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P. Ferstl, S. Gillig, C. Kaufmann, C. Dürr, C. Eder, A. Wierschem, W. Ruß, Ann. N.Y. Acad. Sci. 2010, 1189, 62-67

The melting point of triacylglycerides (TAGs) under atmospheric pressure depends on both the fatty acid composition and crystalline structure of the polymorphic state, which are influenced by the temperature treatment history of the TAG. In this contribution, the additional effect of high hydrostatic pressure is described. Samples were placed in a temperature-controlled cell and pressurized up to 450 MPa. The phase transition was investigated either by perpendicular light scattering and transmission or with a polarized-light microscope. The high-pressure polarized light microscope allows a precise determination of the melting point. The investigated TAGs showed a significant nonlinear increase of the melting point with pressure. Light scattering and transmission were used to observe the phase change in the high-pressure cell. Similar to supercooling in temperature-induced phase transition, we found a dramatic increase of the delay time in our pressure-induced solidification. Even the dependency of this induction time on the control parameter pressure was similar to that in temperature-driven crystallization. We propose that different crystalline structures may be obtained by superpressuring instead of supercooling.

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Y. Kinta, R.W. Hartel, J. Am. Oil Chem. Soc., 2010, 87, 19–27

Bloom on chocolate with different levels of cocoa butter seed addition was investigated. When insufficient cocoa butter seed crystals were added to give proper temper, the chocolate developed bloom as dark brown spheres in lighter color areas, similar to that seen in bloom on untempered chocolate. These dark colored spheres overlapped and the lighter color areas disappeared with increasing seed amount added. The relationship between seed amount and lighter color area (bloom), as quantified by image analysis, showed that over 270 ppm seeds (fat basis) were needed to accomplish good tempering. The cocoa butter crystallization behavior with various amounts of seed was observed by light microscopy. Too few seeds caused sparse beta crystallization and massive betaprime crystallization, which explains the appearance of poorly tempered chocolate bloom. As seed amount increased, beta crystallization of cocoa butter took less time to reach the upper level of solid fat content and the size became smaller. In addition, DSC analysis was carried out to study crystallization and melting behavior of cocoa butter with different seed amounts. Higher levels of added seeds resulted in greater amounts of beta crystal formation and the crystallization temperature increased, which meant crystallization occurred earlier. These results showed that the mechanism of bloom formation on poorly tempered chocolate (insufficient seeds) is due to sufficient time and space for phase (particles and fat) separation as the stable polymorphs grow.

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T. Helgason, T.S. Awad, K. Kristbergsson, D.J. McClements, J. Weiss, 2009, 334, 75-81

The effect of surfactant surface coverage on formation and stability of Tween 20 stabilized tripalmitin solid lipid nanoparticles (SLN) was investigated. A lipid phase (10% w/w tripalmitin) and an aqueous phase (2% w/w Tween 20, 10 mM phosphate buffer, pH 7) were heated to 75°C and then homogenized using a microfluidizer. The resulting oil-in-water emulsion was kept at a temperature (37°C) above the crystallization temperature of the tripalmitin to prevent solidification of emulsion droplets, and additional surfactant at various concentrations (0–5% w/w Tween 20) was added. Droplets were then cooled to 5°C to initiate crystallization and stored at 20°C for 24 h. Particle size and/or aggregation were examined visually and by light scattering, and crystallization behavior was examined by differential scanning calorimetry (DSC). Excess Tween 20 concentration remaining in the aqueous phase was measured by surface tensiometry. Emulsion droplets after homogenization had a mean particle diameter of 134.1 ± 2.0 nm and a polydispersity index of 0.08 ± 0.01. After cooling to 5°C at low Tween 20 concentrations, SLN dispersions rapidly gelled due to aggregation of particles driven by hydrophobic attraction between insufficiently covered lipid crystal surfaces. Upon addition of 1–5% w/w Tween 20, SLN dispersions became increasingly stable. At low added Tween 20 concentration (<1% w/w) the SLN formed gels but only increased slightly at higher surfactant concentrations (>1% w/w). The Tween 20 concentration in the aqueous phase decreased after tripalmitin crystallization suggesting additional surfactant adsorption onto solid surfaces. At higher Tween 20 concentrations, SLN had increasingly complex crystal structures as evidenced by the appearance of additional thermal transition peaks in the DSC. The results suggest that surfactant coverage at the interface may influence crystal structure and stability of solid lipid nanoparticles via surface-mediated crystal growth.

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M.T. dos Santos, G.A.C. Le Roux, X. Joulia, V. Gerbaud, Comp. Aided Chem. Eng., 2009, 27, 885-890

Computer-Aided Mixture Design for product development can take advantage from equilibrium modelling. Systems composed by triacylglycerols (TAG) mixtures are widely used for many applications (foods, cosmetics, pharmaceutical and lubricants) and their end-use properties are very close related to phase behaviour (melting and crystallization). Such molecules can have different polymorphisms in solid state, leading to a lack of intersolubility and consequently formation of multiple solid phases. This work has implemented the solid-liquid equilibrium for TAG mixtures in a two step approach: stability tests and equilibrium compositions computations for two phase mixtures. The Michelsen’s method for stability analysis was adapted to cope with polymorphisms and was successful for phase-split detection. Melting curves for mixtures composed by 9 TAGs in different compositions and molecular structures were simulated revealing good agreement with physical background for such systems. Further implementations of other initialization independents stability tests and robust optimization techniques show, therefore, a great potential for use as auxiliary computational framework for match improved mixtures in structured lipids research.

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