Evaluation Methods of Crystal Behaviors and Structures of Symmetrical Monounsaturated Triacylglycerols

Abstract

Cocoa butters and its equivalents are predominantly comprised of symmetrical monounsaturated triacylglycerols (SUS), namely, SUSfats. The packing of SUS will be changed significantly during crystallization as the polymorphous forms are transferred from α- to β-crystals. The transformation of the SUS-fat phase during crystallization and tempering determines the properties and qualities of final chocolate products. It is important to monitor and determine the crystallization behaviors and crystal structures of SUS-fats at cooling and melting conditions. This review systematically discusses typical tempering procedures of SUS-fats and evaluation methods on their crystallization, mainly crystallization behaviors and kinetics, crystallinity, crystal microstructures, and polymorphism. Differential scanning calorimetry (DSC) is currently widely used to determine crystallization behaviors of SUS-fats, while MultiTherm TC and temperature modulated optical refractometry are typical newly developed approaches for determining their phase transitions. Both crystallization kinetics and crystallinity could be obtained from nuclear magnetic resonance (NMR). Avrami, Gompertz, and Jeziorny are the important models to reveal the number, size, polymorph, and shape of SUS-fat crystals. Polarized light microscopy, scanning electron microscopy, and three-dimensional laser-scanning confocal microscopy are applied to observe morphological changes of fat crystal surfaces, and the order of SUS packing could be extracted from the fractal dimension. Detailed packing and polymorphism information could be obtained from synchrotron X-ray diffraction, DSC, NMR, rheology, infrared spectroscopy, and their combinations. The results contribute to realizing precise control in the manufacture of chocolates, especially tempering and its simplified process.