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

Abstract

The steady and small-deformation shear behaviors of 2–4wt.% cross-linked waxy corn starch (CWCS) and 0.5wt.% κ-carrageenan (κC) mixtures have been studied. In some cases κC gelation was induced by the addition of 20mM KCl and cooling to 25°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°C/min) to 96°C, held for 10min, then cooled (1.5°C/min) to 80°C. Steady tests were carried out at 60°C and oscillatory tests were carried out at 60 and 25°C. Only more concentrated dispersions displayed an anticlockwise up–down shear loop, which was enhanced by κC and KCl. Hysteresis disappeared with continuous shearing, and shear-thinning was observed with CWCS+κC being more viscous than CWCS+κC+KCl and CWCS. Oscillatory tests at 60°C showed that dispersions exhibited a predominantly solid-like behavior which was attenuated at lower starch concentrations and when κC and KCl were added. At 25°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 25.3g/g up to 29.6g/g when κC was added, irrespective of starch concentration. Particle size data also showed higher swelling power in κC-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 κC and KCl were added.