Fermentation vs. Oxidation
Tea sellers often say fermentation when they mean oxidation; true fermentation uses microbes and applies only to dark teas such as shou pu'er, not to black tea or oolong.
Definition
Fermentation and oxidation are different processes, and tea writing mixes them up constantly. Oxidation is a reaction driven by the leaf's own enzymes when its cells are broken, and it is what turns oolong and black tea brown. Fermentation, in the strict sense, is a change caused by microorganisms such as fungi, yeasts and bacteria feeding on the leaf. The confusion comes from older English usage and from Chinese. The Chinese word fajiao (发酵) is used for both, so oolong is often described as "semi-fermented" and black tea as "fully fermented", even though no microbes are involved. Early English tea manuals also called the oxidation room of a black tea factory the fermenting room, and the term stuck in the trade. Genuine microbial fermentation happens in hei cha, the dark teas. Shou pu'er is made by wo dui, piling damp leaf so microbes and heat transform it over weeks. Liu Bao, Anhua dark tea and Fu brick tea also involve microbial activity, and Fu brick is deliberately grown with the golden fungus Eurotium cristatum. Raw sheng pu'er changes slowly during aging through a mix of slow oxidation and microbial activity. Kombucha is a fermented drink made from brewed tea.
In Depth
How to read labels: if a seller says a black tea, Dian Hong or oolong is "fermented", read it as oxidized. If a seller says shou pu'er, Liu Bao or Fu Zhuan is fermented, they mean microbes, and that is correct. "Post-fermented" is the usual term for dark teas, because the microbial step comes after the leaf is first made.
How to tell in the cup: microbial fermentation gives earthy, woody, damp-forest, sometimes medicinal or compost-like aromas, especially in young shou pu'er. Oxidation alone gives fruit, honey, malt and floral notes. A black tea should never smell of earth or storage.
Why it matters: storage advice differs. Oxidized teas like black and oolong are made to be finished products and mostly stale with time. Fermented dark teas are living products that can keep changing for years in the right humidity.
See Also
- Dark Tea (Hei Cha)
- Tea Processing
- Pu'er Fermentation