EFFECT OF ACIDIFICATION AND NEUTRALIZATION OF MILK ON SOME PHYSICOCHEMICAL PROPERTIES OF CASEIN MICELLES

Acidification of milk, at low temperatures, to pH 5.0 or 4.6, followed by neutralization to pH 6.6 (reformed milk), resulted in a reduction in the buffering maximum of milk at pH similar to 5.1; this buffering peak is caused by the solubilization of colloidal calcium phosphate (C CP). The reduced buffering in reformed milk suggests that little refor mation of CCP occurs on neutralization; reformed milks also had an ele vated Ca2+ + activity. Acidification ofmilk to pH > 5.5 followed by ne utralization to pH 6.6, hardly reduced buffering (at pH similar to 5.1 ), suggesting that either little CCP dissolved on acidification or tha t reformation of CCP occurred on neutralization. Acidification of milk to low pH values and neutralization resulted in improved renneting pr operties and a reduction in rennet coagulation time (RCT). Dialysis of reformed milk resulted in a reduction in its renneting properties whi ch became inferior to those of control milk, possibly due to its reduc ed CCP content or to structural changes in the micelles caused by remo val of CCP. Addition of low concentrations of CaCl2 to milk, at a cons tant pH (6.6), improved its renneting properties. Electron micrographs of milk acidified to pH values less than or equal to 5.5 prior to neu tralization showed increased clustering of casein particles, presumabl y caused by the reduction in electrostatic repulsion between casein pa rticles during acidification to low pH values. The original micellar a ppearance was not restored on neutralization or dialysis of reformed m ilk. It is concluded that the micellar system is not readily reversibl e; once disintegrated by acidification, micelles do not reform on neut ralization.