Listeria and the special shelf life considerations of ready-to-eat foods

When it comes to date labels, shelf life and food safety, there’s a narrow range of foods that merit some caution because, after an extended stay in the fridge, they might allow a specific human disease-causing bacteria to grow even while kept cool, and even while they still look and taste fine. These are typically moist, relatively low-acid, ready-to-eat foods served cold. That could include smoked seafood, soft cheeses, deli meats, deli salads, pre-cut fruits and vegetables, like melon, and other foods. You may notice that we’re leaning a lot on “mights” and “coulds”–that’s because many products are designed with protections in place.

In this article, we’re going to focus on a specific type of bacteria that can infamously survive and grow in the fridge, if given the right opportunity. Its name is Listeria monocytogenes. Nice to never meet you!

Check out our Behind the Date Label page for more help assessing food past its printed date. 

“Use by” and “sell by” dates and listeria

In food products where listeria can grow, the “use by” date may be pegged, with a robust safety buffer, to the time bacteria needs to reach problematic levels. But before you panic about listeria in food juuuust past its date, keep these facts in mind: 

(1) Often, though not always, food will be obviously unfit to eat before listeria can reach disease-causing levels.

(2) While listeriosis can be quite serious and has a high death rate, exposure to small amounts of the bacteria rarely makes healthy adults sick; the small portion of the population at high risk from foods where the bacteria easily grows is advised to simply avoid them altogether (here’s a list of foods with higher listeria risk and alternatives from the Centers for Disease Control for pregnant people, the elderly and those otherwise immunocompromised).  

(3) Careful handling of specific foods will significantly reduce what’s already a relatively low risk to healthy adults. In the United States, the CDC estimates that listeriosis strikes only about 1,600 people every year. 

Aside from its mind-bending tendency to rarely cause illness, but to cause serious illness when it does strike, L. monocytogenes has some special and very frustrating features. Here are two of them:

• It is widespread in the environment, commonly residing in soil and on plants and animals. 
• It can grow at refrigeration temperatures (salmonella and pathogenic E. coli, by contrast, struggle to grow in a fridge that’s at the proper temperature of 40 degrees or less). 

So, because listeria is ubiquitous, it’s easy for it to land on food, and if that food provides a comfortable setting (as in, not very acidic, little competition from other microbes, easy access to nutrients and abundant water) listeria can boost its numbers, making it more likely to infect a person. Listeria is killed by heat, but a cooked food that’s handled a lot afterward, like a roasted chicken that’s then chopped for chicken salad, can be contaminated after cooking and then support the bacteria’s growth. That can actually be an ideal scenario from listeria’s perspective because cooking the chicken will kill off other microbes that would have otherwise competed for resources. 

How do regulators, manufacturers and stores reduce the risk of listeria?

Technically, the U.S. Department of Agriculture has a zero tolerance rule for any L. monocytogenes on ready-to-eat foods containing poultry or meat. Practically speaking, however, listeria is so ever-present it inevitably lands on some foods and may never be detected. We’ve all probably consumed it in non-disease-causing quantities; many countries even allow low levels of listeria on food. Kathy Glass, associate director of the Food Research Institute at the University of Wisconsin, said keeping listeria at bay “is a never ending battle for manufacturers (just like dust accumulating in your home).”

Manufacturers also frequently add antimicrobials to foods, particularly lunch meats, so even if bacteria lands on the food, it can’t grow, said Douglas L. Marshall, chief scientific officer at Eurofins Microbiology Laboratories, which helps companies assess the shelf life and safety of their foods. Marshall said it’s common to add salts of lactic acid, which is the same acid you find in fermented foods, and acetic acid, the acid in vinegar.  Words like “lactate” and “acetate” in an ingredient list would be clues that such acids are present.

“The vast majority of commercial refrigerated foods manufacturers have engineered safety into their packaged products, such as adding antimicrobials that prevent psychrotrophic pathogen growth or doing post-packaging pasteurization that kills pathogens in the product,” Marshall said. “Pathogen growth retardation should last for the duration of intended shelflife…As a result, there are few outbreaks associated with packaged deli foods (salads, dips, meats, etc.). Instead, the main risks are those refrigerated products that have no or few barriers – think fresh cut leafy greens, fresh cut fruits, unpasteurized juices, unpasteurized cheeses, or items that are subject to cross contamination at retail deli counters, food service kitchens, or in the home.”

Check out our article on the shelflife of pre-cut produce

How should you handle food associated with listeria?

Always keep your fridge set to 40 degrees or below. The cooler the fridge, the slower listeria will grow.

If you aren’t in a high risk group and would like a guideline, consider that while the U.S. government generally doesn’t require consumer-facing date labels, the Food and Drug Administration’s Food Code says that certain ready-to-eat food prepared on-premises in a retail environment should be properly refrigerated and sold within seven days to prevent the spread of listeria. This guideline is also conservative and accounts for some time in the consumer’s home after it’s sold.

Likely due to the Food Code’s standard, your lunch meat might be stamped with a “sell by” date to help grocery store staff manage inventory. No matter what the “sell by” date says, the USDA advises consumers that lunch meats should be refrigerated and then consumed within three to five days of purchase. 

And again, to protect your sanity, remember that listeria rarely strikes healthy people and that keeping food cool and eating it promptly are best practices for avoiding listeria and spoilage bacteria too. Spoilage bacteria won’t cause disease, but can turn your food into a slimy, gooey, stinky mess. 

Can foodborne pathogens other than listeria grow in the fridge?

