Locust bean gum

Locust bean gum (European Union additive number E410) is a galactomannan vegetable gum extracted from the seeds of the Carob tree. It forms a food reserve for the seeds and helps to retain water under arid conditions. It is used as a thickener and gelling agent in food technology. It is also called Carob Gum or Carubin.

Structural unit


Locust bean gum is a galactomannan similar to guar gum consisting of a (1→4)-linked β-D-mannopyranose backbone with branchpoints from their 6-positions linked to α-D-galactose (i.e. 1→6-linked α-D-galactopyranose). There are about 3.5 (2.8 - 4.9) mannose residues for every galactose residue.

Molecular structure
Locust bean gum is polydisperse, consisting of non-ionic molecules made up of about 2000 residues. Lower galactose substitution also decreases the stiffness (i.e. increases the flexibility) but increases the extensibility of the isolated chains. The galactose residues prevent strong chain interactions but there may be up to 10-11 unsubstituted mannose residues in a row and junction zones may form between such clear areas when they consist of greater than about six residues. These nano-crystalline links dissociate in hot water. If the galactose residues were perfectly randomized or blocked, it is likely that each molecule would have more than four such areas capable of acting as junction zones, so allowing gel formation.

Functionality
Locust bean gum is less soluble and lower viscosity than guar gum as it has fewer galactose branchpoints. It needs heating to dissolve but is soluble in hot water. Locust bean gum differs from guar gum in that it does form thermally-irreversible weak gels by association of the galactose deficient regions and therefore has poorer freeze thaw behavior. These unsubstituted areas also allow increased interaction with cellulose. Being non-ionic, locust bean gum is not affected by ionic strength or pH but will degrade at pH extremes at higher temperatures.

Locust bean gum specifically retards ice crystal growth by forming a structured gel at the solid/liquid interface. This particularly occurs on freeze-thaw cycling, which encourages the frustrated crystallization of the galactomannan, so causing the gel to form. It encourages phase separation with skimmed milk powder showing synergistic viscosity with casein and becoming slightly thixotropic on forming a biphasic system containing casein micelles within a continuous polysaccharide network. To aid this it may usefully be combined with xanthan, with which it shows viscosity synergy, and κ-carrageenan (it adsorbs to super helices of κ-carrageenan, as do cassia and tara gums but not guar gum).

Another galactomannan with lower substitution (with a mannose to galactose ratio of between about 5 - 7) is cassia gum, obtained from Cassia tora (also known as Cassia obtusifolia). It has a fairly regular substitution pattern but even lower solubility in cold water than locust bean gum. Galactomannans may be engineered with lower substitution by the specific removal of some of their pendant galactose groups using certain α-galactosidases.

Copyright notice
This article was copied almost verbatim from Martin Chaplin's page on the LSBU website (below). He has given permission for it to be used, as long as it is attributed, and such attribution is kept when it is used by a third party.