THERMAL INACTIVATION OF ESCHERICHIA COLI 0157:H7 AND INTERNAL TEMPERATURE COMPARISONS IN THE COOKING OF GROUND BEEF PATTIES

Ground beef patties (114 g) inoculated with a five strain mixture of Escherichia coli 0157:H7 were cooked to six predetermined final internal temperatures (55C, 60C, 65C, 68C, 71C, 74C) in an electric skillet. The presence of verotoxin was then determined in enrichments of E. coli 0157:H7 in the patties. Visual clues including lack of pinkness in color of meat and in meat juices were also observed since consumers rely on them as indicators of doneness. No E. coli 0157:H7 or verotoxin was identified in ground beef patties cooked above 61.2C where the inoculum was below 5000 cfu/patty, the probable infective dose associated with previous outbreaks. E. coli 0157:H7 and verotoxin were detected in patties inoculated with 7.7x 105 CFU/patty even when cooked to 74C. As the final internal cooking temperatures increased, the internal color of the ground beef changed from red to pink to brown. Although we did not observe premature browning in our experiments others have documented its occurrence. Instructions for cooking should emphasize that lack of pinkness may be one clue to doneness but is not always reliable. Expressible meat juices always had some red or pink even when cooked to 74C suggesting that this is also not a good visual clue for doneness. In experiments comparing thermocouples and thermistors, taking internal temperatures of ground beef patties by inserting probes from the top, temperature readings varied by as much as 9C from the reference thermocouple. In contrast, when patties were probed from the side, readings were within 3C of each other from the reference. We suggest that doneness of ground beef patties be assessed with a thermocouple or thermistor inserted into the center of the patty from the side and the patty be removed from the heat only when a temperature of 71 C or higher has been reached.