The effect of commercial production and product formulation stresses on the heat resistance of Escherichia coli O157:H7 (NCTC 12900) in beef burgers

The effects of commercial beef burger production and product formulation on the heat resistance of Escherichia coli O157:H7 (NCTC 12900) in beef burgers were investigated. Fresh beef trimmings were inoculated with E. coli O157:H7 to approximately log10 7.0 cfu g−1 and subjected to standard beef burger production processes, including freezing, frozen storage and tempering. The tempered trimmings were processed in line with commercial practice to produce burgers of two formulations, a ‘Quality’ burger containing 100% beef and an ‘Economy’ burger containing 70% beef and 30% other ingredients (salt, seasoning, soya, onion and water). The burgers were then frozen and stored. Control ‘unprocessed’ burgers were produced to each of the above formulations using fresh beef trimmings. All burger types were heat-treated at 55, 60 or 65 °C. Samples were examined by plating on Tryptone Soya Agar (TSA), incubated at 37 °C for 2 h, before overlaying with SMAC (TSA/SMAC) and incubation at 37 °C. The resultant counts were used to derive D-values for E. coli O157:H7. At each treatment temperature, the D-values from each burger formulation using frozen tempered trimmings were significantly lower (P<0.001) than the D-values from that formulation using fresh trimmings. At each treatment temperature, the D-values from Economy burgers using processed trimmings were significantly higher (P<0.001) than the D-values from Quality burgers using processed trimmings. A similar trend of significantly higher (P<0.001) D-values for Economy burgers was observed using fresh trimmings. This study found that commercial processing and product formulation have profound effects on the heat resistance of E. coli O157:H7 in beef burgers.