Influence of Vegetable Varieties on the Growth of Listeria monocytogenes and Escherichia coli O157:H7

Introduction: Packaged fresh-cut vegetables are generally minimally processed without a processing step to eliminate contaminated microorganisms.  The cut or damaged surfaces of vegetables release nutrients and moisture that may provide a favorable environment for microbial growth.

Purpose: The purpose of this study was to determine the influence of four varieties of fresh-cut vegetables on the growth rates of Listeria monocytogenes and Escherichia coli O157:H7 at refrigerated and abuse temperatures.

Methods: Iceberg lettuce, green pepper, carrot, and broccoli were washed with 0.1% chlorine and cut into pieces.  A mixture of 5 strains of L. monocytogenes or E. coli O157:H7 was inoculated on the cut or damaged surfaces (0.5 cm x 0.5 cm) of vegetables pieces to achieve an initial level of 10^2-3 CFU/g.  Samples were placed in sterile containers and stored at 4 (L. monocytogenes only), 8, 12, 15, and 20°C. The populations of L. monocytogenes and E. coli O157:H7 in samples during storage were determined to estimate their growth rates (GR).

Results: At 4°C, the GR of L. monocytogenes were the lowest (P < 0.05) in carrot (no growth), followed by broccoli (0.12 ± 0.07 log/day), green pepper (0.13 ± 0.03 log/day), and iceberg lettuce (0.16 ± 0.04 log/day).  At 8°C, the GR of L. monocytogenes was higher than E. coli O157:H7 in the vegetables. At temperatures ≥ 12°C, the GR of both pathogens on the vegetables were not significantly different (P > 0.05), and the GR were lower (P < 0.05) on carrot and broccoli than green pepper and iceberg lettuce.  

Significance: The data suggest that L. monocytogenes is more of a concern than E. coli O157:H7 in fresh-cut vegetables and carrot is less supportive to growth of both pathogens than lettuce, green pepper and broccoli.