Humidity-driven lactose crystallization on milk powders: a surface-level study

This study introduces in-situ atomic force microscopy (AFM) under controlled humidity ramps as a novel approach for real-time monitoring dairy powder surface evolution. Applied to skim milk powder (SMP) and whole milk powder (WMP) as case-study matrices, the method was combined with dynamic vapor sorption (DVS), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and powder rheology to provide a multi-scale characterization of humidity surface changes. Both powders exhibited sigmoidal sorption isotherms with a critical transition (40-60% RH) associated with moisture-induced plasticization of amorphous lactose. In-situ AFM revealed limited surface changes below 43% RH, followed by a sharp increase in surface roughness above 53% RH consistent with surface restructuring likely associated with lactose crystallization. This restructuring occurred earlier and more extensively in SMP, with rapid roughness growth, while WMP showed delayed and attenuated surface changes possibly due to its lipid-rich surface and its influence on water accessibility and/or lactose mobility. XPS analyses confirmed surface rearrangements with a C/O ratio decrease compatible with lactose crystallization. Increased RH significantly impaired powder flowability, particularly above 60% RH.