Foodborne pathogens contaminate melons during production from farm to fork and cause outbreaks. Understanding contamination risks and devising effective decontamination measures are important to prevent foodborne outbreaks. The objective was to investigate a) the attachment strength (AS) of Salmonella Newport and Listeria monocytogenes to melon rinds and b) efficacy of plant-based antimicrobials against both foodborne pathogens on melon rinds. The AS of S. Newport and L. monocytogenes and efficacy of antimicrobials on 6-7 melon varieties grown in 7 locations covering 6 US states (AZ, IN, NC, GA, TX-Uvalde, TX-Weslaco, and CA) were investigated. Melon discs or pieces were inoculated with overnight cultures of one of the test pathogens (~6-7 log CFU/mL). Inoculated discs were allowed 30 min for bacterial attachment, then vortexed for 15 sec to recover loosely attached and sonicated for 2 min to recover strongly attached cells, which were enumerated to calculate AS using the formula; AS= (strongly attached bacteria) / (loosely attached bacteria + strongly attached bacteria). Melon pieces were immersed in 5% olive extract or 0.5% oregano oil solution for 2 min with gentle agitation, stored at 4°C and survivors enumerated at Days 0 and 3. OC164 and HD150 had the lowest and highest average AS for Salmonella (0.24 and 0.31), respectively. HD150 and Infinite Gold had the lowest and highest average AS for L. monocytogenes (0.17 and 0.27), respectively. In general, AS of Salmonella and L. monocytogenes on cantaloupes (0.25-0.28; 0.21-0.27) was higher than on honeydews (0.24-0.31; 0.17-0.23). Salmonella showed a stronger AS than L. monocytogenes. Plant-based antimicrobials reduced both pathogens on all samples, regardless of melon types, varieties or locations. Compared to control, antimicrobials caused 1.7-3.6 and 1.3-4.0 log reductions in Salmonella and L. monocytogenes, respectively. In most cases, pathogen populations were below detection levels (1 log CFU/g) at Day 3. In general, oregano oil had better antimicrobial activity than olive extract, and both antimicrobials were more effective on Salmonella than L. monocytogenes and on honeydews than cantaloupes. Results will help determine a) melon varieties that could pose a food safety risk due to stronger bacterial attachment in case of a contamination event from pathogens and b) antimicrobials that could potentially be used as sanitizers for decontaminating melons.

Libin Zhu

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