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Milk Ultracentrifugal Opalescent Layer. I. Composition as Influenced by Heat, Temperature, and pH¹

Department of Food Science and Nutrition, University of Minnesota, St. Paul 55101

ABSTRACT

The effect of heating skimmilk to 90 C and adjusting it to temperatures of 0 to 5 and 35 to 40 C and to pH 5.0 and 7.5 upon amount and composition of the three major skimmilk ultracentrifugal fractions, i.e., pellet, supernatant, and opalescent liquid layer located above tne pellet, was determined. Recentrifugation of the crude opalescent layer produced an opalescent layer supernatant and pellet.

The composition of opalescent layer supernatant and pellet fractions was generally intermediate between that of skimmilk supernatant and pellet fractions but contained higher concentrations of certain constituents than either of these two skimmilk fractions. Opalescent layer pellet was more highly solvated (3.65 to 4.03 g H₂O/g dry pellet) than skimmilk pellet (2.85 g H₂O/g dry pellet) at 0 to 5 C. Heating skimmilk caused a marked increase in the calcium/phosphorus ratio and in the disulfide group and sialic acid content of opalescent layer pellet and altered the opalescent layer supernatant composition but, otherwise, had little effect on composition of other ultracentrifugal fractions. Sialic acid and disulfide data indicated that heat induced ß-lactoglobulin--casein complex is concentrated in the opalescent layer fraction. The translucent portion of skimmilk pellet was more highly solvated and contained lower mineral and protein content than the opaque portion of the pellet. Temperature adjustments of milk to 0 to 5 and 35 to 40 C altered skimmilk pellet solvation and casein and mineral dissociation from casein micelles. Adjusting skimmilk pH to 5.0 dissociated substantial amounts of casein from the micelles, thus increasing the casein content of opalescent layer and skimmilk supernatant fractions. Skimmilk pellet fractions 0 to 5 C from pH 6.6 skimmilk had the lowest solvation, i.e., 2.85 g H₂O/g dry pellet compared to 3.5 and 3.8 g H₂O/g dry pellet for pH 7.5 and 5.0. The effect of pH adjustment to 5.0 upon calcium and inorganic phosphorus dissociation from casein micelles at 0 to 5 C also was determined.

¹ Scientific Journal Series Paper 8114, Minnesota Agricultural Experiment Station, St. Paul.