Particle shrinkage and morphology of milk powder made with a monodisperse spray dryer

The drying of monodisperse droplets can produce uniform powders where every particle has experienced similar, predictable air conditions. For the first time, a single-stream dryer was used with monodisperse milk droplets having solids contents as high as those used in industrial spray dryers (over 40 wt% total solids). With size measurement of droplets and powders, the particle shrinkage was precisely determined. Changes in shrinkage and morphology were observed when varying feed solids contents and drying temperatures. The majority of particles had an inwardly buckled morphology and hollow, inflated particles were produced at higher temperatures due to boiling. Drying was modelled with a numerical simulation using the Reaction Engineering Approach (REA), which is a semi-empirical model of moisture removal rate. This model accurately predicted the moisture contents of the powders collected. Several sub-models were used for particle shrinkage. The most accurate shrinkage assumption was found to be isotropic shrinkage from the removal of water (known as perfect shrinkage). This may suggest that the surface crust (initially spherical) is viscoelastic and deforms in response to drying stresses.

► Using same-sized droplets in a small spray dryer, we produced uniform milk powder.
► Drying was accurately modelled with the Reaction Engineering Approach.
► The particle shrinkage is greater for feeds with low initial solids content.
► Particles generally buckle inwards, although they can inflate at higher drying temperatures.