Composition and interfacial functionality of casein-rich powders recovered from dairy side-streams

Casein-rich powders recovered from dairy side-streams may provide functional milk protein ingredients, but their composition-dependent interfacial performance remains insufficiently characterized. This study recovered casein-rich powders from skim milk, spoiled skim milk, whole milk, and antibiotic-withdrawal raw milk by acid precipitation and compared them with commercial casein. The recovered powders were characterized by proximate composition, Fourier-transform infrared spectroscopy with attenuated total reflectance, sodium dodecyl sulfate-PAGE, differential scanning calorimetry, thermogravimetric analysis, emulsifying capacity in 3 vegetable oils over 48 h, foaming performance over 60 min, water- and oil-holding capacity, and preliminary cytocompatibility in NIH/3T3 fibroblasts. Skim milk- and spoiled skim milk-derived powders were strongly enriched in protein and showed amide I and II bands, electrophoretic profiles, and thermal degradation patterns broadly comparable to commercial casein. In contrast, whole milk- and antibiotic-withdrawal raw milk-derived powders contained higher residual lipid fractions, reflected by ester carbonyl and aliphatic C-H bands and lipid-associated thermal transitions. These compositional differences were associated with lower early emulsifying capacity and reduced foam stability, particularly in the whole milk-derived powder, whereas skim-derived powders showed interfacial performance closer to commercial casein. Cell viability remained above the cytotoxicity threshold under the tested in vitro conditions, but these data do not establish food or biomaterial safety. Overall, the results show that acid precipitation can recover casein-rich powders from heterogeneous dairy streams, but their interfacial functionality varied with co-recovered constituents, especially residual lipids and mineral-associated material. These findings support further development of dairy side-stream-derived casein-rich powders as composition-tunable ingredients for colloidal food and related interfacial systems.