After dinner the cup comes up to the lips. One mouthful goes down, and the tongue is left clean. Sencha poured into the same cup made a child pull a face. The packet says no more than “mellow” and “soft astringency”.
What follows traces how the source of astringency rearranges itself in the roast, and how much is known about the taste on the far side.
What settles at the bottom of the cup
Set the cup down and a pale brown gathers at the bottom. Roasting lowers the source of astringency, and changes its line-up as well.
What packets and articles call tannin is, in tea, a group of compounds called catechins. An earlier chapter followed their total falling to about a fifth in the roast. The same chapter noted that some of them rise along the way.
The falling total is the familiar part. The sources go as far as showing that the rising ones have a definite face. That face comes next.
The “epi” falls from the front
Pinch a roasted leaf and it crumbles with a dry crackle and a toasty smell. The compounds that rise are the forms without “epi” at the front of their names.
A prefectural research station roasted tea light, medium and dark, then measured the catechins compound by compound. The most abundant one fell sharply, while others climbed steeply. Before roasting almost all are epi forms; by the light roast, more than a third have switched.
The falling total gets passed around, while this change of address rarely does. The amounts sit in the table below, and the sources go as far as the switch itself. The switch, as it happens, has a name.
Figure 8-1-1The eight catechins and the share of each form, by depth of roast (Kochi, first-flush tea; mg per 100 mL of infusion)
| Compound (first-flush tea) | Aracha | Light roast | Medium roast | Dark roast |
|---|---|---|---|---|
| Epicatechin (EC) | 7.43 | 5.17 | 3.41 | 1.98 |
| Catechin (C)* | 0.17 | 0.28 | 0.20 | 0.09 |
| Epigallocatechin (EGC) | 2.09 | 0.50 | 0.96 | 0.72 |
| Gallocatechin (GC)* | 0.00 | 0.62 | 0.39 | 0.00 |
| Epicatechin gallate (ECg) | 3.50 | 1.88 | 1.25 | 0.53 |
| Catechin gallate (CG)* | 0.86 | 1.89 | 1.70 | 1.11 |
| Epigallocatechin gallate (EGCg) | 20.38 | 6.29 | 3.15 | 1.18 |
| Gallocatechin gallate (GCg)* | 0.46 | 5.23 | 3.53 | 1.38 |
| Total | 34.89 | 21.86 | 14.59 | 6.98 |
| Non-epi share [calculated] | 4.3% | 36.7% | 39.9% | 36.9% |
| Gallate-type share [calculated] | 72.2% | 69.9% | 66.0% | 60.1% |
It happens in kitchen water too
Steam from the kettle clouds the kitchen window. Under heat, a catechin flips at one single point in its shape.
Like a glove turned inside out, each has a fixed partner: the same name with the “epi” taken off. A review of the chemistry shows every pair working the same way, matching the compounds that rose in the station's table. An old report had already written the rearrangement down.
Warming catechins in water is enough to set the flip going. Roasting, though, puts dry leaf over heat, and the sources only go as far as saying the conditions differ. What is left to ask is how the flipped forms taste.
A taste no one has measured yet
A cooling mouthful rolls towards the back of the tongue. How astringent the flipped forms are has not yet been tested.
A review that tasted catechins at tea strength found the strongest tastes among the epi forms. One leaned astringent, the other bitter. Only epi forms were tasted there; the ones that rise in the roast were left out.
No study found here measures the rising forms at the strength of a hojicha infusion. On the taste side, “roasting softens astringency” is short of support, and the sources reach only as far as the switch. Last comes the raw leaf itself.
A dark roast from a later picking
On the shelf stand dark roasts from an early picking and a later one. Change the raw leaf, and the amount moves even at the same depth.
Roasted to the same depth, the second picking kept more catechins than the first. Matching the roast does not match what is inside.
The sources go as far as saying depth alone does not fix the amount. The matching happens further along, and a later part takes up blending. The figures and test conditions are gathered just below.
The numbers in detail
- The trial
- The trial is “Roasting Level and the Quality of Hojicha” by the Tea Research Station of the Kochi Prefectural Agricultural Technology Center. Aracha from first-flush, karibancha (刈番茶) and second-flush crops was roasted twice in a hojicha roaster into light, medium and dark roasts. The catechins were measured as eight compounds.
- Totals and breakdown
- The fall of the total to about a fifth appears in chapter 2, Figure 2-2-1. Epigallocatechin gallate, the most abundant, fell sharply, while one compound rose more than elevenfold. The four that rise are the forms with no “epi” at the front of the name. The values by compound are in Figure 8-1-1.
- Share by form
- The proportions by form are calculated values not printed in the original. In aracha, 95.7% of the catechins are epi-type; by the light roast, non-epi forms hold 36.7%.
- The mechanism
- The mechanism follows a 2007 review by Sano. Heating inverts the catechin ring at position 2 alone, so epigallocatechin gallate becomes gallocatechin gallate. All four pairs work the same way, matching the four that rose in the Kochi table.
- Older report, warm water
- A 1968 report by Nakagawa had already described the rearrangement as epimerisation, polymerisation and decomposition. By the same review, standard catechins steeped for 30 minutes in a buffer at 90°C (pH 6.0) isomerise.
- Taste review
- The taste data come from a 2022 review by Narukawa. Four epi-type catechins were tasted at the concentrations found in tea infusions (300 micromoles per litre and below). The strongest was epicatechin gallate, then epigallocatechin gallate. The first is described as more astringent, the second as more bitter.
- Raw-leaf difference
- The dark-roast second flush held 8.68 mg of catechins in total, more than the 6.98 mg of the dark-roast first flush. Both are per 100 mL of infusion.