The influence of pH on the viscous behavior of starch-poly(ethylene-co-acrylic acid) complexes

Abstract Starch-poly (ethylene-co-acrylic acid) (EAA) complexes were prepared by jet-cooking mixtures of either cornstarch, waxy cornstarch or high amylose cornstarch with aqueous ammonia dispersions of EAA (4% EAA based on the weight of starch). Viscosities (η) were determined at temperatures ranging from 80°C to 22°C, and plots of log η versus 1 T (K −1 ) were prepared. When cooked with EAA, cornstarch and waxy cornstarch showed major changes in viscous behavior between 50°C and 60°C. Above 50–60°C, viscosity increased markedly with a reduction in temperature; however, viscosity increased slowly below 50–60°C with an apparent activation energy for the process approximating that of water itself. The temperature dependence of the measured viscosity from 80°C to 60°C could be attributed to the large increase in size and complexity of the flowing particles as individual amylopectin molecules were bound together by complexed EAA. Apparently, complexing is essentially complete at 50°C. When high amylose cornstarch was cooked in the absence of EAA, retrogradation produced a sharp increase in log η at temperatures below about 50°C. However, if EAA is present, association between amylose molecules apparently takes place via complex formation rather than retrogradation, since log η increases sharply at about 70–80°C. Also, in contrast to cornstarch and waxy cornstarch, log η versus 1 T plots for high amylose cornstarch did not level off at low temperatures. In general, viscosities increased with the pH of the system, particularly when η was measured at high temperatures. This could result from improved complexing ability of EAA under high pH conditions, possibly due to reduced micelle size and maximum extension of polymer chains from micelle surfaces.