Skin layer stratification in drying droplets of dairy colloids

Abstract The evaporation of a colloidal dispersion is characterized by solute accumulation at the air-liquid interface, leading to the gradual formation of a gelled skin. The development of this layer, from preliminary colloid deposit to complete solidification, affects overall drying process and final sample morphology. Despite, progress in the last decades, the mechanisms governing skin formation in drying colloidal suspensions have not yet been fully clarified, especially in complex polydisperse systems. In this work, we investigate this open question in droplets consisting of the two main milk proteins, i.e. whey proteins and casein micelles. Using complementary experimental approaches, we evaluate skin rheological behavior during the different stages of the evaporation process, highlighting the specific role of each colloid. Our results are interpreted in the light of drying-induced protein stratification, whose evidence is provided by the direct observation of dry skin section structure. This study contributes to the understanding of the competitive drying mechanisms occurring in binary colloidal systems. Moreover, our outcomes are potentially valuable for the optimization of milk powder production in dairy industry.