Purpose: We compared the thermotolerance of two representative strains from each of the six serogroups with reference O157:H7 strains at 65.5°C in 15% and 27% fat ground beef. Reference O157:H7 strains included four heat tolerant beef-trim isolates and an outbreak-linked strain (ATCC 43895). Thermal tolerance of a mutant strain of O157:H7 lacking the gene for sigma factor rpoS and a lactic acid bacterium (LAB) was also determined. The LAB (Pediococcus acidilactici) has shown to be an effective surrogate for O157:H7 in beef thermal processing.
Methods: Small Whirl-Pak bags containing 25 g preheated ground beef (65.5°C) were inoculated with 1 ml aliquots of stationary-phase culture (~109 CFU/ml), and immediately transferred to a shaking water-bath (65.5°C). At each sampling time (0 - 180 s), one bag of meat was removed and held on ice to reach ≤4°C. The meat was everted into a stomacher bag and stomached with 99 ml Butterfield’s Phosphate Diluent (BPD) for 30s. Serial dilutions were made in BPD and viable cells enumerated by plating on modified Eosin Methylene Blue agar (37°C for 24h). D65.5°C-values were calculated from log CFU/g data, and compared using ANOVA (n=3 at each fat level).
Results: At a 15% fat level, D65.5°C-values ranged from 69.54 s (E. coli O145:H-) to 37.75 s (E. coli O121:H19), though there were no significant differences between the strains (P ≥ 0.05). For 27% fat, D-values ranged from 78.74 s (E. coliO157:H7 beef trim isolate) to 35.21 s (ATCC 43895).
Significance: These results suggest that levels of fat in meat affect the survival of the pathogen and thermotolerance of the pathogen is affected by the strain in question. This data is important in establishing industry intervention protocols.