Atomic Force Microscopy Nanoscale Analysis: Impact of Storage Conditions on Surface Properties of Whey Protein Powders

Abstract Dairy powders are usually subjected to environmental variations during storage and/or shipment that strongly impact their chemical, nutritional and structural features. Nevertheless, these modifications are rarely investigated at the particle surface level, which represents the interface in contact with air, water, materials or other powders and directly influences powder functionalities. In this paper, atomic force microscopy (AFM) is used in imaging, nanoindentation and force spectroscopy modes to investigate the evolution of the surface properties such as the hydrophobicity and stiffness of whey protein powders at the nanoscale after controlled storage conditions. Our results evidenced that surface modifications are more enhanced by high storage temperature than storage time (for the same storage energy) and correspond to an increase of both surface hydrophobicity and heterogeneity. The strong impact of residual lactose in the powder (around 1.5 % lactose) is also highlighted on these phenomena by performing surface comparisons with a reference powder (β-lactoglobulin) without lactose. This reference powder permitted the discrimination between surface protein denaturation and surface lactosylation.