After dinner you lift the cup. One mouthful goes down and your tongue is left clean. Sencha poured into the same cup made a child at the table pull a face at the first sip. On the back of the packet: “mellow”, “soft astringency”. That is as far as a packet goes.
Where does the source of astringency go when tea is roasted?
Down. And what remains is a different line-up. What packets and articles call tannin is, in tea, a group of compounds called catechins. That the total falls to about a fifth through roasting, and that some of them rise partway, was covered in chapter 2. This section looks at which ones rise.
The ones that rise are the forms without “epi”
The Tea Research Station of the Kochi Prefectural Agricultural Technology Center roasted aracha, or unrefined tea, from first-flush, karibancha (刈番茶) and second-flush crops twice in a hojicha roaster, made a light, a medium and a dark roast of each, and measured the catechins as eight compounds. The direction differs compound by compound. Epigallocatechin gallate, the most abundant of them, fell sharply, while one compound rose more than elevenfold. The four on the rising side are the forms whose names carry no “epi” at the front (Figure 8-1-1). Worked out as proportions [calculated], 95.7% of the catechins in aracha are epi-type, whereas by the light roast the non-epi forms already hold 36.7%. The falling total is what gets passed around. This change of address does not.
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% |
The swap has a name
The swap has a name. A review from 2007 set out the reaction in which heated catechins invert at position 2 of the ring and nowhere else. The destinations are fixed as well: epigallocatechin gallate goes to gallocatechin gallate, the partner with the “epi” taken off the front. All four pairs work the same way, and the four compounds rising in the Kochi table match that list. A report from 1968 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 on their own. Roasting exposes dried leaf to high heat, a different set of conditions from that experiment. A reaction that proceeds in water on the stove goes further over a flame.
No study found tastes the new forms at tea strength
What matters is whether the form on the far side is astringent. A review from 2022 had four catechins tasted at the concentrations found in tea infusions (300 micromoles per litre and below) and reported the strongest in taste to be epicatechin gallate, then epigallocatechin gallate, the first more astringent, the second more bitter. That review tasted only the four epi-type compounds; the four that rise with roasting are not in it. No study found here tests, at the strength and under the conditions of a hojicha infusion, how much the four rising compounds add to astringency. On the taste side, the account of roasting weakening astringency is still short of support.
Change the raw material and the quantities move
Change the raw material and the quantities move even at the dark roast. The dark-roast second-flush tea still held more total catechins than the dark-roast first flush (Figure 8-1-1). If matching the roast does not match what is in the cup, the matching happens further along. Part 4 will take that up, under blending.