Food Structure

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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 of soluble starch are released from the granule were observed by scanning electron microscopy (SEM). Corn starch granules begin to swell radially, then undergo radial contraction and random tangential expansion. They form complex geometrical structures at the midpoint range (67-70°C) unlike the more uniform single-dimensional tangential swelling that occurs with lenticular granules of wheat starch . At higher temperatures, when starch begins to solubilize, corn starch granules lose their distinct ridges and appear to melt into thin flat disks. These progressive configurational changes are reflected in the rheological properties of more concentrated starch dispersions cooked for 75 minutes . At the early stages of gelatinization (6 3-65 °C ) the granules are relatively rigid and at high enough concentration shovv dilatant behavior (viscosity increasing with shear rate). At these temperatures, granules remain rigid and maintain their birefringence but are mechanically sheared by stirring during cooking . Once the granules undergo extensive swelling, develop ridges, and lose their birefringence (67-70°C), they are soft enough to exhibit shear thinning behavior (viscosity decreasing with shear rate ). The extent of shear thinning depends on concentration, because viscosity and shear stress increase with concentration and the granules become more susceptible to deformation. At high enough concentrations (and associated stresses), the ridges are not as clearly defined as they are at lower concentrations . Granules become more flat and flexible when cooked above 75°C.

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