6 Leftovers That Are Risky to Reheat If Not Done Properly, According to Food Safety Experts

Cooked Rice

As rice expands in volume when cooked, it can be easy to accidentally prepare more than you need. In turn, you might find yourself reheating leftover rice—though you should think twice before doing so. “Rice can be a problem because spores from a [bacteria], Bacillus cereus, can remain on the rice even after it’s cooked,” explains food scientist Dr. Bryan Quoc Le, Ph.D. These spores can continue to grow on the rice even after it’s cooled down and refrigerated. If the rice isn’t properly reheated, the spores can stay alive and produce bacteria, potentially paving the way for food poisoning, Dr. Le says.

To safely reheat leftover rice and kill any remaining bacteria, reheat it until it’s steaming hot, at least 165°F, says food safety expert Dr. Vanessa Coffman. Dr. Le also suggests cooking rice in another dish—such as fried rice—to ensure it reaches this temperature. For an extra layer of precaution, cool rice quickly after you first cook it (within an hour or so) and store it in the refrigerator for up to one day, Dr. Coffman says.

Cooked Eggs

Raw eggs can be contaminated with Salmonella, a type of illness-causing bacteria, Dr. Le notes. This is more of a concern if the eggs aren’t stored properly (i.e., in a refrigerator at or below 40°F), or if the eggs are old. And since Salmonella can survive at high temperatures, eggs need to be cooked to 165°F to ensure all the bacteria is killed, Dr. Le says. “If some [of these bacteria] remain and grow while the cooked eggs are cooled in the refrigerator, they could cause food poisoning.” To play it safe, always start with fresh eggs that have been properly stored. When reheating cooked eggs, “make sure they’re steaming hot, at least 165°F as measured by a food thermometer,” Dr. Coffman says.

Cooked Potatoes

Whether you have a leftover baked potato, fries, or mashed potatoes, you’ll want to use caution when reheating cooked potato dishes. Potatoes, like rice, can be contaminated with the bacteria Bacillus cereus, Dr. Le explains. If cooked potatoes are cooled down to room temperature, Bacillus cereus can multiply and grow, increasing the risk of foodborne illness. To avoid this issue, “cooked potatoes need to be fully recooked to avoid this pathogen,” Dr. Le says.

Cooked Spinach

Spinach (and other leafy greens) can be contaminated with E. coli, either from runoff wastewater or wild animals that defecate in the fields, Dr. Le says. “Fully cooked spinach will be free of E. coli, but if the spinach has been improperly cooked, refrigerated, and improperly reheated, this can [cause the pathogen to multiply], causing serious complications and food poisoning symptoms.”

Leftovers Kept for Several Days

Contrary to popular belief, storing leftover food in the fridge doesn’t completely stop bacterial growth. “Bacteria can continue to grow in food over time, even in the fridge,” Dr. Coffman says. What’s more, “by day four, harmful bacteria like Listeria or Staphylococcus aureus could be at high enough levels to make you sick.” That’s why it’s recommended to store leftovers for three to four days max, per experts at the USDA. After this timeframe, leftover food is unsafe to reheat, even if it reaches 165°F. In this case, it’s best to toss the food out.

Cooked Food Left Out for More Than a Few Hours

Another food that’s unsafe to reheat is any food that’s been kept at room temperature for more than two hours. The reason? When food reaches room temperature, it enters the “temperature danger zone,” which is between 40°F to 140°F, Dr. Coffman explains. This is the temperature range that allows bacteria—like E.coli and Salmonella—to quickly grow and multiply. After two hours (or just one hour if the room temperature is higher than 90°F), the food is likely unsafe to reheat. “Unfortunately, reheating won’t kill toxins that bacteria might have produced, [so] it’s safest to discard it,” Dr. Coffman suggests.