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

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Abstract
Raw skim milk was heated to 143 .+-. 1.degree. C by direct steam injection, held for 8-10 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 Ca 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 glycoproteins containing sialic acid and buried sulfhydryls and/or reduction of disulfide bridges in milk proteins. Alteration of the native protein was determined by isolation procedures for specific proteins and electrophoretic and densitometric techniques. Casein micelles were most resistant to alteration of their structure while whey proteins, especially .beta.-lactoglobulins, were very susceptible to structural changes. These structural changes may be related to sedimentation and flavor problems associated with ultra-high-temperature milks.