Surface composition of industrial spray-dried milk powders. 2. Effects of spray drying conditions on the surface composition

The effects of various spray drying conditions (feed solids content, drying temperature, degree of homogenization and initial droplet size) on the surface composition of industrial spray-dried milk powders (skim milk powder and whole milk powder) were investigated. Experiments were performed in a laboratory-scale spray drier and the surface compositions of the powders were measured using electron spectroscopy for chemical analysis (ESCA). For whole milk powder, the amount of surface free-fat was also measured as a complement to the surface composition estimated using ESCA. For both skim milk powder and whole milk powder, the surface composition of the powders was found to be determined to a large extent by the spray drying conditions employed. At higher feed solids content or drying temperature, less fat and protein appeared on the surface of the powders. Less redistribution of components seemed to occur within the drying droplet because of high viscosity and rapid crust formation. Increasing the number of homogenization passes reduced the fat globule size and consequently the amount of fat present on the powder surface. The initial droplet size did not have a significant effect on the range of particle sizes studied in this work. Strong interaction effects between the spray drying conditions were also found. The results suggest that a combination of spray drying conditions is necessary to control the surface composition of milk powders. Possible mechanisms behind the formation of the surface composition of industrial spray-dried milk powders were proposed based on the findings in this work and theoretical considerations.