In 2004, the U.S. National Advisory Committee on Microbiological Criteria For Foods identified L. monocytogenes as the pathogen “most likely to first reach the level of public health concern under refrigeration” for most refrigerated ready-to-eat foods. 

The committee also looked at three other pathogens technically capable of growth at refrigeration temperatures (Clostridium botulinum, Bacillus cereus, and Yersinia entercolitica), but found that in cases where they caused disease, extended time in the fridge was not a central cause; more often poor food handling outside the fridge was at issue.

New Zealand food safety authorities still note these other types of bacteria as potential threats in ready-to-eat foods.

And if your fridge is above 40 degrees other types of foodborne pathogens could grow, and listeria can grow faster. 

“If your refrigerator is operating at more like 50 degrees Fahrenheit, your food’s still relatively cold, right?” said Faith Critzer, a food science professor at the University of Georgia who focuses on produce and microbiology. “And you may not notice anything. But that slight shift is very beneficial to an organism like listeria.”

When people get listeriosis is it typically because they waited too long to eat a food? Or ate it after the “use by” date?

It’s possible that they either waited too long to eat the food, or, perhaps a little more likely, that they ate a food that was heavily contaminated earlier in the supply chain. In the European Union, about a third of invasive listeriosis cases are due to growth in the “consumer phase,” according to quantitative modeling cited by the European Food Safety Authority. 

If a food is contaminated, which is possible even when it still looks fine, then waiting to eat it will increase your odds of getting sick;  the bacteria will have more time to grow to infectious levels. When consumer handling is the problem, only a single person could come down with listeriosis. One case can’t be deemed an “outbreak.” 

When a widespread listeriosis outbreak does occur it’s typically because a food was heavily contaminated (like in this outbreak, linked to a poorly cleaned milkshake machine) and then widely distributed. In those cases, the food most likely looks, smells and tastes normal. And, usually, because healthy individuals rarely get sick from listeria, only a minority of people who consume the food will get sick.

In the U.S., when a company discovers listeria may be present on its food, it issues a recall. But since a recall only means they detected the bacteria and not that anyone has gotten sick, plenty of recalled products never cause a documented outbreak. For example, in 2021, between FDA and USDA records, I counted more than 50 recalls on foods like salads and hot dogs due to possible listeria contamination. During that same time, CDC data accounted for nine listeriosis outbreaks, and, of those, only four were traced to specific foods (ice cream, pre-cooked chicken, leafy greens). According to the Mayo Clinic, symptoms of listeria infection can show up anywhere from several days to a month after eating contaminated food. That range makes it especially tough to track down the offending food. 

How do food manufacturers know if their food is susceptible to listeria growth and if they should apply a “use by” date? 

Companies that manufacture foods associated with listeria risk often work with consultants to make sure they’re covering their food safety bases. Various organizations have also published decision trees to help manufacturers determine whether their food could support the growth of listeria, or other pathogens and should use a date pegged to food safety. Here’s one from the European Food Safety Authority:

You’ll find another, from WRAP, in their Labelling Guidance report. And ReFED, a U.S. nonprofit focused on reducing food waste, offers a slide deck decision tree as part of its Date Label Standardization Tool. 

Check out our article on how companies determine shelf lives for a variety of products.

Tell me again which foods I should be careful with to avoid exposing myself to listeria?

Ready-to-eat foods that are moist, relatively low acid, stored in the fridge for an extended amount of time, and served cold can support listeria growth if they don’t have antimicrobial ingredients. Factors that make a food riskier include: 

• Serving the it cold, straight from the fridge (thorough heating kills listeria)
• Being assembled outside the strict protocols of a factory
• Containing high moisture and low acid levels
• Including ingredients that are cooked (which kills bacteria that might compete with listeria and/or render the food spoiled before listeria grows enough) and then mixed with raw foods and other ingredients that could be contaminated. 
• Not containing antimicrobial ingredients or a post-packaging pastuerization step

ReFED’s decision tree lists a number of foods that could merit a “use by” date label for safety reasons. They include:

• Deli meats
• Pâté and meat spreads
• Unpasteurized milk and soft cheeses
• Smoked seafood
• Cooked ready-to-eat crustaceans
• Prepared salads and sandwiches
• Sushi

But in some cases, those foods may be prepared with anti-microbial ingredients and will stay safe for a long time. Or, at least until heartier-than-listeria spoilage organisms render the food unpalatable. Or other quality changes could crop up before the food becomes unsafe. The bread in a pre-made sandwich, for example, could get so tough or stale that no one would want to eat it anyway. And that could easily happen before listeria gets to problematic levels.

On the CDC’s website, you can find a complete list of foods that people who are extra vulnerable to listeriosis are urged to avoid. The CDC’s table also includes alternatives, like eating deli meats that have been heated to 165 degrees or opting for cream cheese instead of brie. 

And visit other EatOrToss articles in our Behind The Date Label series for more on how to assess “use by” and “best by” dates on food. 

Critzer said that while it’s a good idea to be mindful of listeria, scientists and food companies are doing good work in the background to prevent illness. 

“It’s not that we have this gigantic issue and it’s terrible,” she said. “It’s that we have an issue. We’re trying to reduce the incidence even more and hopefully are making good moves through regulations and the industry acting proactively.”

The image at the top of this post is “a three-dimensional (3D), computer-generated image of a grouping of Listeria monocytogenes bacteria,” sourced from CDC’s image gallery. According to CDC, the artistic recreation by illustrator Jennifer Oosthuizen was based upon scanning electron microscopic imagery.