What koicha is
Koicha (濃茶) means «thick tea». It is made with 4 to 6 g of matcha in only 30 to 40 ml of hot water (80 to 85 °C), giving a paste about the consistency of runny honey, with no foam, a deep dark green.
Koicha is served in the central phase of the chaji, the full ceremony. The same bowl is shared between several guests, which calls for a ritual wiping of the rim before it is passed on.
Why it needs the best matcha
At koicha concentrations, any bitterness or astringency in the matcha is amplified. Only a first-harvest matcha from a premium cultivar (Gokou, Asahi, Samidori), shaded for 25 to 30 days, has enough umami to stay pleasant at that strength. With a second harvest or a lower grade, koicha is undrinkable.
The technique
Unlike usucha, koicha is not whisked in Ws. The chasen turns in slow, deliberate circles that work the powder into the hot water. It can take 2 or 3 minutes. You are after a smooth, lump-free, glossy paste. If the surface is lumpy or matt, keep going.