Tokoroten is a traditional Japanese dish of noodles made of gelatin derived from marine algae. It likely developed during the Heian Period (794-1185) but did not become widespread among the general population until the Edo Period (1603-1868), when it was often eaten as a cooling dish on hot summer days. Tokoroten is made by boiling various species of red algae, known in Japanese as tengusa, so that they release the polysaccharides that support their cell walls. The resulting gelatinous liquid is the basis for agar, the clear jelly that is used in laboratory sciences as a growth medium for microorganisms (the seaweeds that make agar are known collectively as agarophytes). Agar also has numerous culinary applications as a thickener and a vegetarian substitute for gelatin. Known in Japanese as kanten, agar may have been accidentally discovered in Japan in the 17th century when a seaweed soup was left outside during the winter and froze, resulting in a dry substance that was reconstituted through boiling to create a jelly with a pleasant texture.
Tokoroten is made by boiling dried tengusa in water until the liquid starts to thicken, at which point it is strained into a tray and allowed to cool so that it will set, forming a clear gelatin. This is then cut into long blocks and made into noodles using a tentsuki, a rectangular wooden tube with a screen at one end through which the blocks of gelatin are pushed to cut them into noodles. Tokoroten does not have much flavor; the noodles are served cold with sauces or condiments that vary by region: Black honey is popular in Kansai, fish sauce in Kochi, ponzu sauce in Kyushu, or sweet vinegar in Nagoya, for example.
Among the species of red algae most commonly used for tokoroten are Gelidium elegans (known in Japanese as makusa, and considered to be female algae) and Gelidium pacificum (known as obusa, and considered male algae). Makusa makes a soft, sticky gelatin, while obusa makes a gelatin that is thicker and firmer. Tengusa is categorized based on how it has been prepared: Akakusa is rinsed and dried tengusa; sarashi is tengusa that has been repeatedly washed and dried in the sun, giving it a very pale color (the best sarashi, which makes the highest-quality tokoroten, is called tora); kisarashi is tengusa that has not been washed; and aosarashi is tengusa that is left to sit for a while before drying. Tengusa is also classified based on how it was harvested: If picked directly from the sea floor, it is called okigusa, while washed-up tengusa gathered from the beach is known as yorigusa and is considered to be of lesser quality. Traditionally, tengusa was harvested by divers known as ama (literally “women of the sea”). Ama are best known for diving for pearls, but they also collect shellfish (such as abalone and oysters) and seaweed.
One of the areas most closely associated with tengusa harvesting is the Izu Peninsula and Izu Islands on the west coast of Honshu (about 60% of Japan’s Gelidium comes from this area), but it is also harvested in other regions of the country. Unfortunately, there are not many ama divers left—the last woman to practice this tradition in Inatori, a town on the Izu Peninsula, died a few years ago. The future of traditional tokoroten is also at risk because, instead of using dried tengusa to make the noodles at home, many people now buy agar in powder or strips; and because of the general overharvesting of Gelidium worldwide: Japan was the only supplier as recently as the early 20th century, but harvesting ramped up in other parts of the world following WWII, and peaked between the 1960s and 80s. Continuing to produce tokoroten from hand-harvested tengusa is an important way to maintain the direct link between traditional Japanese gastronomy and the sea by preserving the practices and knowledge that constitute these traditions.