Too ordinary to write about
There is very little English-language writing about roasted grain drinks, and the reason is not that they are obscure. It is that where they are drunk they are unremarkable: mugicha in a Japanese fridge, boricha on a Korean table, oksusu-cha from a vending machine. Nobody writes a tasting note about tap water either.
What roasting does
Raw grain gives a cup that tastes of wet flour. Roasting develops the same browning reactions that make bread crust and coffee taste of anything, and the depth of the roast determines almost everything about the result. Under-roasted grain is flat; over-roasted grain is acrid; and the window between them is wide enough that most producers land in it.
The dosing rule that is different
Roasted grain needs two to four grams per hundred millilitres, which is roughly twice what a leaf infusion needs. The common failure with grain drinks is not over-steeping — the material is already cooked and has very little bitterness to give — but using a leaf-sized quantity and wondering why the cup is thin.
Hot in winter, cold in summer
Nearly all of these are drunk both ways, and the switch is seasonal rather than a matter of preference. A jug of barley tea in the fridge is a Japanese and Korean summer fixture; the same material is served hot from the same kitchen in January. Cold brewing works because there is no astringency to manage.
Which grains
Barley is the most widespread — mugicha in Japan, boricha in Korea, and the two are the same material. Corn is the sweetest and is usually blended with barley. Buckwheat comes in two species that taste noticeably different, common buckwheat being round and sweet and tartary being genuinely bitter. Roasted rice is familiar to most people through genmaicha, where it is blended with green tea rather than drunk alone. Job’s tears and roasted soybean are heavier again, and the Korean powdered versions are closer to a thin porridge than an infusion.
Caffeine
None of the grains contains caffeine. Genmaicha does, because it contains green tea alongside the rice — the rice is not the caffeinated part, and that distinction catches people out regularly.