Reduction of Pathogens on Cantaloupes
Salmonella contamination of cantaloupes continues to be a significant concern in the produce industry. The Centers for Disease Control and Prevention has reported nine outbreaks of salmonellosis associated with consumption of cantaloupes from 1990 to 2014, resulting in almost 500 reported illnesses and 5 deaths. Listeria monocytogenes has also been recognized as a pathogen of concern in the cantaloupe industry. In 2011, Colorado-grown cantaloupes were found to be contaminated with L. monocytogenes originating from harvesting and packing facility equipment, resulting in 147 illnesses, 1 miscarriage, and 33 deaths.
Chlorine and chlorine dioxide are frequently used to sanitize cantaloupes after harvest. We have previously compared these sanitizers with a mixture of levulinic acid and sodium dodecyl sulfate (SDS) for their efficacy in reducing Salmonella and Listeria on/in cantaloupe stem scar and netted rind tissues after simulated dump tank treatment. Treatment with levulinic acid/SDS consistently resulted in greater decreases in pathogen contamination compared to treatment with chlorine or chlorine dioxide alone.
We have obtained from growers in south Georgia fresh harvested Eastern variety cantaloupes (Cucumis melo L. var. reticulatus cv. Athena) to evaluate levulinic acid/SDS as a sanitizer for field application to post-harvest cantaloupes. We also evaluated the sanitizer as a dip treatment for cantaloupes in packing facilities. Both treatments were applied in combination with simulated dump tank treatments to determine if increased efficacy of sanitizer application could be achieved.
Field treatment consisted of injecting 7.5% levulinic acid/1.0% SDS solution (200 µl) by pulse needle-free injection at 60 psi, combined with a 30-ml spray treatment with 7.5% levulinic acid/0.5% SDS. After treatment, stem scar and netted rind tissues were spot inoculated (100 µl of either Salmonella Poona or Listeria monocytogenes suspension) or soil inoculated (2.5 g of soil containing pathogens). Following pathogen acclimation and drying, the cantaloupes were subjected to a simulated dump tank treatment with chlorine (200 µg/ml) or 5% levulinic acid/2% SDS. The inoculated stem scar and rind areas were then excised in 2.5 cm x 2.5 cm x 0.5 cm pieces.
The initial population of S. Poona on the rind decreased by 4.74 and 5.31 log CFU/piece (spot inoculation) and 3.27 and 3.36 log CFU/piece (soil inoculation), respectively, after simulated chlorine and levulinic acid/SDS dump tank treatments. Spot and soil inoculation methods resulted in statistically similar (P > 0.05) results for S. Poona on cantaloupe stem scar tissue and the merged data revealed a greater reduction (P < 0.05) in log CFU/piece after treatment with 5% levulinic acid/2% SDS (2.45 log CFU/piece) compared to treatment with 200 µg/ml chlorine (1.36 log CFU piece) for field and tank treatments. Reductions in L. monocytogenes on/in spot- and soil-inoculated stem scar and netted rind tissue were not significantly different (P > 0.05). Reductions of 1.65 and 3.12 (stem scar) and 4.75 and 5.40 log CFU/piece (netted rind) were observed after dump tank treatments with 200 µg/ml chlorine and 5% levulinic acid /2% SDS, respectively.
A second study focused on comparing a dip treatment with 5% levulinic acid/2.5% SDS versus a chlorine (200 µg/ml) dump tank treatment alone and a chlorine dump tank treatment with a dip treatment. Populations of S. Poona (log CFU/piece or number positive by enrichment culture/number analyzed) were 5.25, 3.07, 7/10, and 5/10 (stem scar) and 3.90, 25/40, 28/ 40, and 20/40 (netted rind) on untreated (control) cantaloupes, and cantaloupes treated 10 minutes with 200 µg/ml chlorine in a dump tank alone, 1 min with 5% levulinic acid/2.5% SDS dip alone, and dump tank plus dip treatments, respectively.
Single- and double-hurdle packing shed treatment of cantaloupes initially containing L. monocytogenes at 4.40 log CFU/piece resulted in populations (log CFU/piece or number positive enrichment cultures/number samples analyzed) of 3.3, 1.05, 2.95 (stem scar) and 38/41, 34/41, and 35/41 (netted rind) on cantaloupes treated for 10 min with chlorine (200 µg/ml) in a dump tank, 1 min with a 5% levulinic acid/2.5% SDS dip, and combined dump tank and dip treatments, respectively.
Treatment with the levulinic acid/SDS sanitizer alone or in the field/tank treatment consistently resulted in greater reductions of Salmonella and Listeria on/in cantaloupe stem scar and rind tissues compared to treatment with chlorine. The porous stem scar tissue continues to be a challenge with regard to protection against inactivation of pathogens. Stem scar tissue injection of 7.5% levulinic acid/1.0% SDS did not result in significant decreases in S. Poona or L. monocytogenes. Two-step hurdle approaches such as applying a field sanitizer spray combined with a dump tank treatment or a tank treatment combined with dip treatments, however, provide additional reductions of S. Poona on cantaloupe tissues compared to single-treatment approaches.