Abstract
Several series of modified corn starches covering a wide range of fluidities were prepared from corn starch treated with varying dilutions of hydrochloric acid at 50 °C. The fluidities were measured by means of a standard fluidity funnel, and the effect of acid concentration on the speed of modification was noted. The viscosities of the pastes were measured in the MacMichael viscosimeter and the flow-pressure relations noted. It is shown that the viscosity is dependent on the rate of shear and that the experimental data are well expressed by the exponential relation F = KPn, where F is the flow, P is the pressure, K and n are constants. For pastes of the same concentration, there is a rapid decrease m the value of n with increasing degree of modification. Transitions from one type of flow to another are shown in the same viscosity measurement at different driving pressures. Microscopic examination shows that the dispersed phase of ordinary starch pastes consists of swollen starch granules. The viscosity of a starch paste which has undergone no severe pretreatment depends on the deformability of the swollen granules and on the volume relation between dispersed phase and dispersion medium.
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