Kanten (agar, or agar agar) (寒天)

"Kanten" (agar, or agar agar) is made from frozen and dried mucilage of red algae such as tengusa (agar weed) and ogonori (Chinese moss). Kanten, as sold at markets, is produced through repeated freezing and drying in a naturally cold climate during the winter. It's similar to the gelatin of bovine and swine origin as an ingredient in edible gel (jelly), but chemically it's a different substance.


In 1685, during the Edo period (in the reign of Shogun Ietsuna TOKUGAWA), Tarozaemon MINO, a lord of the Japanese-style inn "Minoya," located in what is now Fushimi Ward, Kyoto Prefecture, found that the tokoroten (gelidium jelly), which he had once thrown outside of his house, had become a dried foodstuff after having frozen and melted repeatedly for several nights and days. He tried to reuse this dried foodstuff and succeeded in making new tokoroten jelly with less of the seaweed odor. He asked INGEN Ryuki Zenji (Ryuki INGEN, a master of Zen Buddhism), the founder of Manpuku-ji Temple, to taste the new tokoroten jelly, and the latter then encouraged Tarozaemon to put it to practical use as an ingredient in vegetarian dishes. When the priest questioned Tarozaemon about its name, he replied that it had no name yet.
Reportedly, Ingen then gave it the name 'Kanten.'

Subsequently, Hanbei MIYATA, in Osaka, made an improvement in the manufacturing process and diffused it. Further, during the Tenpo era Kumezaemon KOBAYASHI, a peddler from Shinano Province, promoted the manufacture of kanten as a second job among farmers in the Suwa area, and established it as kaku-kanten (agar in the shape of a rectangular bar).

In 1881, Robert Koch developed the technology of culturing eubacterium using agar medium, and since then the demand for kanten has increased throughout the world. This made kanten one of the major export items before World War II, but, during the war period the exportation of kanten from Japan was banned for strategic reasons.

The countries that would otherwise have imported kanten from Japan could no longer do so; consequently, they were forced to try producing kanten and eventually developed industrial methods of producing kanten without relying on natural conditions. Thus, powdered kanten became available. Also in Japan, research in the production of powdered kanten started in 1946, and by 1970 the number of its producing companies had reached 35. However, as of 2004 there were only five such companies involved.

Today, most of the kanten sold in markets is a product that's bleached following the removal of any color and typical flavor of seaweed; this is done by dissolving the starchy substance with sulphuric acid, hydrochloric acid and so on.

Outside Japan, good-quality kanten is produced in Morocco, Portugal, Spain, Chile and Argentina.

The manufacturing process

Kanten is produced in a season of severe cold, from December to February of the following year.

Refining the raw seaweed

Spread out tengusa seaweed over a sandy beach, spray fresh water onto it from time to time, and expose it to sunlight for ten or so days, until it becomes a pale yellow. Soak the bleached seaweed in river water to soften it, and then pound it in a water mill. After removing the shell, the sand and other foreign materials from the pounded seaweed, expose it to running water for desalination and depigmentation.


Add seaweed at a rate of 20% to 40%. This is because tengusa seaweed is expensive and because pure tengusa consolidates too hard.

Boiling and aging

Boil the raw seaweed for about three hours in water containing a small amount of sulfuric acid and acetic acid to elute the mucilage of the seaweed. Reduce the heat of the boiling water to 70 to 80 degrees centigrade and maintain that temperature. A solution of approximately 4 kl of water and 30 g of sulfuric acid (diluted with water) is usually used for 12 kg of raw seaweed.


Place the boiled seaweed in a jute bag and squeeze it gently to filter out the liquid, which is then left at rest to obtain the supernatant fluid from it. Again, boil the marc of the squeezed seaweed to obtain the secondary liquid, and mix the liquid with freshly boiled liquid or use it to boil new, raw seaweed.


Transfer the supernatant fluid into a container, and let it stand to cool and coagulate, thus making tokoroten (gelidium jelly). For kaku-kanten (agar in the shape of a rectangular bar), cut the congealed tokoroten jelly into the shape of rectangular columns with sides four centimeters wide. For hoso-kanten (string-type agar), use an extruder to push out strings of tokoroten with square-cut sections five millimeters wide. Lay the cut or extruded pieces of tokoroten side by side on mushiro (straw mats).


