The effect of chymosin on κ-casein-coated polystyrene latex particles and bovine casein micelles

Bovine kappa-casein was adsorbed onto negatively charged polystyrene latex particles of various sizes and the changes in the hydrodynamic diameters of the particles were monitored by photon correlation spectroscopy. The latex particle diameters were increased on addition of small amounts of kappa-casein, and plateaued once a maximum adsorption was attained. The size increase was about 25 nm regardless of the initial size of the latex particle, indicating that the surface arrangement of kappa-casein was similar on all latex sizes. Treatment of the casein micelles in skim milk and the kappa-casein-coated latex particles with chymosin initially decreased the particle diameter by about 10 nm after which the particle diameter rapidly increased as the aggregation reaction proceeded. The action of chymosin on both the casein micelles and the kappa-casein-coated latex particles was retarded by increasing the NaCl concentration. The similarity between the kappa-casein-coated latex particles and the casein micelles in milk, especially their behaviour towards chymosin, indicates that these latex particles may be a simple model system for studying some casein micelle properties under controlled conditions. (C) 1998 Elsevier Science Ltd. All rights reserved.