Roughness analysis of single nanoparticles applied to atomic force microscopy images of hydrated casein micelles

The topography of fully hydrated casein micelles was measured by Atomic Force Microscopy and quantitatively investigated by a custom-made roughness analysis procedure. Casein micelles in ultra filtrated permeate (UFP) were chosen as a typical example of food nanoparticles in order to validate the approach. Casein micelles were first covalently immobilized on solid substrates by carbodiimide chemistry and subsequently a number of individual particles were scanned at high resolution. Parameters for characterizing single particle surface roughness were defined and implemented in MATLAB. An essential feature of the analysis is the discrimination between overall particle shape and short-scale surface roughness. Parameters determined were: Root-mean-square roughness, ratio of area to projected area, particle compression and particle size. The method is generally applicable to particles with radii from ∼15 nm–1 μm and will be useful for monitoring changes in hydrocolloid surface morphology in relation to environmental changes or industrial processing.