All summer a jug sits on the bottom shelf of the fridge. Leaf and water go in at night, and by morning the water has turned a pale amber. It tastes lighter than the same leaf brewed hot.

What follows is what a lower temperature brings out and what it leaves behind: first in measurements on sencha, then in roasted tea, then in the sellers' guidance.

A light cup from the morning jug

Tip the jug and the cold tea fogs the inside of the glass. At a lower temperature, part of the astringency and the caffeine come out less readily, while the amino acids still come out well.

That order, though, was measured in sencha, before any roasting. Roasted tea has almost none of the savoury components a cold brew would be aiming for.

These measurements describe sencha. After the order come the two faces of astringency.

Heavy astringency and light

A mouthful of cold tea feels softer at the back of the tongue. The source of astringency comes in a heavy kind and a light kind, and the heavy one barely dissolves in cold water.

A national research body measured sencha left in cold water against sencha brewed briefly in hot. The heavy kind came out at only about a fifth, clearly apart from the light kind. Caffeine fell in between; in iced water it dropped further, while the savoury amino acid stayed high.

Those proportions did not change with longer steeping, the report says. The figure below shows sencha, before roasting. After astringency comes cold water and caffeine.

Figure 3-3-1What a lower temperature extracts (sample: sencha; 80°C for 2 minutes = 100)

Epigallocatechin (10°C, 1 hour)70% (of the amount extracted at 80°C for 2 minutes)
Epigallocatechin gallate (10°C, 1 hour)20
Caffeine (10°C, 1 hour)50
Caffeine (0.5°C, 1 hour) = under 2020
Theanine (0.5°C, 1 hour)80
Drawn from Monobe M. (2018) JARQ 52(1):1-6. The sample is sencha, and the values are percentages of the amount extracted at 80°C for 2 minutes. The compounds are epigallocatechin (the lighter source of astringency), epigallocatechin gallate (the heavier one), caffeine and theanine. The figures are approximate in the original too, so read them in steps of 20% at most. “Caffeine (0.5°C, 1 hour)” is under 20%, and its bar is drawn at that upper bound of 20. No equivalent measurement uses hojicha as its sample.

Cold water still lets caffeine out

Even in the jug in the fridge, the leaf slowly unfurls. Cold water does not keep the caffeine in.

One report stirred leaf in water at around room temperature and measured as it went. Before long, both caffeine and the lighter astringent kind had dissolved past seven-tenths of what the leaf held. In sencha steeped in still colder water, part of the caffeine came out within a few hours.

Both measurements used sencha. Even in cold water, caffeine was extracted. After sencha comes a cup of roasted tea.

What roasting leaves behind

Roasted leaf smells toasty the moment the packet opens. In roasted tea, the savoury component a low temperature would draw out is nearly gone.

No cold-brew study using roasted tea as its sample could be found. In figures from a regional experiment station, a light roast kept under a tenth of the savoury amino acid in the unroasted leaf, and deeper roasts showed none. Cold water pays off in green tea because it can reach amino acids that dissolve well even cold.

For hojicha, then, lowering the temperature is less a way of drawing things out than a way of letting less out. After the components comes the strength of the sellers' guidance.

Cold-brew guides run thin

Ice clinks in a glass. The sellers' guidance runs two ways: tea steeped in cold water, and tea brewed with boiling water and poured over ice. The cold-water guides sit on the thinner side of the teapot guides.

Line the cold-water guides up and the steeping time runs from tens of minutes to several hours. Where they give the amount of leaf, it is clearly less leaf to water than the teapot guides use. The iced version is built the other way, hot water and a short brew.

So when cold brew is said to have less caffeine, temperature is not the only thing that has moved. The share that dissolves and the amount in one cup are better kept apart. How to keep them apart is taken up in Chapter 1. The detailed figures follow below.

The numbers in detail

Cold-water extraction
Manami Monobe's measurements on sencha, for the National Agriculture and Food Research Organization (JARQ 52(1), 2018). With 80°C for 2 minutes as 100, 10°C water for 1 hour gave about 70 for epigallocatechin, about 20 for epigallocatechin gallate, and about 50 for caffeine.
Iced water
in the same report, iced water at 0.5°C for 1 hour gave caffeine under 20 and theanine about 80. Steeping longer did not change the proportions.
Water at 20°C
Horie and colleagues, Tea Research Journal 91 (2001). 1 g of first-flush aracha was stirred in 500 mL of water at 20°C. After 20 minutes, both the caffeine and the lighter astringent component had dissolved past 70% of the amount in the leaf.
Cold brew at 7–8°C
another measurement cited in the same report found 20–50% of the caffeine dissolved within 2 hours, in sencha steeped cold.
Theanine
the Tea Research Station of the Kochi Prefectural Agricultural Technology Center. First-flush leaf (2013), 1.9 g in 200 mL of boiling water for 5 minutes, compared per 100 mL. Against 21.12 mg in the unroasted aracha, the light roast had 1.27 mg; none was detected in the medium and dark roasts.
Poured over ice
Ippodo Tea's iced hojicha, 10 g of leaf with 240 mL at 100°C for 60 seconds.
The seven cold-water guides: steeping times from 20 minutes to 3 hours. The five that give an amount of leaf use 1.0 g to 1.7 g per 100 mL, thinner than the teapot guides' 1.7 g to 4.2 g.
Keeping it
for cold-brewed green tea, the National Agriculture and Food Research Organization advises keeping it in the fridge and drinking it within a day.