71A-48 |
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N. M. PETERS and R. F. Roberts. Dept. of Food Science, Pennsylvania State Univ., 121 Borland Lab, University Park, PA 16802-2504 The potential of extending milk process runs to longer than 24 hr has prompted renewed interest in staphylococcus. Staphylococcus aureus has been associated with mastitis in dairy cows and can produce a heat-stable toxin that may pose a health risk during extended processing runs. Characterization of S. aureus strains isolated from mastitic cows will provide information to determine the significance of this risk. The objective of the present study was to characterize growth kinetics of S. aureus strains isolated from mastitic cows and to determine the toxin production profile of these strains. Sixteen S. aureus strains previously isolated from mastitic cows were tested for thermonuclease, SEA, SEB, SEC, SED, and SEE genes by PCR. Growth kinetic data for three representative strains, one that occurred frequently in mastitic cows, one that occurred infrequently in mastitic cows, and an ATCC strain (29740), were obtained in milk at temperatures of 21, 26, 31, 37, 40, 45, and 50 ºC. Growth data were analyzed using Arrhenius kinetics to determine activation energy, growth rate, and generation time. The maximum population of the three strains occurred after 10 to 12 hours in the optimum growth temperature range of 37 to 40 ºC. Growth rates were proportional to temperature from 21 to 40 ºC, and declined above 45 ºC. No growth was observed at 50 ºC. Activation energies for all strains ranged from 15.19 to 17.67 kcal/mol and were not significantly different (P < 0.05). All sixteen strains were positive for the thermonuclease gene, which has a high correlation with toxin production. The known toxin producer was positive for sec and another low prevalence strain was positive for SEC and SEA genes. Results from this study indicate S. aureus strains from mastitic cows grow rapidly in milk, and there is potential for toxin production in milk held at growth temperatures for extended periods of time.
Session 71A, Dairy Foods: General
2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana |