We’ve all eaten leftovers that are likely past their prime — opened a container, sniffed and checked to ensure no signs of mold, taken a few bites to “check and see” and then dove in with little regard to the potential sickness that could ensue.
Yet, according to the FDA, leftovers are ultra-time-sensitive items, to be cooked and eaten three to four days after preparation. To get a more realistic idea of how long leftovers are OK to eat, we spoke with microbiologist Andrea Casero and food scientist Guy Crosby. It turns out that in many cases, there’s some wiggle room.
What The FDA Guidelines Say
The FDA notes that cooked leftovers should be refrigerated or frozen two hours after preparation, and that leftovers should be eaten or thrown out four days after refrigeration.
While food safety is a serious and important concern, unless you’re marking all your containers with “Best By” dates, there is likely some room for error.
According to Peter Cassell, the press officer at the FDA, “It’s a general recommendation that is based on the growth rates of bacteria and proper storage. Generally, use by dates, with the exception of infant formula, are not exact safety dates and are more about the quality from the manufacturer.”
What The Experts Say
According to our experts, it’s not quite as simple as a single rule to guide the whole process. Guy Crosby, a certified food scientist and adjunct associate professor at the Harvard T.H. Chan School of Public Health, explained to HuffPost that “the guidelines as stated [by the FDA] are too general ― one size fits all.” Instead, he recommends considering each item based on how the food was prepared, how it was stored and what the item in question is.
To understand why, we’ll need to briefly go through some chemistry. Foods differ in pH levels, moisture content and potential ingredients that bind to water. Cooked foods with a lower pH levels ― think tomato sauce or lemon pie ― typically will last longer, up to one week, as the acidic condition is more inhospitable to bacteria. Leftovers with higher pH levels ― such as fish or eggs -– can harbor and grow bacteria more quickly and should be eaten or thrown out at the three-day mark.
Other considerations to note include foods with ingredients that bind water and foods with high moisture contents. Foods containing high levels of sugar, such as cookies and muffins, bind water and prevent the growth of bacteria and mold. These foods can be kept unrefrigerated and stored in an airtight container for longer periods safely, up to a week. On the other hand, foods with a high moisture content — for example, cooked meat and cooked vegetables — can grow bacteria more quickly, even when stored in the refrigerator. These should be eaten within the three-day guidelines.
Crosby explained to HuffPost that “a fruit pie that contains a high level of sugar [and binds water] and also contains acidic fruits can actually be stored at room temperature in a closed container for up to about one week because the high level of sugar and acidic conditions prevent the growth of bacteria.”
Realistic Guidelines You Can Follow
To ensure that you’re safely and adequately storing your food, Crosby recommends keeping food in airtight containers, as oxygen can encourage the growth of bacteria.
Check the temperature of your refrigerator to ensure that it is keeping food cold enough ― the general recommendation is to maintain the temperature of 35-40 degrees Fahrenheit, according to Crosby.
The back of the fridge tends to be cooler than the front and the door of the fridge, which are usually warmest, so store foods accordingly.
Grab some painter’s tape and label leftovers with the date they went into the refrigerator.
If you really want to preserve your leftovers safely past that one-week mark, Crosby recommends freezing them.
For an additional preventative measure, use our science lesson as a guide. “If leftover food is still warm when ready for the refrigerator, package it in multiple small containers so it will cool faster and reduce the rate of bacterial growth. Adding a little acid like vinegar and lemon juice will also help,” Crosby said.
Why Haven’t I Gotten Sick From ‘Old’ Leftovers?
Never gotten sick from potentially passé leftovers? It’s unlikely it has to do with your “iron stomach” and more to do with having a healthy and strong immune system, according to microbiologist and founder of Biobusters Andrea Casero, “Our immune systems play a key role in preventing food poisoning. Eating a lot of fruits, nuts and vegetables, having a healthy and active life can boost your immune system,” he said.
Casero also noted that a major cause of food poisoning can be a lack of cleanliness. “Bacteria biofilm (a group of microbes that accumulate) are breeding grounds for organisms like E. coli, listeria, salmonella and more, which thrive mostly in warm, moist places — like kitchen sponges, refrigerator or a food container not properly cleaned. These bacteria growing in a biofilm are highly resistant to disinfectants, up to 1,000 times more resistant than the same bacteria not growing in a biofilm, and can contaminate your leftovers,” Casero told HuffPost. This means that you could be contaminating your food before it even hits the refrigerator, thanks to improperly washed surfaces or bacteria biofilm multiplying on your tools.
According to Casero, some best practices include washing your hands carefully before and after touching raw foods and ensuring that foods are stored in properly cleaned containers. This means changing up old kitchen sponges or other cleaning tools every week or two.
And checking to see whether food has “gone off” with the help of sensory cues won’t necessarily protect against poisoning, either. Both Crosby and Casero agree that odor or the growth of mold won’t be reliable indicators unless the foods are very old. Your best bet is to bin food that is older than a week.