Rheological properties and phase transition of red algal polysaccharide-starch composites

The rheological properties and phase transition temperatures of 1.0 or 2.0% agarose, kappa- and iota-carrageenans in the presence of 0.0-2.0% starches were investigated to understand the changes in these properties with the degree of gelatinisation, concentration, and swelling-solubility of coexisting starches. Generally, the addition of completely gelatinised rice starch resulted in significant decrements in maximal storage modulus (G(max)', an index of the susceptibility of syneresis), initial rate of G' increase (dG'/dt, an index of initial gelation rate), and gel-melting temperature (T-m) of agarose composites. These decrements were closely correlated with the concentrations of agarose and soluble starch in the continuous matrices (C-x,C-red and C-x,C-st, respectively), and depending on the sample preparation used. In kappa-canrageenan composites, the increased gelling temperature (T-gel), G(max)', dG'/dt and T-m values were obtained by adding starches, where the G(max)', and T-m values were significantly correlated with C-x,C- (red) more than C-x,C- (st). The influences of the starches on the gel properties, except for T-gel, of iota-carrageenan resembled those on the K-carrageenan. The gel strengths of all composites decreased by the addition of starch. It is concluded that the interference of soluble starch, the exclusion effect of swollen granules, the molecular incompatibility and the coupling interactions in the matrices appeared to give certain influences on the gel properties of these composites. (C) 1999 Elsevier Science Ltd. All rights reserved.