The Effect of Corn Oil as an Additive to Sequester Phenolic Compounds in Spices, and Increase Salmonella Recovery: A Comparison between TSB and mBPW

Introduction: Oregano essential oils and phenolic compounds such as carvacrol and thymol inhibit the detection of pathogenic microorganisms in spices. In this study, corn oil is added to the pre-enrichment broth as an additive to neutralize the effect of these phenolic compounds.

Purpose: The increased recovery of Salmonella was evaluated by comparing two pre-enrichment broths: modified Buffer Peptone water (mBPW) and Trypticase Soy Broth (TSB). 

Methods: Oregano samples (25 g) were artificially contaminated with 10³ lyopholized Salmonella serovar Montevideo and aged for 2 weeks. Oregano samples were pre-enriched in mBPW and TSB with and without 2% (vol/vol) corn oil, respectively, and incubated overnight at 37°C. Samples were transferred to selective enrichment broths Rappaport-Vassiliadis (RV) and tetrathionate (TT) and plated on Xylose-Lysine-Tergitol 4 (XLT-4) agar at 24 h. Typical Salmonella colonies were examined as described in the FDA Bacteriological Analytical Manual (BAM), and suspect colonies were confirmed using the Vitek® 2 Compact.

Results: The samples with corn oil had ≥10 fold more recovery than the samples without the corn oil.  The average recovery of Salmonella was 323 CFU with corn oil compared to 24 CFU without corn oil. There were no significant differences between mBPW and TSB, thus the results suggest that the increase in recovery was due to the corn oil.

Significance: Detection of pathogens from oregano samples is repressed by the release of the phenolic compounds, so finding an approach to increase recovery and neutralize the effect of the antimicrobial phenolic compound is a key for a detection method.