Protein interactions in milk protein concentrate powders

Solutions (5%, w/w, pH 6.9), made from six different pilot-plant milk protein concentrate (MPC) powders, were centrifuged (700g for 10min at 20 degrees C to isolate the insoluble material. The sediment (insoluble material) was characterized using one-dimensional (1D) and two-dimensional (2D) sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and transmission electron microscopy (TEM). The levels of solubility in MPC powders varied between 32% and 98%. The TEM results showed that the insoluble material consisted of large particles (similar to 100 mu m) where the casein micelles are fused together by some form of protein-protein interactions. The ID PAGE results showed that the amounts of insoluble material in MPC powders increased with storage time at elevated temperatures. This material consisted predominantly of alpha- and beta-caseins. This material was formed largely by weak non-covalent (hydrophobic) protein-protein interactions that were dissociable under SDS-PAGE conditions. The 2D PAGE results demonstrated that the disulphide-linked protein aggregates present in MPC powders consisted of kappa-casein, beta-lactoglobulin and some alpha(s2)-casein, and not all of this material was sedimented. These aggregates were not considered to play a major role in the formation of the insoluble material. (c) 2005 Elsevier Ltd. All rights reserved.