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Effect of Ultra-High-Temperature Steam Injection Upon Constituents of Skim Milk¹

Department of Food Science, North Carolina State University, Raleigh 27607

ABSTRACT

Raw skim milk was heated to 143 ± 1 C by direct steam injection, held for 8 to 10 s, and vacuum cooled. Analyses of different chemical characteristics of ultra-high-temperature processed skim milk showed a decrease in pH, titratable acidity, and ionized calcium as compared to the control; sialic acid and active sulfhydryls increased. Increases in sialic acid and sulfhydryls might be due to unfolding of protein moieties thus exposing: 1) glycoproteins containing sialic acid and 2) buried sulfhydryls and/or reduction of disulfide bridges in milk proteins. Alteration of the native protein was determined by two techniques: 1) isolation procedures for specific proteins and 2) electro-phoretic and densitometric techniques. Casein micelles were most resistant to alteration of their structure while whey proteins, especially ß-lactoglobulins, were very susceptible to structural changes. These structural changes may be related to sedimentation and flavor problems associated with ultra-high-temperature milks.

¹ Paper No. 5144 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh. The use of tradenames in this publication does not imply endorsement by the North Carolina Agricultural Experiment Station nor criticism of similar ones not mentioned.