Moorhead - About Agar

How it Began

The use of seaweed as food has been traced back to the fourth century in Japan and the sixth century in China. Today those two countries and the Republic of Korea are the largest consumers of seaweed as food. However, as nationals from these countries have migrated to other parts of the world, the demand for seaweed for food has followed them, as, for example, in some parts of the United States of America and South America. Increasing demand over the last fifty years outstripped the ability to supply requirements from natural (wild) stocks. Research into the life cycles of these seaweeds has led to the development of cultivation industries that now produce more than 90 percent of the market's demand.

China is the largest producer of edible seaweeds, harvesting about 5 million wet tons annually. The greater part of this is for kombu, produced from hundreds of hectares of the brown seaweed, Laminaria japonica, that is grown on suspended ropes in the ocean. The Republic of Korea grows about 800,000 wet tons of three different species, and about 50 percent of this is for wakame, produced from a different brown seaweed, Undaria pinnatifida, grown in a similar fashion to Laminaria in China. Japanese production is around 600,000 wet tons and 75 percent of this is for nori, the thin dark seaweed wrapped around a rice ball in sushi. Nori is produced from a red seaweed - a species of Porphyra.

Various red and brown seaweeds are used to produce three hydrocolloids: agar, alginate and carrageenan. A hydrocolloid is a non-crystalline substance with very large molecules and which dissolves in water to give a thickened (viscous) solution. Alginate, agar and carrageenan are water-soluble carbohydrates that are used to thicken (increase the viscosity of) aqueous solutions, to form gels (jellies) of varying degrees of firmness, to form water-soluble films, and to stabilize some products, such as ice cream (they inhibit the formation of large ice crystals so that the ice cream can retain a smooth texture).

Japanese first discovered the gelling properties of agar, extracted with hot water from a red seaweed as a source of hydrocolloids dating back to 1658. Extracts of Irish Moss, another red seaweed, contain carrageenan and were popular as thickening agents in the nineteenth century. It was not until the 1930s that extracts of brown seaweeds, containing alginate, were produced commercially and sold as thickening and gelling agents. Industrial uses of seaweed extracts expanded rapidly after the Second World War, but were sometimes limited by the availability of raw materials. Once again, research into life cycles has led to the development of cultivation industries that now supply a high proportion of the raw material for some hydrocolloids.

Seaweed used as fertilizer dates back at least to the nineteenth century. Early usage was by coastal dwellers, which collected storm-cast seaweed, usually large brown seaweeds, and dug it into local soils. The high fiber content of the seaweed acts as a soil conditioner and assists moisture retention, while the mineral content is a useful fertilizer and source of trace elements. In the early twentieth century, a small industry developed based on the drying and milling of mainly storm-cast material, but it dwindled with the advent of synthetic chemical fertilizers. Today, with the rising popularity of organic farming, there has been some revival of the industry, but not yet on a large scale; the combined costs of drying and transportation have confined usage to sunnier climates where the buyers are not too far from the coast.

Cosmetic products, such as creams and lotions, sometimes claim on their labels that the contents include "marine extract", "extract of alga", "seaweed extract" or similar. Usually this means that one of the hydrocolloids extracted from seaweed has been added. Alginate or carrageenan could improve the skin moisture retention properties of the product. Pastes of seaweed, made by cold grinding or freeze crushing, are used in thalassotherapy, where they are applied to the person's body and then warmed under infrared radiation. This treatment, in conjunction with seawater hydrotherapy, is said to provide relief for rheumatism and osteoporosis.

Agar-Agar

Two genera, Gelidium and Gracilaria, account for most of the raw material used for the extraction of agar. Extraction of Gelidium species gives the higher quality agar (as measured by the gel strength: the strength of a jelly formed by a 1.5 percent solution). Agar-Agar is one of the most ancient stabilizers used in food. It is said that during the XVI century the Japanese people, who named it “Kanten”, used it.

The use of agar became widespread in Indonesia, Philippines, Malaysia, China, and Korea, where people called it Agar-Agar, which means seaweed. Its introduction to Europe dates from 1859, almost two hundred years later. Agar has been used in food preparations for over three hundred years.

The best quality agar is extracted from species of the red algal genera Pterocladia, Pterocladiella and Gelidium, and is harvested around the world. Agars of lesser quality are extracted from Gracilaria and Hypnea species. Agar quality is seasonal in Pterocladiella species; low in the colder months and high in the warmer months. There is currently a shortage of agar-producing seaweeds (Gelidium) thus causing a shortage and forcing prices to constantly rise.

