(64h) Characterization of Swelling Behavior of Cross Linked Waxy Maize Starch Dispersions

The mechanism of equilibrium swelling of starch at constant temperature, rheological behavior as well as the applicability of polymer solution thermodynamics to swelling and pasting behavior of starch were investigated. Waxy maize and cross-linked waxy maize starch dispersions (2% w/w) were isothermally heated at 70, 75, 80, 85 and 90°C for 80 minutes, and swelling power determined for each sample. Swelling power was constant for cross-linked waxy maize starch at all temperatures investigated, whereas it doubled for waxy maize starch at 70°C, and continued to increase with temperature. Frequency sweep measurements showed that the storage modulus for cross-linked waxy maize starch samples remained constant with frequency at about 1000 Pa, and that for waxy maize increased steadily from 50 to 100 Pa, with the data displaying noisy scatter. Pasting behavior was measured with the Rapid Visco Analyzer at a heating rate of 10°C per minute, holding at 95°C and a cooling rate of 10°C. The pasting profile showed a lower peak viscosity for waxy maize starch, with a significant level of breakdown. On the other hand, cross-linked waxy maize starch showed a higher peak viscosity and an increase in viscosity during cooling, with no breakdown. Osmotic pressure measurements were carried out on starch dispersions at room temperature, and the results used to calculate the Flory-Huggins ÷ parameter, which is a measure of the interaction between starch and water. For all samples, the Flory-Huggins ÷ parameter was calculated to be 0.5, which means water is an athermal solvent for both waxy maize starch and cross-linked waxy maize starch. Intrinsic viscosity measurements of starch dispersions were made at different temperatures in order to infer the nature of the polymer chains from Mark-Houwink parameters. Light scattering measurements at different temperatures were carried out to characterize the second virial coefficient. Atomic force microscopy measurements of stress vs strain for a single starch granule were also carried out to determine the elastic free energy of the starch polymer network. The validity of polymer swelling theory for waxy and cross linked waxy maize starch in water were then evaluated.