The effect of crisping, misting, and storage temperature on the survival or growth of Listeria monocytogenes and natural psychrotrophic bacteria on romaine lettuce.

Abstract

L. monocytogenes has been linked to fresh produce and detected in the retail environment. This study simulated the retail practices (crisping, misting, and storage) of unbagged whole heads of romaine lettuce to determine the growth of L. monocytogenes and natural psychrotrophic microflora. Three nalidixic acid-resistant strains of L. monocytogenes strains were inoculated to each head of lettuce (≈5 log10 CFU/g). For crisping, 24 heads of romaine lettuce were immersed in tap water or electrolyzed water (EW; free chlorine: 55 ppm) for 5 min, followed by holding at 5 °C for 2 h. The water-crisped (WC), EW crisped (EWC), or non-crisped (NC) lettuces were placed in a commercial refrigerated cabinet for misting at 5 °C. After 24-h misting, heads of lettuce were placed in perforated drain boxes with cover at 5 °C or 15 °C. The tap water and EW crisping achieved 1.3 and 2.9 log10 CFU/g reduction of L. monocytogenes, respectively. Approximately 1 log additional reduction of L. monocytogenes in the non-crisped lettuce was shown after misting (p < 0.05), but no significant effect of misting on the population of L. monocytogenes was observed on WC or EWC lettuces (p > 0.05). Regardless of the storage temperature or misting, L. monocytogenes populations remained significantly (p < 0.05) lower on EWC lettuce than NC and WC lettuce. On days 4 and 7 of storage, the natural psychrotrophic bacteria on lettuce stored at 5 °C was significantly lower than stored at 15 °C, and its population was not affected by crisping and misting. These provide insight into the influence of retail lettuce handling practices on the risk of L. monocytogenes.