Tracing leafy green storage and display temperatures in schools

Ellen Thomas, PhD candidate in the Department of Food, Bioprocessing and Nutrition Sciences at North Carolina State University writes,

In 2006, an E. coli O157:H7 outbreak associated with bagged spinach resulted in 205 illnesses and three deaths. Investigators cited many factors, including feral swine, that resulted in contamination of spinach in the field. It is also suspected that temperature abuse of the spinach (meaning that the spinach wasn’t kept cold enough) during transportation and storage also allowed the E. coli to reproduce, boosting the population of bacteria on the spinach  Numerous additional studies have shown that temperature abuse of cut leafy greens can result in growth of pathogens such as E. coli O157:H7.

product_butterlettuceOne of the great parts about working in the field of food safety and extension is that you often get a chance to help protect public health by answering questions about current policies for different groups. Two years ago, the North Carolina Department of Public Instruction (DPI) voiced concerns that their current systems for serving salads as part of school lunch programs might not be holding produce at cold enough temperatures, as recommended by the FDA Food Code (5°C or below). DPI approached our group to request help in addressing this issue.

The North Carolina school Hazard Analysis and Critical Control Points HACCP program defines two methods for the storage and serving of salads. The first system is “time in lieu of temperature” (TILT), which sets a time limit on how long food can be held on the serving line; any food remaining after that time must be discarded. The second system is the three-day rule, in which a salad may be displayed for up to three days and must be discarded by the end of day three.

In 2012, I visited 24 schools across North Carolina in order to determine the temperatures at which salads were actually being stored and served. I did this by placing a data logger in a “test salad” that was held with other salads and held in the same conditions (it was labeled to prevent accidental sale). The data logger recorded temperature every five minutes for the entire time that the salad was put on the serving line. This allowed me to determine how long temperatures might have exceeded 5°C.

I found that the majority of the serving systems were out of compliance- most salads were reaching temperatures above 5°C for long periods of time. After observing this trend, we made the recommendation to DPI that schools should adopt the TILT system to replace the three day rule in order to reduce the risk of foodborne illness through temperature abuse – and they did.

“Tracing Temperature Patterns of Cut Leafy Greens during Service in North Carolina School Food Service” was published in the September issue of Journal of Food Protection.


Contaminated fresh produce has been increasingly identified as a cause of foodborne illnesses. Because of concerns about pathogen growth on these food items at retail, the 2009 U.S. Food and Drug Administration Food Code established that cut leafy greens (lettuce, spinach, spring mix, cabbage, arugula, and kale) must have time and temperature controls for safety and hence should be kept at refrigerated temperatures (5°C or lower). The purpose of this study was to determine the temperature profiles of cut leafy greens in single-serving clamshell containers provided as part of the North Carolina School Lunch Program and to compare the two policies that North Carolina has in place to control the temperature of these products (the 3-day rule and time in lieu of temperature). Temperatures were recorded with data loggers in 24 schools during a 3-day period. In all cases, substantial temperature variability was found for these products, including temperatures above 5°C for at least 1 h on each of the 3 days. In some cases, temperatures reached above 5°C for more than 3 h throughout the serving time. The results demonstrate the importance of developing a protocol for continuous temperature monitoring of leafy greens served in school lunch programs.

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About Ben Chapman

Dr. Ben Chapman is a professor and food safety extension specialist at North Carolina State University. As a teenager, a Saturday afternoon viewing of the classic cable movie, Outbreak, sparked his interest in pathogens and public health. With the goal of less foodborne illness, his group designs, implements, and evaluates food safety strategies, messages, and media from farm-to-fork. Through reality-based research, Chapman investigates behaviors and creates interventions aimed at amateur and professional food handlers, managers, and organizational decision-makers; the gate keepers of safe food. Ben co-hosts a biweekly podcast called Food Safety Talk and tries to further engage folks online through Instagram, Twitter, Facebook, YouTube and, maybe not surprisingly, Pinterest. Follow on Twitter @benjaminchapman.