Degradation and assembly of β-casein micelles during proteolysis by trypsin

Abstract Proteolytic action of trypsin on β-casein molecules includes both degradation and assembly of β-casein micelles. These parallel processes were characterised by atomic force microscopy (AFM) and fluorescence spectroscopy (FS). Micelles were dried in air on the surface of mica and their diameters and heights were measured by AFM for sets of several tens or hundreds of micelles. Proteolytically modified small micelles in the diameter range of 8 to 50 nm had a narrow diameter distribution compared with original ones. In the ranges of medium (50–100 nm) and large (>100 nm) micelles, the assembly gave an increase in the ratio of height to diameter for modified micelles. A delay in red shift of tryptophan fluorescence was found at the beginning of proteolysis, indicating the contribution of assembly process. Degradation was specific for small micelles while, for medium and large ones, a dominance of the assembly process was apparent.