Evaluation of tripalmitin crystallization in sesame oil through a modified Avrami equation

Tripalmitin (TP) crystallization in sesame oil solutions (0.98, 1.80, and 2.62%, wt/vol) was investigated by utilizing a modification of the Avrami equation. The modified equation retains the original correspondence to the nucleation process (i.e., n) and crystal growth and simply corrects the value of th, crystallization rate constant (z) by eliminating the influence of n The energy of activation (E-a) values for TP crystallization in sesame oil solution, calculated with the modified z, were quite similar to those calculated with the reciprocal of time required to achieve 50% of TP crystallization (t(F=0.50)(-1)) However, E-a values calculated with z from Avrami's original equation were quite different from those obtained with t(F=0.50)(-1) Thus, z and E-a values calculated through the Avrami equation yield erroneous results, especially when comparing crystallization processes having different magnitudes of n, as in this study. Additional analysis that considered the viscosity of the TP oil solutions concluded that, at equal supercooling conditions (e.g., 22.0-22.5), the magnitude of z and E-a became more dependent upon the crystal growth process as oil viscosity decreased. In contrast, as viscosity of the oil phase increased, the main crystallization process, evaluated through z and E-a, was nucleation. Furthermore, within the super cooling interval achieved at the temperatures utilized, the increase in supercooling at constant viscosity conditions (e.g., 5.25-5.5 dynes/cm(2)) would produce a higher degree of nucleation without an appreciable effect on TP crystal size. The results obtained indicate that investigating the effects of supercooling, molecular diffusion (i.e., viscosity) and TP concentration on the magnitude of z and E-a during TP crystallization in sesame oil requires a multiple variable statistical approach.