Purpose: This study compared survival of non-O157 STEC (6 strains from serogroups O26, O45, O103, O111, O121 and O145) and the known heat-acid-tolerant O157:H7 strain ATCC 43895 in 15% and 27% fat ground beef during sequential refrigeration, partial cooking, and incubation in a simulated gastric environment.
Methods: Inoculated ground beef (25 g) was refrigerated (4°C) for 16-17 h and then heated 30 min at 54.4°C to mimic partial cooking. After cooling in ice 10 min, the inoculated beef was combined with pre-warmed (37 °C) Synthetic Gastric Fluid (pH 2.0; 70 ml) and held at 37°C. Surviving cells were enumerated before and after heating and after 0.5, 1, 2, 3, 4, and 6 h of incubation in SGF. Overall changes in population (D-log CFU/g) were calculated for each strain separately at the 6-h sampling time, relative to the level before heating.
Results: STEC populations decreased by 1.5 – 3.0 log CFU/g or by 0.8 – 3.5 log CFU/g during cooking in 15% and 27% fat ground beef, respectively, and by 1.4 – 5.7 log CFU/g during subsequent 6-h incubation in SGF in both 15% and 27% fat ground beef. In 27% fat ground beef there were very significant (P <0.01) inter-strain differences in survival. One strain each from serogroups O26, O103, and O111, and the O157:H7 reference strain all survived significantly better than other strains tested. There was no statistical difference in D-log CFU/g when results for 15% and 27% fat ground beef were compared, except for one strain of serogroup O26 which exhibited significantly lower D-log CFU/g values in 15% ground beef.
Significance: Several non-O157 STEC strains were shown to survive as well as O157:H7 in ground beef when exposed to sequential refrigeration, partial cooking, and synthetic gastric fluid. Fat level, 15% or 27%, did not affect STEC survival.