Purpose: To determine the heat resistance of E. coli O157:H7 and L. monocytogenes on inoculated almonds.
Methods: Almond kernels were inoculated with Salmonella Enteritidis PT 30 (control), E. coli O157:H7 (five isolates from produce- or low-moisture-food–associated outbreaks), or L. monocytogenes (five isolates from produce-associated outbreaks) at 9 log CFU/g, and dried for 72 h and equilibrated (48 - 72 h) to a moisture content of 4.5 to 5%. Almonds were exposed to hot oil (121°C for 30 and 60 s), hot water (80°C for 30 and 60 s), or a dry oven (138°C for 15 min). Survivors were enumerated by plating on tryptic soy agar and appropriate selective media.
Results: During desiccation, average population decreases of 0.6, 1.8 (range: 1.1 - 2.7 log CFU/g), and 1.3 log CFU/g (range: 1.2 - 1.4 log CFU/g) were observed for Salmonella, E. coli O157:H7, and L. monocytogenes, respectively. Reductions of E. coli O157:H7 (3.1 - 5.8 and 1.1 - 4.8 log CFU/g) and L. monocytogenes (4.5 - 5.0 and 2.3 - 2.7 log CFU/g) were similar to or greater than those of Salmonella (3.1 and 1.4 log CFU/g) after the hot oil (60 s) and dry oven treatments, respectively. Among the E. coli O157:H7 isolates, NML#11-1865, isolated from walnuts, was consistently more resistant to desiccation and to hot oil and dry oven treatments. Reductions in hot water (60 s) were similar among all strains evaluated (2.6–3.9 log CFU/g).
Significance: Among the isolates evaluated, E. coli O157:H7 and L. monocytogenes display similar or lesser thermal tolerance than Salmonella Enteritidis PT 30 on inoculated almonds.