Thermal Resistance of Heat-, Cold-, and Starvation-Injured Salmonella in Irradiated Comminuted Turkey

Abstract

To investigate the effects of sublethal stress on Salmonella thermal inactivation kinetics, an eight-strain Salmonella cocktail was subjected to heat shock (30 min at 54°C), cold shock (2 h at 4°C), and starvation stress (10 days in phosphate buffer at 4°C), harvested by centrifugation, and inoculated into irradiated comminuted turkey. Immediately after stressing, the Salmonella cocktails contained 89.1% heat-injured, 44.7% cold-injured, and 67.7% starvation-injured cells, as determined by plating on selective and nonselective media. D60°C-values for the heat-shocked cocktail (0.64 min on Trypticase soy agar containing 0.6% yeast extract [TSAYE], 0.35 min on xylose lysine desoxycholate [XLD] agar) were higher (P < 0.05) than those for the unshocked control (0.41 min on TSAYE, 0.17 min on XLD), whereas D60°C-values for the cold-shocked cocktail (0.38 min on TSAYE, 0.17 min on XLD) were not significantly different from those for the control. Starved cells had the same D60°C-value on TSAYE as did the unshocked cocktail, but the D60°C-value on XLD was significantly lower (0.14 min). Although starvation and cold shock were not thermally protective, heat shock increased thermal resistance, indicating that product history and the physiological state of the Salmonella cells should be considered when developing and validating thermal processes. D60°C-values observed on selective media were significantly lower than those observed on nonselective media for all stress treatments and for the control. Therefore, nonselective culture media should be used to assess the response of microorganisms to a thermal challenge when developing performance standards for lethality.