Carrageenan is a polysaccharide extracted from red seaweed. D-galactose has a structure consisting of alternating α-1,3 or β-1, 4 bonds. Depending on the number of sulfate groups and the presence or absence of anhydro bonds, it is classified into three types: kappa, iota, and lambda. Each type has different solubility, gelation properties, optimal ions for gelation, and gel properties.