49-7

J. Tattiyakul and M. A. RAO. Department of Food Science and Technology, Cornell University-Geneva, P. O. Box 462, Geneva, NY 14456-0462

Justification. Cross-linked waxy maize (CWM) starch dispersions heated in excess water exhibit relatively stable viscosity because the starch granules do not rupture as easily as in the case of unmodified starches. Previous studies in our laboratory showed that the granule volume fraction and morphology play important roles in the dispersion's rheology.

Objective. The main objectives of this study were: to obtain a functional viscosity model of a 5% (w/w) CWM starch dispersion during gelatinization for use in thermal process calculation, and to examine heated CWM starch dispersion’s shear rate vs. shear stress behavior with respect to thixotropic and antithixotropic behavior.

Methods. Complex viscosity data were obtained on 5% CWM starch dispersions over the frequency range: 1.26 to 31.38 rad/s and temperature range: 55-95°C using a parallel plate geometry of a rheometer (AR 1000, TA Instruments, Inc.). Stepped shear flow tests were conducted with a cone-plate geometry to study the flow behavior of a dispersion heated at 75° for 5 min.

Results. The 5% CWM starch dispersions had an onset gelatinization temperature at 60°C with a peak complex viscosity of 6.9 Pa s at 64°C. The lowest post-gelatinization complex viscosity was about 0.85 x peak viscosity recorded at about 80°C. A modified Cox-Merz relationship was derived to relate the complex viscosity to the apparent viscosity. Flow data on the gelatinized 5% CWM starch dispersions showed that the sample exhibited a combined time-dependent shear-thickening (antithixotropic) and time-dependent shear-thinning (thixotropic) behavior.

Significance. A mathematical model was developed to describe both the increasing and decreasing segments of complex viscosity versus temperature data over a wide range of oscillatory shear rates. The antithixotropic behavior, that appears to have resulted from a shear-induced structure formation, predominated when the level of shear stress imposed on the sample reached values less than about 120-150 Pa.