Serum protein and casein concentration: effect on pH and freezing point of milk with added CO2.

The objective of this study was to determine the effect of protein concentration and protein type [i.e., casein (CN) and serum protein (SP)] on pH (0 degree C) and freezing point (FP) of skim milk upon CO2 injection at 0 degree C. CN-free skim milks with increasing SP content (0, 3, and 6%) and skim milks with the same SP content (0.6%) but increasing CN content (2.4, 4.8, and 7.2%) were prepared using a combination of microfiltration and ultrafiltration processes. CO2 was injected into milks at 0 degree C using a continuous flow carbonation unit (230 ml/min). Increasing SP or CN increased milk buffering capacity and protein-bound mineral content. At the same CO2 concentration at 0 degree C, a milk with a higher SP or a higher CN concentration had more resistance to pH change and a greater extent of FP decrease. The buffering capacity provided by an increase of CN was contributed by both the CN itself and the colloidal salts solublized into the serum phase from CN upon carbonation. Skim milks with the same true protein content (3%), one with 2.4% CN plus 0.6% SP and one with 3% SP, were compared. At the same true protein content (3%), increasing the proportion of CN increased milk buffering capacity and protein-bound mineral content. Milk with a higher proportion of CN had more resistance to pH change and a greater extent of FP decrease at the same carbonation level at 0 degree C. Once CO2 was dissolved in the skim portion of a milk, the extent of pH reduction and FP depression depended on protein concentration and protein type (i.e., CN and SP).