Flow Behavior of Milk Chocolate Melt and the Application to Coating Flow

The rheological properties of chocolate, especially shear viscosity and yield stress, are important control parameters for enrobing processes in confectionery manufacture. The rheological parameters of molten milk chocolate were measured at 42°C during steady pipe flow using a magnetic resonance imaging (MRI) viscometric method. The experimental method combines shear rate values obtained from an MR velocity image and shear stress values obtained from an independent pressure drop measurement. The experimental factors were emulsifier type and emulsifier level. The rheogram data were fit by the Casson model to yield the Casson yield stress and plastic viscosity. The Casson yield stress ranged from 1.9 to 15.0 Pa; the Casson viscosity ranged from 6.0 to 14.6 Pa s as a function of emulsifier content. The rheological parameters were incorporated into a drainage theory model to predict coating thicknesses in the enrobing process. The model was solved numerically and yielded good approximations to the experimental values that were between 1.1 to 2.7 mm.