Polymorphism and solidification kinetics of the binary system POS-SOS |
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Authors: | Ph Rousset M Rappaz E Minner |
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Affiliation: | (1) Département des Matériaux, Ecole Polytechnique Fédérale de Lausanne, MX-G, CH-1015 Lausanne, Switzerland;(2) Nestec S.A., Nestlé Research Center, 1000 Lausanne 26, Switzerland |
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Abstract: | As a first approach to modeling the crystallization behavior of cocoa butter, the binary system of its two major components,
1-palmitoyl-2-oleoyl-3-stearoyl-s n-glycerol (POS) and 1,3-distearoyl-2-oleoyl-s n-glycerol (SOS), was studied. Differential scanning calorimetry, coupled with polarized light microscopy, was used to determine
the phase diagrams of the various polymorphic forms (the most metastable sub-α and α, intermediate δ and β′, and stable β).
Associated theoretical phase diagrams were also built. The presence of a solid solution for β and the ideal behavior of α
were confirmed. Kinetics of isothermal solidification as a function of temperature were studied for three compositions of
the POS-SOS system. Results were displayed as time-temperature-transformation (TTT) diagrams and crystal morphology maps.
Dependence of the crystallization kinetics on composition of the binary system was interpreted in terms of nucleation and
growth mechanisms. The asymmetry of the POS molecule induces a slower growth rate of the mixture when the concentration of
this triacylglycerol is increased. Solidification kinetics during continuous cooling were studied at various cooling rates
for the system POS-SOS 25:75. Experimental results were compared with numerical predictions of a solidification model based
upon TTT data and an additivity principle. The calculated and measured volume fractions of the different phases formed and
times of onset and finish of the solidification were in good agreement. |
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Keywords: | Binary system cocoa butter crystallization DSC kinetics polarized light microscopy polymorphism POS SOS triacylglycerol |
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