Effect of Psychrotrophic Bacteria from Raw Milk on Milk Proteins and Stability of Milk Proteins to Ultrahigh Temperature Treatment

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Abstract
The effects of psychrotroph [Pseudomonas spp.] 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. .kappa.- and .beta.-casein were most susceptible although some of the isolates also attacked the whey proteins. Detectable proteolysis did not require large psychrotroph populations. A 10-20% decrease in .kappa.-casein during 2 days at 5.degree. 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 .beta.-lactoglobulin by 21% and 3, respectively. Milk that had undergone proteolysis exhibited decreased detectable .kappa.-, .beta.-, and .alpha.s-caseins and increased loss of .beta.-lactoglobulin as a result of ultrahigh temperature treatment. Milk suffering extensive .kappa.-casein degradation coagulated during ultrahigh temperature treatment. Coagulation during or shortly after heating increased with severity of heat treatment and size of psychrotroph population.