Agar / kanten

What agar is

Agar is a jelly-making substance extracted from the cell walls of red seaweeds. It is a mixture of two polysaccharides, agarose, a linear polymer that makes up about 70 per cent and does the gelling, and agaropectin, a more varied group of smaller molecules with acidic side groups such as sulfate, which makes up about 30 per cent. The backbone of agarose is a repeating unit of 1,3-linked beta-D-galactopyranose and 1,4-linked 3,6-anhydro-alpha-L-galactopyranose. Agarose can be separated from the agaropectin and purified, and that fraction, a small corner of the market at about 0.2 per cent of sales, goes to laboratories.

It reaches the kitchen in two forms. Natural agar, the product of the Japanese tradition, is sold as white, semi-translucent strips or squares made by freezing, thawing, draining and drying the extract. Industrial agar is a fine powder, generally 60 to 100 mesh. The Japanese name for the strips, kanten (寒天), comes from kanzarashi tokoroten, meaning cold-exposed tokoroten, the seaweed jelly the agar was first made from; agar itself is a Malay word that Europeans adopted.

Agar is the seaweed counterpart of gelatin, which is an animal protein. It differs from carrageenan, another red-seaweed gel, in the shape of its helices: agar’s double helix is left-handed, carrageenan’s right-handed. And it differs from konnyaku, the konjac gel, in that agar melts again when reheated and sets again on cooling, while konnyaku does not.

A Japanese winter invention

Agar is attributed to an innkeeper, Minoya Tarozaemon, in Japan in 1658. The story is that he put out leftover tokoroten, a seaweed jelly, in the winter cold, and found that, frozen and dried, it gave a whiter and lighter product. Whatever the truth of the legend, the technique it describes is the real method of making kanten. Industrial dry production began in the early eighteenth century, and the product became known as kanten.

Agar then moved into science. The French chemist Anselme Payen made the first chemical analysis in 1859. Robert Koch began to use agar for solid culture media in 1881 or 1882, depending on the account, following the suggestion of Fanny Hesse, wife of his assistant Walther Hesse. Agar replaced gelatin as the medium because of its higher melting temperature, which let cultures be incubated near body temperature. In the twentieth century Matsuoka registered freezing-process patents in the United States in 1921 and 1922, and H. H. Selby and C. K. Tseng developed the classic American method in California before the Second World War.

Japan made most of the world’s agar until the war. Production was about 2,500 tonnes a year around the war and reached about 10,000 tonnes by the mid-1970s, since when it has fluctuated because wild seaweed stocks are unstable and sometimes over-harvested. A 1984 table counts 6,683 tonnes in total, 3,158 of them from Gelidium, with Japan at 2,440 tonnes, Spain at 890 and Chile at 820.

The seaweeds

Gelidium is regarded as the genuine source of agar, and the other agarophytes, the seaweeds that carry agar, are Gracilaria, Pterocladia and Gelidiella. The two main kinds of seaweed divide the market: Gelidium agar is used mainly for bacteriological plates, and Gracilaria agar mainly for food. A cultivated genus, Hydropuntia (formerly placed in Gracilaria), supplies most of the world’s agar: its agar content is 19 to 30 per cent of dry weight, about 3.6 million tonnes of the fresh seaweed were harvested in 2019, which put it third among seaweed genera, and Hydropuntia alone accounts for 66 to 80 per cent of global agar production. It is grown on rafts and nets, with nets harvested after about three months at 15 to 20 kilograms a net, and it can grow in abandoned shrimp ponds, where it takes up dissolved nutrients from the feed. Red seaweed production reached 18.3 million tonnes a year after 2019.

How agar is made

Making agar means freeing the polysaccharide from the cell wall, clarifying it and removing the water. The seaweed is first treated with alkali. Gelidium is typically diffused in sodium carbonate at about 80 to 95 degrees Celsius, while Gracilaria is given a sodium hydroxide treatment, commonly 0.1 molar, to raise the gel strength. In experiments on other Gelidium species, gel strength rose as the alkali concentration rose, and a strong alkali treatment also lowered the sulfate content of the agar, to 0.175 per cent at 10 per cent sodium hydroxide, and raised its melting temperature to 96.5 degrees.

The extract is then kept near boiling, with its pH adjusted, at a concentration of about 0.8 to 1.5 per cent agar. It is filtered at 85 to 100 degrees, usually in pressure filters, and set into a gel. Concentration by freezing and thawing follows: the frozen gel is thawed, the water drains off and the gel reaches about 11 to 12 per cent agar, at most about 15 per cent; syneresis under pressure can reach about 25 per cent. Drying completes the product. Freezing needs about 11,477 kilocalories per kilogram of agar against 53,361 for evaporating the same water, which explains why freezing is used for concentration.

Quality is measured as gel strength: the load in grams per square centimetre that breaks a standard 1.5 per cent gel in 20 seconds. Industrial agar ranges from 600 to 1,100 grams per square centimetre, with normal quality at 700 to 800. Oriental natural agar, in strip or square form, typically ranges from 150 to 400. Food-grade agar should have moisture below 18 per cent, ash below 5 per cent, gel strength above 750 and fewer than 10,000 bacteria per gram.

