What to Do With Old Cooking Oil

Pouring liquid into bowl.

star5112 / Flickr / CC BY 2.0

Here are some suggestions that will, hopefully, prevent the formation of future fatbergs.

One can’t help but feel terribly sorry for the eight employees of Thames Water in London who are working non-stop to clear away the enormous ‘fatberg’ currently clogging pipes beneath Whitechapel. While pipe blockages are a regular occurrence, this is the biggest yet – a whopping 145-ton mass the size of 11 double-decker buses (or a blue whale!), made from a strange combination of solidified cooking oil and wet wipes. “Yuck” is a serious understatement.

This problem, however, goes to show that London residents – and, doubtless, many others around the world – persist in their cluelessness about how to dispose of these everyday products. We’ve written many times on TreeHugger about the perils of so-called disposable wipes; one could say we’re flush with posts on the topic. But cooking oil is one that has not been discussed in as much detail, so here we go with tips on how to use, reuse, and discard old oil.

Cooking oil should never be flushed down the toilet or poured into a sink, no matter how much hot water or soap chases it down.

Cook Wisely

Cooking oil takes on the flavor of whatever it has cooked, so try to cook like with like. Think, too, about the order in which you fry foods. Breaded items tend to leave a lot of residue, whereas vegetables (with or without batters) are much cleaner; cook in order of cleanest to messiest. If you’re frying meat like chicken, the fat will render during the frying process and mingle with the cooking oil, which can shorten its lifespan.

Use Solid Oils

I make this statement from a disposal point of view. Cook with oils that solidify once they cool, such as coconut oil, lard, vegetable shortening, or bacon fat. These are easiest to dispose of, since you can scrape them into the garbage directly. (Read further about the environmental and ethical effects of different cooking oils here.) Of course, it’s harder to use these solid oils in the large quantities required for deep-frying, which leads to the next point ...

Use Less Oil

The main reason I don’t own a deep fryer is because I don’t want to deal with old cooking oil. It’s too much hassle, and strikes me as wasteful, not to mention unhealthy. When a recipe requires frying (like latkes or falafel), then I use much less oil than it calls for. Sure, the texture may not be perfect, but then I don’t have surplus kicking around and can look forward to the real thing at a restaurant sometime.


You should reuse old oil as much as possible. Cool the oil, strain through cheesecloth to get rid of food bits, and store in a glass jar (or the original container) in a dark cupboard.

There is no limit to the number of times you can reuse old cooking oil, but you should watch for signs of degradation, such as a murky appearance, foam, or an odor that’s off.

Mix With New

Food52 says it’s possible to mix small quantities of old oil with new for better frying.

“As oil breaks down, the molecules become less hydrophobic, which means they can come in closer contact with the food; thus, frying can happen more efficiently! (We learned this, too, from Kenji at Serious Eats.) This is why you'll hear that some people reserve old oil for mixing with new oil. At some point, however, used oil becomes so much less hydrophobic than in its original state that it enters the food too quickly, which leads to sog and grease.”

Discard Wisely

There are a few recommendations for disposing of old oil. You should see if your city or municipality accepts cooking oil for recycling. (This is what fast-food venues typically do, as old oil now has value as biofuel.)

If you cannot recycle or reuse, you can pour the old oil into a non-recyclable sealable container and dump in the trash. This is the official recommendation from Thames Water.

Personally, I dislike the idea of tossing oil in the trash. I prefer to dig a hole in a corner of the yard near the compost bin and pour it in. It’s cleaner, simpler, and really no different than sending it to a landfill.