Anaerobic-held oolong
An ordinary oolong line with the air taken out of one slot. Where a maker would shake and rest the leaf through a night, this route seals it into a tank and displaces the oxygen.
What makes this route what it is
The defining step is an absence. Every other route is defined by an operation — a heat, a pressure, a rupture, an organism — and this one is defined by removing a gas from a vessel and waiting. It is also the clearest case in the catalogue of a route that exists because a measurement exists: the manufacture is designed around a number a laboratory reports, not around a cup.
The route, in order
Each note describes what the step is doing in this route. The same operation does a different job in a different chain.
An ordinary open oolong shoot. Nothing about the material anticipates the sealed hold, and the same intake could be taken down any oolong route.
A short sun wither where the weather allows it. Some producers skip straight to the indoor stage, which is a schedule decision rather than a style one.
Factory production working to a fixed line frequently omits the sun entirely and withers indoors under forced air throughout.
Indoor withering to the moisture the sealed vessel needs. This matters more here than in a shaken route: once the tank is closed nothing can be adjusted, so the leaf has to be right before it goes in.
The step the route exists for, occupying the position where a rolled oolong would be shaking and resting through the night. Hours in a sealed vessel with the oxygen displaced, commonly repeated with airings in between, and the number and length of the holds is the recipe.
Ordinary fixation once the holds are finished. It plays its usual role of ending the leaf's own chemistry, and unusually little judgement is exercised at this point because the preceding step was run to a clock rather than to a nose.
Cloth-wrapped compression into a pellet, as for a Taiwanese rolled oolong. The shape is a market convention here rather than a requirement of the method.
Interleaved with the rolling in the usual way, taking the pellet down to storage dryness.
A light finishing bake. Heavier bakes are used on some lots and push the tea towards a dried-fruit register that sits more comfortably with the sourness than a light bake does.
Not every lot is baked. The preceding staged drying already takes the pellet to storage dryness, so a producer who stops there ships the tea as the hold and the fixation left it; the bake is a finishing decision taken lot by lot rather than a fixed station on the line.
How it differs from its neighbours
A route described on its own teaches very little. The useful fact is always which other route it is nearly identical to, and where it leaves it.
One position in the route is filled by a different named method. The shape of the route is unchanged.
One slot, two philosophies. The rolled route fills the position between withering and fixation with shaking, resting and a maker's nose across a whole night; this fills it with a sealed tank and a timer. Everything the two teas do not share in the cup follows from that substitution.
The two routes are the same tea up to a point. Everything before the divergence is shared; everything after it is not.
Both routes reach step four with withered leaf and both then do something no ordinary oolong does — but in opposite directions. Oriental Beauty's difference was made in an open field by an insect and cannot be scheduled; this one is made in a closed vessel by a gas supply and is nothing but schedule. They are the two ends of how much of a tea's character can be engineered.
Where it goes wrong
The sourness is the method working, and the commonest failure is not a fault in the tea but a mismatch in expectation — a buyer who has been sold it as an oolong and meets stewed fruit. Within the manufacture, an incomplete seal produces a lot that is neither one thing nor the other, part browned and part held, and no later step recovers the aroma an ordinary oolong night would have built.
In full
This is a modern industrial manufacture rather than a traditional one, and it is unusually easy to describe because it changes exactly one thing. Withered leaf, instead of being shaken and rested through the alternating cycles that build an oolong's aroma, is loaded into a sealed vessel from which the air is displaced — commonly with nitrogen, sometimes by drawing a vacuum — and held there under temperature control for hours. The hold is often repeated, with the leaf aired between rounds, and the schedule of holds and airings is what a producer actually varies. Afterwards the leaf rejoins a conventional route: fixation, ball rolling, drying, sometimes a light bake. The result is immediately recognisable and quite unlike its neighbours — a reddish liquor and a stewed-apple, tamarind, dried-apricot sourness where a rolled oolong of the same cultivar would be floral. The trade name refers to a compound the leaf is reported to accumulate when it is held without oxygen, and lots are graded and sold on measured content of it, which makes this the only route in the catalogue whose product is specified by an assay rather than by a taster.