Gelidium grows best where there is rapid water movement, which is in the eulittoral and sublittoral zones. Depending on the species, it can be found in water from 2m to 20m in depth. Gelidium prefers rocky areas with steep slopes, and is rarely found on muddy or sandy bottoms. It prefers partial shade and may be bleached by full intensity light in tropical latitudes. It usually grows best at 15-20°C, but can tolerate higher temperatures. It can survive in low nutrient conditions and some species adapt to low or high salinity.

For Gelidium, the harvesting methods used in Spain and Portugal are typical of the industry. A high percentage of the harvest comes from the gathering of storm-cast seaweed. Two people dragging a net to collect the seaweed; for cast material that has settled to the bottom in shallow bays, boats may be used to drag the nets. Sometimes significant quantities of cast seaweed collect in depressions in the sea floor and this is collected using a suction tube, put in place by a diver, to draw the material up and into a boat.

The seaweed is held onto the rock by a holdfast, a structure that often consists of many finger-like pieces that are called rhizoids. It is important for some rhizoids to be left on the rock so the Gelidium can re-grow. Harvesting is done by plucking the plant and placing in a net or basket. It is then transported to a boat. If a diver removes the entire plant then regeneration of the seaweed beds will be much slower. Machines designed to help the diver cut or “mow” the bed have been devised, which allow for less damage to the rhizoids. These machines also have a vacuum to help transport the cut plant to a harvest boat.

In several countries, such as Chile and Indonesia, most of the harvest is from attached weed that is picked by hand either at low tide or by snorkeling in shallow waters.

More than 50% of food grade Gracilaria is extracted from Gracilaria gracilis and Gracilaria chilensis. Gracilaria species were once considered unsuitable for agar production because the quality of the agar was poor (gel strength too low). In the ‘50s, it was found that pre-treatment of the seaweed with alkali before extraction lowered the yield but gave a good quality agar. This allowed expansion of the agar industry, and led to the harvesting of a variety of wild species of Gracilaria in countries such as Argentina, Chile, Indonesia and Namibia.

Over-harvesting of the wild crop Chilean Gracilaria created a need for new methods of cultivation. Ponds and open waters of protected bays were used. These methods have spread beyond Chile to other countries, such as Indonesia, Namibia China, the Republic of Korea, the Philippines and Viet Nam using species of Gracilaria native to each particular country. Obviously, Gracilaria species can be grown in both cold and warm but the best quality is grown in cool/cold water. Gracilaria is also an ideal vegetable substitute for animal gelatin in recipes.

Cultivating

Gracilaria is grown on the bottom of the sea. As the plant enlarges it provides more resistance to water movement and could eventually break off, leaving some of the plant in the sediment or sand to grow again. The broken pieces drift and may be collected by nets or are picked up after they wash onto the shore. Raking the beds from a boat is also practiced, but care is needed or the sea bottom may be damaged, leaving little residual plant to grow again.

Line or rope farming was pioneered in China in the 1950s for the cultivation of brown seaweeds, and the method has been adapted to several other genera, including Gracilaria. Pieces of Gracilaria are fixed to a rope or monofilament line such as nylon. The rope needs to be stable when exposed to sunlight and salt water for long periods; polypropylene rope is often used. Untwisting the rope to open the lay, inserting the plant and then allowing the rope to twist back to its natural position can attach the seaweed.

Pond cultivation of Gracilaria is less labor intensive than rope farming (no need to fix many pieces to a rope or net) and has been quite successful. One disadvantage of Gracilaria from ponds is that the agar extracted from it is often of low gel strength and perceived lower quality thus lower prices.

Ponds are usually no larger than a hectare, 0.5-1.0. Pieces of fresh seaweed, either gathered from natural beds or nearby ponds, are distributed evenly over the pond surface and allowed to sink to the bottom. Because the Gracilaria is not fixed in any way, any wind motion of the water will drive the plants to one side of the pond. This even occurs in small ponds, so an area that is not exposed to strong winds is preferable. Ponds need access to both salt and freshwater so that the salinity can be adjusted and so that the water can be changed every 2-3 days. Water change is usually made using tidal flows, with gates to control the water flow in and out.

Harvesting is possible every 35-45 days depending on the seasonal growth rate. About half the seaweed is harvested, usually by people wading through the pond, scooping it off the bottom into nets and placing it on a wooden raft or floating basket. The plants remaining form seed material for the next crop, and are broken into smaller pieces and broadcast over the pond. The harvested material is often laid around the banks of the pond to dry in the sun for 2-3 days. A cleaner product is obtained by drying away from the pound.

Processing Flakes/Powder

Gelidium is simply washed to remove sand, salts, shells and other foreign matter and is then placed in tanks for extraction with hot water. The agar dissolves in the water and the mixture is filtered to remove the residual seaweed. The hot filtrate is cooled and forms a gel (jelly), which contains about 1 percent agar. The gel is broken into pieces, and sometimes washed to remove soluble salts, and, if necessary, it can be treated with bleach to reduce the color. Then the water is removed from the gel, either by a freeze-thaw process or by squeezing it out using pressure. After this treatment, the remaining water is removed by drying in a hot-air oven. The product is then milled to a suitable and uniform particle size.

