Heat Resistance in Escherichia coli and its Implications on Ground Beef Cooking Recommendations in Canada
Authors: Yang X. , Tran F. , Klasse M. 
Published: July 2021

ObjectivesRecent reports of an extremely heat resistant but non-pathogenic beef Escherichia coli strain, AW 1.7, raised concerns over the adequacy of cooking ground beef to 71°C in Canada. The objective of this study was to assess the adequacy of the current cooking recommendations for ground beef in relation to heat resistant E. coli.Materials and MethodsIn total, 8 potentially heat resistant E. coli strains (4 generic and 4 E. coli O157:H7) from beef along with E. coli AW1.7 were included in this study. Heat resistance of the strains was first evaluated by decimal reductions at 60°C (D60°C-value), the time required to have a log reduction of the bacterial population at 60°C. The more heat resistant strains of each group (E. coli 62 and 68, and E. coli O157 J3 and C37) were further assessed for their heat resistance when grown in Lennox Broth without salt (LB-NS), LB + 2% NaCl and Meat Juice (MJ). Then, the two most heat resistant E. coli O157 strains (J3 and C37) and E. coli AW 1.7 were each introduced to extra lean ground beef (100 g) in vacuum pouches for determination of their D-values at three temperatures, 54, 57, and 60°C, from which a z-value for each strain was derived. The thermal characteristics of all three strains were fed into a predictive model to determine the process lethality of cooking burgers to 71°C with resting for up to 5 min. Finally, inactivation of the most heat resistant E. coli strain AW1.7, assessed in this study and reported in the literature, in ground beef was validated by grilling burgers containing 6.20 ± 0.24 log CFU/g of the organism to 71°C without or with a resting of 3 or 5 min.ResultsThe D60°C-values for these strains varied from 1.3 to 9.0 min, with J3 and AW1.7 being the least and most heat resistant strains, respectively. The D60°C-values for E. coli 62 and 68 were similar and were not affected by growth medium, while the heat resistance of C37, J3 and AW1.7 varied with the growth medium. When heated in extra lean ground beef (100 g) in vacuum pouches, the mean D54°C, D57°C, and D60°C-values were 44.8, 18.6 and 2.9 min for C37, 13.8, 6.9 and 0.9 min for J3, and 40.5, 9.1 6.1 min for AW1.7. The derived z- and D71°C-values were, respectively, 5.0, 5.1 and 7.3°C; and 0.022, 0.008, and 0.156 min. Burger temperatures continued to rise after being removed from heat when the target temperature was reached, by up to 5°C, and resting of 1 min would result in a destruction of 133, 374, and 14 log C37, J3 and AW1.7, estimated from process lethality. When burgers inoculated with AW1.7 were cooked to 71°C, 14 of the 15 burgers yielded no E. coli, while the 15th had a reduction of 4.5 log. Additional resting of 3 or 5 min resulted in complete elimination of AW 1.7.ConclusionIt has been predicted that 2% of E. coli from beef may carry heat resistant genes. The findings in this study, along with the very low level of total E. coli expected in ground beef in Canada, suggest that cooking ground beef to 71°C should be adequate to ensure the safety of such products.