The Microbiologist’s Guide to Safe Leftovers

November 8, 2023

Abundance is the point of Thanksgiving. It’s a holiday created to celebrate the harvest and be grateful for the richness in our lives.With abundance comes leftovers, if only to give the cooks a break for a day. And with leftovers comes the risk of food poisoning.

Leftovers are a dynamic environment where microorganisms, primarily bacteria, play a significant role,” says microbiologist Anne Moscona, MD. “All food can become a breeding ground for bacteria when it's left out at room temperature for too long. Eating food contaminated with bacteria can make you sick.”

We spoke with Moscona to find out more about microorganisms in leftovers and how to prevent getting sick.

How Bacteria Affects Food

Bacteria are microorganisms that are a natural part of the environment. They are everywhere. Cooking typically kills most bacteria, making the food safe for consumption. But, cooking does not eliminate all bacteria. And even after food is cooked to a safe internal temperature, bacteria can be reintroduced and reproduce.

Bacteria can be beneficial or harmful. The type of bacteria and conditions in which they grow makes all the difference.

Good Bacteria vs. Bad Bacteria

Some bacteria play important roles in food production and preservation. In food preservation (canning, pickling, fermenting), certain bacteria are intentionally used to preserve and flavor the food (jam, yogurt, cheese, sauerkraut, kimchi). These preservation processes create an environment that is inhospitable to harmful bacteria while allowing beneficial bacteria to thrive.

However, some bacteria have the potential to cause foodborne illnesses when consumed. These harmful bacteria—pathogenic, like salmonella, escherichia coli (E. coli), listeria, and campylobacter—can multiply rapidly if food is mishandled or stored improperly.

Bacteria in food thrive and multiply under specific conditions. Their favorite condition is temperature range is between 40°F and 140°F (4°C to 60°C), known as “The Danger Zone.”

Proper cooking, refrigeration, and re-heating help control bacterial growth.

Is the Smell Test Enough?

Sensory evaluation—smell, appearance, and taste tests—are useful indicators of food freshness and safety. However, they do not provide a complete assessment of what is safe to eat. “Smelling OK” should not make you eat a leftover that has been left out too long. Use these basic guidelines instead:

  • If leftovers smell or look weird (unlike themselves, foul, discolored, or textured), discard them.
  • If you are unsure about the food’s safety, discard it.

There is wastefulness connected to this method, but your health and that of your family come first. You can also purchase and prepare less.

Sharpies and Labels Are Your Friends

Write or label dates on leftovers to help you keep track of time. Leftovers are safest when consumed within 3-4 days. The longer leftovers are stored, the higher the risk of bacterial growth.

If the food looks or smells off, it should be discarded, even if it falls within the recommended time limits.

How to Store Leftovers

This is a story about leftovers, but storing food means anything you do with it besides cooking and eating: sitting on a plate or platter or table.

Cooked food needs to be refrigerated within two hours of cooking, whether you’ve already finished the meal or are waiting for it to start. After two hours, bacteria can start multiplying rapidly. Refrigerate leftovers within 1 hour of preparation if the food is exposed to temperatures above 90°F, like a hot car or picnic.

Within two hours of cooking, store food:

  • In the refrigerator, at or below 40°F
  • In or on a hot surface, above 140°F
  • Storing food at these temperature ranges slows down bacterial growth.

Freezing leftovers

If you can’t eat leftovers within 3-4 days, discard or freeze them. Properly packaged leftovers (airtight) can be safely stored for an extended period and enjoyed later. Some food, like seafood, dairy products, and cooked rice, spoil easily and should be discarded or frozen if you can’t eat within 1-2 days.

How to reheat leftovers

Leftovers and pre-cooked food should be heated to at least 165°F to kill bacteria that may have grown during storage. That means heat thoroughly, all the way through.

To see more about this topic, check out the CDC’s guide to food safety.