Heat-Induced Protein Changes in Milk Processed by Vat and Continuous Heating Systems

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Abstract
This study investigated and compared the effects of heating system and residence times on the physicochemical properties and interactions of casein and whey proteins in heated milk. Milk was processed by vat heating (85.degree. C for 10 to 40 min), HTST heating (98.degree. C for .5 to 1.87 min), UHT heating (140.degree. C for 2 to 8 s), cooled, and fractionated into casein and whey by isoelectric precipitation. .beta.-Lactoglobulin A and B variants were partially denatured by HTST and UHT heating and totally denatured by vat heating. Increasing residence time caused significant (P < .01) increases in denaturation of both .beta.-lactoglobulin variants in UHT and HTST heating systems and of .alpha.-lactalbumin the vat heating system. Surface hydrophobicity and sulfhydryl content were negatively correlated with whey protein denaturation. Sodium dodecyl sulfate electrophoresis of the casein fraction of heated milk indicated the presence of a high molecular weight component that would not enter the gel. Addition of 2-mercaptoethanol to heated casein samples dissociated this component, which the concurrent appearance .beta.-lactoglobulin and .alpha.-lactalbumin bands. In HTST and UHT heating systems, ratio of .beta.-lactoglobulin to .kappa.-casein increased linearly from the complex with increasing residence time.