Effect of pH on the viscosity of heated reconstituted skim milk

Reconstituted skim milk was adjusted to pH values between 6.5 and 6.7 and heated (90degreesC) for up to 30 min. The change in the viscosity, as well as the casein micelle size and the level of whey protein association with the micelles, was monitored for each milk sample. The change in the viscosity was dependent on the pH of the milk at heating. At pH 6.5, the viscosity increased markedly during the early stages of heating, and plateaued on prolonged heating. The magnitude of the change in viscosity decreased as the pH of the milk was increased, so that only small changes in viscosity were observed on heating at pH 6.7. The change in viscosity was linearly correlated with the change in particle volume (calculated from the particle size). This indicates that the viscosity changes were related to a change in the Volume fraction of the casein micelles as a consequence of heating. The level of denatured whey proteins associating with the micelles was also markedly dependent on the pH at heating. High levels associated at pH 6.5, and the level associated was lower as the pH was increased to pH 6.7. The changes in viscosity and casein micelle volume were correlated with the levels of denatured whey proteins that were associated with the casein micelles, although there was some deviation from linearity at low levels of association (< 15%). (C) 2003 Elsevier Ltd. All rights reserved.