Carrageenan, is a family of linear sulphated polysaccharides typically obtained by alkali extraction from red seaweeds. It is named after Irish moss (Chondrus crispus, also known as Carrageen moss), which is called carraigín in Irish. It was originally isolated from this alga in 1844.

These chemicals are large, highly flexible molecules which curl around each other forming double-helical structures. This gives them the ability to form a variety of different gels at room temperature. They are widely used in the food and other industries as thickening and stabilising agents. A particular advantage is that they are thixotropic—they thin under shear stress and recover their viscosity once the stress is removed. This means that they are easy to pump but stiffen again afterwards.

For example, they can be used in:

• Desserts, ice cream, milk shakes, sauces - gel to increase viscosity
• Pates and processed meat - Substitute fat to increase water retention and increase volume
• Toothpaste - stabilizer to prevent constituents separating
• Fire fighting foam - thickener to cause foam to become sticky
• Shampoo and cosmetic creams - thickener
• Air freshener gels
• Shoe polish - gel to increase viscosity

There are three classifications of carrageenan:

• Kappa - strong, rigid gels. Produced from Kappaphycus cottonii
• Iota - soft gels. Produced from Eucheuma spinosum
• Lambda - form gels when mixed with proteins rather than water, used to thicken dairy products. The most common source is Gigartina from Southern Europe.

It is interesting to note, however, that a lot of red algal species produce different types of carrageenans during their developmental history. For instance, the genera Gigartina produces mainly Kappa carrageenans during its gametophytic stage, and Lambda carrageenans during its sporophytic stage.

• See also: Alternation of generations

All are soluble in hot water, but in cold water only the Lambda form (and the sodium salts of the other two) are soluble.

When used in food products, carrageenan has the EU additive E-number E407. Although introduced on an industrial scale in the 1930s, the first use was in China around 600 BC (where Gigartina was used) and in Ireland around 400 AD. There is evidence from animal studies which indicates that degraded carrageenan might cause ulcerations in the gastro-intestinal tract and gastro-intestinal cancer. Chemical studies indicate that degraded carrageenan can easily be produced from undegraded carrageenan.

The largest producer is the Philippines, where cultivated seaweed produces about 80% of the World supply. The most commonly used are Cottonii (Eucheuma cottonii) and Spinosum (Eucheuma spinosum), which together provide about three quarters of the World production. These grow at sea level down to about 2 metres. The seaweed is normally grown on nylon lines strung between bamboo floats and harvested after three months or so when each plant weighs around 1 kg.

Interestingly, it should be noted that the Cottonii variety has been reclassified as Kappaphycus cottonii by Maxwell Doty (1988), thereby introducing the genus Kappaphycus, on the basis of the phycocolloids produced (namely kappa carrageenan).