Gracilaria is also washed, but it must be treated with alkali before extraction; this alkaline pre-treatment causes a chemical change in the agar from Gracilaria, resulting in an agar with increased gel strength. Without this alkaline pre-treatment, most Gracilaria species yield an agar with a gel strength that is too low for commercial use. For the alkali treatment, the seaweed is heated in 2-5 percent sodium hydroxide at 85-90°C for 1 hour; the strength of the alkali varies with the species and is determined by testing on a small scale. After removal of the alkali, the seaweed is washed with water, and sometimes with very weak acid to neutralize any residual alkali.

A large and reliable freshwater supply is a requirement for an agar factory. Water consumption is high and the processing of Gracilaria requires more than for Gelidium. Higher water consumption also means larger quantities for waste disposal, so recycling of water is becoming more necessary, depending on the location of the factory.

Agar strips

Agar for use in food is sold in two forms: strip agar and agar powder. The powder is produced by the method previously described above. Agar strip, sometimes called natural agar, is produced on a small scale in the Republic of Korea, Japan, and China by the old, traditional method. Gelidium must be used; it was the only raw material used before the WWII.

Gelidium is boiled for several hours in water, acidified by the addition of either vinegar or dilute mineral acid. The hot extract is filtered through cotton cloth, and then poured into wooden trays to cool, which will form a gel. The gel is extruded to produce spaghetti-type strips about 30 cm long.

The strips are frozen and then thawed repeatedly (which causes water to release) leaving a more concentrated gel. The strips are then dried. Strips are assembled into bundles and sold for domestic use. Consumers should soak the strips to make them easier to dissolve in boiling water.

Bacteriological agar

This can only be made from species of Gelidium because the resulting agar has a low gelling temperature (34-36°C) that allows the addition of other materials to the agar with a minimum risk of heat damage. Gracilaria and Gelidiella produce agars that gel at 41°C or higher. Bacterial agars must not contain anything that might inhibit the growth of bacteria, such as trace metals, soluble carbohydrates or proteins, nor should they contain any bacterial spores. They must not interact with any materials that must be added as nutrients for the bacteria under study. The gels must be strong and have good clarity.

Conclusion

The use of agar is dependent on the unique properties of its ability to form gels. Agar dissolves in boiling water and when cooled it forms a gel between 32° and 43°C, depending on the seaweed source of the agar. In contrast to gelatin gels, that melt around 37°C, agar gels do not melt until heated to 85°C or higher. In food applications, this means there is no requirement to keep them refrigerated in hot climates. At the same time, they have a mouth feel different from gelatin since they do not melt or dissolve in the mouth, as gelatin does. This large difference between the temperature at which a gel is formed and the temperature at which it melts is unusual, and unique to agar. Many of its applications take advantage of this difference.

About 90 percent of the agar produced is for food applications, the remaining 10 percent being for bacteriological and other biotechnology uses. Agar has been classified as GRAS (Generally Recognized As Safe) by the United States of America Food and Drug Administration, which has set maximum usage levels depending on the application. In the baked goods industry, the ability of agar gels to withstand high temperatures means agar can be used as a stabilizer and thickener in pie fillings, icings and meringues. Cakes, buns, etc., are often pre-packed in various kinds of modern wrapping materials and often stick to them, especially in hot weather; by reducing the quantity of water and adding some agar, a more stable, smoother, non-stick icing is obtained.

Some agars, especially those extracted from Gracilaria chilensis, can be used in confectionery with a very high sugar content, such as fruit candies. These agars are said to be "sugar reactive" because the sugar (sucrose) increases the strength of the gel. Because agar is tasteless, it does not interfere with the flavors of foodstuffs; this is in contrast to some of its competitive gums that require the addition of calcium or potassium salts to form gels. In Asian countries, it is a popular component of jellies; this has its origin in the early practice of boiling seaweed, straining it and adding flavors to the liquid before it cooled and formed a jelly. A popular Japanese sweet dish is mitsumame; this consists of cubes of agar gel containing fruit and added colors. It can be canned and sterilized without the cubes melting. Agar is also used in gelled meat and fish products, and is preferred to gelatin because of its higher melting temperature and gel strength.

In combination with other gums, agar has been used to stabilize sherbets and ices. It improves the texture of dairy products like cream cheese and yogurt. It has been used to clarify wines, especially plum wine, which is difficult to clarify by traditional methods. Unlike starch, agar is not readily digested and so adds little caloric value to food. It is used in vegetarian foods such as meat substitutes.