Characterization of calcium phosphate nanoparticles sequestered by phosphopeptides in response to heat treatment

The presence of phosphorylated proteins or peptides prohibits a transition of amorphous calcium phosphate (ACP) into a more crystalline phase by forming calcium phosphate nanoparticles (CPN) complexes, which enables the co-existence of soft and hard materials in a biofluid (e.g. milk), also is relevant to the stability and biological function of the colloidal system. In this study, the chemical composition, structure and stability of CPN complexes were investigated in response to different temperature. The CPN complexes sequestered by phosphopeptides were amorphous nano-spheres and had a similar salt composition with that of native micellar calcium phosphate (MCP) in raw milk but with a lower solubility. Moderate heating did not change the amorphous structure, merely caused an aggregation between these complexes. After heating above 90 °C, the phosphopeptides could no longer sequester all calcium phosphate salts transferred from the ultrafiltrate forming nanoparticles. On the contrary, a transformation of amorphous CPN into apatite crystalline was observed, which reveals the instability mechanism of calcium phosphate associated with phosphopeptides induced by high-heating.