More than 90 percent of Japanese seaweed comes from the northern island of Hokkaido, where it’s harvested, dried, and packaged in small family operations that have been in the business for generations. According to the Hokkaido Marine Products Grading Corporation, there are twelve kombu species grown in the area, and each species has a different shape, flavor, and texture.
The harvesting is done by hand. Wild kombu (which makes up around 65 percent of Japanese kombu production; the rest is farmed) is harvested between May and September. Harvesters use a rod specifically designed not to damage the type of seaweed they are gathering. For instance, a V-shaped rod is used for species with wide leaves, like ma, rishiri, and rausu. The length of the rod ranges from two to ten meters, depending on the depth of the ocean floor. With the rod, the hunters grab kombu right above the root, then twist and pull the whole thing up.
Once harvested, each seaweed blade is cut above the root, washed, and laid to dry on a clean bed of rocks. Kombu can grow to more than five meters; the popular naga kombu can reach as long as ten meters. Dried kombu is cut to the standard lengths specified by the inspection agency for grading. In Japan, kombu must be flat, to make it presentable and easy to use. If the kombu is not flat enough, it is slightly rehydrated and then put through a presser. (Watching the kombu press in action reminds me of Anthony Hopkins ironing newspapers in The Remains of the Day.) The kombu is then trimmed with small scissors to remove damaged or discolored parts. Finally, it’s aged in a warehouse for improved color and umami, for a period ranging from a week to a year, depending on its type and quality.
Packaging is done strictly following the rules set by the inspection agency for each kombu species. Rishiri (a long-cut kombu from northern Hokkaido) has to be “covered by a high-quality cardboard that meets the Japanese industrial standards, tied twice around with high-quality bands horizontally in three places, and once around vertically.” One of the horizontal bands is colored to indicate the grade (from the highest to the lowest: green, red, purple, and brown), and placed either at the top or the middle part of the package, depending on the timing of the harvest. There are no national standards for grading kombu, and the criteria differ from region to region (and sub-region to sub-region). Each species has its own grading criteria, too. In general, kombu is inspected for color, shininess, weight, thickness, width, shape, damage, dryness, and the amount of white powder that has formed on the surface. (This is a substance called mannitol, and is actually an umami component, but it’s not appreciated aesthetically).
Chefs buy kombu not just by grades, but also for the different characteristics each kombu has. Rishiri is ideal for making clear, delicate dashi, but it may lack the soft texture of hidaka, which can cook much faster to make dishes such as kobumaki (fish wrapped in kombu and cooked with soy, sugar, and mirin) or kobujime (raw fish sandwiched between sheets of kombu, which work as a preservative). Shiroita kombu is the core of ma or rishiri kombu that has been softened by a soak in vinegar, before the surface is shaved down. Shiroita kombu is typically used to top battera, a traditional mackerel sushi. Since mackerel is highly perishable, Japanese cooks in the days before refrigeration covered the fish with shiroita to prevent if from drying or rotting. This had the added benefit of imparting shiroita’s umami to the already umami-rich fish.
Kombu can be very expensive: one kilogram of the highest-quality kombu can cost around $200. But Japanese chefs don’t seem to mind paying a premium for their dried kelp. “Kombu is the most important foundation of umami in Japanese cuisine,” says Kimio Nonaga, the third-generation chef at Nihonbashi Yukari, in Tokyo. The Japanese commitment to kombu runs deep. After kombu is shipped from the producer, some distributors keep it in their warehouse to age for a few years, like wine. Yoshihiro Takahashi, the fifteenth-generation chef at Hyotei, in Kyoto, says, “We use three-year-aged kombu, because the strong oceanic flavor disappears after aging and the dashi becomes more transparent. If it is aged over three years, the flavor is too concentrated to make dashi, which needs to stay as a background for other ingredients.”