After days of planning and hours of cooking, the last thing you want to do is toss out your lavish Thanksgiving meal. It’s practically become tradition to feast on leftover turkey for days after the holiday. But how long can you really get away with eating that meat, or those extra mashed potatoes or slices of pumpkin pie, without risking an upset stomach—or worse? What are the best ways to preserve food to keep flavors fresh?
Scientific American spoke with food scientists about the health and safety of storing leftovers, as well as other Thanksgiving Day cooking and exposure factors that can speed up the time it takes food to reach what public health agencies call “the danger zone.”
What are some guidelines for storing leftovers?
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The U.S. Department of Agriculture’s Food Safety and Inspection Service recommends throwing out refrigerated leftovers after three to four days. If foods are kept in the freezer, they can usually still be consumed for three to four months. There is some flexibility in these rules, however.
“I like to go by the USDA’s guidelines, but I am not a stickler to it,” says Bryan Quoc Le, a food scientist and a faculty research fellow at Pacific Lutheran University and author of 150 Food Science Questions Answered. “Personally, I sometimes push [refrigerated leftovers] to four or five days, depending on the type of food.”
Smell is a common indicator people use to see if food has gone bad. A foul odor may suggest that certain bacteria, mold or other microorganisms have gotten the best of your leftovers. But surprisingly, spoilage bacteria generally don’t cause illness. (Although people might feel sick from the revolting scent and taste.) Disease-causing bacteria typically do not change food’s smell, flavor or appearance. That’s why a sniff test isn’t always reliable, says Jennifer Quinlan, a food safety expert and a professor of nutrition sciences at Drexel University. Instead Quinlan emphasizes the importance of taking preventive steps when cooking, such as avoiding cross contamination and not leaving raw or cooked food sitting out for too long.
Why do leftovers seem to go bad so quickly?
Le explains there are several factors that influence how long certain items stay fresh. Animal-derived foods such as meat and dairy have a higher likelihood of containing pathogens that, given enough time, can grow on leftovers and make people sick. The most common pathogenic bacteria are Staphylococcus aureus, Salmonellaand Escherichia coli. Le recommends throwing away dairy and meat products by the end of the third day after initially cooking them.
The food’s composition also matters. Water or moisturecan helpbacteria, yeast and other microorganisms grow faster, while high sugar or salt content can slow down some microbial growth. Such sweet or salty foods are more likely to get contaminated by spoilage microorganisms that make your food smell and look rotten—but they are less likely to cause life-threatening infections, Le explains. These foods are a little safer to eat for longer periods of time, compared with dairy or meat products without such content.
Acidity is another factor. Canned cranberry sauce, for example, has a pH of about 2.4. Le says only a few pathogens can survive in such acidic conditions. Meat and dairy, on the other hand, have a neutral pH, making them a more hospitable environment for microorganisms. Additionally, meat and dairy products are rich in nutrients—fats, sugars, proteins and minerals—that microorganisms need to grow. Fresh vegetables or other plant-based dishes do not have such an array of microbe-supporting nutrients, so only a select number grow in these types of foods. Baked goods also tend to lack such nutrients and to have a high sugar content.
When should one start storing food?
Food safety experts say dishes should be sealed and stored in the fridge immediately—ideally, under two hours after the food is prepared, at which point the risk of illness begins to increase. Bacteria and other microorganisms thrive at room temperature (between 68 and 74 degrees Fahrenheit), Le says. Some bacteria such as Clostridium perfringens, one of the most common culprits of food poisoning,can withstand the oven’s heat when a turkey is baking and can potentially cross contaminate other foods, including stuffing and gravy. “That’s a really bad [bacterium] that you don’t want to leave out for too long because it grows at room temperature and produces toxins,” Quinlan warns.
The USDA says leaving food that has an internal temperature of 40 to 140 degrees Fahrenheit out on the table is typically considered the “danger zone,” Le notes, because harmful bacteria can rapidly divide and contaminate the food. “Every 20 minutes the food is left out in room temperature, microorganisms are doubling in number.”
The Centers for Disease Control and Prevention recommends refrigerating perishable food in under two hours after preparation. If the food has been sitting at temperatures above 90 degrees F, such as in a hot car, the agency recommends refrigerating or discarding it within an hour.
Le warns leaving out food to cool to room temperaturemayput it in the danger zonefor a longer period of time,increasing the risk of bacterial growth. He advises putting food in the fridge even if it is still warm. Dividing large amounts of food into smaller portions and putting them in shallow containers canhelp hot food cool down more quickly.
How does refrigeration help preserve leftovers?
When food temperature drops to refrigeration level—at or below 40 degrees F—it decreases the metabolism of most microorganisms, Le says. Lower temperatures slow down many of the chemical processes that normally allow microbes to reproduce and extract energy from food sources. But this does not mean the fridge is killing the bacteria. Rather it keeps them in suspended animation, Le says. Once you reheat the food, he adds, “they're going to come back to life.”
Freezing leftovers can make them last about two to three months, depending on the fat content. As with refrigeration, the colder temperature temporarily stops pathogens from dividing and reproducing. Although frozen foods can be stored longer, Le says that there is a trade-off: supercold temperatures can cause freezer burn, which changes the flavor of the food the longer it’s stored.
What’s the best way to package and store leftovers?
Limiting the oxygen accessible to pathogens such as bacteria and mold can help slow their growth. Quinlan advises packing leftovers with vacuum-sealed food storage containers or wrapping them tightly.
For wrapping, Le recommends using double layers of aluminum foil and plastic wrap for extra protection. Plastic wrap is permeable to oxygen but not to water. Aluminum foil blocks both oxygen and water, making it better at depriving microorganisms of the resources they need to divide.
Where you place your leftovers in the fridge is also important. “The temperature in the fridge fluctuates throughout the night, even when it’s closed,” Le says. He recommends putting leftovers like meats and dairy, which are the first to go bad, closer to the center of the fridge. This makes them more likely to remain near a constant temperature. Put food that is the slowest to spoil, such as high-acid cranberry sauce or sugary sweet potatoes, by the door or the edge of the fridge. Experts say these tips can help extend the shelf life of leftovers, allowing you to spend more time enjoying them instead of throwing them away.
