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A comprehensive study of the isothermal crystallization kinetics of tripalmitin-tristearin mixtures was carried out using DSC, with data fitted to the Avrami equation. Polymorphs were identified by subsequent melting of samples in the differential scanning calorimeter, with additional confirmatory information obtained from wide-angle X-ray diffraction. It was found that α-, β′-, and β-forms require small (<1.0°C), moderate (3.5-8.5°C), and large (9.0-13.0°C) amounts of subcooling below their respective polymorph melting temperatures for nucleation to occur. Concurrent crystallization of β and β′ polymorphs was not observed. The β polymorphs exhibited sharper heat flow exotherms than β′, due to the higher crystallization driving forces experienced. Analysis of apparent induction times shows that the activation free energy of nucleation for the β-form is significantly higher than for the β′-form. Samples rich in either species crystallized faster (both shorter apparent induction times and sharper peaks) than samples with equivalent compositions. Driving-force arguments do not fully explain this behavior, strongly suggesting that mass transfer resistances (greatest for equivalent compositions) have a significant effect on kinetics. Multiple crystallization events were observed for 50-80% tristearin samples between 56 and 60°C and were attributed to a demixing of tripalmitin-rich and tristearin-rich β phases, in line with established phase diagrams.The phase behavior and crystallization kinetics of TG have been extensively studied (1), and these studies have generally concentrated on either single TG or natural fats. Natural fats are complex multicomponent mixtures of TG and other minor components, and their crystallization behavior is poorly understood in comparison with well-defined systems of pure TG. A better understanding is required. One approach to the behavior of real fats is to build up an understanding of the interactions between individual TG components. A number of workers have studied the equilibrium phase behavior of mixed TG, and this has resulted in the publication of phase diagrams for a number of binary and ternary systems (2,3). Much rarer, however, are studies where the crystallization kinetics of mixed systems are examined (4,5). This paper seeks to study such behavior in tripalmitin-tristearin binary mixtures.The polymorphic and phase behavior as well as the crystallization kinetics of pure tripalmitin and tristearin has been reported previously in the literature (6-10). However, relatively few studies have been performed on blends of tripalmitin and tristearin, although a phase diagram is well established (11-13). In this work, the isothermal crystallization and subsequent melting of tripalmitin-tristearin mixtures covering the whole composition range were studied using DSC. This technique is able to provide accurate and reproducible kinetic data and also yields information on the identity of the resulting polymorphs by a subsequent melting of the crystallized material, which can be compared wi...

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The effect of composition and cooling rate on polymorphic occurrence and crystallization rates in binary mixtures of tripalmitoylglycerol (PPP) and tristearoylglycerol (SSS) has been studied using differential scanning calorimetry (DSC), light microscopy and X-ray diffraction under constant cooling rates. In general, b and b' polymorphs were observed at cooling rates of 0.5 K/min and less, and the a form at higher cooling rates. Double peaks were observed in the DSC exotherms of mixtures containing more than 30% SSS. The X-ray data showed that, when cooling at 0.5 K/min, the double peaks correspond to the crystallization of the a and b' forms. However, when cooling at 1 K/min and higher, the double peaks appear to arise from the crystallization of two a phases -one PPP rich and the other SSS rich. Light microscopy experiments also show two distinct morphologies, which corroborates this view. The apparent SSS-rich phase crystallizes as irregular spherulites whereas the apparent PPP-rich phase forms a fine crystal mass. Analyses of thermograms relating to the a form show that the amount of supercooling at the onset of crystallization is virtually independent of composition. However, peak width information shows that, once started, pure-component samples crystallize faster than mixtures.

A novel algorithm for the computer-based analysis of a sequence of optical microscope images of a crystallization has been developed to extract growth and nucleation data. The algorithm subtracts grayscale pixel values in corresponding positions on successive images, thereby locating pixels relating to new growth which are then either assigned to newly identified (nucleated) crystals or digitally "grown" onto existing crystals. Thus, the algorithm tracks the natural processes of nucleation and growth. The result is a series of maps that identify pixels with specific crystals by an assigned number (label) which remains the same for each crystal from image to image, thereby enabling the growth of any crystal (which can be of any shape) to be tracked. These maps are able to be analyzed (again by computer) to extract unimpinged crystal size and number information, and hence provide crystal growth rate, nucleation rate, and solid fraction data, and crystal size distributions. The method is mainly demonstrated for the isothermal crystallization of tripalmitin from the melt, but an example of isothermal crystallization of sucrose from aqueous solution is also presented.