Effect of temperature on the rheological properties of starch/carrageenan mixtures

The aim of this study was to describe the effect of pasting procedures on the rheological properties of the starch/carrageenan system in relation to the starch granule size, measured by laser scattering. The rheological behaviour of 3% crosslinked waxy corn starch/0.5 % kappa-carrageenan mixtures with or without KCl was studied using steady (at 60 degreesC) and oscillatory (at 60 degrees and 25 degreesC) shear tests. The pasting temperatures ranged from 96 degreesC (undercooked starch granules) to 125 degreesC (overcooked starch granules) to obtain different degrees of starch granule swelling. All dispersions exhibited a shear-thinning behaviour. For the starch pastes, the apparent viscosity, the storage modulus and the median diameter of the swollen granules increased as the pasting temperature increased and the apparent viscosity could be related to the size of the swollen granules. In all cases it was confirmed that starch granules remained intact after thermal treatment. The viscoelastic behaviour of the starch pastes was also governed by the swollen granules leading to a solid-like behaviour at 25 degrees or 60 degreesC. Addition of carrageenan increased significantly the apparent viscosity compared to starch alone, while the mechanical spectra of starch/carrageenan mixtures indicated a more fluid-like system. It is noteworthy that the median diameter of the starch granules increased in the presence of carrageenan. The effect seems to be more complex than expected and would need a more thorough characterisation of the continuous phase.