How the gel works

Agar gels because of a coil-to-helix transition. Heated above its melting range of 85 to 95 degrees Celsius, the agarose chains lie as loose coils in solution. As the solution cools, the chains twist into ordered double helices stabilised by hydrogen bonds, and as cooling continues the helices aggregate into a three-dimensional network that traps a large amount of water. A 1.5 per cent solution gels at about 32 to 43 degrees and does not melt below 85. That gap, called hysteresis, between setting and melting is the property that matters: the gel stays solid at room temperature and well above it, and a gel that has set can be incubated near body temperature, where gelatin would melt.

Several conditions shift the result. A higher concentration gives a stronger, denser gel, sugars strengthen it, and strongly acid conditions can hydrolyse agarose chains and weaken it. Agar gels are stable across pH 5 to 8 and need no added cations to set. The gel is clear, rigid, loses little water by syneresis and holds water well, and it resists enzymatic degradation. Because most organisms cannot digest agar, the gel stays intact where microbes grow on it. That is the other reason bacteriology adopted it.

Agar in fermentation and food

Agar has a double life in fermentation. In the laboratory it is the solid medium on which microbes are isolated and counted, sold as bacteriological agar, which makes up 4 to 5 per cent of agar sales; the agarose fraction is about 0.2 per cent of the market. In the kitchen it is a texture ingredient: as a food it is an additive, not a nutrient, used at up to 1 per cent for gelling and at a hundredth of that or less for controlling viscosity.

Food uses include confectionery, jellies, marmalade, mitsumame, bakery icings, yoghurt, meat products and aspics. In dairy it is a texture enhancer, and it reduces syneresis in drinks of grass jelly, where replacing 20 per cent of the original gel with agar markedly cut the water that separated. It also forms edible films that have been tested as wrappers. It is about 80 per cent dietary fibre and swells in the stomach, which fuelled the Asian kanten fad diets. Cooks have also blended it with gelatin to make mixed gels with a medium texture, and agar is used alone in formulations that need a gel that holds up at low pH and binds moisture.

On the plate, the choice of medium matters as much as the gel. Agar plates can be selective or indicator media, such as MacConkey agar and TCBS agar, in which the agar carries the nutrients and reagents that decide what grows and how it looks.

Regional uses and varieties

In Japan, agar is a pantry ingredient with a clear division of labour. Tokoroten, the noodle-like strands of seaweed jelly, is made by boiling agarophyte seaweed, traditionally tengusa (Gelidium amansii), letting it set and pressing it through an extruding device. It has been eaten in Japan for over a thousand years, is thought to have come from China in the Nara period, and in the Edo period was sold around factories as a summer snack. Today it is dressed with vinegar and soy sauce, sometimes with nori, hot pepper or sesame, and in the Kansai region it is eaten as a dessert with kuromitsu syrup. Commercial kanten is now mostly made from ogonori (Gracilaria) rather than tengusa.

Kanten also sets sweet jellies. Mizu yokan, the water yokan, has more water than usual and is usually chilled and eaten in summer, although Fukui Prefecture traditionally eats it in winter. Red bean paste is the typical base, white kidney bean paste gives a milder, translucent version that can be flavoured with green tea powder, and other versions use chestnut, persimmon or sweet potato. Agar is one of its main ingredients, with sugar and bean paste, and its invention in the 1600s was essential to the modern sweet. Anmitsu is a dessert of agar cubes served with fruit, and Japanese kanten later reached Manchuria, Korea and Taiwan.

In the Philippines the same job is done by gulaman, sold as foot-long bars, plain or coloured, or as powder, and made from agar or from carrageenan. Unlike gelatin, it needs boiling water to dissolve, and it sets at room temperature. It sets sago at gulaman, the roadside refreshment, and goes into buko pandan, agar flan, halo-halo and fruit salads. Vietnam has thach, or rau cau, and agar jellies are used in Taiwanese bubble tea.

Choosing and using it

Agar is dissolved in boiling water. It does not dissolve in cold water, and that is the main practical difference from gelatin. The gelling level is up to 1 per cent of the mixture: a 1.5 per cent solution gels at about 32 to 43 degrees Celsius, and bacteriological agar at 10 to 11 grams per litre gels at 32 to 36 degrees and does not melt below 85 to 86. Sweeteners and flavourings are stirred into the hot solution, which is then poured into moulds and set by cooling into a clear, firm gel. Because the gap between setting and melting is so wide, a jelly sets at room temperature without a refrigerator and keeps its form on a warm table, and a set gel can be reheated to a liquid and set again.

Agar needs no added calcium or other cations to set, and the gel is stable across pH 5 to 8. Sugar strengthens it, while strongly acid conditions can hydrolyse the agarose chains and weaken it, so a very sour mixture boiled for a long time gives a softer gel than a neutral one. Its texture is firmer than that of gelatin desserts, and cooks have blended the two to get a medium texture. The form matters: oriental natural agar in strips or squares rates 150 to 400 grams per square centimetre in the standard gel-strength test against 600 to 1,100 for industrial powder, with 700 to 800 normal, and the powder is ground to 60 to 100 mesh. A recipe written for strips therefore cannot be assumed to work gram for gram with powder. Agar is not an animal product, so it serves as the vegetarian counterpart of gelatin. No allergen is documented for agar, and no household shelf life is established.

Notes

The date of Koch’s first use of agar is given as 1881, or as 1882 with Walther Hesse introducing it. Gelling temperatures are quoted as 32 to 42, 32 to 43 and 32 to 45 degrees Celsius, and the spelling of the legendary innkeeper’s name varies.