On how the sol-gel transition and the internal flows drive the skin formation in the course of drying of dairy colloids

The production of dairy powders, involving high added value products such as high protein powders and infant milk formulas, has nowadays become strategic for the dairy industry. Due to their complex composition in terms of proteins and macromolecules, shedding light on the underlying mechanisms governing the evaporation of such dairy products constitutes a main industrial concern and a scientific challenge at the same time. In this study, we used the single droplet approach to investigate the evaporation of dairy solutions consisting of the two major milk protein components (whey proteins and native micellar caseins), alone or in mixes with different whey protein to casein ratio. In the works performed in our group in the last decade[1-3], we demonstrated that studying the drying of such biocolloidal systems at the single droplet stage is consistent to predict the behavior at larger industrial drying scale. We observed that the droplet-to-particle transition that characterizes the evaporation of dairy mixes consists of three main phases: pure shrinkage, skin formation and buckling, and finally end of the process. Here, we mainly focused on the gradual development of the droplet gelled skin due to the continuous sol-gel transition at the air-liquid interface (buckling stage). In particular, we aimed at understanding how the different physico-chemical properties of whey proteins (rigid globular shape) and casein micelles (deformable sponge-like structure) affect the mechanisms of formation of such a skin and therefore the particle final shape. To this end, we estimated in real-time the internal flows in drying droplets of whey protein and casein mixes by optical microscopy (particle tracking and particle image velocimetry). Simultaneously, the development of the skin due to the continuous solute migration was visualized by both optical and confocal microscopy. In addition, an elemental analysis by scanning electron microscopy was realized on the dried particle skin of the different mixes to extract further information about the specific deposition of the two colloidal components during the desiccation process. Our outcomes highlight the strong impact of both chemical composition and physical structure of whey proteins and casein micelles on the duration and the intensity of the internal flows during the drying of mixes, as well as on the mechanisms of organization of the external skin throughout the evaporation process. These results represent a promising progress in order to improve the control of the functional properties of milk powders. Furthermore, they contribute to improve the understanding of the drying mechanisms in binary colloidal systems, which are still far from being fully elucidated.