Hydrocolloids and rheology: Regulation of visco-elastic characteristics of waxy rice starch in mixtures with galactomannans

Texture and sensory properties of starch solutions or gels, resp., can be influenced by regulating the visco-elastic properties that cannot be adjusted by simply varying the waxy rice starch concentration. Therefore, the hydrocolloid galactomannans guar gum and locust bean gum were used for addition. A total polymer concentration of c = 5% was maintained through all tests, the hydrocolloid fractions varied (c(HK) = 0.5%, 1%, 2%). The mixtures were entirely decomposed in an autoclave at 130 degrees C resulting in an almost complete destruction of the granular starch structure. With rheomechanical oscillation measurements in the linear-visco-elastic range the influence of the galactomannans onto viscosity yield and visco-elasticity was determined quantitatively. It was found that the influence on the viscosity is depending on the deformation rate. At low Frequencies and with increasing galactomannan Fraction the viscosity decreases, while above a critical frequency omega(kr) an increase in viscosity could be detected compared to pure starch. The mixtures also displayed an increase in viscous fractions versus elastic fractions compared to pure starch. With increasing galactomannan fraction, the ratio storage modulus/loss modulus is reduced continuously from G':G '' > 10 (definitely mainly elastic) to G':G '' approximate to 1 with simultaneously decreasing influence of the frequency. Based on the presented data and knowledge on solution structure of the individual components, a model for describing network structures in mixtures was developed. Changes in viscous-elastic properties are based on a thermodynamic incompatibility between starch and galactomannan fractions. The mixtures contain mixed network structures, with waxy rice starch (permanent network points) and galactomannans (temporary network points) contributing individually and characteristically to the resulting flow behaviour.