Tea leaf chemistry

The taste, smell and lift of a cup come from a handful of compound groups in the leaf. Their shares in dry tea have been measured, and they explain where much brewing advice comes from.

By dry weight, the largest group in tea leaf is polyphenols, at 18–36%, and catechins make up 70–80% of them. They account for astringency, liquor color and the haze in cooled tea. Caffeine runs at 2–5%, highest in the bud and first leaf, and only part of it reaches the cup: a 30-second rinse removes about 9%. The Japanese chemist Sakato Yajiro described theanine in 1949. Shaded gyokuro and matcha have the most, pu-erh almost none.

Aroma is harder to pin down. Researchers have found about 600 volatile compounds in tea, yet the recognizable aroma of a given tea rests on a few dozen, and almost all of them form during withering, roasting and other processing rather than in the fresh leaf.

Where tea safety is concerned, the measured risks come less from the leaf than from drinking habits. Temperature comes first: IARC classes drinks above 65 °C as Group 2A. Next comes fluoride, for people who drink liters of brick tea made from mature leaf, and then pesticides. Depending on solubility, anywhere from 1–4% to 90–100% of the residue passes into the liquor.