Phase separation and composition of coacervates of lactoferrin and caseins

Abstract

The bovine milk casein proteins (αs-casein (ACN), β-casein (BCN) and κ-casein (KCN)) form complexes with lactoferrin (LF), an anionic protein also present in bovine milk that has an important bacteriostatic function especially in infant food. LF and each casein form complexes at near neutral pH, which is in between the respective pI's (≈8.3 and ≈ 4.6). The stoichiometry of the complexes is determined by the net charge of the proteins. Optimum complexation occurs at charge neutrality and is characterized by a maximum in turbidity. Mixing LF with each casein at a ratio where charge neutrality is obtained leads to a new complex coacervate phase. The kinetics of complex formation for LF/BCN and LF/KCN is rapid and appears to occur through a nucleation and coalescence process. However, the kinetics of complex formation between LF and ACN is much slower and therefore a nucleation and growth process is proposed, based on model calculations of the turbidity. The composition of the complexes is the same as the experimental mixing ratio if mixing ratio leads to a neutral complex. On standing and light centrifugation a complex-coacervate phase is formed, which is a viscous liquid.