The effects of ionic strength, pH and milk mineral composition on the adsorption of caseins and whey proteins on to particles of hydroxyapatite (HA) were studied by determining the amounts of adsorbed proteins on HA using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and by zeta-potential measurements. The amount of adsorbed proteins increased with increasing ionic strength from 0 to 0.1 M and decreasing pH from 8 to 6, for both sodium caseinate (SC) and whey protein isolate (WPI). In general, when the absolute value of the zeta-potential decreased because of changes in pH or ionic strength, more protein was found to adsorb on to the HA particles. This may be attributed to a decrease in the electrostatic repulsions between the HA particles and the protein species. The effects of the composition of the milk serum were investigated using simulated milk ultrafiltrate (SMUF). Both caseins and whey proteins adsorbed less on to HA particles in SMUF than on to HA particles in water. However, the effect of the composition of SMUF on the adsorption was less pronounced for caseins than for whey proteins. This may be related to the phosphorylated nature of the caseins. As the phosphate groups of the caseins bind more strongly to the HA binding sites than the carboxyl groups of the whey proteins, caseins might compete better with the phosphate and citrate ions in the milk serum for adsorption.
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The authors gratefully thank the Fonterra Research and Development Centre for financial support. Ran Gao and Graeme Gillies are thanked for their helpful discussions and Claire Woodhall for proof-reading the manuscript.