CHANGES IN RHEOLOGICAL PROPERTIES OF HYDROXYPROPYL POTATO STARCH PASTES DURING FREEZE-THAW TREATMENTS .2. EFFECT OF MOLAR SUBSTITUTION AND CROSS-LINKING

The effects of molar substitution (M. S.) and cross-linking on the fre eze-thaw stability of hydroxypropyl potato starch pastes were studied by following the changes in the rheological behaviour during heat trea tment of the thawed starch pastes, and by determination of syneresis. In addition, differential scanning calorimetry (DSC) was used to inves tigate the thermal transition properties of the starch pastes that exh ibited syneresis. With increasing M. S. , the changes in both complex modulus (G) and phase angle (delta) indicative of destabilization, an d starch paste syneresis were delayed with regard to the number of fre eze-thaw cycles. When a starch paste showed a peak in G, related to s yneresis, it also showed a thermal transition peak in the DSC measurem ents. The enthalpy value decreased with increasing M. S. , and it appe ared to be related to the extent of syneresis and the magnitude of the peak in G. Comparison of the rheological behaviour of hydroxypropyl potato starch pastes and hydroxypropyl and cross-linked potato starch pastes, suggested that the former were gradually transformed into coar sely aggregated structures while the latter became diluted starch disp ersions, due to shrinkage of swollen starch granules and their disrupt ion.