Kombu

A thick, dark sea kelp — Saccharina japonica, mostly — dried and aged on wooden racks off the coast of Hokkaido for a year or longer. The white bloom on its surface is not mold. It is mannitol, a natural sugar that surfaces during drying, and it is the visible sign that the kombu is ready to work.

Kombu is the first half of dashi and the structural floor of most of Japanese cuisine. It releases glutamates into cold water without being heated — which is why traditional dashi is cold-soaked, not boiled. Boiling kombu makes it bitter and slimy. The instruction do not boil is not a stylistic preference. It is a chemical one.

Different grades are used for different purposes. Ma-kombu gives a clean, slightly sweet broth and is the kombu used for kaiseki. Rishiri-kombu is sharper and more savory and is the kombu most home cooks reach for. Hidaka-kombu is the softest and is the one you can also eat after using it for stock.

Kombu is one of the most exact ingredients in the Japanese pantry — and one of the most translatable. A piece of kombu, cold-soaked in water with a few dried shiitake, will give an Italian soup the depth that ordinarily takes a parmesan rind and four hours of simmering. The principle is identical. The route is shorter.