Tea processing biotech

Every category of tea comes from the same plant, Camellia sinensis. What separates green from black from oolong is not the leaf but what is done to one enzyme in the hours after picking. The industry’s traditional word for that process is “fermentation”, and it is wrong in a way worth understanding, because the correct account explains the whole product range.

What actually happens

Fresh tea leaves are rich in catechins — flavan-3-ols including epigallocatechin gallate — and they also contain polyphenol oxidase and peroxidase. In an intact leaf these are kept apart: the enzymes sit in the cytoplasm and plastids, the catechins in the vacuole. Nothing reacts.

Rolling or macerating the withered leaf ruptures cell membranes and brings them together. Polyphenol oxidase, using atmospheric oxygen, oxidises catechins to quinones, which then couple with each other and with other leaf constituents.

This is enzymatic oxidation carried out by the plant’s own enzymes. No microorganism is involved, nothing is anaerobic, and no substrate is being converted to alcohol or acid by a living culture. It is the same browning chemistry seen in a cut apple, run deliberately and to completion.

What the products are

Two classes of pigment result. Theaflavins form when a pair of catechins — one simple, one with a trihydroxy ring — couple into a seven-membered benzotropolone ring. They are orange-red, and they carry briskness and brightness in the cup.

Thearubigins are the larger, heterogeneous, poorly characterised polymers formed by further oxidation and coupling. They are reddish-brown, make up a much larger fraction of a black tea’s solids, and supply colour depth and body. The theaflavin-to-thearubigin ratio is a real quality index in the trade.

The categories, restated correctly

Green tea is made by stopping the enzyme immediately. The picked leaf is steamed or pan-fired within hours — “fixing” — which denatures polyphenol oxidase before it can act. Catechins therefore survive largely intact, which is why green tea’s catechin content is high and its liquor pale. The choice of steaming or pan-firing accounts for much of the difference between Japanese and Chinese green teas.

Black tea allows oxidation to run essentially to completion before firing halts it.

Oolong is partial: the leaf is bruised at the edges rather than fully macerated, so oxidation proceeds in the damaged regions only, and is stopped part-way.

White tea is barely handled — withered and dried — so a slow, uneven oxidation occurs without deliberate maceration.

Pu-erh is the exception, and it is the real thing

Pu-erh is genuinely microbial, and the irony is that it is the one tea whose Chinese name has always distinguished it as post-fermented.

Ripened pu-erh is made by wet piling: fixed and dried leaf is heaped, moistened and held warm for weeks. Aspergillus species, other fungi, yeasts and bacteria colonise the pile and metabolise the leaf, with the pile’s own heat sustaining the process. The chemistry is different in kind from oxidation — microbial enzymes degrade catechins and cell-wall polysaccharides and generate new compounds, producing the earthy, mellow character and the very dark liquor. Raw pu-erh achieves something similar far more slowly over years of storage.

So the tea everyone calls fermented is not, and the tea everyone calls aged is.