ACID-BASE BUFFERING PROPERTIES OF HEATED MILK

Unheated milk has a buffering peak at pH 5 during acid titration of mi lk and during the back titration of alkalized milk due to solubilizati on of CCP. These buffering peaks increase with an increasing severity of heat treatment due to precipitation of calcium phosphate. Heating a t very high temperatures, i.e. 120-degrees-C for 10 min, resulted in a shift in the pH of buffering maximum from pH 5 to 4.4, probably due t o the formation of hydroxyapatite. Little solubilization of indigenous CCP or heat-precipitated calcium phosphate occurred during cold stora ge and solubilization of calcium phosphate is probably not responsible for the phenomenon of rennet hysteresis. The buffering properties of unheated or heated (100-degrees-C for 10 min) milks which were acidifi ed to pH 4.6 and reneutralized were similar with reduced buffering at pH 5, probably because indigenous CCP and heat-precipitated calcium ph osphate were solubilized on acidification and were not reformed on neu tralization. Acidification to pH 4.6 did not solubilize all the heat-p recipitated calcium phosphate in severely heated milk, 120-degrees-C f or 10 min, as a buffering peak occurred at pH 4.4. The buffering curve s for heated (100-degrees-C for 10 min) milk which was acidified to pH 5.6 and reneutralized was similar to unheated milk.