A STUDY OF STARCH GELATINIZATION USING DIFFERENTIAL SCANNING CALORIMETRY, X-RAY, AND BIREFRINGENCE MEASUREMENTS

The order-disorder transition that occurs on heating an aqueous suspension of starch granules has been investigated using differential scanning calorimetry (d.s.c.), X-ray crystallinity, and birefringence methods. Starches from wheat, corn, rice, and waxy maize starches were used for the study. Measurements on dilute suspensions (2wt.% starch), showed that decreases in crystallinity occur both before the birefringence of granules starts to disappear and after all birefringence is lost. At such concentrations the ordered domains in single granules melt over a temperature span of about 10-degrees, indicating that the order-disorder transition is not a highly cooperative process. The crystallinity values of more-concentrated suspensions (approximately 50wt.% starch), suggest that a melting process accounts for the two main peaks evident in the corresponding d.s.c. traces. The X-ray data do not support the concept that the specific heat change in the d.s.c. traces is attributable to a glass transition in the initial stages of gelatinization. Firstly, the X-ray measurements do not show that a significant endothermic transition occurs without a corresponding change in crystallinity. Secondly, the X-ray data suggest that the volume expansion functions measured by other investigators using thermomechanical analysis are attributable to the increase in the quantity of amorphous starch polymer with temperature rather than to a glass transition followed by a melt.