Milk protein concentrate (MPC) powders, ranging from 35 (MPC35) to 87 (MPC90)% protein, were reconstituted to 8.5% protein and assessed for heat stability at 120 degrees C, Ca-ion activity, heat-induced dissociation of kappa-casein, and heat-induced gelation of serum-phase proteins in ultracentrifugal supernatants of unheated MPC suspensions. Heat stability of MPC suspensions depended on the protein content of the powder from which the suspensions were prepared. MPC70 had excellent heat stability compared with MPC35; however, MPC80, MPC85 and MPC90 were highly unstable to heating. Ca-ion activity increased with increasing protein content of the MPCs, whereas the extent of heat-induced dissociation of kappa-casein and gelation of serum-phase proteins decreased. Increased heat stability with increasing protein content from MPC35 to MPC70 was attributed to decreased kappa-casein dissociation and reduced gelation of serum-phase proteins. Despite these stabilising factors, excessively high Ca-ion activity caused MPC80, MPC85 and MPC90 to have very poor heat stability at pH 6.3-6.8, 6.3-7.1 and 6.3-7.3, respectively. (C) 2015 Elsevier Ltd. All rights reserved.