Thermal isoelectric precipitation of α‐lactalbumin from a whey protein concentrate: Influence of protein–calcium complexation

The selective precipitation of α‐lactalbumin (α‐LA) at a pH around its isoelectric point (4.2) under heat treatment is the basis for a fractionation process of whey proteins. As precipitation is a phenomenon dependent on the protein hydrophobicity, and as the release of the tightly bound calcium occurring at pH around 4 modifies the α‐LA hydrophobicity, the specific role of calcium on isoelectric precipitation is investigated. A study of the extent of α‐LA precipitation in a whey protein concentrate under various operating conditions of pH, temperature, protein concentration, and calcium content is presented. We propose a mechanism for this phenomenon as a combination of a complexation equilibrium and of an irreversible precipitation, to account for the influence of temperature, α‐LA concentration total ionic content, and calcium concentration, and also to estimate the complexation equilibrium constant. © 1995 John Wiley & Sons, Inc.