Thermal resistance of Escherichia coli O157:H7 in laboratory media, milk, and beef extracts during non-isothermal processing at various heating rates

Abstract

This study investigated the effect of non-isothermal treatments with different heating rates (HRs) on inactivating Escherichia coli O157:H7 in various heating media. E. coli O157:H7 in phosphate-buffered saline (pH 7; PBS), Luria-Bertani broth with (LB_S) or without (LB) 1% NaCl, milk, 3% beef extract (beef_3%), and 30% beef extract (beef_30%) were inactivated under non-isothermal conditions (up to 50 - 60 °C) using various HRs between 1.32 and 10.78 °C/min. After treatment with an HR of 1.32 °C/min to a target temperature of 60 °C, the reduction level of E. coli O157:H7 were 6.49, 2.28, 1.20, 1.30, 5.55 and 5.02 log CFU/mL in PBS, LB, LB_S, milk, beef_3%, and beef_30%, respectively. Contrastingly, E. coli O157:H7 treated in PBS, LB, milk, beef_3%, and beef_30% with an HR of 10.78 °C/min to the same target temperature were completely inactivated to not-detectable level. Moreover, E. coli O157:H7 treated at 10.63 °C/min exhibited more severe morphological injuries than those at 2.23 °C/min. Interestingly, inactivation through non-isothermal treatment is potentially involved in either ribonucleic acid or lipid synthesis, as E. coli O157:H7 treated at 10.63 °C/min for 3.5 min less recovered on TSA with rifampicin and streptomycin. These observations highlight the ability of the E. coli O157:H7 to survive for long periods at a potentially sub-lethal temperature, which may be enhanced concomitantly with a decrease in the HR of the non-isothermal treatment.

Keywords: Escherichia coli O157:H7; Heating media; Heating rate; Non-isothermal inactivation; Stress response.