High pressure effects on the structure of casein micelles in milk as studied by cryo-transmission electron microscopy

Milk was processed with high hydrostatic pressure in order to modify the casein micelles. Images, that in details showed the casein micelle structure in untreated and pressure-treated skim milk, were obtained by using cryo-transmission electron microscopy (cryo-TEM). Sizes and shapes adopted by casein micelles in pressurised milk are concluded to be a result of an equilibrium distribution between self-assembling casein molecules in the serum phase and caseins adsorbed to surfaces of casein micelles and are governed by an initial pressure-dependent displacement of caseins into the serum phase. Pressurisation of milk at moderately high pressure, in the range 150–300 MPa, favoured formation of a large number of small micelles that coexisted with a fraction of large micelles, and which appeared perfectly spherical with smooth and well-defined surfaces, features which are suggested to originate from secondary adsorption of caseins. Pressurisation of milk at 400 MPa favoured formation of smaller casein assemblies, with sizes between 30 nm and 100 nm. Measurements of free calcium concentration [Ca2+] showed that calcium was rebound to casein micelles after pressurisation of milk. Furthermore, the electron microscopy images indicated that the substructures were similar for pressure-modified casein micelles and casein micelles in untreated milk.