A RHEOLOGICAL CHARACTERIZATION OF KAPPA-CARRAGEENAN GALACTOMANNAN MIXED GELS - A COMPARISON OF LOCUST BEAN GUM SAMPLES

Mixed gels of kappa-carrageenan and locust bean gum (LBG) obtained from different varieties of Portuguese carob trees and commercial gums were compared. The viscoelastic properties of the gels were measured using dynamic parallel-plate geometry. Mixed gels at 1.0% of total polysaccharide concentration without addition of KCl showed, whatever the LBG sample, a synergistic maximum when the ratio of kappa-carrageenan to LBG was 80/20. The amplitude of this maximum varied with the LBG sample. The gels prepared at 0.3% total concentration with KCl added, showed a synergistic maximum at the same mixing ratio and the amplitude varied in a similar manner. Each sample was fractionated into the fraction soluble at 25-degrees-C and the fraction soluble at 90-degrees-C. Mixed gels of kappa-carrageenan with cold-water-soluble and hot-water-soluble fractions, and also with tara gum and guar gum were prepared at the 80/20 ratio. It was found that the synergistic maxima were related to the intrinsic viscosity and the M/G ratio. A linear relationship between the storage modulus G'(max) at the synergistic maximum and the product of the intrinsic viscosity and the square of the mannose to galactose ratio was found, suggesting that the synergistic mechanism can be ascribed to both the unsubstituted (galactose-free) regions of the galactomannan and the molecular weight.