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Serum Protein and Casein Concentration: Effect on pH and Freezing Point of Milk with Added CO₂¹

Northeast Dairy Foods Research Center, Department of Food Science, Cornell University, Ithaca, NY 14853

Corresponding author: D. M. Barbano; e-mail:
dmb37{at}cornell.edu.

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°C) and freezing point (FP) of skim milk upon CO₂ injection at 0°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°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°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°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).

Key Words: carbon dioxide • casein • freezing point • pH • serum protein

Abbreviation key: CCP = colloidal calcium phosphate, CN2.4, CN4.8 and CN7.2 = skim milks with, respectively, 2.4, 4.8, and 7.2% casein in the fat-free portion and 0.6% serum protein in the fat-free, casein-free portion, FP = freezing point, MF = microfiltration, NCN = noncasein nitrogen, NPN = nonprotein nitrogen, SP = serum protein, SP0, SP3, and SP6 = skim milks with, respectively, 0, 3, and 6% serum protein in the fat-free portion and 0% casein, TN = total nitrogen, TP = true protein

This article has been cited by other articles:

				Y. Ma and D. M. Barbano Milk pH as a Function of CO2 Concentration, Temperature, and Pressure in a Heat Exchanger J Dairy Sci, December 1, 2003; 86(12): 3822 - 3830. [Abstract] [Full Text] [PDF]