The Microbiologist’s Guide to Safe Leftovers

Abundance is a classic part of Thanksgiving, a holiday to celebrate the harvest and be grateful for the richness in our lives. But with abundance comes leftovers—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.”

Dr. Moscona explains microorganisms in leftovers and the best ways to avoid ilness.

How bacteria affect food

Bacteria are microorganisms that are everywhere as a natural part of the environment. Cooking typically kills most bacteria, making the food safe for consumption. But, cooking does not eliminate all bacteria. 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 make 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 foods such as jam, yogurt, cheese, sauerkraut, and 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 (pathogenic) bacteria—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 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 evaluations—smell, sight, and taste tests—are useful indicators of freshness and safety. But they do not provide a complete assessment of what is safe to eat.

There are a few reasons the ‘smell test’ by itself isn’t enough. People have varying levels of sensitivity to odors and may not detect subtle changes, so one person’s “smells good” can be different from another person's perception. What’s more, strong seasonings, spices, and sauces can mask the smell of spoiled food.

In addition to smell, if foods look weird (discolored or oddly textured), discard them. But keep in mind that some harmful bacteria are not detectable by smell, appearance, or taste.

In general, if you are unsure about a food’s safety for any reason, discard it. Although there is wastefulness to this approach, your health and that of your family come first. Wastefulness could be counteracted by purchasing or preparing less food.

How to store leftovers

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. Within two hours of cooking, store food at these temperatures to slow bacterial growth:

• In the refrigerator, at or below 40°F
• In or on a hot surface, above 140°F

If food is left out at temperatures above 90°F, like a hot car or picnic, refrigerate leftovers within one hour of preparation.

Labeling leftovers

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

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

Freezing leftovers

If you can’t eat leftovers within three to four days, discard or freeze them. Properly packaged leftovers (airtight) can be safely stored for an extended period and enjoyed later. Some foods, like seafood, dairy products, and cooked rice, spoil easily and should be discarded or frozen if you can’t eat within one to two 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.