation involves living microorganisms. Oxidation involves none — it is a chemical reaction driven by the leaf’s own enzymes and a gas. The words are used interchangeably across tea writing, including by producers, and getting them the wrong way round makes half the category descriptions incoherent. TeaHQ keeps them in separate fields because a tea can be both: an aged oolong was oxidised and then ferments.
The scale, and where each category sits
Green tea: none, stopped deliberately. White tea: a little, acquired slowly across a long wither with nothing stopping it — which is why calling white tea unoxidised is not quite right. Yellow tea: essentially none, since the yellowing step is a separate non-enzymatic change. Oolong: partial, anywhere from barely started to nearly complete, and the maker chooses. Black tea: complete. Dark tea: not applicable, because its defining change is microbial.
Why percentages are a trap
Oxidation figures are quoted everywhere — 15% for a light oolong, 70% for Oriental Beauty — and measured almost nowhere. There is no standard method, no instrument in routine use, and the numbers circulate because they sound precise. TeaHQ records oxidation as a descriptor rather than a percentage for that reason, and where a producer states a figure it goes in a note with the source attached.
What it means for brewing
More oxidation means more tolerance of heat. A fully oxidised black tea takes boiling water without complaint; an unoxidised green tea turns bitter and thin at the same temperature, because the compounds that carry bitterness extract far more readily in hot water than those carrying sweetness. Working out roughly where a tea sits on the oxidation scale tells you roughly how hot to brew it.