Effect of cooking temperature and stirring speed on rheological propertiesand microstructure of cornstarch and oat flour gels

Effects of cooking temperature (70, 75, 80, 85, 90, 95 degrees C) and stirr ing speed (60, 160, 260 rpm) on the gel/paste properties of cornstarch and oat flour were investigated. Pasting properties during cooking were evaluat ed using an Rapid Visco-Analyser-4 (RVA). Rheological properties and micros tructure of cooled gels/pastes were evaluated using dynamic mechanical anal ysis (DMA) and stress relaxation and scanning electron microscopy (SEM). Fo r both cornstarch and oat flour, the storage modulus (G') of gels measured at 25 degrees C increased with cooking temperature up to about 85 degrees C at 160 rpm and 260 rpm and up to about 90 degrees C at 60 rpm and decrease d dramatically thereafter for all stirring speeds. G' values for oat flour gels were lower than those for cornstarch gels at all temperatures, but par ticularly at 85 degrees C and 90 degrees C. Higher G' values of the cornsta rch gels at these temperatures were associated with a composite gel structu re containing collapsed starch granules. This phenomenon was not as apparen t for the oat flour gels. The scanning electron micrographs of cornstarch a nd oat flour gels prepared at the different cook temperatures highlighted t he importance of changes in the starch granules on both the pasting curves and rheological properties of the corresponding gels. Results for stress re laxation indicated that the cornstarch gels were more solidlike for all RVA cook temperatures. These results may be useful in preparing customized pre gelatinized starches for specific functions in foods.