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A continuous flow high-temperature short-time pasteurization system was used to determine kinetic parameters (D- and z-values) for thermal destruction of the bacterial pathogens, Escherichia coli and Klebsiella pneumoniae , in mature human milk. D-and z-alues of each bacterium were determined from data on survivors enumerated on both selective media, Violet Red Bile agar or MacConkey's, and on a non-selective medium, nutrient agar (NA). For E. coli , D-values were determined at 58, 60, 62 and 64°C. The predicted value of D at 60°C is 31.5 s. The z-value for E. coli is 3.2°C. D-values for K. pneumoniae were determined at 52, 56 and 58°C. Based on these data the predicted value of D at 60°C is 1.3 s. The z-value for K. pneumoniae , is 2.8°C. For both E. coli and K. pneumoniae , counts on NA tend to be higher than on selective media. This is undoubtedly due to the inhibitory nature of the selective media. This also suggests that some degree of thermal injury may occur for each organism.

Rates of thermal destruction of Staphylococcus aureus were determined in mature human milk using a continuous flow high-temperature short-time pasteurization system. D and z values for inactiviation of S. aureus were determined from data on survivors capable of forming colonies in an appropriate selective medium. The effects of thermal injury on D and z values were assessed by survivor colony forming units (CFU) on Staphylococcus medium 110 (SMI 10), nutrient agar (NA), Trypticase Soy Agar (TSA), Trypticase Soy Agar with 7.5% NaCi (TSAS) and Baird-Parker medium (BP) (Difco Laboratories, Detroit, MI). D values for inactivation of S. aureus at 52, 58 and 60 and 62°C were used to predict D at 60°C of 15.3 s and 24.3 s when based on survivor CFU in SM 110 and nutrient agar, respectively. The z value was 4.9°C in either medium. D-values for inactivation of S. aureus at 60, 62, 64 and 67°C were used to predict D at 60°C of 41.2 s, 41.0 s and 34.7 s when based on survivor CFU in BP, TSA and TSAS, respectively. The z values were 6.5, 6.5 and 6.4°C, respectively.