Steady flow and viscoelastic behavior of crosslinked waxy corn starch-κ-carrageenan pastes and gels

The steady and small-deformation shear behaviors of 2-4 wt.% cross-linked waxy corn starch (CWCS) and 0.5 wt.% K-carrageenan (KC) mixtures have been studied. In some cases kappa C gelation was induced by the addition of 20 mM KCl and cooling to 25 degrees C. Rheological behavior was interpreted considering swelling-solubility as well as laser diffraction particle size and size distribution data. CWCS was chosen because it lacks amylose and its granules can swell without disruption under appropriate thermal conditions; thus the condition of particles dispersed in a continuous phase can be best fulfilled. Dispersions were heated (1.5 degrees C/min) to 96 degrees C, held for 10 min, then cooled (1.5 degrees C/min) to 80 degrees C. Steady tests were carried out at 60 degrees C and oscillatory tests were carried out at 60 and 25 degrees C. Only more concentrated dispersions displayed an anticlockwise up-down shear loop, which was enhanced by KC and KCI. Hysteresis disappeared with continuous shearing, and shear-thinning was observed with CWCS + kappa C being more viscous than CWCS + kappa C + kappa Cl and CWCS. Oscillatory tests at 60 degrees C showed that dispersions exhibited a predominantly solid-like behavior which was attenuated at lower starch concentrations and when KC and KCI were added. At 25 degrees C, trends were similar but with slightly greater G' and accentuated elastic character at higher starch concentrations. KCl-containing media formed gels whose rigidity depended on starch concentration. Solubility was low (<2%) and swelling increased from 25.3 g/g up to 29.6 g/g when KC was added, irrespective of starch concentration. Particle size data also showed higher swelling power in kappa C-containing media. Results could be interpreted in terms of two types of systems; particles suspended in a macromolecular solution and composite gels of particles embedded in a network matrix when both KC and KCI were added. (C) 1999 Elsevier Science Ltd. All rights reserved.