Multistage melt crystallization of a variety of plant-based polyunsaturated fatty acid (PUFA) mixtures was conducted to obtain an overview on their liquid-solid equilibrium behavior. Crystal fraction and mother liquor samples were analyzed at every stage. Phase behavior diagrams at low saturated fatty acid concentrations were investigated in detail as they are of significant technical relevance with respect to efficient depletion of the saturated content from the samples. A high oleic acid content in the PUFA mixtures led to difficulties in depleting the mother liquor from the saturated content by melt crystallization. Thermograms from the differential scanning calorimetry were supplemented with illustrations which allow a novel interpretation of results and verification of conclusions.
Important parameters like supercooling and cooling rates affecting the melt crystallization of a polyunsaturated fatty acid (PUFA) mixture obtained from sunflower oil were investigated and compared via high-resolution polarized-light microscopy. PUFA was thermally characterized in a differential scanning calorimeter and the significant liquid-solid-phase transition temperatures determined were then implemented in the development of specific temperature profiles. Analyzed between two glass slides, induction times were found to decrease with low crystallization temperatures and the number of nuclei per unit area increased with higher supercooling and cooling rates. A comparison between the linear crystal growth rates of pure standards of each of the main fatty acids present in the PUFA mixture and the PUFA mixture itself indicated that the latter is much slower than that of each of its pure components.
Nucleation kinetics of high linoleic and high oleic polyunsaturated fatty acid (PUFA) mixtures were investigated during melt crystallization by means of a focused beam reflectance measurement (FBRM) sensor. Samples were analyzed thermally and chemically in the differential scanning calorimeter (DSC) and gas chromatograph (GC), respectively. Investigations showed that a high oleic content in the PUFA mixtures lead to higher induction times, τ i . Unlike previous investigations, a clear relation between τ i and cooling rates was established: τ i increases with increasing cooling rates and decreasing supercooling. Activation energies of nucleation, ΔG c ′, were determined by means of the Fisher Turnbull equation wherein ΔG c ′ decreases exponentially with increasing supercooling. Investigations and further evaluations conclude that while the plot establishes the effect of supercooling on ΔG c ′, it does not incorporate the effects of molecular diffusion or the cooling rate. Hence, 3D modeling was performed to visualize the combined effects of cooling rate and supercooling on induction time.
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