ASK THE EXPERT - The Effects of Time and Temperature on Pathogen Growth
By: Dr. Benjamin Chapman, Department Head of Agricultural & Human Sciences and Professor at North Carolina State University
My phone is often blowing up with food safety texts from friends around the world. Whether it's a question about refrigerating hot sauce, leaving leftovers from a restaurant in the car overnight, or a power outage leading to cool to the touch food in the fridge, my responses are often looking for more information: do you know the time and temperature?
Time and temperature together are linked factors in the growth of pathogens - the harmful bacteria that can lead to a foodborne illness.
One of the mantras of the podcast I cohost with my friend and colleague Don Schaffner, Food Safety Talk, is that assessing the safety of food boils down to "it's complicated and it depends" and often it's the variable time and temperature that is at the root of that saying. Whether someone has forgotten roasted vegetables in the oven overnight or left groceries on in their trunk, what really determines the safety of the food is quantitative - the raw numbers on how long and at what temperature.
Or to get even more specific, what was the starting temperature, what was the end temperature when the mistake was discovered and how long did it take to get there (or sit there).
Sometimes people say things like, 'it seemed pretty cool when I touched it.' I'm a fan of temperature data. Just saying something is cool when touched doesn't provide enough information to judge the safety of the food. The difference in risks with some foods and some pathogens when comparing between 55°F for 6 hours and 42°F for 6 hours is significant. A little bit of contamination, with maybe just 20 cells of a pathogen can grow to millions over that time increasing the chance of illness. And not all pathogens grow at the same rate under all conditions. Water content, pH level (or acidity of the food), and salt content are all important factors to consider when determining pathogen growth rate.
One of the tools I use (but often don't tell those texting friends) is a pathogen growth modeling resource called Combase. This tool allows me to make some guesses at the food environment and see growth (or death) of pathogens. But none of this really works without the time and temperature information.
Throughout the pandemic (and really starting before) there has been a shift away from traditional food purchasing habits - so many more are ordering food, either as meals, pre-portioned recipe kits or groceries and food boxes from a variety of services. The growth of these services also has led to an increase of texts from those same individuals in my contact list - stuff like 'I came home to find my box in the sun and the gel pack had fully melted' or 'there was a delay in delivery, we went away for the weekend and my food sat on my front step for an extra two days.' These questions are answered with the same query for more time and temperature information. The best tool consumers must assess is some sort of temperature taking device - like a thermometer. Not only do they help with all the 'I left food out' questions but also provide the best way to figure out doneness (for both safety and quality reasons).
This is a systematically formatted database of quantified microbial responses to the food environment.
The ComBase Browser enables you to search thousands of microbial growth and survival curves that have been collated in research establishments and from publications.
The ComBase Predictive Models are a collection of software tools based on ComBase data to predict the growth or inactivation of microorganisms.
Not all pathogens grow at the same rate under all conditions.
Water content, pH level (or acidity of the food), and salt content are all important factors to consider when determining pathogen growth rate.
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 gatekeepers of safe food. Ben co-hosts a bi-weekly 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.
Areas of Expertise
- Consumer, retail and food safety culture
- Home food preservation
- Communicating food safety risk reduction messages
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