Links between particle surface hardening and rehydration impairment during micellar casein powder storage

Storage is an unavoidable critical phase regarding dairy powder reconstitution abilities, particularly for high casein content powders, which generally present a poor rehydration behavior. The ability of micellar casein powders to completely rehydrate can thus be particularly affected by storage time and temperature. To implement best practices for the optimization of storage conditions, understanding changes occurring is a crucial point. For the first time, biophysical techniques were used to investigate powder surface at the nanoscale. Atomic force microscopy revealed that particle surface became rougher during storage, associated with the formation of hollow zones (around 500 nm) holes when stored for 10 months at 40 °C. Mechanical properties of micellar casein particle surface during powder storage was quantified using AFM nanoindentation. Spatially-resolved force/indentation curves evidenced a significant stiffer surface for aged powder (Young modulus of ∼20 GPa) in comparison with the fresh one (∼0.2 GPa). These findings were fully consistent with the formation of a crust at the powder surface observed by high-resolution field-emission scanning electron microscopy during powder rehydration. Finally, alterations of the rehydration process can be related to modifications occurring at the particle surface during storage.