Carrageenan

Source

Carrageenan (E407) is a collective term for polysaccharides prepared by alkaline extraction (and modification) from red seaweed (Rhodophycae), mostly of genus Chondrus, Eucheuma, Gigartina and Iridaea. Different seaweeds produce different carrageenans.

Structural unit

Molecular structure

Carrageenans are linear polymers of about 25,000 galactose derivatives with regular but imprecise structures, dependent on the source and extraction conditions [292]. Idealized structures are given below and κ-carrageenan, for example, has been found to contain a small proportion of the dimer associated with ι-carrageenan.

The torsion angles phi (φ_H, H₁C₁OC₄ or H₁C₁OC₃), psi (ψ_H, C₁OC₄H₄ or C₁OC₃C₃) have been determined for λ-, μ- and ν-carrageenans [1095]. All carrageenans are highly flexible molecules which, at higher concentrations, wind around each other to form double-helical zones. Gel formation in κ- and ι-carrageenans involves helix formation on cooling from a hot solution together with gel-inducing and gel-strengthening K⁺ or Ca²⁺ cations respectively (not Na⁺, although Na⁺ does take part in an aggregation process to form weak gels with κ-carrageenan due to phase separation [723]), which not only aid helix formation but subsequently support aggregating linkages between the helices so forming the junction zones. The strongest gels of κ-carrageenan are formed with K⁺ rather than Li⁺, Na⁺, Mg²⁺, Ca²⁺, or Sr²⁺ [1077]. Incomplete formation of ¹C₄ 3,6-anhydro-links will reduce the extent of helix formation as the unbridged α-linked galactose residues may flip to the ⁴C₁ conformation.

Note that the gelling hydrocolloid agar is produced from the same family of seaweeds, the major difference being the presence of L- rather than D-3,6-anhydro-α-galactopyranose units but still forming double helical junction zones.

Functionality

Carrageenans are used mainly for thickening, suspending and gelling. κ- and ι-carrageenans form thermoreversible gels on cooling in the presence of appropriate counterions.  κ-Carrageenan forms a firm clear, if brittle, gel with poor freeze-thaw stability; the coil-double helix transition being followed by a K⁺-induced aggregation of the helices [516]. κ-Carrageenan gels may be softened (and is generally regarded to be synergistically strengthened^a) with locust bean gum. ι-Carrageenan has less specific ionic binding but increased ionic strength allows helices to form junction zones in soft elastic gels with good freeze-thaw stability. λ-Carrageenan is non gelling as the lack of the ¹C₄ 3,6-anhydro-link allows the galactose residues to revert to their ⁴C₁ conformation which does not allow the initial double helix formation required for gelling. Additionally, the high density of charged sulfate groups encourages an extensive conformation. λ-Carrageenan has been found to act as a cryoprotectant and improves the freeze-thaw behavior of locust bean gum.

κ-Carrageenan stabilizes milk κ-casein products due to its charge interaction with the casein micelles (~200 nm diameter); their incorporation into the network preventing whey separation. Such complexes are soluble when both have same charge and are held together by counterions or oppositely charged patches. Carrageenan is also used as a binder in cooked meats, to firm sausages and as a thickener in toothpaste and puddings.

It may be noted that a cancer health scare concerning degraded carrageenan has recently been examined by the European Commission Scientific Committee on Food which found no evidence in support and states that carrageenan is safe to use in foods.

Interactive structures are available (Chime, 4 KB).

Footnotes

^a However, a recent paper found no such synergy [290]. [Back]