Purpose: Evaluate the effect of hydro-cooling on L. monocytogenes ability to infiltrate into cantaloupes and develop large bacterial population in edible portions of the fruit.
Methods: Cantaloupes, both freshly harvested and purchased from a local grocery marker, were used in the study. Fruit temperatures were calibrated to 40°C for 24 h. Hydro-cooling was conducted at 6°C for 40 min by a submersion of cantaloupes into water containing Acid Blue 9 dye (100 mg/ml) and 3 Jensen farm outbreak L. monocytogenes strains (6 log CFU/ml). After hydro-cooling fruits were stored at 3°C. On day 1, 7, and 18 after hydro-cooling, 3 - 5 fruits of each cultivar were surface disinfected by a submersion in 0.5% sodium hypochlorite solution for 15 min. Different areas of fruit mesocarp (variable depths and distances from stem scar) were aseptically extracted and analyzed for bacteria internalization.
Results: Dye uptake by cantaloupes in the process of hydro-cooling demonstrated that from the stem scar water is distributed within the fruit through the vascular system primarily in hypodermal mesocarp, and reaches the calix. Water influx spreads through the mesocarp via secondary xylem vessels. Under experimental conditions L. monocytogenes internalized into cantaloupes during hydro-cooling. Populations of internalized L. monocytogenes colonized edible portions of the fruit reaching 8 log/g within two weeks after hydro-cooling at storage temperature of 3°C.
Significance: Identification of postharvest practices leading to melon contamination will allow significant reduction in foodborne Listeriosis associated with consumption of cantaloupes