Potato Starch Dispersions: Structure and Rheology
The influence of heating, shearing and addition of amylose on the structure and rheology of potato starch dispersions has been evaluated. The methods used were dynamic viscoelastic measurements, light microscopy and transmission electron microscopy (TEM). The gelatinized granules were completely close-packed in the concentration range between 4% and 10%. Dispersions with intact swollen granules showed a viscoelastic behaviour typical of weak colloidal networks at temperatures above 60°C. The structural character and the viscoelastic behaviour were completely changed when shear forces were applied during heating. Starch was solubilized and granules were disrupted. Sheared dispersions showed a similar type of viscoelastic behaviour to that of starch in macro-molecular solution. The transition in viscoelastic behaviour that was caused by shearing corresponded with the point where fragments of granules were dispersed in a continuous phase of solubilized starch. A microscale demixing of amylose and amylopectin occurred within the phase of solubilized starch. Shearing did not change the type of viscoelastic behaviour of cereal starch pastes such as wheat and maize starch.
Pure potato starch dispersions did not form gels in the time scale studied. Cereal starch systems showed a fast gelation pattern similar to that of mixed systems with potato starch granules with added aqueous amylose.
The gel strength of mixed systems with potato starch and added amylose was evaluated by a model applicable to biphasic aqueous gels. The theory and experiments were in good agreeement, implying that the added amylose and potato starch could be regarded as phase-separated components, and that the inherent amylose of potato starch did not contribute to the gel strength resulting from the added amylose. The granules act as a gel reinforcer by increasing the effective concentration of the amylose network. When a high proportion of amylose was added to systems with molecularly dispersed starch, the resulting gel strength was similar to that in granular systems.
TEM studies revealed a number of structure types both in systems with gelatinized granules and in molecularly dispersed starch. The added amylose gave rise to a typical network structure not found in pure potato starch systems. The structure within gelatinized granules was discovered to be quite dense at the surface, which may explain the restraints on the diffusion.