Properties of milk protein gels formed by phosphates

We investigated the properties of gels that were formed by adding emulsifying salts, such as tetrasodium pyrophosphate (TSPP), to reconstituted milk protein concentrate solution. The pH of a 51 g/L milk protein concentrate solution was adjusted to 5.8 after adding TSPP. Milk protein concentrate solutions were placed in glass jars and allowed to stand at 25 degrees C for 24 h. Gels with the highest breaking force were formed when TSPP was added at a concentration of 6.7 mM, whereas no gel was formed when TSPP was added at concentrations of < or =2.9 or > or =10.5 mM. Several other phosphate-based emulsifying salts were tested but for these emulsifying salts, gelation only occurred after several days or at greater gelation temperatures. No gelation was observed for trisodium citrate. Gelation induced by TSPP was dependent on pH, and the breaking force of gel was greatest at pH 6.0. Furthermore, when the concentration of milk protein concentrate in solution was increased to 103 g/L, the breaking force of the gel increased, and a clearly defined network between caseins could be observed by using confocal scanning laser microscopy. These results suggest that TSPP-induced gelation occurs when the added TSPP acts with calcium as a cross-linking agent between dispersed caseins and when the balance between (a reduced) electrostatic repulsion and (enhanced) attractive (hydrophobic) interactions becomes suitable for aggregation and eventual gelation of casein molecules.