Edible seaweed, or sea vegetables, are algae that can be eaten and used in the preparation of food. They typically contain high amounts of fiber. They may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae.
Seaweeds are also harvested or cultivated for the extraction of polysaccharides  such as alginate, agar and carrageenan, gelatinous substances collectively known as hydrocolloids or phycocolloids. Hydrocolloids have attained commercial significance, especially in food production as food additives. The food industry exploits the gelling, water-retention, emulsifying and other physical properties of these hydrocolloids.
Most edible seaweeds are marine algae whereas most freshwater algae are toxic. Some marine algae contain acids that irritate the digestion canal, while some others can have a laxative and electrolyte-balancing effect. Most marine macroalgae are nontoxic in normal quantities, but members of the genus Lyngbya are potentially lethal. Typically poisoning is caused by eating fish which have fed on Lyngbya or on other fish which have done so. This is called ciguatura poisoning. Handling Lyngbya majuscula can also cause seaweed dermatitis. Some species of Desmarestia are highly acidic, with vacuoles of sulfuric acid that can cause severe gastrointestinal problems.
The dish often served in western Chinese restaurants as 'Crispy Seaweed' is not seaweed but cabbage that has been dried and then fried.
Seaweeds are used extensively as food in coastal cuisines around the world. Seaweed has been a part of diets in China, Japan, and Korea since prehistoric times. Seaweed is also consumed in many traditional European societies, in Iceland and western Norway, the Atlantic coast of France, northern and western Ireland, Wales and some coastal parts of South West England, as well as Nova Scotia and Newfoundland and Labrador. The Māori people of New Zealand traditionally used a few species of red and green seaweed.
Nutrition and uses
Seaweed contains high levels of iodine relative to other foods. In the Philippines, Tiwi, Albay residents created a new pancit or noodles made from seaweed, which can be cooked into pancit canton, pancit luglug, spaghetti or carbonara and is claimed to have health benefits such as being rich in calcium, magnesium and iodine.
One study in 2014 pointed to certain species of seaweed as being a possible vegan source of biologically-active Vitamin B12. The study noted that B-12 was found in both raw and roasted seaweed, the latter containing about half as much—but still a sufficient amount. A mere 4 grams of dried purple laver is considered sufficient to meet the RDA for B-12.
Polysaccharides in seaweed may be metabolized in humans through the action of bacterial gut enzymes. Such enzymes are frequently produced in Japanese population due to their consumption of seaweeds.
In some parts of Asia, nori 海苔 (in Japan), zicai 紫菜 (in China), and gim 김 (in Korea), sheets of the dried red alga Porphyra are used in soups or to wrap sushi or onigiri. Chondrus crispus (commonly known as Irish moss) is another red alga used in producing various food additives, along with Kappaphycus and various gigartinoid seaweeds.
Japanese cuisine has seven types of seaweed identified by name, and thus the term for seaweed in Japanese is used primarily in scientific applications, and not in reference to food.
Seaweed oil, also called Algae oil, is used for making food. Seaweed oil is also used as a source of fatty acid dietary supplement, as it contains mono- and polyunsaturated fats, in particular EPA and DHA, both of them Omega-3 fatty acids . Its DHA content is roughly equivalent to that of salmon based fish oil.
Seaweed oil is also used for biofuel, massage oil, soaps, and lotions.
Common edible seaweeds
Common edible seaweeds include:
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