How they are formed
When leaf cells are ruptured by rolling or by the crushing action of CTC manufacture, oxidative enzymes held separately inside the cell meet the catechins and atmospheric oxygen. The catechins are converted into reactive intermediates which then combine, most simply in pairs, to form theaflavins — a small group of orange-red compounds with a distinctive structure formed from two catechin units. As oxidation continues, further combination produces thearubigins, a much larger, browner and chemically heterogeneous group. The whole sequence runs over a few hours in a warm, humid room and is halted by heat, which destroys the enzymes. This is what the firing step at the end of black tea manufacture is for, and it is why a black tea’s character is set by when the maker chose to stop rather than by any fixed schedule. The physical form of the leaf sets the pace as much as the enzymes do. Orthodox rolling ruptures cells progressively and unevenly, so oxidation proceeds gradually and the maker can stop it at a chosen point; CTC maceration ruptures everything at once and drives the sequence quickly to completion. That is why the two manufactures produce such different balances of the compounds described here, and why the choice of machinery is effectively a choice about where on the sequence the tea will end up.
What theaflavins are associated with
Brightness and briskness — two words with precise meanings in professional black tea evaluation. Brightness is the clear, luminous orange-red quality of a good liquor, as distinct from a dull or muddy one, and it is visible before anything is tasted. Briskness is the lively, clean, refreshing grip in the mouth that a good Assam or Ceylon has and a flabby tea lacks. Theaflavins are also associated with the coppery ring at the edge of a well-made black tea in a white cup, which tasters read routinely. Because they are formed early in the oxidation sequence and consumed as it continues, a tea oxidised carefully and stopped at the right point retains more of them, which is one of the things distinguishing a well-made orthodox black tea from an over-oxidised one. Milk interacts with this directly, which is one reason the British and Indian black tea traditions developed as they did. Dairy softens the grip that briskness depends on, so a tea selected for a milk-drinking market is chosen for colour, body and strength rather than for the qualities that a bright liquor offers when drunk alone. A tea admired for its brightness is being admired for something milk will remove, which is why the same leaf can be excellent and inappropriate at once.
What thearubigins are, and how honest to be about them
Thearubigins are the larger, later, darker products of continued oxidation, and they make up the bulk of the oxidised polyphenol material in most black tea. They are associated with the depth of colour, with body and with a rounder, less brisk character. They are also genuinely poorly characterised: the name covers a heterogeneous mixture of compounds rather than a defined molecule, they have resisted full structural description, and published accounts of them are correspondingly vaguer than accounts of theaflavins. This is worth stating rather than glossing, because a great deal of tea writing describes thearubigins as though they were as well understood as theaflavins. What is well established is the direction — more of them means darker, deeper and less brisk — and that is what this catalogue asserts. The honest position matters practically as well as intellectually. Because the group is not well defined, figures quoted for its content depend heavily on the analytical method used and are not straightforwardly comparable between studies or between teas. This is one of several places in tea chemistry where a number circulating in popular writing has more precision than the underlying measurement supports, and it is the reason this catalogue describes directions rather than reproducing values.
The balance between them, and why the trade tracks it
Because the two families are formed sequentially, the ratio between them is effectively a record of how far oxidation was allowed to run, and it corresponds to differences a taster can detect. A liquor weighted towards the earlier products is brighter, brisker and often more aromatic; one weighted towards the later products is darker, heavier, smoother and duller. Neither is correct in the abstract: a tea intended for milk benefits from depth and colour, and a tea intended to be drunk alone benefits from brightness. Black tea evaluation in the producing industries has long attended to this balance under the vocabulary of brightness, briskness, colour and body, which are cupping terms rather than chemical measurements, and which map onto the underlying chemistry closely enough to be useful. The same balance is partly why a black tea’s liquor colour is a poor guide to its strength. A dark liquor indicates how far the sequence ran rather than how much material is dissolved, so a pale, brisk, well-made tea can be considerably stronger in the mouth than a darker one — and a very dark cup from a broken grade may be mostly colour. Judging strength by eye is one of the commonest brewing errors and this is the mechanism behind it.
Tea cream, and what it tells you
A well-made strong black tea goes cloudy as it cools, and the cause is a genuinely interesting piece of tea chemistry: caffeine and the larger oxidation products form complexes that stay dissolved while hot and come out of solution as the liquor cools, producing a haze known in the trade as cream. It is a positive quality signal rather than a fault — creaming down is associated with a liquor rich in exactly the compounds a good black tea should have — and it is also the cause of the cloudy iced tea that troubles people who chill hot-brewed tea. That is why cold-brewed iced tea stays clear, and why the standard remedies for cloudiness involve cooling quickly, brewing for cold from the start, or accepting the haze as evidence of a well-extracted cup. There is a practical corollary for anyone making iced tea. Brewing hot and chilling quickly, brewing at double strength over ice so the drop is fast, or brewing cold from the start all avoid the slow cooling during which the haze forms. Adding a little acid, as lemon, also clears it. None of these improves the tea; they address an appearance, and it is worth knowing that the cloudy version is the better-extracted one.
Why this is not the whole black tea story
These compounds explain colour, briskness and body, and they do not explain aroma at all. The malt of an Assam, the muscatel of a second flush Darjeeling, the cocoa and orchid of a Keemun and the sweet-potato character of a Dian Hong come from the volatile fraction, which is generated by an overlapping but distinct set of reactions during withering, oxidation and firing. A black tea can be textbook in colour and briskness and dull in the nose, which is exactly what a great deal of commodity tea is. So the oxidation-product story is one axis of black tea quality, and aroma is another, and the two are only loosely coupled — which is why professional cupping scores them separately rather than reading one from the other. There is a further axis again, which is body, and it comes largely from a different set of components — the polysaccharides and other structural material released during manufacture. A black tea can therefore be bright, aromatic and thin, or dark, dull and thick, in any combination. Reading the four dimensions separately, as professional cupping vocabulary does with brightness, briskness, colour and body, is far more informative than the single scale of strength that consumer writing usually offers.