Survival and growth of Listeria monocytogenes on whole cantaloupes is dependent on site of contamination and storage temperature

Abstract

Whole cantaloupes (Cucumis melo L.), marketed as ‘Rocky Ford’, were implicated in a large multi-state outbreak of listeriosis in the United States in 2011; however, survival and growth of Listeria monocytogenes on whole cantaloupes remains relatively unexplored. The research presented here evaluated three different storage temperatures, two sites of contamination of cantaloupes, and two cantaloupe varieties to determine their effect on the survival of L. monocytogenes. ‘Athena’ and ‘Rocky Ford’ cantaloupe cultivars were grown in soil and harvested, and individual melons subsequently received a multi-strain inoculum of L. monocytogenes (6 log CFU/melon), which were then stored at 4°C, 10°C, and 25°C. Changes in L. monocytogenes populations on the rinds and stem scars of cantaloupes stored at each temperature were determined at selected times for up to 15days. An analysis of variance revealed that inoculation site and storage temperature significantly affected survival of L. monocytogenes on cantaloupes during storage (p<0.05), but cultivar did not influence L. monocytogenes (p>0.05). Populations of L. monocytogenes on stem scars of cantaloupes stored at 25°C increased by 1–2 log CFU/melon on day 1, and were significantly greater than those on cantaloupes stored at 4°C or 10°C (p<0.05), which remained constant or increased by approximately 0.3 log CFU/melon, respectively, over the same time period. A decrease of 2–5 log CFU/melon of L. monocytogenes occurred on the rinds of cantaloupes during storage by day 7, and were not significantly different at the three different storage temperatures (p>0.05). In trials performed in rind juice extracts, populations of L. monocytogenes decreased by 3 log CFU/mL when stored at 25°C by day 3, but grew by 3–4 log CFU/mL when stored at 4°C over 7days. Overall, site of contamination and storage temperature influenced the survival of L. monocytogenes on cantaloupes more than cantaloupe cultivar type.