What is actually happening in an oxidising leaf
Intact plant cells keep their contents separated into compartments. Rolling, bruising or crushing the leaf breaks that separation, and oxidative enzymes — polyphenol oxidase chiefly, with peroxidase alongside — come into contact with the catechins while atmospheric oxygen is available. The enzymes catalyse the conversion of catechins into reactive quinones, which combine into theaflavins and, as the process continues, into the larger thearubigins. Colour shifts from green through copper to dark brown, aroma is transformed as other reactions run alongside, and astringency and bitterness give way to malt, dried fruit and body. It runs over hours in a warm humid room and is stopped by heat, which destroys the enzymes. There are no microorganisms involved at any point, and nothing is being consumed and converted by anything living. The maker’s controls are worth naming because they make the process legible. Temperature and humidity in the oxidation room set the rate; the degree of rupture achieved in rolling sets how much of the leaf is participating; and the timing of firing sets where it stops. None of these is automated in orthodox manufacture, which is why the same leaf in two makers’ hands produces two teas and why oxidation is described as a judgement rather than a step.
Why the wrong word is so entrenched
Chiefly because the Chinese term used across the tea industry for this process translates literally as fermentation, and the terminology travelled into English with the tea rather than being coined in it. Producers, exporters and reference works have used it for well over a century, so the usage is not a popular error to be corrected so much as a translation artefact that hardened. The practical damage is that English-language tea writing then applies the same word to two genuinely different processes — enzymatic oxidation and microbial post-fermentation — and readers reasonably conclude that black tea and pu-erh are made the same way, or that a partially oxidised oolong is partly rotten. TeaHQ keeps them in separate fields on every tea record for this reason, so that a record can state both independently rather than choosing between them. There is a second contributor, which is that the visible outcomes look alike. Both processes darken the leaf, soften it, deepen the liquor and produce a sweeter, rounder cup, so an observer without a laboratory has little reason to suspect two mechanisms. It took the chemistry to separate them, and the vocabulary has never caught up — which is a good example of how a term can be entrenched by observation as well as by translation.
Where each category sits, and why the scale is a descriptor
Green tea: oxidation is stopped deliberately at the outset by the kill-green step, so essentially none. 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, chosen by the maker. Black tea: complete. Dark tea: not applicable in this sense, because its defining change is elsewhere. This catalogue records the position as a descriptor rather than a percentage, because oxidation figures are quoted everywhere and measured almost nowhere — there is no standard method and no instrument in routine use, and the numbers circulate because they sound precise. The scale also flattens something important, which is that oxidation is uneven within a leaf and within a batch. Oolong manufacture deliberately produces leaves oxidised at the edges and green at the centre, and the classic red rim on a green leaf is visible in the spent leaf of a well-made tea. A single position on a scale cannot express that, which is one more reason the descriptor is paired with a free-text note where a producer has said something specific.
The genuine exception: microbial post-fermentation
Dark tea is the case where the word fermentation is correct, and it is a genuinely different process. In the manufacture of ripe pu-erh, damp maocha is heaped in large piles under controlled temperature and humidity for a period of weeks, and a microbial community — fungi and yeasts, with Aspergillus species commonly reported as prominent — develops and drives the transformation of the leaf. In Fu brick tea, a deliberate fungal flowering produces the golden specks known as jin hua, identified in the literature as a Eurotium species. Raw pu-erh ages naturally over years by a slower combination of ongoing oxidation and microbial activity. In every case something living is doing the work, which is what makes fermentation the right word here and the wrong word for a black tea. The two dark-tea routes are worth keeping apart as well, since they produce different teas. Wet piling compresses years of change into weeks under managed conditions and yields the dark, smooth, earthy character of ripe pu-erh; natural ageing runs slowly over decades and yields something quite different, retaining more aromatic complexity and more of the original leaf’s character. This catalogue records them as separate values on the same field precisely because the outcomes are not interchangeable.
Why the distinction has practical consequences
It is not a terminological nicety. Because dark tea depends on microbial activity, its storage requirements invert the ordinary rules: it needs air exchange and a certain ambient humidity, and sealing it airtight arrests the process it was bought for. Because black tea depends on an enzymatic reaction that was halted by firing, sealing it is exactly right. A reader who believes both are fermented has no way to derive that difference and will store one of them wrongly. The same confusion produces the belief that pu-erh is simply very old black tea, that ageing any tea will deepen it, and that an oolong is an incompletely made black tea. Each of those errors leads to a specific practical mistake, which is why this catalogue models the two processes as separate fields rather than as one scale. There is a further consequence for brewing. Because dark tea’s transformation happened through a different route, it tolerates and often rewards boiling water and long steeps that would ruin a partially oxidised tea, and compressed dark tea in particular is frequently boiled rather than infused. A reader who has collapsed the two processes into one has no basis for that difference either, and will brew a cake as though it were a strong black tea.
The cases that need both fields
Some teas have undergone both processes and cannot be described by either alone. A traditionally roasted oolong that has been aged for decades was partially oxidised during manufacture and has since undergone slow ongoing change including microbial activity. An aged white tea cake was minimally oxidised and is now ageing. Liu Bao and the other hei cha have their own combinations. This is precisely why the catalogue carries an oxidation descriptor and a separate post-fermentation field with values for none, wet piling, natural ageing and both, rather than a single scale from green to dark. A model that forces one answer would have to misdescribe an entire class of teas, and the classes it would misdescribe are the interesting ones. There is a modelling lesson in this that generalises past tea. Where two processes can co-occur, a single scale forces a choice that misrepresents every record at the overlap, and the overlap is usually where the interesting material is. Splitting the field costs one column and buys the ability to describe an aged roasted oolong accurately — which is the sort of trade this catalogue makes repeatedly, and states in the schema so that the reasoning survives the person who made it.