Effects of temperature and storage time on inactivation of Listeria monocytogenes, Salmonella enterica, and Escherichia coli 0157:H7 populations by sophorolipid and sanitizer in-vitro and on tomato

Authors: Olanya, Modesto; Ukuku, Dike; Solaiman, Daniel; Ashby, Richard - Rick; Niemira, Brendan; Mukhopadhyay, Sudarsan

Submitted to: International Journal of Food Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2017
Publication Date: 4/16/2018
Citation: Olanya, O.M., Ukuku, D.O., Solaiman, D., Ashby, R.D., Niemira, B.A., Mukhopadhyay, S. 2018. Effects of temperature and storage time on inactivation of Listeria monocytogenes, Salmonella enterica, and Escherichia coli 0157:H7 populations by sophorolipid and sanitizer in-vitro and on tomato. International Journal of Food Science and Technology. 53:1303-1315. http://doi.org/10.1111/ijfs.13711.
DOI: https://doi.org/10.1111/ijfs.13711

Interpretive Summary: Pathogenic bacteria continue to pose significant food safety risks to consumers even though considerable measures for foodborne control exist. There are limited published data on the combined application of sophorolipid (SL-p), an environmentally friendly antimicrobial compound and sanitizer treatments to inactivate pathogens on produce. We evaluated the potential of different concentrations of SL-p for its antimicrobial activity against foodborne pathogens in-vitro. SL-p, sanitizer and combinations of SL-p and sanitizer were assessed for inactivation of Listeria monocytogenes, Salmonella enterica, and Escherichia coli O157:H7 on grape tomatoes. Temperatures and storage time of SL-p application affected pathogen populations. Among the three pathogens, Listeria was the most sensitive to SL-p treatment, as reductions of 5 logs relative to the untreated control were recorded at 0.12% of SL-p. The reductions of Salmonella and E. coli were attained at SL-p concentrations of 2-5% in in-vitro assays. On grape tomatoes, pathogen inactivation increased as SL-p levels increased from 0.12% to 5%, but the reductions were less than observed in in-vitro assays. This may be attributed to the hydrophobic nature of tomatoes that may retain less sophorolipid on grape tomato surface to sufficiently impact bacterial populations, and perhaps due to lower antimicrobial activity for the sophorolipid level used. In contrast, sanitizer and combinations of SL-p plus sanitizer showed remarkable results as greater than 5 log pathogen reductions were recorded. Due to the biodegradable nature of sophorolipid and generally regarded as safe sanitizer applied in this study, they offer great potential for application to control pathogenic bacteria. Combining sophorolipid with sanitizer or other antimicrobials will greatly reduce foodborne pathogen populations and enhance post-harvest produce safety.

Technical Abstract: The increased consumption of produce by consumers has been reported due to perceived health benefits of post-harvest produce. However, periodic pathogen occurrences and contamination of tomatoes and leafy greens have exacerbated food safety risks for consumers; and therefore pathogen control is crucial. We investigated the effects of temperatures (5 and 25C), storage time (30min and 24hrs) for inactivation of Listeria monocytogens, Salmonella enterica, and Escherichia coli O157:H7 by sophorolipid (SL-p) produced fermentatively using palmitic acid as a co-substrate at different concentrations in in-vitro. Inactivation of pathogenic bacteria on grape tomatoes by SL-p, sanitizer (Lovit) and combinations of SL-p and sanitizer were also determined in replicated experiments. Temperature and storage time significantly (P<0.05) affected pathogen inactivation by SL-p as pathogen reductions were greater at 25C and 24hr than at 5C and 30 minutes of storage. L. monocytogenes was the most sensitive to SL-p treatment as reductions of 5 log relative to untreated controls were attained at SL-p concentrations of 0.12%. Significant reductions of S. enterica (1.91-3.85 logs) and E. coli O157:H7 (0.87-4.09 logs) were recorded at 2-5% of SL-p. In contrast, lower populations of Salmonella and E. coli O157:H7 were inactivated than L. monocytogenes. On grape tomatoes, pathogen populations inactivated increased at SL-p levels of 0.12-5% at 25C. Treatments of pathogen-inoculated tomato by sanitizer as well as sanitizer plus SL-p reduced bacterial populations by 5.29-5.76 logs and 0.71-3.3.66 logs, respectively. These results imply that combining SL-p with sanitizer will reduce pathogen populations and enhance grape tomato safety at post-harvest.