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

Abstract

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.

Thermal Transitions of Cocoa Butter: A Novel Characterization Method by Temperature Modulation

“…Afterwards, the sample was cooled down to 4 °C with a cooling rate of 0.5 K min −1 , modulation period of 60 s (f MOD = 17 mHz) and amplitude of 0.5 K. After 8 minutes at 4 °C, the sample was heated again to 55 K with a heating rate of 0.5 °C min −1 and same modulation frequency and amplitude (17 mHz and 0.5 K, respectively), and kept at this temperature for 8 minutes. The cooling and heating rate (0.5 K min −1 , respectively) and the modulation frequency (17 mHz) were chosen based on the parameters reported at [17,18].…”

“…Albeit initially proposed to investigate transparent and isotropic materials, Häupler and Flöter recently employed TMOR technique to investigate the crystallization and melting behavior of palm oil in comparison to conventional DSC analysis [17]. Results revealed systematic differences between the two techniques.…”

Feasibility Study on the Determination of the Solid Fat Content of Fats Using Temperature‐Modulated Optical Refractometry

“…This is far from practical. Because heat transfer into the small sample volume subject to the measurement is very good it is fair to assume that “local thermal equilibrium” is established rapidly . The question is hence whether other less time‐consuming thermal procedures to generate a solid suspension in the sample also result in a representative inhomogeneous coverage of the prism.…”