Topographical changes in high-protein, milk powders as a function of moisture sorption using amplitude-modulation atomic force microscopy

Abstract

This study aimed to examine how microscopic morphological developments, such as lactose crystallisation, swelling of particles and changes in surface roughness, occur as a function of moisture sorption in skimmed-milk (SMP), milk protein concentrate (MPC) and whey protein isolate (WPI) powders. Atomic force microscopy (AFM) has the potential to identify high-resolution, microstructural changes in high-protein, milk powder particles. A sample preparation technique was developed, which allowed a single-layer of uniformly distributed powder particles to be applied to a mica surface for subsequent AFM analysis. An amplitude modulation (AM)-AFM technique was used, and analysis showed that equilibration of powders under conditions of increasing relative humidity (RH) causes changes in topography and increased surface roughness. In SMP, significant surface changes were observed due to the development of lactose crystallisation and eventual stacking of crystal layers with increased moisture sorption. MPC, however, showed characteristic ‘dimples and folds', which may have been due to shrinkage and compaction of surfaces. With higher moisture content, the number of surface-folds and height-ranges increased, with MPC powders, held at 85% RH appearing highly jagged. The surfaces of WPI powders were smooth but were characterised by the presence of broken powder fragments. Such fragments were absent in SMP and MPC powders, suggesting that WPI powders were the most friable. WPI powders appeared not to change as a function of moisture sorption. AM-AFM was used to provide high-resolution, three-dimensional images of HPMP particles at nano- and micrometre length scales.