title: Escherichia coli O157:H7 reduction in hamburgers with regard to premature browning of minced beef, colour score and method for determining doneness.
abstract: This study investigated the effect of premature browning (PMB) on the survival of Escherichia coli O157:H7 in beef hamburgers after cooking with respect to interior colour of the hamburger and recommendations to cook hamburgers to a core temperature of 71 °C. Assessment of doneness by visual inspection or measurement of internal temperature was compared in terms of survival and the increased relative risk of illness due to PMB was estimated. At the last consume-by-day, hamburgers made from minced meat packaged in 80/20 O2/CO2 (MAP hamburger) and from meat minced at retail packaged in atmospheric condition (control hamburger) were inoculated with a gfp-tagged strain of E. coli O157:H7 (E. coli O157:H7gfp+). Hamburgers were cooked for different times during assessment of the core temperature every 30s and cut in halves after cooking. Doneness was evaluated based on visual judgement of the internal colour using a score chart (C-score) from 'uncooked' (score 1) to 'tan with no evidence of pink' (score 5). An alternative five point score chart (TCC-score) including texture of the meat, clarity of meat juice and internal colour was also developed. Enumeration of viable E. coli O157:H7gfp+ in cooked hamburgers was based on fluorescent colonies recovered from plates. Results showed that MAP hamburgers developed PMB when compared with controls (P=0.0003) and that the shortest cooking time for the highest C-score was 6 and 11 min for MAP and control hamburgers, respectively. The mean temperature in the MAP hamburger was then 60.3 °C. The TCC-score reduced the difference between MAP and control hamburgers. It was also shown that the survival of E. coli O157:H7gfp+ was highest in MAP hamburgers. The predicted absolute risks for illness were highest for MAP hamburgers for all C-scores and the relative risk associated with PMB increased with doneness. For a C-score of 4 (slightly pink) the predicted relative risk for illness was 300 times higher for MAP hamburger than for controls. A variable pathogen reduction was observed when cooking hamburgers to temperatures of 70-76 °C (the 5th and 95th percentile range was around 3.3 log CFU). The lower reductions, at the 5th percentile, may, depending on initial contamination levels, not be enough to ensure sufficient and safe inactivation of E. coli O157:H7. Efforts to inform consumers about PMB in minced meat packaged in high oxygen packages (≥60% O2) are needed with the aim to make consumers use thermometers correctly or at least not determine doneness based only on meat colour.
authors.affiliation: Department of Biomedical Sciences and Veterinary Public Health, Swedish University of Agricultural Sciences, P.O. Box 7028, SE-75007 Uppsala, Sweden. Electronic address: Sofia.Boqvist@slu.se.
authors.forename: Sofia
authors.initials: S
authors.lastname: Boqvist
authors.affiliation: Department of Biomedical Sciences and Veterinary Public Health, Swedish University of Agricultural Sciences, P.O. Box 7028, SE-75007 Uppsala, Sweden. Electronic address: Lise-Lotte.Fernstrom@slu.se.
authors.forename: Lise-Lotte
authors.initials: LL
authors.lastname: Fernström
authors.affiliation: Department of Biosystem and Technology, Swedish University of Agricultural Sciences, P.O. Box 103, SE-230 53 Alnarp, Sweden. Electronic address: Beatrix.Alsanius@slu.se.
authors.forename: Beatrix W
authors.initials: BW
authors.lastname: Alsanius
authors.affiliation: Division of Risk and Benefit Assessment, National Food Agency, SE-75126 Uppsala, Sweden; Department of Microbiology, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden. Electronic address: roland.lindqvist@slv.se.
authors.forename: Roland
authors.initials: R
authors.lastname: Lindqvist
mesh_terms: D000818:Animals; D002417:Cattle; D003116:Color; D003296:Cooking / Q000379:methods*; D019453:Escherichia coli O157 / Q000254:growth & development*; D005516:Food Microbiology; D049452:Green Fluorescent Proteins / Q000235:genetics; D006358:Hot Temperature; D008461:Meat Products / Q000382:microbiology*; D050296:Microbial Viability; D010100:Oxygen; D000069466:Red Meat / Q000382:microbiology*
chemical_list: D049452:Green Fluorescent Proteins; D010100:Oxygen
keywords: Bacterial inactivation; Doneness evaluation; Food safety; Modified atmosphere; Relative risk