Growth probability of Escherichia coli O157:H7 and Listeria monocytogenes in packaged fresh-cut romaine mix at fluctuating temperatures during simulated transport, retail storage and display

Temperature abuse during commercial transport and retail sale of leafy greens negatively impacts both microbial safety and product quality. Consequently, the effect of fluctuating temperatures on Escherichia coli O157:H7 and Listeria monocytogenes growth in commercially bagged salad greens was assessed during transport, retail storage, and display. Over a 16-month period, a series of time/temperature profiles for bagged salad greens were obtained from 16 transportation routes covering four geographic regions (432 profiles), as well as during retail storage (4867 profiles) and display (3799 profiles) at nine supermarkets. Five different time/temperature profiles collected during 2 to 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. monocytogenes growth in commercial bags of romaine lettuce mix. Microbial growth predictions using the Baranyi/Ratkowsky and McKellar models 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 8,122,127,472 scenarios during transport, cold room storage, and retail display. Using inoculated bags of retail salad, E. coli O157:H7 and L. monocytogenes populations increased a maximum of 3.1 and 3.0 log CFU/g at retail storage. Both models yielded < 1 log CFU/g RMSEs and biases with the Baranyi model fitting slightly better to the laboratory data. Based on the simulation results, both pathogens generally increased < 2 log CFU/g. However, retail storage duration can significantly impact the extent of E. coli O157:H7 and L. monocytogenes growth. Greatest growth of E. coli O157:H7 would be expected in product located at the bottom of the pallet during summer transport. This large-scale U. S. 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.