The Avrami Index and the Fractal Dimension in Vegetable Oil Crystallization

The behavior of the Avrami plot during TAG crystallization was studied by DSC and rheological measurements in oil blends of palm stearin (26 and 80%) in sesame oil, using different crystallization temperatures (TCr) attained under several cooling rate conditions (1, 10, and 30C/min). In the same way, the relationship between the growth mechanisms of TAG, measured by the Avrami index (n), and the mass fractal dimension (D) of the crystal network was investigated. This last parameter was measured as TAG crystallized in the oil blend under isothermal conditions. Results showed that TAG crystallization in a vegetable oil involves the process of TAG lamellar development, nucleation, and crystal growth. Each event occurred at a different rate and extent as affected by cooling rate and TCr, and as a function of crystallization time under isothermal conditions at a given cooling rate. Within this framework, we proposed that n calculated from the second region of the Avrami plot is a parameter mainly associated with crystal growth, whereas n from the first region is associated more with nucleation. On the other hand, changes in D values followed the different polymorphic states developed by TAG as a function of TCr. Additionally, it was shown that, independent of the concentration of palm stearin in the oil blend, at cooling rates of 1 and 10C/min the increase in n from ?3 to ?4 produced a curvilinear increase in D from ?1.75 to ?3.0. The growth mechanism of the TAG crystals (i.e., n), also affected the magnitude of D. However, different behavior was observed in the nD relationship when n < 2.7 and at 30C/min. Paper no. J10192 in JAOCS 79, 855866 (September 2002).