The migration of lactose, protein and fat within milk droplets and particles in a spray dryer is investigated with a view to eventually modelling this process using computational fluid dynamics. Both protein and fat accumulate preferentially at the surface of the milk particles as they dry, at the expense of lactose. This has repercussions for the rate of particle agglomeration and wall deposition within the spray dryer, and the functional properties of the dried powder, because the fat and lactose surface concentrations affect the stickiness of the milk particles. The surface fat coverage, and hence the particle stickiness, is particularly sensitive to small changes in fat content between 0% and 5%, which is likely to be important for the control of powder properties and the operation of spray drying equipment in skim milk production. In addition, a higher drying temperature favours the appearance of lactose over protein at the surface of the milk particle. We postulate that higher temperatures hasten the formation of a surface skin, which hinders the migration of surface-active protein towards the surface. Finally, we have confirmed observations made by various other researchers on the morphological evolution of a milk droplet as it dries, which involves the formation of a skin and a vacuole, and the inflation and subsequent shrinkage of the particle.

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A mini spray dryer has been used to investigate morphological changes that occur to milk particles during the spray drying process. We have found that the mini spray dryer is ideal for such investigations, because phenomena such as skin and vacuole formation in particles can be analyzed without the added complication of particle agglomeration, which only occurs in much larger spray dryers where particle number concentrations are higher. We have confirmed observations made by various researchers that the bulk density of spray-dried milk powder is greatly affected by the drying temperature, due to the strong influence of the latter on the porosity of the particles. In addition, we have attempted to explain observations made by various workers that fat accumulates preferentially at the surface of a particle during drying by postulating that fluid fat is transported towards the surface, via a network of cracks and pores, by the development of a vacuole overpressure which is also responsible for the inflation of the particle. Finally, we have shown that milk powders can be spray dried a second time, by reconstitution with water, with no change to the thermodynamic characteristics of the resultant powder. Thus, milk concentrates for spray drying research can be prepared from already-spray-dried milk powders rather than using the more arduous evaporation method to concentrate unprocessed milk.

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