Is It Safe to Reuse Cooking Oil? Here’s What a Food Scientist Says

There’s something undeniably delicious about the golden-brown crunch of fried chicken, the delicate crisp of tempura vegetables and the pillowy texture of homemade doughnuts. We can, of course, make healthier versions of these foods in an air fryer or oven. But the irresistible qualities of particular fried delights are incomparably delicious when subjected to the magic of cooking oil. But in the aftermath of frying, a practical question remains: What should we do with the remaining (often large quantity of) oil?

Many of us reuse it to save money, reduce waste and enhance flavor. Some dishes even taste better when cooked in previously used oil, which has developed character. But is this common practice actually safe? We spoke with Jennifer Pallian, B.Sc., RD, to help solve this cooking conundrum. An accomplished food science writer with more than 15 years of test-kitchen experience, she had much to say about whether or not we should be reusing our cooking oil.

Is It Safe to Reuse Cooking Oil?

The short answer is yes—but with some important caveats. “Reusing cooking oil can be safe if you do it properly, but the risks increase the more times you use it, especially if it’s not stored correctly,” Pallian explains. This process of repeatedly heating oil “leads to some pretty big changes,” she cautions. Not only can it produce toxic and carcinogenic compounds that increase your risk for “inflammation, high blood pressure and even certain cancers, but it can also lose its health-promoting benefits, losing healthy fats, including omega-3s, and gaining harmful ones, like trans fats,” Pallian explains. “So while you can reuse oil a few times, it’s important to know when to stop.” Thankfully, the best practices are straightforward, easy to understand and accessible. 

What Are the Best Practices for Reusing Cooking Oils?

Follow these tips to keep your fried foods tasty and reduce possible health risks.

• Cook at lower temperatures. For the best deep-frying results, maintain your oil temperature between 350°F and 375°F (175°C and 190°C). Moreover, avoid heating oil beyond its smoke point, which, Pallian cautions, can accelerate degradation and produce harmful compounds that may contribute to inflammation, heart disease and even DNA mutations linked to cancer. In addition, “the fumes from overheated oil can contain toxic particles, which could pose respiratory risks,” she says.
• Use a thermometer. This affordable tool helps keep temperatures consistent and prevents overheating, especially when deep-frying.
• Choose the right oils. “Oils rich in polyunsaturated fats, like soybean or sunflower oil, break down faster than oils with more saturated fat, like coconut oil,” says Pallian. For deep-frying, choose oils that combine high heat stability, neutral flavor and good reusability, like peanut, avocado, rice bran, palm, vegetable, canola and safflower oils, all of which have other pros and cons but stand up well to high heat. If properly filtered and stored, these oils can typically be reused three to four times, though this varies based on what foods you’re frying and how well you maintain the oil. “Even the more stable oils will degrade after multiple frying cycles,” warns Pallian, “so no matter what you’re using, there’s a limit to how many times you can reuse it before it becomes unsafe.”
• Top up the oil after use. Rather than completely replacing it, which can be costly, topping up the oil lost after deep-frying helps maintain the oil’s volume, helps dilute the accumulation of degraded compounds formed during frying and potentially extends the oil’s usability, which helps maintain uniform frying temperatures and higher-quality cooking results.
• Use fresh oils, when affordable, for each deep-frying cycle. While reusing oil can save money, using fresh oil for each cooking session is optimal for food quality and health, when your budget allows.
• Prevent flavor transfer. Designate specific oils for similar foods to prevent flavors from bleeding into one another. Oil used for fish, for example, shouldn’t be reused for neutral foods like doughnuts, as the flavors will transfer and potentially ruin your food. “Imagine a doughnut fried in the same oil … used for fried fish. No, thank you!” Pallian emphasizes.
• Strain oil thoroughly after each use. This is critical for removing food particles that accelerate spoilage. To give this more context, Pallian explains that battered and breaded foods, or those with high sugar content, “tend to break down the oil more quickly because they leave behind burnt particles that speed up degradation.” 
• Cap it. “Heating oil repeatedly (especially more than four or five times) depletes polyunsaturated fatty acids, vitamins and antioxidants while increasing the formation of harmful compounds” that can lead to health problems, says Pallian. Even after following all the above practices, don’t reuse it more than three times. 

How to Safely Store Cooking Oil

Beyond how you should cook with it, proper storage is essential for preserving the oil quality between uses. Pallian recommends this filtering method: First, let the oil cool to a just-warm temperature to avoid burn risks. Then, pour it through a fine-mesh strainer lined with cheesecloth or paper towels to catch any debris. “Do this over a large heatproof bowl or directly into a clean container if you are confident in your pouring skills.”

The type of container also matters. Choose an airtight, opaque, light-proof container; if there are still a lot of leftover particles, strain it again! Now, label it with the date and previous use to keep track of how many times you’ve used it and what you used it for to avoid cross-contamination. Finally, store it in a cool, dark place, like a pantry, basement or even a refrigerator, to slow down oxidation and rancidity. The latter may cause some oils to become cloudy, but it will clear when the oil returns to room temperature.

When (and How) to Throw Your Oil Away

Knowing when your oil has reached the end of its usable life is just as important as knowing how to store it. If the oil emits a rancid or sour smell, appears excessively dark or cloudy, develops a sticky texture, smokes at lower temperatures than usual, starts foaming when you heat it or contains too many leftover food particles, it’s time to throw it out. “A strong burnt … smell is another giveaway, as is excessive smoking at normal frying temperatures. When oil reaches this stage, it’s best to toss it,” Pallian says.

To dispose of it properly, mix cooled oil with an absorbent material like paper towels, coffee grounds or cat litter to solidify it. Once it’s solidified, you can place it in a sealed, nonbreakable container and dispose of it with your regular household trash. Alternatively, Pallian advises pouring it into a sealable container, like an old coffee can or milk jug, and throwing it away with your trash. For an eco-friendly alternative, “some areas even have recycling programs where used cooking oil is turned into biodiesel or soap,” Pallian enthuses. To dispose of it responsibly and reduce waste, check for local drop-off points in your area. Whatever you do, don’t pour it down the sink or dump it in the yard. Pouring it down drains or toilets will clog your pipes and lead to “serious plumbing issues,” says Pallian. Discarding it outside can attract pests, harm plants and cause other environmental harm. 

The Bottom Line

Reusing cooking oil can be safe, but it requires attentive care and knowing when to call it quits. By filtering it thoroughly, storing it correctly and watching for signs of degradation, you can safely reuse oil a few times. As Pallian reminds us, “No matter how well you store it, you still shouldn’t reuse it indefinitely because harmful compounds build up over time.” When in doubt about your oil’s quality, it’s always better to start fresh rather than risk using degraded oil that could affect both the flavor of your food and potentially your health.