On the addition of protein (casein) to aqueous lactose as a drying aid in spray drying-theoretical surface composition
Spray drying is the primary process for the formation of dairy powders. In multistage drying operations, the post-spray-drying stage-for example, a fluidized bed drying stage-may not alter much of the surface composition formed earlier due to the extremely high surface viscosity. Spray drying of high-sugar-content product without a retardant on the surface to prevent sticky powder deposition in drying chamber may produce very low yields due to drying chamber wall deposition or cyclone deposition. The powder products are not flow able. A functional spray-dried product begins with its efficient incorporation into water; hence, surface composition plays a key role. This study is an attempt to explore the solid formation around the outermost layer of a single droplet of protein-sugar solution during the drying process using a continuum approach (diffusion-convection equations). The main feature of this model is that the multicomponent effect is lumped into the viscosity of the fluid at the surface, which inversely affects the diffusivities of individual components in the solution droplets or suspension droplets. The trend (and the order of magnitudes) of surface composition development of the caseinate/lactose system (from a 20/80 ratio down to 0.01/99.99) has been explored and the phenomeno has been explained with the aid of the continuum model, coupled with a simplified, geometrical, molecular-level interpretation. Based on the current study, we were able to make some useful conclusions.