Heat stability of milk: interrelationship between assay temperature, pH and agitation

As determined by the standard subjective assay procedure, the minimum in the heat stability--pH curve of milk persisted down to at least 116 degrees C. However when samples were not agitated during heating the minimum became progressively less pronounced as the assay temperature was lowered and it disappeared at approximately 116 degrees C. Activation energies (Ea) for unagitated samples were approximately 30 Kcal/mole at pH 6.87 (maximum) and at pH 7.28, throughout the temperature range 116--145 degrees C and for the pH 6.95 (minimum) sample at 116--125 degrees C; however Ea for the pH 6.95 sample increased to approximately 100 Kcal/mole in the range 127--135 degrees C suggesting that some highly temperature-dependent reaction had occurred and caused premature coagulation at certain pH values, i.e. to a heat stability minimum. The stability of concentrated milk (20% total solids) was very low at pH values above 6.9, regardless of whether the samples were agitated or not during heating and the maximum/minimum in the heat stability--pH curve persisted down to at least 90 degrees C in both agitated and quiescent samples.