Correlation of Microscopic Structure of Corn Starch Granules with Rheological Properties of Cooked Pastes

The progressive geometric changes that occur in swelling of corn starch granules during heating throughout the range of gelatinization (63-72°C) and at higher temperatures when substantial amount s o f soluble starch are released from the granule were observed by scanning electron micro s copy (SEM). Corn starch granules beg in to swell radially, then undergo radial contraction and random tangentia l expansion. They f orm complex geome trica l structures at the midpoint range (67-70°C) unlike the more uni f orm s ingle-dimensional tangential swelling that occurs with lenticular granules of wheat s t arch . At higher temperatures, when starch begins to solubilize, corn s tarch granules los e the ir distinct ridges and appear to melt into thin flat disks. These progressive configurational changes are reflected in the rheological proper ties of more concentrated s t arch dispersions cooked for 75 minut es . At the earl y stages of gelatinization (6 3-65 °C ) the granules are r e l a tive l y rigid and at high e nough concentration shovv dilatant behavior (viscosity increasing with shear rate). At these t emperatures, granules r emain rigid and maintain their birefringence but are mechanically s h eared by s tirrin g during cookin g . Once the granules undergo ex tensive swelling, develop ridges, and los e their birefringence (67-70°C), they are soft enough to exhibit shear thinning behavior (viscosity decreasing with shear rat e ). The extent of shear thinning d epends on concentration, b e cause viscosity and shear s tress increase with concentration and the granules become more susceptible to d eformation. At high enough concentrations (and associated s t resses), the ridges are not as clearly defined as they are at lower conc entrations . Granules become more flat and flexible when cooked above 75°C. Initial paper received February 4, 1982. Revised manuscript received April 20, 1982. For this paper contact D.O. Christianson, (telephone number 309 685 4011 X290).