At pH 6.2-6.4, about half of the lactoferrin of bovine skim milk was retained in the colloidal (micellar) phase. Decreasing the pH to 5.4 or increasing the pH to 7.8 increased the fraction of lactoferrin bound to the casein micelles, as did cooling skim milk from 35 degrees C to 5 degrees C. In contrast, salt addition to milk released lactoferrin into the serum phase. At natural pH, CaCl2 and NaCl had similar effects on partitioning of lactoferrin, as long as total ionic strength in milk was conserved. However, in skim milk at pH 5.6 supplemented with NaCl, the fraction of lactoferrin retained in the colloidal phase increased upon addition of CaCl2. These results supported the hypothesis that lactoferrin is bound to casein micelles mainly through electrostatic interactions, but that some binding sites are probably located in the interior of casein micelles that are more or less accessible, depending on physico-chemical conditions.
