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Think twice before you heat up that week-old lasagna.

Leftovers—they're every meal planner's best friend. What's leftover from a delicious home-cooked dinner or a fancy restaurant visit makes the best work lunch the next day. But as much as we might try to stretch out Monday night's dinner over the course of several days, life happens. And now you can't help but feel a little uneasy about those week-old leftovers that suddenly resurfaced in your fridge.

Avoid wasting money, food, and most importantly: avoid getting food poisoning. Learn the rules of when to keep leftovers and when to toss them, according to the food safety experts at the United States Department of Agriculture.

Person scooping leftovers into tupperware
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Can You Tell If Leftovers Have Gone Bad?

You can't be sure your leftovers are still safe to eat based on sight, smell, or even taste. This is because foodborne illness is the result of contamination from bacteria, and bacteria often doesn't change the appearance of food. With time, you may notice a change in your food such as mold, change in color, texture, or smell—but if you don't see these changes in your food, it doesn't necessarily mean you're in the clear. It's best to stick to the old rule: when in doubt, throw it out.

The bottom line: the ol' sniff test isn't going to be enough to determine whether food is safe to eat. It's important to know the guidelines on how to properly store and reheat leftovers, as well as some general rules on how long to keep your leftovers.

Tips for Safely Handling Leftovers

How long leftovers last is also dependent on how you handle and reheat leftovers. Stick to these guidelines to preserve your leftovers for as long as possible without risk of foodborne illness.

1. Keep Food at a Safe Temperature

According to the USDA, bacteria can rapidly grow between the temperatures of 40 degrees F and 140 degrees F. This means you don't want to leave food in this temperature "danger zone" for an extended period of time.

To prevent bacteria growth, hot food should be kept at a temperature of 140 degrees F or warmer, and cold food should be kept at 40 degrees F or cooler. Any dish that has been left at room temperature for more than two hours should be thrown out. Dishes that have been left at a temperature of 90 degrees F or higher should be thrown out after an hour (sorry, summer potluck leftovers).

2. Store Leftovers with Care

When it comes time to refrigerate your leftovers, you'll want to cool food as quickly as possible. The longer you allow food to sit at above 40 degrees F, the more bacteria you invite to contaminate your food. Despite what you may have heard elsewhere, the folks at the USDA say hot foods can be placed directly in the fridge. They recommend dividing up larger quantities into smaller containers so they can cool even faster.

Be sure to wrap leftovers tightly in plastic wrap or store them in an airtight container to retain moisture and keep bacteria out. Label them with the date so you can know when it's time to toss them.

3. Reheat to Reach a Safe Internal Temperature

Ideally, reheated leftovers should reach an internal temperature of 165 degrees F to ensure that bacteria is killed. It's never too late to start using a digital thermometer—like this #1 Amazon best seller! If you're using a microwave, use a microwave safe covering with a vent to help retain heat and destroy harmful bacteria. Make sure soups, sauces, and gravies are brought to a boil, to ensure any trace of harmful bacteria is killed. Slow cookers aren't recommended for reheating foods, as they leave the food at an unsafe temperature for too long.

How Long Do Leftovers Last in the Fridge?

Now for the real question: can last week's dinner become this week's lunch? Here's what the USDA says:

"Leftovers can be kept in the refrigerator for 3 to 4 days or frozen for 3 to 4 months."

So if you've passed the four day mark, it's best to go ahead and throw it out. I know, whomp whomp. But better that than flirting with food poisoning, right? This goes for really any type of leftover including: salads, cooked meats, casseroles, or pizza. If you don't think you'll be able to eat your leftovers within four days, go ahead and freeze them. You'll thank yourself when you need to make dinner on the fly, and you'll help to reduce food waste.