Relationship between crystallization behavior, microstructure, and mechanical properties in a palm oil-based shortening

摘要

In this study, the effects of cooling rate, degree of supercooling, and storage time on the microstructure and rheological properties of a vegetable shortening composed of soybean and palm oils were examined. The solid fat content vs. temperature profile displayed two distinct regions: from 5 to 25degreesC, and from 25degreesC to the end of melt at 45-50degreesC. A peak melting temperature of 42.7degreesC was determined by DSC. Discontinuity in the crystallization induction time (determined by pulsed NMR) VS. temperature plot at 27degreesC also suggested the existence of two separate groups of crystallizing material. Isothermal crystallization kinetics were characterized using the Avrami and Fisher-Turnbull models. In using DSC and powder X-ray diffraction, the a polymorph formed upon fast cooling (>5degreesC/min), and the beta' form predominated at lower cooling rates (<1degreesC/min). An a to beta' transition took place upon storage. Fractal dimensions (D-f) obtained by microscopy and image analysis showed no dependence on the degree of supercooling since D-f remained constant (similar to1.89) at crystallization temperatures of 5, 22, and 27degreesC. Crystallization at 22degreesC at 1degreesC/min and 15degreesC/min yielded D-f values of 1.98 and 1.93, respectively. Differences in microstructure were observed, and changes in particle properties increased the parameter lambda higher degrees of supercooling.