Study of milk/κ-carrageenan mixtures by atomic force microscopy

Structural characteristics of milk components and milk/κ-carrageenan mixtures were studied by atomic force microscopy (AFM). Samples were adsorbed on highly ordered pyrolytic graphite (HOPG) and mica substrates. Sample attachment by physisorption and imaging in air by AFM resulted in a very promising technique to investigate structures and interactions between biological macromolecules under “near-native” conditions. Differences in adsorption of casein micelles on HOPG and mica surfaces were observed. Casein micelles were adsorbed satisfactorily on HOPG while poor adsorption was obtained on mica surface. Topographical images of casein micelles adsorbed on HOPG denoted pseudo-spherical structures forming aggregates which were polydisperse in size and shape. In the case of milk/κ-carrageenan mixtures, it was observed that as κ-carrageenan concentration increased, the amount of casein micelles attached on HOPG surface decreased. An opposite behavior was observed when samples were adsorbed on mica. Hydrophobic and hydrophilic interactions between samples and substrates helped in improving the understanding of the interactions between the macromolecules studied. Our findings agreed with previous studies suggesting a surface level interaction between casein micelles and κ-carrageenan, the structures observed being dependent on polysaccharide concentration. It can also be concluded that mica may be a suitable substrate for imaging milk/κ-carrageenan mixtures.