Slam Dump (Tank) for Cantaloupe Listeria Internalization

In US food safety regulations, cantaloupe melons are a recognized source of illness caused by eating contaminated food. They accounted for 19 out of the 34 cases of foodborne disease (FBD) outbreaks recorded between 1973 and 2011 where the Food and Drug Administration (FDA) identified melons as the source. Because cantaloupes are fresh produce, there are several potential steps in the farm to fork cycle that can increase risk of contamination. Macarisin et al. (2017) investigated pathogen internalization parameters in the fruit following dump tank washing to find out if this stage could be a risk factor in FBD.¹

Pathogen internalization is a risk in fresh produce and takes place when FBD microbes establish themselves within the food matrix. Once internalized, they resist traditional sterilization procedures such as surface washing and other treatments. Furthermore, they present a particular hazard in fresh produce, since these foodstuffs are usually eaten with minimal preparation or heat treatments that might otherwise render the pathogens harmless to consumers. Various studies have shown that internalization is facilitated through cuts, abrasions and other injuries to the produce surface, allowing easy entry for microbes to colonize.

Cantaloupe melons are ground-grown crops that must be separated from the vine before packaging for consumer distribution. There are two ways to harvest the fruits. Full slip, where agricultural workers lightly twist the cantaloupe stem from the vine, takes place when the melon is ripe. However, in order to extend shelf life, some producers prefer to clip the melons from the vine before peak ripeness. Full slip tears the stem from the fruit and leaves a scar, whereas clipping leaves a short stem.

Harvesting takes place in the field. Producers then transport the melons to packaging facilities where they undergo washing and hydrocooling in a dump tank to remove surface contamination. Since the cantaloupes grow on the ground, they are susceptible to a range of contamination from animal waste, surface water contamination and other risks; dump tank washing is carried out to reduce the contaminants from the rind.

Macarisin and coauthors suspected that the dump tank wash could potentially be a source of contamination in cantaloupes. They used conditions that closely followed those encountered during the post-harvest pre-packaging wash and investigated dye infiltration within the fruit’s edible mesocarp to track water uptake.

The research team also investigated the effect of simulated FBD pathogen contamination in the dump tank water using commercial Listeria monocytogenes cultures. The team grew the L. monocytogenes cultures in Buffered Listeria Enrichment Broth (Thermo Scientific) before creating suspensions of 4 log and 6 log cfu per milliliter of dump tank water. 

The researchers obtained cantaloupe melon cultivars (Athena and Rocky Ford) harvested by full slip or clipped methods. Prior to dump tank washes at either 6°C or 18°C, they raised fruit temperatures to replicate a variety of harvesting and processing conditions. Macarison et al. then stored the fruit at 4°C until analysis as an approximation of storage before consumer purchase and consumption. Each of the two cultivars was subjected to the different washing and temperature regimes for comparison.

At day 7 post wash, the investigators cut the melons open and examined the melons visually for dye uptake. They noted the distribution of the dye within the fruit matrix and also its quantity. For microbial analysis, Macarisin et al. subjected the fruits to a rigorous and effective surface decontamination to remove microbial surface contamination before slicing to collect mesocarp for L. monocytogenes evaluation by direct plating and culture. Colony confirmation was carried out using molecular testing.

All variations of treatment (temperature, dump tank temperature, harvesting method) resulted in dye uptake by the cantaloupes. However, clipped melons took up less dye than those harvested by full slip. Dye infiltrated under the rind and into the mesocarp, and onward toward the seeds contained in the center of the fruit.

The research team found that the microbial analyses largely replicated the dye findings. Microbial culture found evidence of L. monocytogenes contamination within all fruits, with those harvested by full slip showing the heaviest concentrations. Moreover, melon flesh devoid of dye infiltration was also negative for L. monocytogenes.

The results showed that under experimental conditions, cantaloupes would take up water from dump tank washing that could contribute to L. monocytogenes internalization. The contamination and water uptake was not affected by cultivar or water/fruit temperature differential but was modified by harvesting method. In separate studies, the team showed that isothermal dunk tank washing, where fruit and water were at the same temperature, did not affect uptake or pathogen internalization.

Although the study design captured only experimental conditions, Macarisin et al. suggest that the results indicate that pathogen internalization is possible under field handling. However, before making suggestions to modify harvesting protocols, the authors advise that larger-scale trials under field conditions should be completed. Studies should also confirm pathogenic burden and potential for FBD transmission in these fruits arising from dump tank contamination.

Learn more about testing for L. monocytogenes.

Reference

1. Macarisin, D., et al. (2017) “Internalization of Listeria monocytogenes in cantaloupes during dump tank washing and hydrocooling,” International Journal of Food Microbiology, 257 (pp.165–175).

 

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