Thermal Tolerance of Acid-Adapted and Non-adapted Escherichia coli O157:H7 and Salmonella in Ground Beef During Storage

Thermal tolerance of acid-adapted Escherichia coli O157:H7 or Salmonella in ground beef was evaluated during storage at 4°C or –20°C. Both pathogens were adapted to acidic conditions (pH ∼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 ∼6.6) and TSB + 1% glucose for 24 h at 37°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°C were subjected to heat treatment at 62°C or 65°C on days 1, 7, 14, 21, and 28, and the samples stored at –20°C were subjected to heat treatment at 62°C or 65°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 D62 values were observed on days 21 and 28 for non-adapted E. coli O157:H7 stored at 4°C and on days 90 and 120 for non-adapted E. coli O157:H7 stored at –20°C (P \u3c 0.05). Higher D62 values were observed on days 21 and 28 among non-adapted Salmonella strains stored at 4°C and on day 28 for acid-adapted strains of Salmonella stored at 4°C (P \u3c 0.05). Higher D62 values for acid-adapted strains of Salmonella stored at –20°C were observed on days 30, 60, and 90 (P \u3c 0.05), when while no differences were observed in the D65 values of acid-adapted and non-adapted strains of E. coli O157:H7 and Salmonella throughout storage at both temperatures (P \u3e 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