It usually happens when the refrigeration system is temporarily shut down during cleaning and sanitizing of the cold room.
Temperature sensors were placed in trucks during transport, retail storage rooms, and retail display cases to monitor temperature fluctuations and collect time/temperature profiles.
Both pathogens increased less than 2 log CFU/g based on mathematical models, found the study.
Microbial growth predictions using the Koseki-Isobe and McKellar-Delaquis models were validated by comparing the root mean square error (RMSE), bias, and the acceptable prediction zone between the laboratory growth data and model predictions.
Tracing distribution temperatures during the three stages and mathematically predicting potential for growth and/or survival of E. coli O157:H7 and L. monocytogenes have been identified as major data gaps in current risk assessments.
Time and temperature readings were recorded at 5-min intervals during two to four days of transport.
Over a 16-month period, time-temperature profiles for bagged salads came from five transportation routes covering four geographic regions (432 profiles), and during retail storage (4,867 profiles) and display (3,799 profiles) at nine supermarkets.
Five different time temperature profiles collected during 2 to 3 days of transport, 1 and 3 days of retail storage, and 3 days of retail display were then duplicated in a programmable incubator to assess E. coli O157:H7 and L. monocytogenes growth in commercial bags of romaine lettuce mix.
Monte Carlo simulations were performed to calculate the probability distribution of microbial growth from 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 log CFU/g at retail storage.
“The opening of truck doors during loading and unloading, outside temperature extremes, and retail storage and display conditions can contribute to temperature fluctuations,” said the researchers.
Within each refrigerated cargo trailer, 24 to 30 sensors were placed in three key zones (front, midway, and rear, midway up the sidewall of the truck) and at three pallet levels (top, middle, and bottom).
Sensors were placed midway up the sidewalls at the front, midsection, and rear of each trailer (unpublished data).
In bags of inoculated romaine mix, E. coli O157:H7 populations remained at less than 3 log CFU/g
during 48 to 52 hours of incubation at the five selected and simulated transport temperatures.
Simulated retail storage led to E. coli O157:H7 population increases of 0.1 to 3.1 log CFU/g, with no significant growth seen during 72 hours at retail display temperatures.
Listeria monocytogenes populations increased less than 0.6 log CFU/g at the five simulated transport temperatures.
Greater growth was seen during 72 hours at retail storage and display temperatures, with populations increasing up to 3 and 1.1 log CFU/g, respectively.
Source: Journal of Food Protection, Vol. 77, No. 2, 2014, Pages 197–206
Online ahead of print, DOI: 10.4315/0362-028X.JFP-13-117
“Growth of Escherichia coli O157:H7 and Listeria monocytogenes in Packaged Fresh-Cut Romaine Mix at Fluctuating Temperatures during Commercial Transport, Retail Storage, and Display”
Authors: Wenting Zeng, Keith Vorst, Wyatt Brown, Bradley P Marks, Sanghyup Jeong, Fernando Perez-Rodriguez and Elliott Ryser