Conformation and association of κ-carrageenan in the presence of locust bean gum in mixed NaI/CsI solutions from rheology and cryo-TEM

Abstract

Mixtures of locust bean gum (LBG) with κ-carrageenan (KC) in 0.1 M aqueous solutions of the mixed salts NaI/CsI were investigated by cryo-transmission electron microscopy (cryo-TEM) and dynamic viscoelastic measurements. Previous studies have shown that as the cesium content is increased in such mixed salt solutions, a transition occurs from molecularly dispersed helices to ‘superhelical rods’ of KC. We now found that LBG stabilises the superhelical rods, shifting the transition to a lower content of Cs for the mixtures than for KC alone. The formation of superhelical rods was evidenced both by cryo-TEM images and by an onset of thermal hysteresis in the coil–helix transition of KC. In the mixtures, the transition temperatures on cooling and heating were insensitive to the proportions of LBG and KC present at all cesium contents. Under conditions where no helix aggregation occurred (no hysteresis) the mixtures showed high tan δ values and low storage moduli. Under aggregated conditions, gels formed, and gels with added LBG had enhanced moduli compared to gels with KC alone. On the basis of these results we propose that LBG associates to the super-helical rods of KC.