Thermal tolerance of acid-adapted Escherichia coli O157:H7 or Salmonella in ground beef was evaluated during storage at 4 degrees C or -20 degrees C. Both pathogens were adapted to acidic conditions (pH approximately 4.6) by growing in tryptic soy broth supplemented with 1% glucose. A five-strain cocktail of E. coli O157:H7 or Salmonella was grown separately in TSB (pH approximately 6.6) and TSB + 1% glucose for 24 h at 37 degrees C to provide cells with or without acid adaptation. Irradiated ground beef was inoculated with either acid-adapted or non-adapted E. coli O157:H7 or Salmonella; the samples stored at 4 degrees C were subjected to heat treatment at 62 degrees C or 65 degrees C on days 1, 7, 14, 21, and 28, and the samples stored at -20 degrees C were subjected to heat treatment at 62 degrees C or 65 degrees C on days 1, 30, 60, 90, and 120. Decimal reduction time (D values) of the pathogens was determined as an indicator of thermal tolerance. Significantly higher D(62) values were observed on days 21 and 28 for non-adapted E. coli O157:H7 stored at 4 degrees C and on days 90 and 120 for non-adapted E. coli O157:H7 stored at -20 degrees C (P < 0.05). Higher D(62) values were observed on days 21 and 28 among non-adapted Salmonella strains stored at 4 degrees C and on day 28 for acid-adapted strains of Salmonella stored at 4 degrees C (P < 0.05). Higher D(62) values for acid-adapted strains of Salmonella stored at -20 degrees C were observed on days 30, 60, and 90 (P < 0.05), when while no differences were observed in the D(65) values of acid-adapted and non-adapted strains of E. coli O157:H7 and Salmonella throughout storage at both temperatures (P > 0.05). This suggests that acid adaptation of foodborne pathogens provides a certain level of protection against heat treatment at lower cooking temperatures, while at higher temperatures there were no observed differences between the sensitivity of acid-adapted and non-adapted strains in an actual food system over an extended period of refrigerated and frozen storage.

Escherichia coli O157:H7 and Salmonella were adapted to acidic environments by growing in tryptic soy broth with 1% glucose (TSB + 1%G). Thermal tolerance of acid‐adapted pathogens was evaluated in meat serum. Five‐strain cocktail of both bacteria were grown separately in TSB and TSB + 1%G for 24 h at 37C. Meat serum was prepared from irradiated ground beef, inoculated with either acid‐adapted or nonadapted E. coli O157:H7 and Salmonella, and heated at 58, 62 and 65C to determine D values. D values of acid‐adapted E. coli O157:H7 were higher (P < 0.05) than nonadapted cells at 58, 62 and 65C. Significantly higher (P < 0.05) D values of acid‐adapted Salmonella were observed at 58 and 62C, but not at 65C. D values were 22.46 and 10.59 min at 58C, 3.58 and 1.38 min at 62C, 1.02 and 0.75 min at 65C for acid‐adapted and nonadapted E. coli O157:H7, respectively. D values of acid‐adapted Salmonella were 9.36 min at 58C, 1.66 min at 62C and 1.14 min at 65C, whereas the nonadapted counterparts had D values of 6.44, 0.88 and 0.95 min at 58, 62 and 65C, respectively. PRACTICAL APPLICATIONS Foodborne pathogens exist in the environment in healthy and/or stressed forms and have the potential to contaminate food products. This research was conducted to determine the effectiveness of commonly used heat treatments to eliminate acid‐adapted Escherichia coli O157:H7 and Salmonella in ground beef. There is limited literature on the thermal tolerance of acid adaptation of foodborne pathogens in actual food systems and studies in the past have been conducted at mild heating temperatures, whereas effects of heating to temperatures that are more indicative of cooking temperatures have rarely been researched. This study stresses upon the need for regulatory agencies and large‐scale meat manufacturers to take into consideration the increased thermal tolerance of acid‐adapted pathogens, as healthy growing cultures in a laboratory medium may inaccurately represent their survival in natural food environment.

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