Effect of Milk Concentration on Heat-Induced, pH-Dependent Dissociation of Casein from Micelles in Reconstituted Skim Milk at Temperatures between 20 and 120 °C

The effect of pH, milk concentration, heating temperature, and heating time on the dissociation of casein from the micelles in reconstituted skim milk was investigated. For the 10% total solids milk samples, low levels of casein were rendered soluble at pH below 6.7 regardless of heating temperature, whereas increasing levels of casein were solubilized as the pH was increased from 6.7 to 7.1. The quantity of soluble casein increased with temperature to a maximum dissociation at about 60−80 °C, decreased as the temperature was raised to 100 °C, and then increased as the temperature was raised further. The dissociation behavior of α s -casein and β-casein showed similar dependence on temperature to that of the total casein. In contrast, the dissociation of κ-casein increased with increasing temperature over the entire temperature range studied. Most casein dissociation occurred during the first minutes of heating, with little further change on prolonged heating. As the milk was concentrated, a higher level of dissociation of casein, particularly κ-casein, was observed at lower pH and at higher temperatures, particularly above 80 °C.