Kennedy J, Blair I S, McDowell D A, Bolton D J
Foods Safety Department, Teagasc-- The National Food Centre, Ashtown, Ireland.
J Appl Microbiol. 2005;99(5):1229-35. doi: 10.1111/j.1365-2672.2005.02697.x.
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.
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.
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.
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.
本研究的目的是：（i）提供金黄色葡萄球菌的热失活数据；（ii）研究描述金黄色葡萄球菌热失活的动力学，包括十进制减少时间（D值）和速率常数（k），并比较两种计算D值的不同方法；（iii）确定冷藏是否会使这些微生物变得坚韧从而导致耐热性增加。
从家用冰箱中分离出的金黄色葡萄球菌菌株在37℃振荡培养8小时，回收后通过离心洗涤，并合并形成五种菌株的混合菌液。该混合菌液的样品（a）在50、55和60℃下进行热处理，或（b）在模拟家用冷藏条件下保存72小时，然后按上述方法进行热处理。通过直接选择性接种到贝尔德·帕克琼脂（BPA）上并随后在胰蛋白胨大豆琼脂（TSA）上进行回收接种，然后覆盖BPA，对热处理和冷藏后再热处理样品中的金黄色葡萄球菌数量进行计数。使用两种不同的方法获得D值，当热失活遵循一级动力学时，这两种方法均可使用。在第一种方法中，通过将存活细胞的对数（10）与时间作图并使用方程D = -1/斜率来获得D值。第二种方法使用速率常数（k），该常数从ln N/N(0)对时间的图的斜率获得，D使用方程D = 2.303 k(-1)获得。D(50)、D(55)和D(60)值范围为94.3至127.9分钟、13至21.7分钟和4.8至6.5分钟。预冷并未增强耐热性。计算方法不影响获得的D值，因为本研究中金黄色葡萄球菌的热失活遵循一级动力学，r(2)值为0.91 - 0.99。
胰蛋白胨大豆肉汤（TSB）中金黄色葡萄球菌的热失活遵循一级动力学，一般来说，这些细菌的冷藏不会增加其在后续烹饪等热过程中的热破坏抗性。
本研究提供了关于金黄色葡萄球菌热抗性急需的数据，并验证了冷藏作为一种食品储存活动不会导致微生物在后续烹饪中变得坚韧。然而，所产生的数据强烈表明，金黄色葡萄球菌比单核细胞增生李斯特菌更耐热，在设计食品温和热处理时应将其用作目标微生物，在这种情况下，目前关于巴氏杀菌的建议（70℃持续2分钟，最低）应予以修订。
