Experimental and modelling studies of the flow properties of maize and waxy maize starch pastes

Methods have been developed for characterising the time-dependent and non-Newtonian rheological behaviour of gelatinized maize starch pastes as a function of starch concentration, temperature and shearing conditions. Two types of commercial maize starch were used in this study: a normal maize with 30% amylose, and a waxy maize consisting mainly of amylopectin. Results from transient flow property measurements using a concentric cylinder viscometer show that the starch pastes are strongly thixotropic and shear thinning as characterised by a viscosity that decreases with time and rate of shear. The rheology of the starch pastes studied can be satisfactorily modelled using the structural kinetic approach which postulates that after gelatinization and cooling, the polysaccharide polymers and the swollen starch granules form network structures which are irreversibly broken under shear leading to the time- and shear-dependent change in the flow properties. At equilibrium, the starch pastes exhibit pseudoplastic behaviour which can be described by the power law model. While pastes of the two maize starches studied exhibit similar rheological behaviour at the same starch concentration and temperature, the normal maize starch paste is more viscous initially in the gel state, but breaks down more rapidly and becomes thinner than the waxy maize starch paste under the steady-state sheared condition. This difference is due to the different gel structures formed on cooling, and to the different swelling characteristics of the granules of the two maize starches studied. (C) 1998 Elsevier Science S.A. All rights reserved.