A Comparison of Salmonella Survival and Detection Using an Enrichment Technique in Dry- and Wet-Inoculated Rendered Chicken Fat Treated with Sodium Bisulfate

Abstract

ABSTRACT The differences in the recovery of Salmonella from rendered chicken fat treated with sodium bisulfate (SBS) when inoculated with a dry versus wet inoculum were evaluated. Food-grade rendered chicken fat was inoculated with a dry inoculum and a wet inoculum containing a cocktail of Salmonella serovars (Enteritidis, Heidelberg, and Typhimurium). In addition, the effect of an antimicrobial treatment (SBS) against Salmonella in both the aqueous phase and fat phase of the chicken fat was evaluated. The untreated control samples in the aqueous phase had a consistent level of Salmonella (∼7 log) when both the dry and wet inocula were used. In the SBS-treated aqueous phase, Salmonella pathogens were not detectable after 6 h when the wet inoculum was used; when the dry inoculum was used, Salmonella pathogens were not detectable at 24 h. Salmonella pathogens were detected for up to 6 h in the SBS-treated fat phase when the dry inoculum was used compared with 2 h with the wet inoculum. The 24-h fat samples that failed to show growth on Trypticase soy agar were enriched for Salmonella isolation, followed by confirmation by PCR using primers for the invA gene. SBS-treated and control samples from the dry-inoculated rendered chicken fat and the inoculated control from the wet-inoculated rendered chicken fat tested positive for Salmonella. However, the SBS-treated sample from the wet-inoculated fat was negative for Salmonella. The use of dry SBS powder against dry Salmonella inoculum in the fat matrix caused only ∼2.8-log reduction after 24 h compared with ∼2.2-log reduction in the positive control. However, the recovery of Salmonella from untreated control fat was lower and was not different (P > 0.05) from that recovered from the SBS-treated fat. The results suggest that viable but nonculturable states of Salmonella may develop in rendered chicken fat or that injured cells may be present, which indicates that testing should include an enrichment and appropriate molecular confirmation instead of agar plating alone. HIGHLIGHTS