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

Mixtures of locust bean gum (LBG) with ic-carrageenan (KC) in 0.1 M aqueous solutions of the mixed salts NaI/CsI were investigated by cryo-transmissio n electron microscopy (cryo-TEM) and dynamic viscoelastic measurements. Pre vious studies have shown that as the cesium content is increased in such mi xed salt solutions, a transition occurs from molecularly dispersed helices to 'superhelical rods' of KC. We now found that LEG stabilises the superhel ical rods, shifting the transition to a lower content of Cs for the mixture s 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 a nd heating were insensitive to the proportions of LEG and KC present at all cesium contents. Under conditions where no helix aggregation occurred (no hysteresis) the mixtures showed high tan delta values and low storage modul i. Under aggregated conditions, gels formed, and gels with added LEG had en hanced moduli compared to gels with KC alone. On the basis of these results we propose that LEG associates to the super-helical rods of KC. (C) 1999 E lsevier Science B.V. All rights reserved.