The freezing field is surrounded by shelter hedges about 1.8 m high, where stakes about 50 cm long are piled on the east and west ends to support crossbars on which straw mats with tokoroten jelly are laid to freeze in the cold night air. Kaku-kanten, which is made after logging two nights of complete freezing, is regarded as being the best in quality. If it's cold enough to freeze the kanten so quickly in just one night, the agar polymers will become separated from the ice, resulting in poor shape and quality. If it is mild enough to require four or five nights to freeze the kanten, the resulting product will tend to decay and be colored. Compared with kaku-kanten, hoso-kanten requires less pain in the freezing process.


Frozen kanten is to be exposed to sunlight on the next morning in order to melt the ice portion into water and let the water dribble out. It will then dry in the sun for several more days, resulting in the final product.


Most kanten consists of dietary fiber (a mixture of polysaccharides such as agarose and agaropectin), which can't be dissolved by human digestive enzymes. According to recent research, however, some constituents of kanten may be dissolved by gastric acid into agaro-oligosaccharide, which is absorbed into the human body to produce certain physiological actions.

The coagulation of kanten is caused by polysaccharide. Accordingly, the process of coagulation can't be blocked by the protease (proteolytic enzyme) contained in such fruits as pineapple and kiwi.


Funmatsu kanten (powdered agar)

Its high purity and consistent quality are maintained. It has superior solubility.

Flake kanten (flake agar):

Its minimal precipitability makes it hard to burn dry. It's applied to high-class Japanese confectionery.

Kokei-kanten (solid agar):

Because of the batch production system, no weighing process is required. It is consistent in quality, too.

Kaku-kanten (agar in the shape of a rectangular bar)

It must be rinsed in water and then pureed. It is not consistent in quality. It's used in homemade cuisine.

Ito-kanten (string-type agar):

Its quality is nearly equal to kaku-kanten. It's used for various kinds of Japanese confections.


In addition to being used as an ingredient in cakes, it has attracted attention as a diet food because it has very few calories and deters the intestinum from absorbing oil and sugar. It has also drawn attention as a healthy food because of the above-mentioned agaro-oligosaccharide.

Kanten is cut into cubes for use in anmitsu (a bowl of sweets containing boiled beans, cubes of agar gelatin and bits of fruit with a sweet syrup poured over them). Moreover, a solid processed food, called gyunyu-kan (a sweet milk jelly) is made by adding funmatsu-kanten (powdered agar) into milk and allowing it to solidify. This is preferably used for annindofu (an almond jelly) of Chinese cuisine.


Kanten is used to make solid media for culturing plant tissues and microorganisms. Any medium solidified by kanten is called "agar medium," and this term has become a synonym for all culture media. Especially, high-purity kanten is used for the cataphoresis of nucleic acid (cataphoresis of agarose gel).

Dental treatment
A tooth that has been damaged by dental caries will not recover its form, except in very rare cases. In order to mend the crown of a tooth (corona dentis), the tooth is cut into a shape that allows a dentist to easily fabricate a cover (prosthetic appliance) to fit it. The fabricated prosthetic is then attached to the patient's tooth, completing the process. Since this prosthetic, which is mostly made of metal, can't be fabricated inside the patient's mouth due to its melting point, it becomes necessary to prepare a plaster figure that precisely duplicates the patient's tooth alignment. The plaster figure is fashioned by pouring dental plaster into a duplicate made of such materials (impression materials) and thereby provide elasticity sufficient to recreate the details of the patient's tooth alignment. Once the plaster has solidified, the final adjustments are made to the shape of the plaster figure. One of the impression materials used for dental treatment is made of agar. The agar material offers the excellent elasticity needed to reproduce details, but it's inferior in dimensional stability so that it swells by absorbing moisture and contracts due to evaporation when exposed to the air for a prolonged period of time. Thus the plaster figure must be quickly produced from the agar impression material.

[Original Japanese]