EFFECTS OF THERMOMECHANICAL PROCESSING ON VISCOSITY BEHAVIOR OF CORN STARCHES

A waxy, a dent, and a high amylose starch were subjected to thermomechanical processing of varying degrees of severity. These treatments ran ed from gentle dissolution in 90%DMSO to autoclaving, at temperatures up to 180 degrees C, to jet-cooking. The treatments had major effects on viscosities of the starch dispersions and in particular on the intrinsic viscosities, which were determined to provide a relative measure of the molecular sizes of the processed starches. The intrinsic viscosities, from the least severe treatment to the most severe treatment, were 220 and 98; 181 and 104; 105-80 ml/g, respectively, for the waxy, dent, and high amylose starches. An unexpected characteristic of processed waxy maize was that the viscosity of 6% dispersions was extremely sensitive to intrinsic viscosity changes in the narrow range of 180-165 ml/g. In contrast, viscosities of waxy maize dispersions were relatively insensitive to changes in the lower intrinsic viscosity values ( < 160 ml/g) caused by more severe treatments. Dispersions of jet-cooked waxy maize exhibited Newtonian flow; jet-cooked dispersions of high amylose starch did not.