Effect of acidification and neutralization of milk on some physico-chemical properties of casein micelles

Abstract

Abstract 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 ~5.1; this buffering peak is caused by the solubilization of colloidal calcium phosphate (CCP). The reduced buffering in reformed milk suggests that little reformation of CCP occurs on neutralization; reformed milks also had an elevated Ca 2+ activity. Acidification of milk to pH > 5.5, followed by neutralization to pH 6.6, hardly reduced buffering (at pH ~5.1), suggesting that either little CCP dissolved on acidification or that reformation of CCP occurred on neutralization. Acidification of milk to low pH values and neutralization resulted in improved renneting properties and a reduction in rennet coagulation time (RCT). Dialysis of reformed milk resulted in a reduction in its renneting properties which became inferior to those of control milk, possibly due to its reduced CCP content or to structural changes in the micelles caused by removal of CCP. Addition of low concentrations of CaCl 2 to milk, at a constant pH (6.6), improved its renneting properties. Electron micrographs of milk acidified to pH values ⩽5.5 prior to neutralization showed increased clustering of casein particles, presumably caused by the reduction in electrostatic repulsion between casein particles during acidification to low pH values. The original micellar appearance was not restored on neutralization or dialysis of reformed milk. It is concluded that the micellar system is not readily reversible; once disintegrated by acidification, micelles do not reform on neutralization.