Nanorheological Studies of Caseins

Abstract: To be able to improve the quality of dairy products and dairy processes, knowledge of casein molecules, casein micelles and casein aggregates is important. The rheological properties of casein products, casein gels and adsorbed layers of casein, have been widely investigated. However, the micellar properties of casein could, until recently, only be studied by ensemble methods such as dynamic light scattering and microscopic techniques, using dried or frozen samples. Nano-techniques, such as atomic force microscopy (AFM) have now made it possible to make complementary nanorheological measurements in a liquid environment. In this thesis, the observed nanorheological properties of single casein micelles, adsorbed thin casein layers and rennet-induced casein-retentate gels, are presented and discussed. The ultimate objective of these studies was to investigate the relationship between the local and the macroscopic rheological properties of cheese. The renneted casein gels were produced from casein retentate from ultrafiltration (UF), as another objective of the work was to increase our knowledge on cheese production from concentrated milk. Natural rennet-induced cheese is essentially a particulate calcium phosphate-para-casein matrix, composed of interconnected and overlapping strands of partially fused para-casein aggregates, which are in turn made up of para-casein micelles. To increase our knowledge on the nanorheological behaviour of the building blocks of the casein matrix, the first step was to investigate single adsorbed casein micelles. In the AFM measurements, the nanorheological properties of adsorbed casein micelles could not be explained by any model describing an elastic material (Paper I). Preliminary results led to investigations of the possibility that surface tension phenomena could explain the rheological behaviour. Results from computer modelling confirmed that essential features of such model corresponded to the relation between force and indentation from AFM measurements. A surface tension of 10 mNm-1 was calculated for a casein micelle attached to a hydrophobic graphite surfaces in a Ca-imidazole buffer environment (Paper III).