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Journal of Dairy Science Vol. 59 No. 5 823-827
© 1976 by American Dairy Science Association ®
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Effect of Psychrotrophic Bacteria from Raw Milk on Milk Proteins and Stability of Milk Proteins to Ultrahigh Temperature Treatment1

D. M. Adams, J. T. Barach and M. L. Speck

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

ABSTRACT

The effects of psychrotroph growth in raw milk on proteins of milk and on the response of milk proteins to heat treatments with ultrahigh temperature were studied. Ten gram-negative psychrotrophs isolated from raw milk readily attacked raw milk proteins. kcy- and ß-casein were most susceptible although some of the isolates also attacked the whey proteins. Detectable proteolysis did not require large psychrotroph populations. A 10 to 20% decrease in kcy-casein during 2 days at 5 C accompanied growth of one isolate to a population of only 10,000/ml. Growth of psychrotrophs in raw milk predisposed the proteins to deleterious effects of ultrahigh temperature treatments. Ultrahigh temperature treatment by direct steam injection had little effect on raw milk caseins and decreased {alpha}-lactalbumin and ß-lactoglobulin by 21% and 34%, respectively. Milk that had undergone proteolysis exhibited decreased detectable kcy-, ß-, and {alpha}s-caseins and increased loss of ß-lactoglobulin as a result of ultrahigh temperature treatment. Milk suffering extensive kcy-casein degradation coagulated during ultrahigh temperature treatment. Coagulation during or shortly after heating increased with severity of heat treatment and size of psychrotroph population.

FOOTNOTES

1 Paper Number 4746 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh.






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Copyright © 1976 by the American Dairy Science Association ®.