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Application of Temperature Modulated Optical Refractometry for the Characterization of the Crystallization Behavior of Palm Oil

Häupler, Michaela, Savitri, Ratna Ayu, Hutschenreuter, Valeska, Flöter, Eckhard
European journal of lipid science and technology 2018 v.120 no.5 pp. e1700511
crystallization, differential scanning calorimetry, heat transfer, lipids, melting, refractive index, surface area, temperature, thermal expansion, thermal properties
The phase behavior of fats is mainly determined using DSC. Here, the application of temperature modulated optical refractometry (TMOR) is examined to monitor the phase transitions of palm oil with different degrees of saturation. Studying the phase behavior by both methods revealed systematic differences. At identical scan rates, TMOR yielded up to 2 °C higher crystallization temperatures and identified consistently lower temperatures for melting phenomena. Because the prism serves as heating surface and defines the sample volume considered for the measurement a more direct heat transfer with TMOR is assumed. The sample depth above the prism relevant for the determination is only one micron. Hence, a direct heat transfer is ensured and thermal lag is practically eliminated causing the above‐mentioned differences. Because the TMOR signal is averaged over a defined prism surface area data for inhomogeneous samples can be generated. Although actual values for thermal expansion coefficients appear meaningless the combination of the TMOR signals allows to accurately determine the relevant phase transitions. The identification of different polymorphic forms and levels of solids in palm oil will be studied prospectively building on the promising results reported to identify if TMOR can become a valuable extension of the fat technologists' toolbox. Practical Applications: The new temperature modulated optical refractometry can extend the mainly used differential scanning calorimetry. It works highly accurate at small scan rates (<5 °C min⁻¹) in comparison to the DSC. The new method can provide a deeper insight into samples during heating and cooling due to additional temperature undulation as well as the possibility to perform quasi‐isothermal measurements. It is possible to investigate the crystallization and melting behavior of fats with the new temperature modulated optical refractometry (TMOR). An undulated temperature is applied and the time‐delayed answer of the refractive index is measured. Subsequently, the real and imaginary part of the thermal expansion coefficient α are calculated based on the refractive index of the sample and the occurring phase shift. A plot of the real and imaginary part against the temperature leads to peaks at the phase transition temperatures and a resulting thermogram for the sample.