Carrageenans are polysaccharides made of galactose units with different levels of sulfate substitution. Their sulfate content is between 15 and 40%. Many red seaweeds contain mixtures of such polysaccharides. In carrageenans, all the galactose units occur in the D form.

Depending on the number of sulfate groups per galactose molecule, three types of carrageenan molecules can be distinguished:

• Kappa carrageenan: 1 sulfate unit for two galactose molecules
• Iota carrageenan: 2 sulfate units for two galactose molecules
• Lambda carrageenan: 3 sulfate units for two galactose molecules

It should be noted that carrageenan macromolecules are not homogeneous chains made up of the same repetitive units, but they are heterogeneous, either due to differing molecular structures within the chains or due to differing chains within the seaweed. For example, some carrageenans extracted from South American seaweeds are kappa and iota hybrids, also called kappa 2 or weak kappa carrageenans.

Lambda carrageenan is cold soluble and acts simply as a thickening agent.

Gelling carrageenans require heat treatment for dissolution. When cooling down, the macromolecules have a tendency to spontaneously associate, thus creating junction zones required for a gel.

Being very diverse, carrageenans can broadly be classified into three main “ideal” types of carrageenan, split into two groups:

• Gelling carrageenans: products rich in kappa and iota fractions
• Thickening carrageenans: products rich in lambda fractions

Gelation of kappa carrageenan is particularly enhanced by the potassium ion. It induces gel formation at very low concentrations. Because of its small hydro-dynamic size when hydrated, the potassium ion fits into the coil and partially neutralizes the sulfate groups. Thus, the double-helices can cluster together and form aggregates that create a strong, brittle, gel.

Kappa (k) 1 sulfate for 2 sugar units

The iota carrageenan network is formed by a series of double-helices and kinks that form a transparent, elastic gel. This loosely connected network can easily be destroyed by mechanical action. However, it reforms quickly once the mechanical action has stopped. This property is called " thixotropy " and is very useful in certain applications, such as cold-filled dairy desserts.

Iota (i) 2 sulfate for 2 sugar units

Lambda carrageenans can be used as thickening agents only, since due to electrostatic repulsion, the chains of lambda carrageenan do not have a tendency to self-associate. They can thus be easily separated from each other.

Lambda (l) 3 sulfate for 2 sugar units