An investigation of the thermal inactivation of Staphylococcus aureus and the potential for increased thermotolerance as a result of chilled storage

AIMS: The aims of this study were; (i) to provide thermal inactivation data for Staphylococcus aureus; (ii) to examine the kinetics, including decimal reduction times (D-value) and rate constants (k), that describe the thermal inactivation of Staph. aureus and to compare two different methods of calculating D-values and (iii) to determine whether or not chilled storage would toughen these microorganisms resulting in increased thermotolerance. METHODS AND RESULTS: Isolates of Staph. aureus recovered from domestic refrigerators were grown in shaken culture for 8 h at 37 degrees C, recovered and washed by centrifugation and combined to form a cocktail of five strains. Samples from this cocktail were (a) heat treated at 50, 55 and 60 degrees C or (b) held under simulated domestic refrigeration conditions for 72 h and then heat treated as above. The numbers of Staph. aureus in heat treated and chill held, heat treated samples were enumerated by direct selective plating onto Baird Parker Agar (BPA) and recovery plating on Tryptone Soya Agar (TSA) subsequently overlaid with BPA. D-values were obtained using two different methods both of which may be used when the thermal inactivation follows first order kinetics. In the first method D-values are obtained by plotting the Log(10) of the surviving cells against time and using the equation D = -1/slope. The second method uses the rate constant (k) which is obtained from the slope of a plot of ln N/N(0)vs time and D is obtained using the equation D = 2.303 k(-1). D(50), D(55) and D(60) values ranged from 94.3 to 127.9 min, 13 to 21.7 min and 4.8 to 6.5 min. Prechilling did not enhance thermal resistance. The method of calculation did not affect the D-values obtained because the thermal inactivation of Staph. aureus in this study followed first order kinetics with r(2) values of 0.91-0.99. CONCLUSIONS: The thermal inactivation of Staph. aureus in tryptone soya broth (TSB) follows first order kinetics and in general chilling of these bacteria does not increase the resistance to thermal destruction during subsequent thermal processes such as cooking. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides much needed data on the thermal resistance of Staph. aureus and validates chilling as a food storage activity which does not cause toughening of the microorganisms to subsequent cooking. However, the data generated strongly suggests that Staph. aureus is more thermotolerant than Listeria monocytogenes and should be used as the target microorganism in designing mild thermal treatments for food, in which case the current recommendations for pasteurization (70 degrees C for 2 min, minimum) should be revised.