3.2 Seaweeds as sources of hydrocolloids 3
3.3 Brown seaweeds as sources of alginate 4
3.4 Red seaweeds as sources of agar 5
3.5 Red seaweeds as sources of carrageenan 5
Seaweeds have been used as a human food since ancient times, particularly in China, the Korean Peninsula and Japan. As people from these countries have migrated to other regions, they have taken this use of seaweed to their new countries so that dried and wet, salted seaweed products can now be found in most parts of the world. These seaweeds form the commercial side of the seaweed food industry.
Seaweed has also been eaten by the coastal populations of many countries, sometimes as part of a subsistence living, sometimes as a regular ingredient of salad-type preparations, the latter especially in Hawaii and the warmer countries of Southeast Asia such as Indonesia, Malaysia, Philippines and Thailand. These products are collected and sold on a local basis and their volume and value are not known.
The three most important seaweeds used as human food are species of Porphyra (common Japanese name, nori), Laminaria (kombu) and Undaria (wakame). In recent years, Porphyra has frequently appeared in the Japanese Fisheries Statistics as the third largest catch. All three algae were originally obtained from wild crops but today it is only possible to meet demand by large-scale cultivation methods. Porphyra is classed as a red seaweed, Laminaria and Undaria are brown seaweeds.
The Porphyra life cycle is complex and was only elucidated in the 1950s by a British phycologist. This led to the rapid expansion of the nori industry, at first in Japan, later in China and the Republic of Korea. Nori is sold in packets (about 30 g) of thin sheets, 10-12 cm square, usually uncooked or lightly baked and used to form the outer wrapping on sushi. Sometimes it is cooked and salted and eaten as a snack or sprinkled over rice or noodles. It has a high content of useful edible protein.
Annual production is estimated at 90 000 tonnes (dry), valued US$ 1 460 million.
Laminaria species were first cultivated in Japan but in the 1950s Chinese scientists were able to reduce the cropping time from two years to one year and Chinese production grew to exceed 1.5 million wet tonnes per annum. Most of this was dried and eaten as kombu in the coastal provinces, the surplus was used for alginate production. However the cost of cultivation is high and while this can be absorbed by the price obtained as food (about US$ 3 000 per tonnes (dry), it is not competitive in an open alginate market where the cost of raw material must be around US$ 500 tonnes (dry) so the current Chinese alginate industry needs to use wild crops, often imported. Kombu is used in a wide variety of dishes such as soups, as an ingredient of a Japanese hotpotch, to make pickles and as a tea.
Large-scale cultivation is carried out especially in Japan and China, to a smaller extent in the Republic of Korea, yielding a world output estimated at slightly more than 1 000 000 tonnes (dry) with a value of US$ 3 000 million.
Undaria is especially valued in the Republic of Korea where cultivation is on a larger scale than other countries. It is a thinner, more delicate seaweed than kombu. It is often prepared and marketed as a blanched and salted product, which is stored at -10 degrees Celsius before sale. In use, it is desalted by washing and used mostly in soups; in the Republic of Korea more wakame is used giving a much thicker soup than in Japan. Some processed wakame goods have been marketed as instant foods.
Total harvests of Undaria, most cultivated but some wild, is about 33 000 tonnes (dry) with a value of US$ 230 million.
In the past decade, some French research and development institutions have placed considerable effort into the development of edible seaweed products with a view to their introduction to the European diet and market.
The cell walls of seaweeds contain long chain polysaccharides, which give flexibility to the algae and allow then to adapt to the variety of water movements in which they grow. For example, some brown seaweeds grow attached to rocks in very turbulent waters, requiring maximum flexibility to survive, and these contain a higher amount of these polysaccharides than brown seaweeds growing in calm waters. These polysaccharides are referred to as hydrocolloids because they will disperse in water to give a solution with colloidal properties. Polysaccharides from other sources such as land plants behave in a similar way so sometimes the term “phycocolloid” is used to distinguish the hydrocolloids that are derived from seaweed (from the term, phycology, the study of algae including seaweeds).
When hydrocolloids are dispersed in water they increase the viscosity and so find many uses as thickening agents. Under some conditions they will also form gels and this property is useful for other applications. Their colloidal properties can lead to other uses where their mode of action is less easily defined; for example the hydrocolloid from brown seaweeds is often added to ice cream where it inhibits the formation of ice crystals if the ice cream partly melts and is refrozen (on the way from the supermarket to home).
The hydrocolloids from seaweeds, of commercial importance, are alginate, agar and carrageenan.
The polysaccharide in brown seaweed is alginic acid, present as its sodium, potassium, magnesium and calcium salts (for the chemically minded, alginic acid is a carboxylic acid). Red seaweeds contain a variety of polysaccharides but the ones of commercial importance are agar and carrageenan; they are called sulfated polysaccharides because they contain negatively charged sulfate groups and in the seaweed are combined with a positively charged ion such as those found with alginic acid.
All brown seaweeds contain alginate, but there is a large variation in the quantity and quality of the alginate present. A commercial seaweed needs to contain around 20 percent alginate based on the dry weight of the seaweed. Quality of the alginate is based on the viscosity that it will produce as a one percent solution in water; high viscosity is regarded as high quality. Brown seaweeds that grow in cold waters usually produce a good quality alginate but those growing in temperate to tropical waters often yield low viscosity alginates.
The main commercial sources are species of Ascophyllum and Laminaria (Europe), Lessonia (South America), Ecklonia (South Africa), Durvillaea (Australia and Chile) and Macrocystis (California and Baja California). Species of Sargassum and Turbinaria are harvested from warmer waters but usually provide only low yields of lower quality alginate.
Alginates are used as thickening agents in food, pharmaceuticals and textile printing. Addition of a calcium salt to a solution of sodium alginate causes the formation of a gel and this property finds applications in the food and other industries. Calcium alginate can also be made in the form of fibres and these are used to manufacture surgical dressings.
All raw materials for alginate production are wild seaweeds with the exception of some used in China where the surplus of Laminaria japonica, cultivated for use as food, is used for alginate extraction. Approximately 85 000 tonnes, dry weight (tonnes, dry) are harvested to yield 23 000 tonnes of alginate with a value of US$ 211 million. There are nine known producers with probably 20 other smaller producers, many based in China. However two producers account for at least 60 percent of the total output. The historic growth rate of the industry has been three-four percent per annum over the last 20 years but with fluctuations down to zero in some years because of raw material shortages (El Niño) and competition from other hydrocolloids. Increasing use in the cosmetic, pharmaceutical and biotechnology industries may allow this growth rate to be maintained.
There are two main sources of seaweed for the world agar industry, species of Gelidium and Gracilaria. Species of Gelidium were the original source, historically from Japan, but shortages during World War II gave rise to a search for other raw materials. It was found that Gracilaria species were suitable if the seaweed was first treated with alkali. Gelidium gives the better quality agar (higher gel strength) but is only available from wild species; it is a small alga, grows slowly and attempts at cultivation have not proved to be commercially viable. Gracilaria species are larger algae and have been successfully cultivated so that it is now the major source (about 65 percent) of agar.
Gracilaria cultivation has been particularly successful in Chile but both wild and cultivated material is available in Argentina, South Africa, Japan, Indonesia, Philippines, China and India. Gelidium is always in high demand so that natural resources are collected wherever possible, the principal countries being Spain, Portugal, Morocco, Japan, Republic of Korea, China, Chile and South Africa. Other minor sources of raw material for agar production are species of Pterocladia (a small alga similar to Gelidium, harvested in the Azores and New Zealand) and Gelidiella (India, Egypt and Madagascar).
About 55 000 tonnes (dry) of seaweed are extracted annually to produce 7 500 tonnes of agar with a value of US$ 132 million. Chile, Spain and Japan produce 60 percent of the total agar output. There are 30 known producers and it is estimated there may be 20 other small producers. Development of new applications is slow and the estimated growth rate for the agar industry is one-two percent per annum, much the same as it has been for the last thirty years.
Irish Moss (Chondrus crispus) was the original source of carrageenan and until the late 1960s expansion of the industry was limited by the availability of the wild sources of this alga which grows best in cold waters such as the coasts of Ireland and Nova Scotia. Cultivation of Chondrus in tanks has been found to be too expensive but since the 1970s other warm water species, Kappaphycus alvarezii (also called “cottonii”) and Eucheuma denticulatum (“spinosum”) have been very successfully cultivated so that they are now the main raw materials used for carrageenan production.
Cultivation of the latter two species started in the Philippines but has since spread to other warm water countries with low labour costs including Indonesia and United Republic of Tanzania (Zanzibar). The companies involved in carrageenan extraction are actively promoting cultivation in other areas such as India, Africa and the Pacific Islands. These two species now comprise about 85 percent of the raw materials used by the industry, Chondrus use is five percent (from Canada, France, Spain, Portugal and Republic of Korea), while species of Gigartina, from Chile, Morocco and Mexico account for the remaining 10 percent.
The total raw material consumption is about 150 000 tonnes (dry) of seaweed, yielding 28 000 tonnes of carrageenan with a value of US$ 270 million. There are 24 known producers of carrageenan with perhaps another 10 smaller producers. However three companies account for 65 percent of the total production. Producers are very active in promoting new uses and the growth in the last 15 years has been around eight percent per annum. The estimated growth in the next five years is around five percent per annum.
Seaweeds have long been used as additives to soils, mainly in coastal areas where the wet or partly dried seaweed is easily transported to the area to be fertilised. The high fibre content of the seaweed acts as a soil conditioner and the mineral content as a fertiliser. The large brown seaweeds (species of Laminaria and Ascophyllum in Europe, Sargassum in warmer countries such as the Philippines) have found the greatest use but the advent of synthetic chemical fertilisers has reduced their market. In more recent times, liquid seaweed extracts have been marketed for use on more expensive crops such as vegetables and berry fruits; faster growth and better products are obtained and the results have been linked to the presence of auxin-type plant hormones in the extracts; about 500 wet tonnes of Ecklonia are used annually in South Africa for such an extract but similar products from other brown seaweeds are also made in the United Kingdom and New Zealand.
The brown seaweed, Ascophyllum nodosum, grows abundantly in the colder waters of Ireland, Scotland, Norway and Nova Scotia. Some of it is used for alginate productions but an industry has also been established, based on its use as an additive to animal feed. The dried seaweed is ground to a fine powder and sold as seaweed meal. Artificial drying is used so the production costs fluctuate with the cost of crude oil and since the market can only bear a certain cost, the output has varied over the years. At present it is estimated to be about 1 000 tonnes (dry) per year with a value of US$ five million.
