DEFINITION

Carrageenan is the hydrocolloid obtained by extraction with water or aqueous alkali, from some members of the class Rhodophyceae (red seaweeds). Carrageenan consists chiefly of potassium, sodium, calcium, magnesium, and ammonium sulfate esters of galactose and 3,6-anhydrogalactose copolymers. These hexoses are alternately linked α-1,3 and β-1,4 in the polymer. The prevalent copolymers in the hydrocolloid are designated kappa-, iota-, and lambda-carrageenan. kappa-Carrageenan is mostly the alternating polymer of d-galactose-4-sulfate and 3,6-anhydro-d-galactose. iota-Carrageenan is similar, except that the 3,6-anhydrogalactose is sulfated at carbon 2. Between kappa-carrageenan and iota-carrageenan there is a continuum of intermediate compositions differing in degree of sulfation at carbon 2. In lambda-carrageenan, the alternating monomeric units are mostly d-galactose-2-sulfate (1,3-linked) and d-galactose-2,6-disulfate (1,4-linked). The ester sulfate content for Carrageenan ranges from 18% to 40%. In addition, it contains inorganic salts that originate from the seaweed and from the process of recovery from the extract.