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

Fat crystallization plays a critical role in determining the macroscopic properties of fat containing food products, such as chocolate, butter and margarine. However, most of the currently used techniques to monitor fat crystallization are off-line techniques, while on-line techniques could have considerable financial benefits by avoiding expensive rework or disposal of out-of-specification products. The aim of this PhD research was therefore to study the potential of an advanced ultrasonic shear wave reflection (USWR) technique based on low intensity ultrasound which has the potential to measure the crystallization behavior on-line. This reflection set-up was chosen to avoid problems with the high attenuation of fats and shear waves were used to obtain information about the microstructure of fats. The proposed technique was developed under both static (chapter 3) and dynamic (stirring) conditions (chapter 6) using cocoa butter as a substrate. The validation of both techniques (chapter 5 and 8) was conducted by varying process conditions and adding minor components to cocoa butter. Limonene and lecithin were chosen as minor components and their effects on the crystallization behavior of cocoa butter were also studied in depth by conventional techniques (chapter 4 and 7). In chapter 3, an USWR technique to monitor the isothermal crystallization process of cocoa butter under static conditions was developed. The custom-built experimental set-up basically consists of a cylindrical sample holder, which is open at both sides, with the bottom side connected to a thick plexiglass plate. A shear wave transducer is positioned at the bottom of the plexiglass plate. The remarkable oscillatory damped response in the shear wave reflection coefficient (swRC) as function of the crystallization time could be explained by constructive and destructive interference of a first reflection at the boundary between the 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. An inverse model was developed based on the oscillatory damped response including four parameters: tind (induction time), K (crystallization rate constant), vs2 (shear ultrasonic velocity) and as2 (shear ultrasonic attenuation coefficient). Before this technique was validated, the effect of limonene on the crystallization behavior