Protein Composition of Different Sized Casein Micelles in Milk after the Binding of Lactoferrin or Lysozyme

Casein micelles with bound lactoferrin or lysozyme were fractionated into sizes ranging in radius from similar to 50 to 100 run. The kappa-casein content decreased markedly and the alpha(s)-casein/beta-casein content increased slightly as micelle size increased. For lactoferrin, higher levels were bound to smaller micelles. The lactoferrin/kappa-casein ratio was constant for all micelle sizes, whereas the lactoferrin/alpha(s)-casein and lactoferrin/beta-casein ratio decreased with increasing micelle size. This indicates that the lactoferrin was binding to the surface of the casein micelles. For lysozyme, higher levels bound to larger casein micelles. The lysozyme/alpha(s)-casein and lysozyme/beta-casein ratios were nearly constant, whereas the lysozyme/kappa-casein ratio increased with increasing micelle size, indicating that lysozyme bound to alpha(s)-casein and beta-casein in the micelle core. Lactoferrin is a large protein that cannot enter the casein protein mesh; therefore, it binds to the micelle surface. The smaller lysozyme can enter the protein mesh and therefore binds to the more charged alpha(s)-casein and beta-casein.