Colloidal properties of protein complexes formed in β-casein concentrate solutions as influenced by heating and cooling in the presence of different solutes.

Abstract

Monomeric bovine β-casein self-associates into micelles under appropriate conditions of protein concentration, serum composition and temperature. The present study investigated self-association characteristics of a β-casein concentrate (BCC) prepared from milk at pilot-scale using membrane filtration. The BCC had a casein:whey protein ratio of 77:23, with ∼95% of casein consisting of β-casein, and the remainder being mostly κ-CN. BCC was reconstituted to 1.2% protein (a typical level in infant formula) in various liquid media at pH 6.8 and incubated at different temperatures from 4 to 63 °C for 30 min. Self-association of β-casein on heating was thermo-reversible in deionised water, lactose (4, 6 or 8%) or calcium (9 mM) solutions. In most serum phases, BCC became highly opaque after incubation at 63 °C, but clarified rapidly during cooling to 25 °C. However, in simulated milk ultrafiltrate (SMUF), which has a high ionic strength and is supersaturated in calcium phosphate (CaP), BCC remained opaque during cooling to 25 °C, and retained residual turbidity after 15 h of holding at 4 °C; if SMUF was prepared without phosphate then turbidity development in BCC solutions was markedly reduced. The complexes responsible for this turbidity development were successfully dissociated with 50 mM trisodium citrate. Analysis of pH during heating and holding at 60 °C indicated that SMUF acidified continuously under the period of study, while acidification in BCC/SMUF mixtures terminated after a short period, indicating that the type of CaP formed on heating is altered in the presence of BCC. This study demonstrates that BCC ingredients exhibit pronounced temperature-dependant changes in colloidal properties that are strongly affected by the presence of minerals commonly found in nutritional product formulations.