Fundamentals of the spray freezing of foods—microstructure of frozen droplets

The surface microstructure will determine a number of functional properties of a frozen powder such as stickiness and flowability. In this study, cryo-SEM images were used to observe the influence of various solutes and freezing conditions had on the internal and surface microstructure of frozen droplets. A single droplet freezing method was used to simulate the physical changes taking place during a spray-freezing process. Sucrose solutions with added anhydrous milk fat (AMF) and whey protein concentrate (WPC) were investigated. The level of supercooling (nucleation temperature) significantly influenced the final microstructure of a frozen droplet. The greater the supercooling level, the larger the proportion of the droplet that has a fine cellular ice crystal structure. It was observed that a sucrose layer formed at the outer surface of sucrose solution droplets upon freezing. This could be the product of water evaporation or redistribution of sucrose during freezing. The concentrated sucrose layer showed a tendency to increase in thickness when nucleation was forced at the droplet surface. The presence of AMF and WPC at a high freezing rate, with a high droplet supercooling inhibited the formation of the surface sucrose layer, whereas at a low freezing rate with low droplet supercooling, a sucrose layer was formed with AMF fat globules and or WPC protein aggregates distributed within it. The inhibition of the formation of the sucrose surface layer could reduce the stickiness and increase the flowability of the frozen powder. Whereas, the redistribution of fat to the surface may also be a means of reducing the fat content of a frozen food powder without effecting its sensory impact. The results illustrate that both the composition and the freezing conditions will influence the functional properties of spray-frozen food powders.