Particle size measurements and scanning electron microscopy (SEM) of cocoa particles refined/conched by conical and cylindrical roller stone melangers

Abstract

The particle size distribution (PSD) and the microstructure of the non-fat particles in chocolates are very important to the appearance, mouth feel and texture of the chocolate products. Refining/conching is critical to chocolate processing because most particle size reduction is made and the microstructure of the particles is developed in this process. Currently, the most commonly used comprehensive measurement method for chocolate products is using a laser scattering particle size analyzer, which is relatively expensive and inaccessible to small manufacturers. In this study, three potential alternative methods (micrometer, light microscopy image analysis and Hegman gauge) for the laser scattering method were used to measure the size of cocoa particles at different stages of refining/conching. Also, the influences of the sample weights of cocoa and melanger configuration (conical roller and cylindrical roller) on the efficiency of refining/conching were studied. The performances of the two melangers were similar, however, cylindrical roller stones tend to generate thin and flat particles during the first 4 h of refining. Varying sample weights only influenced particle size reduction at early stages of refining. From the SEM images, a decreasing trend in the number of large spherical particles and an increasing trend in the number of small flat irregularly shaped particles were observed. The particle size measurements by the micrometer of cocoa particles refined for 0.5 h was greater than all other methods. However, the particle size measurements by the micrometer of particles refined for 2 h, 4 h, and 24 h were smaller than the measurements made by any other methods. On the other hand, except for cocoa being refined for 0.5 h, the particle size measurements tested by light microscopy image analysis for all cocoa samples were much greater than other measurement methods. Both the micrometer and light microscopy image analysis can be used to monitor the PSD of cocoa particles during refining/conching. However, the Hegman gauge failed to provide meaningful measurements.