Effects of Low-Frequency Ultrasound on the Drying Process, Microstructure, Surface Chemistry, and Crystallinity of Milk Powders with Varying Casein-to-Whey Ratios and Calcium Addition

This study investigates the effects of casein-to-whey protein ratios, ultrasound pre-treatment, and calcium addition on the surface composition, microstructure, and crystallinity of spray-dried milk powders. X-ray photoelectron spectroscopy (XPS) showed that protein is the predominant component on the powder surface, with sonication enhancing protein surface activity while reducing lactose segregation. Calcium addition increased the presence of oxygen-containing functional groups and stabilised the surface structure. Sonication reduced particle size and smoothed surface morphology, as observed by scanning electron microscopy (SEM), particularly in whey protein samples, whereas casein-rich formulations showed rougher, more irregular structures, making them more resistant to ultrasound-induced fragmentation. X-ray diffraction (XRD) analysis indicated the lactose predominantly remained in an amorphous state across all samples. Calcium addition further promoted the amorphous structure by forming complexes with proteins and lactose. Sonication had a minimal impact on lactose crystallinity, but it played an important role in enhancing protein surface activity and modifying surface composition.