Anaerobic holding (sealed vessel, air displaced)
The oxidation slot, run with the air taken out. Withered leaf is sealed into a vessel from which oxygen has been displaced and held there for hours, and the schedule of holds and airings is the manufacture.
The categories this step defines
In tea, processing is not a modifier applied to a category — it is what creates the category. These are the types that exist because this step exists.
- Oolong — Deliberate bruising to initiate edge oxidation, halted partway, frequently followed by roasting.
What it is
This is an industrial operation that belongs to an ordinary oolong line and interrupts it. Withered leaf is loaded into a sealed tank or bag and the air inside is displaced — commonly with nitrogen, sometimes by drawing a vacuum — and the leaf is then held at a controlled temperature for a period measured in hours rather than minutes. The hold is frequently repeated, with the vessel opened and the leaf aired between rounds, and the number of rounds and the length of each is the producer's recipe. Afterwards the leaf goes back into a conventional route: fixation, rolling, drying. The name the trade uses for the resulting teas refers to gamma-aminobutyric acid, a compound reported to accumulate in leaf held without oxygen, and the teas are graded and sold on measured content of it. TeaHQ records the operation and the compound name because both are facts about manufacture; what drinking the resulting tea does is a separate question this table does not answer and does not imply an answer to.
Also transliterated as: yan yang chu li, yanyang chuli.
What it does to the leaf
Holds intact withered leaf in an atmosphere with oxygen displaced, at controlled temperature, so that the leaf's own metabolism proceeds without the browning that contact with air would otherwise drive.
What it does to the cup
Characteristically sour-fruited — stewed apple, tamarind, dried apricot — with a reddish liquor and very little of the floral top note an ordinary oolong of the same cultivar would carry. The sourness is a signature of the method rather than a fault, which is the single most useful thing to know before buying one.
Variants that matter
Ways this step is carried out that produce genuinely different results, rather than regional names for the same thing.
- Single long hold
- One extended sealed period. Simpler to run and associated with a heavier, flatter result.
- Cycled holds
- Several shorter sealed periods with the leaf aired between them. More equipment time and more handling, and the version producers describe as giving a livelier cup.
- Aged afterwards
- Some of this material is subsequently stored and re-fired like an aged oolong, which layers a second and unrelated set of changes on top of the sealed hold.
Where this gets misdescribed
That excluding air makes this a fermentation. No organism is cultivated and nothing is inoculated — the vessel is sealed precisely so that a chemistry inside the leaf can run undisturbed, which is the opposite intent from a pu-erh pile or a pickling barrel. The word anaerobic is doing honest work here and the word fermentation would not be.
Other steps at the Oxidation stage
- Controlled-environment oxidation — The room and the equipment in which oxidation is actually run — cool, saturated, ventilated, and judged by nose rather than clock.
- Heaped rest before fixation (dui qing) — A short heaping of the leaf at the end of the shaking cycles, used to push the last of the oxidation quickly before the heat goes on.
- Oxidation — The enzymatic browning of ruptured leaf that turns green tea material into black tea material.
- Standing rest between shakes (jing zhi) — The quiet hours between one shaking and the next, which is when oolong actually oxidises — the shaking only decides where.
Sources
- TeaHQ editorial synthesis — TeaHQ
This page is TeaHQ’s own synthesis of well-attested production practice. No external technical document stands behind it, so it describes what producers do rather than asserting measured process parameters.