Three zones of heat
Below 40C, umami leaves kombu very slowly. Around 60C, it flows out well. Above 80C, slime and a strong sea smell join in.
What the slime is
Kombu holds soft fibers called alginate. They protect the kelp in the ocean. Hot water melts them into the stock. The dashi turns thick and cloudy.
Why one hour
Kyoto chefs and scientists tested this in 2002. At 60C, one hour gave the most glutamate. Longer steeping added little.
It is a guide, not a law
Some kombu shops say taste matters more than numbers. A cold overnight soak also works well. The real rule is simple. Never boil kombu.
Ryota says:
No thermometer? Watch the pot edge. Pull the kombu before the first tiny bubbles.
