How long can you keep used frying oil

I would not reuse that oil. The frying-oil reuse that occurs in fast food chains is reuse for the same day or over just a few days. They also store it properly and not just sitting exposed in the fryer.

justkt is correct that more saturated fats are less sensitive to breakdown, this is why bacon grease can be kept for a rather long time.

Three factors play a significant role in oil degradation:

Oxidation

Any contact with air causes oxidation in oil. High temperatures, metal alloys, surface exposure, and even UV light act as catalysts to this reaction.

Hydrolysis

When water interacts with oil it causes it to taste tainted or acidic. This is exacerbated by high temperatures, heating/cooling cycles, and oxidation products.

Polymerizaton

When frying oil deteriorates, the resulting products form both volatile (or reactive) and non-volatile compounds. Non-volatile compounds remain within the frying oil, and can produce polymerization at frying oil temperatures above 200°C (390 F) or in isolated hot spots within the frying system. These molecules bond together to form large, different-sized clusters that accumulate on the oil’s surface. Since they don’t dissolve, they cause foaming; trapping air under the oil, and increase the possibility of hydrolysis.

Given all these, the best environment for any oil is an cold, dark, airtight place. Cold temperatures may cause clouding, but this is not a cause for worry.

Fat is a necessary part of cooking. When tossing vegetables with olive oil for flavor before roasting or deep-frying Buffalo wings to create a crispy, crunchy exterior, most cooks regularly use oil in some capacity. So whether you’re left with grease-slicked pans after searing and sauteing or find yourself with cups of oil after making fried chicken or french fries, you need to do something with that leftover fat. Pouring it down the sink, though tempting, can damage your plumbing, and tossing it in the compost might ruin your compost.

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Knowing what to do with used oil and grease is a requirement of being a cook, so here’s a quick guide to reusing and properly disposing of it.

Start with cold oil for crisper, golden french fries you’ll crave

For smaller amounts of oil: I typically wipe pots, pans and any other dishes with a paper towel and then toss it in the trash. You can also designate a few dish towels for light grease cleanup and set those aside for regular kitchen laundry loads. Another option is to pour very small amounts of oil directly into a trash bag as long as there is other debris that will help soak it up. You should remove any residual grease from your dishes before putting them in the sink or dishwasher to prevent it from entering the sewer system. (We don’t need any more fatbergs.)

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For larger amounts of oil: Start by trying to limit the amount of oil you use by choosing an appropriately sized pan for the quantity of food you’re preparing, particularly when it comes to frying. Beyond that, oil can be reused by letting it cool, straining it through a fine mesh strainer, coffee filter or several layers of cheesecloth, and then storing it in a dark place to fry another day.

Oil starts to degrade once you cook with it and will continue to deteriorate with each use, so there’s a limit to how many times you can reuse it. “In a restaurant, no more than two days,” chef and cookbook author Preeti Mistry told The Washington Post. “Maybe some restaurants give it more days, but we did a lot of frying at [the now-closed] Juhu Beach Club, so it was important to keep it clean.” Oil past its prime won’t be able to reach frying temperatures without smoking and can impart bad flavors to whatever food it touches. “For home use, I would use fryer oil about four times since the amount of items fried is not a huge amount,” Mistry said.

Leftover bacon grease is liquid gold. Here’s how to put it to good use.

According to America’s Test Kitchen (ATK), what you fry can be more important than the number of times the oil is used in determining how long it lasts: “With breaded and battered foods, reuse oil three or four times. With cleaner frying items such as potato chips, it’s fine to reuse oil at least eight times — and likely far longer, especially if you’re replenishing it with some fresh oil.” ATK made this determination by using kits that tested for degradation, but for the home cook, the easiest way to test if oil can still be used is to give it a whiff to check if it smells off or rancid. And for Mistry: “If it sits for more than a week or two, I would also just dispose of it no matter how many uses.”

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To dispose of larger quantities of cooking and frying oil, always let it cool down first to prevent injury and keep whatever you transfer it to from melting. Then pour it into a disposable, preferably non-recyclable, container with a tightfitting lid. To dispose of it, Mistry suggests searching for companies that collect oil near you. “That might be, like in Berkeley, an actual biodiesel station for cars that run on fryer oil or it might be a local restaurant or grocery store,” they said. If that is not an option where you live, you can discard it with your regular garbage.

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