Vol. XXVII · The Cuisines of China Plate 11 of 12tea, 茶, brewing, extraction, water temperature, green tea, 绿茶, oolong, 乌龙, black tea, 红茶, pu'er, 普洱, caffeine, catechins, EGCG, astringency, gongfu, 功夫茶, ISO 3103, leaf ratio, steeping timeMT

Brewing Tea.

Pour boiling water on a delicate green tea and it turns bitter in a minute. The same leaf at 80 °C stays sweet for three.

All true tea is the leaf of Camellia sinensis; its types come from processing (XIV.9). Green tea (绿茶) is heated soon after picking — pan-fired in China, steamed in Japan — to destroy the enzyme polyphenol oxidase (VIII.6), so its catechins stay as they are: grassy, sweet, but bitter and astringent if over-extracted. Black tea (红茶 — "red tea" in Chinese) is rolled and fully oxidised, turning catechins into theaflavins and thearubigins: red-brown, malty, brisk. Oolong (乌龙) is partly oxidised, white tea (白茶) barely processed, and pu'er (普洱) fermented and aged.

Brewing is extraction (III.8). Caffeine is small and very soluble: most of it comes out in the first few minutes. Catechins come out more slowly, and their extraction rises steeply with temperature. Amino acids such as theanine, which give green tea its sweet, savoury taste, come out early. So the three dials are temperature, time and the ratio of leaf to water. Western brewing uses little leaf (about 1 g per 100 mL) for 2–5 minutes; the ISO tasting standard uses 2 g per 100 mL of boiling water for 6 minutes; Chinese gongfu brewing (功夫茶) fills a small pot or 盖碗 with 5–7 g per 100 mL and pours off infusion after infusion of 10–60 seconds. The workshop brews a cup.

Below: caffeine and catechins (polyphenols) extracted into the cup over time at the temperature, leaf and tea you choose (teaching first-order extraction with temperature-dependent rates, leaf contents from the literature), and how the cup will taste.

What you will learn
Workshop
Teaching model: extracted = leaf × content × 0.9 × (1 − e^(−k t)), with k rising with temperature by Arrhenius (caffeine k ≈ 0.5 per minute at 95 °C, activation energy 30 kJ/mol; catechins k ≈ 0.2 per minute, 60 kJ/mol). Leaf contents (dry): caffeine 3 %; catechins about 20 % in green, 10 % in oolong, 5 % in black tea, whose polyphenols are mostly theaflavins and thearubigins. Taste bands (teaching): under 50 mg catechins per 100 mL smooth, 50–90 brisk, above 90 bitter and astringent.

Cooler for green, hotter for black, more leaf for gongfu

In this teaching model, 1 g of green tea per 100 mL for three minutes gives about 41 mg of catechins per 100 mL at 80 °C but 98 at 100 °C — the difference between sweet and harsh — while caffeine changes much less with temperature. A Western mug (1 g per 100 mL, boiling, 4 minutes) holds about 24 mg of caffeine per 100 mL; the ISO tasting brew about 52. A 30-second gongfu infusion with 6 g per 100 mL of oolong extracts about 38 mg of caffeine per 100 mL in the first pour, and the leaf gives several more.

Practical brewing: heat water to the boil and let it cool — about a minute for 95 °C, three or four for 80 °C in a kettle with the lid open — or use a kettle with settings. Warm the pot or cup first. Green tea: 2–3 g in 250 mL at 75–85 °C for 2–3 minutes, or in a glass, leaves floating. Black tea: 2–3 g in 250 mL just off the boil for 3–5 minutes. Gongfu: rinse 5–7 g of oolong or pu'er in a 100 mL 盖碗 with boiling water and discard; then infuse 10–20 seconds and pour off completely, lengthening each infusion a little. Soft water brews clearer, sweeter tea than hard water, which dulls it and leaves a scum.

At the stove

Brew the same green tea at 80 °C and at 100 °C for three minutes each. Taste them side by side, then try the 100 °C one at one minute.

陆羽's 《茶经》, written in the eighth century, judges water before leaf: mountain spring water is best, river water next, well water last, and water should be taken off the fire at the second boil, when it "bubbles like a fountain, like strings of pearls" (涌泉连珠).

Terms

Extraction
Hot water dissolving compounds out of the leaf, each at its own rate.
Catechins
Polyphenols of green tea: bitter, astringent when over-extracted.
功夫茶
Gongfu brewing: much leaf, many short infusions.
盖碗
A lidded bowl for brewing tea.
Tea brewing is extraction: caffeine comes out fast, catechins slowly and much more with heat. Green tea 75–85 °C, 2–3 min; black 95–100 °C, 3–5 min; gongfu much leaf, short infusions.

Sources

  1. D. A. Balentine, S. A. Wiseman and L. C. M. Bouwens, "The chemistry of tea flavonoids", Critical Reviews in Food Science and Nutrition 37 (1997) 693–704; M. E. Harbowy and D. A. Balentine, "Tea chemistry", Critical Reviews in Plant Sciences 16 (1997) 415–480: dry tea leaf holds about 2–4 % caffeine and, in green tea, about 15–30 % catechins; caffeine extracts quickly, catechins more slowly and more strongly with temperature.
  2. ISO 3103:1980, Tea — Preparation of liquor for use in sensory tests: 2 g of leaf per 100 mL of freshly boiling water, steeped 6 minutes.
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.