Purpose: Using a series of commercial time/temperature profiles collected across the United States, the goal of this study was to simulate the probability distributions for growth of Escherichia coli O157:H7 and Listeria monocytogenesin commercially bagged salad greens during transport, retail storage and display.
Methods: Over a 16-month period, a series of time/temperature profiles were obtained from bagged salad greens in 16 transport routes covering four geographic regions (432 profiles), as well as during retail storage (4,867 profiles) and display (3,799 profiles) at nine supermarkets. Five different time/temperature profiles collected during 2 – 3 days of transport, 1 and 3 days of cold room storage and 3 days of retail display (including the best and worst scenarios) were then duplicated in a programmable incubator to assess E. coli O157:H7 and L. monocytogenesgrowth in commercial bags of a Romaine lettuce salad mix. Microbial growth predictions using the Baranyi/Ratkowsky model were validated by comparing the root mean square error (RMSE) and bias between the laboratory growth data and model predictions. Monte Carlo simulations (~100,000 iterations) were then performed to calculate the probability distribution of microbial growth from a total of 432 × 4867 × 3799 (7,987,564,656) scenarios during transport, cold room storage, and retail display.
Results: Using inoculated bags of retail salad, E. coli O157:H7 populations increased a maximum of 0, 0.6, and 0.3 log CFU/g during transport, retail storage, and display, respectively, with L. monocytogenes populations increasing 0.6, 1.9, 1.1 log CFU/g during the same three periods. The RMSEs between the experimental and predicted populations were all acceptable (E. coli O157:H7 < 0.8 log CFU/g, Listeria < 1.2 log CFU/g). Based on the simulation results, both pathogens generally increased < 1 log CFU/g. Storing the product for 1 and 3 days before display yielded overall growth (> 1 log CFU/g) probabilities of 0.02 and 0.34 for Listeria, respectively.
Significance: This large-scale US study—the first using commercial time/temperature profiles to assess the microbial risk of leafy greens during transport, retail storage, and display—should be useful in filling some of the data gaps in current risk assessments for leafy greens.