The Leftover Rule Food Safety Experts Want You to Follow

It turns out your nose may not be the best judge of whether last night's dinner is still fit to eat. Here's what the science says about storing, cooling and finishing off holiday dishes safely.

Thanksgiving is around the corner, and so are the delicious leftovers everyone loves to dig into the next morning. In fact, the same goes for the extras from pretty much any tasty meal you'd happily enjoy a second time.

Sometimes there's enough to last for days after the turkey holiday. But how long can you still safely enjoy the meat, mashed potatoes and pumpkin pie without risking an upset stomach?

Food safety gets plenty of attention, but it's still a topic many of us only partly grasp. On top of that, research in recent years may have challenged some of the advice we picked up growing up.

For example, you may believe or have been told that leftovers are safe to eat as long as they don't look or smell funny. For many people, smell is the go-to indicator of whether food has gone bad, since a foul odor may suggest that bacteria, mold or other microorganisms have gotten the best of your leftovers.

However, according to Bryan Quoc Le, Ph.D., a food science consultant and faculty research fellow at Pacific Lutheran University, this isn't a reliable way to determine whether leftovers are truly safe to consume.

"Some toxins and bacteria, notably Clostridium botulinum, which is responsible for botulism, do not produce a change in odor or appearance in the food because very little toxin is needed to cause severe illness and death," he explained.

Surprisingly, spoilage bacteria generally don't cause illness, though people might feel sick from the revolting scent and taste.

Jennifer Quinlan, a food safety expert and professor of nutrition sciences at Drexel University, also weighed in, agreeing that disease-causing bacteria typically don't alter a food's smell, flavor or appearance.

That's why a sniff test isn't always trustworthy, she says. Instead, Quinlan stresses the importance of taking preventive steps while cooking, such as avoiding cross-contamination and not leaving raw or cooked food sitting out for too long.

Rather than relying on looks or smells, Dr. Le advises following food safety guidance from the USDA. The U.S. Department of Agriculture's Food Safety and Inspection Service recommends tossing refrigerated leftovers after three to four days.

Frozen foods, on the other hand, can usually still be eaten for three to four months. That said, there's some flexibility in these rules. "I like to go by the USDA's guidelines, but I am not a stickler to it," says Dr. Le. "Personally, I sometimes push [refrigerated leftovers] to four or five days, depending on the type of food."

The Centers for Disease Control and Prevention recommends refrigerating perishable food within two hours after preparation. If the food has been sitting in temperatures above 90 degrees F, the agency advises refrigerating or discarding it within an hour.

According to Dr. Le, letting food sit out until it reaches room temperature can prolong its time in the danger zone, giving bacteria more opportunity to multiply. His advice is to refrigerate food right away, even if it hasn't fully cooled.

Splitting big batches of food into smaller portions and transferring them to shallow containers can help hot dishes cool faster. Quinlan also recommends storing leftovers in vacuum-sealed containers or wrapping them up tightly.

This limits the oxygen available to pathogens such as bacteria and mold, which can help slow their growth.