POLYMORPHISM AND SOLIDIFICATION KINETICS OF THE BINARY-SYSTEM POS-SOS

As a first approach to modeling the crystallization behavior of cocoa butter, the binary system of its two major components, 1-palmitoyl-2-o leoyl-3-stearoyl-sn-glycerol (POS) and 1,3-distearoyl-2-oleoyl-sn-glyc erol (SOS), was studied. Differential scanning calorimetry, coupled wi th polarized light microscopy, was used to determine the phase diagram s of the various polymorphic forms (the most metastable sub-alpha and alpha, intermediate delta and beta', and stable beta). Associated theo retical phase diagrams were also built. The presence of a solid soluti on for beta and the ideal behavior of alpha were confirmed. Kinetics o f isothermal solidification as a function of temperature were studied for three compositions of the POS-SOS system. Results were displayed a s time-temperature-transformation (TTT) diagrams and crystal morpholog y maps. Dependence of the crystallization kinetics on composition of t he binary system was interpreted in terms of nucleation and growth mec hanisms. The asymmetry of the POS molecule induces a slower growth rat e of the mixture when the concentration of this triacylglycerol is inc reased. Solidification kinetics during continuous cooling were studied at various cooling rates for the system POS-SQS 25.75. Experimental r esults were compared with numerical predictions of a solidification mo del based upon TTT data and an additivity principle. The calculated an d measured volume fractions of the different phases formed and times o f onset and finish of the solidification were in good agreement.