Effect of Storage Temperature on the Survival and Growth of Listeria monocytogenes Populations in the Presence of Indigenous Surface Microflora of Fresh-cut Cantaloupes

Introduction:  The most recent outbreak of listerosis linked to consumption of fresh-cut cantaloupes contaminated by L. monocytogenes suggests the need to investigate the behavior of Listeria monocytogenesin the presence of native microflora of cantaloupe pieces during storage.

Purpose:  The behavior of L. monocytogenesin the presence of native microflora of cantaloupe pieces during storage and the effect of waiting period before refrigeration on microbial populations on fresh-cut cantaloupes was investigated.

Methods:   Whole cantaloupes were inoculated with L. monocytogenes (10⁸ CFU/ml of suspension) for 10 min and air dried in a biosafety cabinet for 1 h and then were treated (unwashed, water washed and 2.5 % hydrogen peroxide (H₂O₂)). Fresh-cut pieces (~3 cm) prepared from these melons were left at 5 °C and 10 °C for 72 h and room temperature (20 °C) for 48 h. Some fresh-cut pieces were stored at 5 °C after 2 h and 4 h of storage at 20 °C. Microbial populations of fresh-cut pieces were determined immediately by the plate count or enrichment method after preparation and during storage.

Results:   Aerobic mesophilic bacteria and yeast and mold of whole melon and inoculated populations of L. monocytogenes on cantaloupe rind surfaces averaged 6.5 log CFU/cm², 3.3 log CFU/cm² and 4.6 log CFU/cm², respectively. Among the treatments, only H₂O₂ (2.5%) reduced the aerobic mesophilic bacteria, yeast and mold and L. monocytogenes populations to 2.8, 1.3 and 1.8 log CFU/cm², respectively. The populations of L. monocytogenes transferred from melon rinds to fresh-cut pieces were below detection (< 2 CFU/g). Storage temperatures enhanced the lag phases and growth of L. monocytogenes as evidenced by the increase in generation time, implying that these conditions could be a risk factor for produce contamination. Fresh-cut pieces with low populations of L. monocytogenes and approximately 2.8 log CFU/g mesophilic aerobic bacteria had an extended lag phase of 6 h at 10 °C and 4 h at 20 °C before growth of the pathogen could be detected.

Significance:  These results suggest that a waiting period of 4 h at 20 °C before refrigeration of prepared fresh-cut cantaloupes enhanced growth of indigenous microflora and L. monocytogenes transferred to fresh-cut pieces suggesting the need for immediate refrigeration of prepared fresh-cut pieces at 5 °C.