Annelien Rigolle

Development of an ultrasonic shear reflection technique to monitor the crystallization of cocoa butter

The quasi-isothermal crystallization process of cocoa butter was monitored by an ultrasonic shear reflection technique utilizing a custom-built experimental set-up in a temperature controlled environment. To facilitate the interpretation of the measurement results, the propagation of shear waves was first theoretically studied in different configurations of gas, liquid or solid layers with varying thickness for the case of normal incidence, yielding theoretical equations of the shear wave reflection coefficient (swRC) for different layering conditions. The typical experimentally observed pattern of the swRC during quasi-isothermal cocoa butter crystallization was subsequently linked to the theoretical equations. The remarkable oscillatory damped response in the swRC as function of the crystallization time could be explained by constructive and destructive interference of a first reflection at the boundary between a plexiglass delay line and the crystallized cocoa butter and a second reflection occurring at the interface between crystallized and liquid substance. This hypothesis was supported by the excitation frequency dependence of the oscillations. The quality of the fit of the theoretical model to the experimental results was very good and also the reproducibility between different independent measurements was acceptable. Finally, measurements at different temperatures (18°C and 20°C) suggested that the technique was able to detect differences in crystallization behavior, as measurements at 18°C displayed faster oscillations compared to measurements at 20°C. Moreover, this was also confirmed by the theoretical model, as a higher value of the crystallization rate parameter K, exhibited more rapid oscillations.

Ultrasonic wave propagation in cocoa butter during crystallization

In the production of fat containing food products, insight in the crystallization behaviour of fats is of utmost importance to obtain the desired product functionality and product quality. The aim of this research is to study the potential of advanced ultrasonic techniques to monitor fat crystallization. For reasons of sensitivity and signal quality during the entire process, we concentrate on shear wave reflectivity measurements at the interface between a fat sample and a plexiglass delay line. Preliminary experiments show that interesting evolutions can be observed in the experimentally obtained reflection coefficient during the crystallization process of cocoa butter, and we conjecture that the observed features can be linked to particular phases in the process. Additional measurements have been performed to check the influence of sample properties and of the experimental set-up, such as the viscosity or thickness of the cocoa butter layer. It is found that the thickness of the layer influences the reflection coefficient up to a frequency dependent limiting thickness value. In order to eliminate this influence and avoid its contamination in the analysis of the crystallization phases, it is required to use sufficiently thick fat layers. To support the experiments, a shear reflectivity simulation model is proposed, which is coupled to the evolution of sample parameters.