Vol. XIV · Fermentation Plate 9 of 11tea, green tea, oolong, black tea, white tea, dark tea, pu-erh, oxidation, polyphenol oxidase, kill-green, 杀青, 发酵, theaflavinsMT

Tea: Oxidised and Fermented.

Black tea is called "fully fermented" in the trade, but no microbe is involved. The leaf browns itself, like a cut apple.

Fresh tea leaves are full of catechins and of polyphenol oxidase, the enzyme that browns cut apples (VIII.6). Wither the leaves, roll or bruise them to break the cells, and the enzyme meets the catechins and oxygen: they oxidise into orange-red theaflavins and darker thearubigins, and grassy flavours give way to malty, fruity and honeyed ones. Heat — steaming or pan-firing, the step Chinese tea-makers call 杀青, "killing the green" — destroys the enzyme and fixes the leaf at whatever point it has reached.

So the families of tea are degrees of oxidation. Green tea is killed almost at once (Japanese teas by steaming, most Chinese by pan-firing): little oxidation, green and fresh. White and yellow teas oxidise slightly as they wither or are gently heaped. Oolong is bruised and oxidised partway, from 15 % to 70 %. Black tea (Chinese 红茶, "red tea") is rolled and fully oxidised. Only dark teas (黑茶), such as shou pu-erh, are truly fermented: their heaped, moistened leaves are worked on by microbes for weeks (渥堆). The workshop oxidises a leaf.

Below: the share of catechins oxidised against minutes of oxidation after rolling, at the room temperature you choose (a teaching first-order rate with Arrhenius dependence), with the ranges of green, oolong and black teas.

What you will learn
Workshop
Teaching model: oxidation approaches completion at 0.025 per minute at 25 °C after rolling (activation energy 50 kJ/mol), stopped by 杀青; tea classes by approximate oxidation: green under 5 %, light oolong 15–40 %, dark oolong 40–70 %, black over 80 %.

One leaf, many teas

In this model a rolled leaf at 25 °C is about 39 % oxidised after twenty minutes — an oolong — and passes 80 %, black-tea territory, after about 64 minutes; in a warm 32 °C room, after 41. Tea-makers judge it by colour and smell rather than the clock: the leaf turns copper and the aroma moves from grassy to fruity to malty. Then the heat of drying stops the enzyme.

Brew by type. Green teas, whose flavours are delicate and whose catechins turn bitter in very hot water, are brewed at about 70–80 °C for one to three minutes; oolongs at 85–95 °C, often in short repeated infusions (功夫茶); black teas at the boil for three to five minutes. Dark teas, fermented by microbes after processing, are rinsed briefly and brewed near the boil, and improve with age like wine. And none of these "fermented" teas is fermented in the same sense as wine or kimchi except the dark teas.

At the stove

Steep the same weight of a green tea at 75 °C and at 100 °C for three minutes each, and taste. The boiling-water cup is noticeably more bitter and astringent.

The English name "black tea" and the Chinese 红茶, "red tea", describe the same thing: the English named the dry leaf, the Chinese the colour of the brewed cup.

Terms

杀青
"Killing the green": heating fresh tea leaves to destroy their enzymes.
Theaflavins
Orange-red pigments formed when catechins oxidise.
Oolong
Partly oxidised tea.
渥堆
Piling moistened tea so microbes ferment it, for dark teas.
Tea types are degrees of enzymatic oxidation, stopped by 杀青: green little, oolong partway, black fully. Only dark teas are fermented by microbes.

Sources

  1. Enzymatic browning by polyphenol oxidase: inhibited by acid (activity falls steeply below about pH 4), by cold, by excluding oxygen and by heat; blanching inactivates it (M. Y. Lee and J. R. Whitaker, eds., Enzymatic Browning and Its Prevention, ACS Symposium Series 600, 1995). The course's rates are teaching values.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  3. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.