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.

a b s t r a c tSpray drying trials were carried out to produce amorphous sucrose powder. Firstly, pure sucrose solutions were prepared and spray dried at inlet and outlet temperatures of 160°C and 70°C, respectively. No amorphous powder was obtained and only 18% of the feed solids were recovered in a crystalline form, with the remaining solids lost as wall deposits. Secondly, sodium caseinate (Na-C) and hydrolyzed whey protein isolate (WPI) were added in sucrose:protein solid ratios of (99.5:0.5) and (99.0:1.0) and drying trials were conducted maintaining the initial drying conditions. In both these cases, greater than 80% of the feed solids were recovered in an amorphous form. The increase in protein concentration from 0.5% to 1% on dry solid basis did not further improve the recovery. The remarkable increase in recovery from a small addition of protein is attributed to preferential migration of protein molecules to the droplet-air interface, and the subsequent transformation of the thin, protein-rich film into a non-sticky glassy state upon drying. This film overcomes both the particle-to-particle and particle-to-wall stickiness. The measured bulk glass rubber transition temperature (T g-r ) values of the bulk mixtures at various moisture contents were very close to the corresponding mean glass transition temperature (T g ) of the pure sucrose indicating that surface layer T g rather than the bulk T g is responsible for this. Electron spectroscopy for chemical analysis (ESCA) studies revealed that the particle surface was covered by 50-58% (by mass) proteins. The calculated glass transition temperature of the surface layer (T g,surface layer ), based on the surface elemental compositions, showed that the T g,surface layer has increased to the extent that it remained within the safe drying envelope of spray drying.

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