Pre-nucleation Structuring of Triacylglycerols and Its Effect on the Activation Energy of Nucleation

Abstract

Based in a previous study involving anisotropy measurements (rs) and molecular mechanics simulations, we investigated the molecular interaction occurring among triacylglycerols (TAGS) before their nucleation, and its possible involvement in determining the activation free energy to develop a stable nucleus (ΔGc). The Fisher-Turnbull equation was used to calculate the ΔGc involved in the nucleation of tripalmitin (TP) or tristearin (TS) blended in a 25:75 ratio with triolein (TO), high oleic safflower oil (HOSfO), and soybean oil (SBO). The crystallization temperatures used were selected to obtain similar supercooling conditions in all blends investigated. As our previous study showed, the anisotropy measurements (rs) showed that TAGS with at least one chain of palmitic acid (i.e., POO, LLP, PLO) induce TAGS’ pre-nucleation structuring in both the TP and TS blends. However, for TS blends, the molecular interaction occurred well before attaining supercooling conditions required for crystallization (i.e., during the cooling stage). Once these supercooling conditions were achieved, TAGS with palmitic acid acted as template for TS crystallization. As a result, ΔGc in TS/HOSfO and TS/SBO blends were lower than the ΔGc in the TS/TO blend. In contrast, in the TP blends TAGS structuring (i.e., an increase in rs) occurred as a function of time under isothermal conditions as a direct function of the concentration of TAGS containing at least one chain of palmitic acid. We postulate that the molecular interactions occurring in the liquid phase between TP and TAGS with palmitic acid, resulted in the development of a mixed TAGS lamellar liquid structure. This delayed TP nucleation until segregation of TP from the mixed TAGS lamellar liquid structure occurred. This resulted in a higher ΔGc for TP nucleation as the concentration of LLP, POO, PLO, and PLO increased in the blends. The molecular interactions occurring among TAGS in the liquid phase under supercooling conditions must be understood, since they directly relate to the TAGS crystallization process that in turns determines the macroscopic properties evaluated by consumer in lipid-based products. The results presented in this manuscript are in this line of investigation.