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. (c) 2005 Elsevier Ltd. All rights reserved.