E f f e c t of P s y c h r o t r o p h i c Bacteria f r o m R a w M i l k on M i l k Proteins and S t a b i l i t y o f M i l k Proteins to Ultrahigh Temperature Treatment 1 D. M. ADAMS, J. T. BARACH, and M. L. SPECK Department of Food Science North Carolina State University Raleigh 27607 ABSTRACT

Control of bacterial growth during this storage period depends primarily on refrigeration. However, psychrotrophic bacteria, which are in most raw milk supplies (4), can grow readily at refrigeration temperatures. These organisms produce proteolytic enzymes which can attack milk proteins. Several Pseudomonas species have degraded casein and whey proteins during refrigerated storage (8). With longer periods of refrigerated storage before processing, psychrotrophs may be even more important than in the past to the quality of fluid milk and other dairy products. The proteolytic action of psychrotrophs in raw milk may be particularly important to the sterilization of milk at ultrahigh temperatures (UHT). Although micellar casein is not strictly denaturable by heat, sterile milk occasionally undergoes gelation or coagulation (7). Mild proteolysis can enhance greatly coagulation of casein by heat (10). These phenomena are intensified in milk heated at higher temperatures. Our objectives were to determine the effects of psychrotroph growth in raw milk at refrigeration temperatures on raw milk proteins and the stability of these proteins to UHT treatments.

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, to- and /3casein 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 K-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 ~-lactalbumin and ~3-1actoglobulin by 21% and 34%, respectively. Milk that had undergone proteolysis exhibited decreased detectable K-, /3-, and as-Caseins and increased loss of /3-1actoglobulin as a result of ultrahigh temperature treatment. Milk suffering extensive K-casein degradation coagulated during ultrahigh temperature treatment. Coagulation during or shortly after heating increased with severity of heat treatment and size of psychrotroph population.

MATERIALS AND METHODS Test Cultures

INTRODUCTION

Milk processing operations are becoming increasingly centralized. This means that raw milk must be held longer before processing. Received August 21, 1975. i Paper Number 4746 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh.

Psychrotroph cultures isolated from raw milk (1) were identified as Pseudomonas spp. and designated as MC7, MC32, MC35, MC43, MC44, MC50, MC56, MC60, MC62, and MC64. The cultures were maintained in sterile reconstituted nonfat dry milk (NFDM) at 2 C. Transfers of a 1% inoculation were every 4 wk. Growth of Psychrotrophs in Milk

Test milk was drawn aseptically from cows of the University herd. The milk was cooled 823

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rapidly, stored overnight at 4 C to allow cream separation and the cream aspirated off. A .1 ml portion of the stock culture was inoculated into 10 ml of sterile 10% NFDM and incubated for 24 h at room temperature (ca. 23 C). This culture was inoculated into the precooled raw skim milk samples to give initial populations of about 106/ml or 103/ml. The standard plate count (5) of the uninoculated milk was always less than 100/ml. Inoculated and uninoeulated milk in 4 liter or 200 ml quantities was held in 6 liter or 500 ml Erlenmeyer flasks, respectively, in a refrigerated water bath or a forced air refrigerated incubator. The cultures were incubated statically but were shaken vigorously by hand once

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determination of psychrotroph count and protein composition. The remainder was UHT treated and protein composition determined.

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Milk samples were heated by direct steam injection or by the capillary tube method as described (1, 3).

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Effect of psychrotrophic bacteria from raw milk on milk proteins and stability of milk proteins to ultrahigh temperature treatment.

E f f e c t of P s y c h r o t r o p h i c Bacteria f r o m R a w M i l k on M i l k Proteins and S t a b i l i t y o f M i l k Proteins to Ultrahigh...
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