Thermal, structural and rheological characteristics of dark chocolate with different compositions Original Research Article

The effect of different cocoa composition of dark chocolate samples and their ingredients on their thermal, structural and rheological characteristics was investigated. Thermal behavior was evaluated by means of differential scanning calorimetry (DSC), while the crystal morphology was observed by polarized light optical microscopy. The rheological measurements were carried out using both continuous and oscillatory experiments. The formation of more stable polymorphic structures was time and temperature dependent; and it was not affected by either cocoa composition or particle sizes. The kinetics of crystallization was determined by a step crystallization method and modeled by the Avrami equation, it was accelerated by solid particles concentration, lower particle size and lower crystallization temperatures. Negative spherulites with featherlike microstructures defined the time dependent crystal growth and were consistent with Avrami indexes of 3–4. Under continuous shear, cocoa butter was well described by the Bingham rheological model, while Casson and Carreau equations modeled the flow of cocoa liquor and chocolate samples. However the Carreau model was preferred for presenting better fittings and for predicting apparent viscosities at low and at high shear rates. From both, continuous and oscillatory experiments, it was found that composition of chocolate samples in terms of fat and nonfat cocoa solids, and sugar content, affected their rheological behavior. The solid liquid transition of chocolate samples and cocoa liquor was obtained at a yield stress of around 1 Pa from both continuous and oscillatory shear experiments.