Coalescence and agglomeration of individual particles of skim milk during convective drying

• New method to investigate coalescence and agglomeration of particles.
• Evaluation of coalescence and agglomeration behaviour during drying.
• Prediction of coalescence by using surface Ohnesorge's number of particles.
• Prediction of stickiness using surface glass transition temperature of particles.
• Simulation of wet and dry shell formation in a particle during drying.

This work presents a methodology, which combines experiments and modelling, for investigating the coalescence and agglomeration ability of a product and to support the modelling of product properties during spray drying. Two particles were dried simultaneously and contact tests were performed along the drying time. A validated mathematical model describing the drying kinetics of milk particles was used to predict surface conditions during contact tests. Three major mechanisms were observed, coalescence, stickiness, and non-stickiness, which were related to adhesion and cohesion forces. The simulation model allowed evaluation of the surface Ohnesorge dimensionless number and surface glass transition temperature, which showed to be good parameters for predicting contact mechanisms. The model was also used to predict shell formation in drying particles. Wet and dry shell formation was simulated over the drying time, to improve understanding of observed contact behaviour.