The manufacture of milk powders involves heating the milk, usually known as preheating, then concentrating the milk solids by evaporation and spray drying. In the manufacture of high protein milk powders, ultrafiltration/diafiltration is normally applied prior to evaporation and drying. These processes cause interactions between the various components in milk that influence powder functionality. The structure and stability of casein micelles are fairly well known under conditions as in milk. However, the details of the effects of milk powder processes on casein micelle structure and interactions are not much understood. Preheat treatment applied to milk prior to evaporation aims to produce powders with specific functional properties and applications. A major effect of preheating is the denaturation of whey proteins and their association with casein micelles. The nature and extent of association of whey proteins with the casein micelles, involving beta-casein, influence how the casein micelles behave during further processing. During evaporation, casein micelle size increases due mainly to aggregation of some of the micelles and increased association of whey proteins with the micelles. There has been little work examining the specific effects of spray drying on casein micelles in milk. In the manufacture of high protein powders, concentration of milk by ultrafiltration, in particular diafiltration prior to drying, can cause dissolution of colloidal calcium phosphate, resulting in loosening of the casein micelle structure, and possibly swelling of casein micelles. Increasing the degree of concentration causes progressive breakdown of micellar structure from an intact micelles to a swollen diffuse micelle and finally to a smaller fragmented micellar structures. These changes in casein micelles predispose the milk system to further protein-protein interactions during spray drying, and consequently impact on the functionality of the product.
