Microbiological Examination of Frozen Corn Linked to a National Salmonellosis Outbreak Reveals Gaps in Product Hygiene and Thermal Inactivation.

A nationwide outbreak of Salmonella enterica ser. Enteritidis linked for the first time to frozen corn occurred in Canada from September 2021 to January 2022. To identify potential contributing factors, contaminated corn samples were analyzed and compared to unrelated retail samples. Contaminated corn samples had significantly lower sugar content (9% vs. 12%) and higher levels of background microbiota (total aerobic mesophiles, Enterobacteriaceae, and total coliforms) compared to retail samples. S. enterica ser. Enteritidis was recovered from the contaminated corn samples at low levels (average = 5 most probable number [MPN]/100 g). These levels were stable over 3 years of frozen storage (average = 3 MPN/100 g). The response of the outbreak strain to heat was comparable to laboratory strains of the same serovar. Heating retail corn samples artificially inoculated with 1 × 108 colony-forming units of S. enterica ser. Enteritidis by immersion in a 90°C water bath for 7 min resulted in a linear decrease of cells over time as the temperature of the food sample increased. Extrapolation of the data suggests a five-log reduction would require a cooking time of 8 min, corresponding to a food sample temperature of 66°C. However, cooking naturally contaminated corn samples beyond these parameters to a temperature of 74°C failed to completely eliminate Salmonella. The results of this study highlight the hazards associated with the presence of Salmonella in frozen vegetables and emphasize the need for improved hygiene during processing and the development of clear, validated cooking procedures to mitigate risks associated with frozen vegetables.