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What are some guidelines or rules with regard to filtering and reusing (vegetable/canola/sunflower) oil that has been used for deep frying?

Is it safe and acceptable under some circumstances to filter and keep oil? If so, does it depend on which foods were fried? How long can the filtered oil for be kept for?

Or, is it never appropriate? If not, are the reasons health or flavour related? or both?

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4 Answers

up vote 26 down vote accepted

It is absolutely OK to filter and reuse deep-fry oil.

It's not uncommon at some short-order restaurants for them to filter the oil daily and only change it once a week. Of course, it does start to taste a little "off" when you reuse it that many times.

There's also the matter of impurities lowering the smoke point; even when you filter, the result is obviously not "pure" oil; the more you reuse it, the lower the smoke point gets, and eventually it will actually start to smoke at deep-fry temperatures (i.e. become unusable).

For home use, I'd recommend no more than 3 or 4 reuses. Check the oil to see if it needs to be changed sooner than that - if the colour or smell is off, don't use it again. Best to compare it against a sample of the same "fresh" oil; sometimes it's hard to just eyeball it without a frame of reference. If it looks totally clear and smells fresh, you could probably go up to 5 or 6 reuses - but definitely not more than that.

P.S. There will be some people who tell you that you should never reuse oil for general health (not safety) reasons. My response to that tends to be that if you're eating deep-fried food, you're probably not all that concerned about long-term health risks. Rest assured that if you do a lot of eating out, you've eaten plenty of food fried in "leftover" oil.

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That's my rule of thumb too: If it smells like an ugly french-fries joint, throw it away... –  Vinz Jul 23 '10 at 16:16
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In my Navy days, we'd reuse the oil for about six meals. Bear in mind that this was a 20 litre deep-fryer making food for 100-200 hungry sailors though. If we weren't using it too often, the oil would last about two weeks in the fryer. Just remember this rule of flavour transference: If you've deep-fried egg-plant, the oil will taste of it for ever. –  Carmi Jul 24 '10 at 4:26
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Your cooking oil breaks down because of particulate that suspends in the oil as you cook in it. The ways that you can tell if the oil is bad is by visibility (at my restaurant we change at two inches but you could pull it sooner than that) and excessive smoking (because as noted above, particulate lowers smoke point and combustion point, and nobody wants to deal with a deep fryer on fire). The way to test visibility is put a shiny disk on the end of a ruler and shine a light through the oil at a depth of two inches, if you can see the light your oil is still good.

For a home cook, flavor transference is the most likely problem when reusing oil. In a restaurant this is less of a concern because you tend to have segregated fryers for different types of foods. To prevent that, you should never use oil between different types, but you can pretty much just keep fish and meat segregated to prevent the worst of it.

Your oil can also break down over time even while stored so you have a hard storage time of about six months after first use as long as you filter between uses. You can filter using coffee filters or cheese cloth and a strainer. If you use coffee filters or similar paper filters you will most likely need to double or triple them up to get as much of the particulate out as possible.

Bottom line... you really shouldn't throw out your oil after one use, it's just too expensive for that when a few minutes can save it.

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+1 especially for the point about long-term storage; most people don't do a lot of deep-frying at home, so that could bite them. I've experienced it once and it wasn't pretty. –  Aaronut Jul 23 '10 at 19:15
    
+1 for giving a timeline - good to know - I'll now date my oil for reuse with an expiration date. We got some bad upset stomachs from old oil once, and I've been scared to reuse it ever since. –  JustRightMenus Jul 23 '10 at 19:34
    
+1 for mentioning segregating different foods. Thanks! :) –  beetlefeet Jul 25 '10 at 3:52
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Cooks Illustrated has tested and found that you can store the oil for longer in the freezer. cooksillustrated.com/howto/detail.asp?docid=31290 –  derobert Aug 10 '11 at 2:17
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Reusing your frying oil can actually add to the flavor, to a point. You can get a few reuses out of the oil, you'll be able to notice when it starts going bad.

What I do with my frying oil when I'm done using is is filter it through some coffee filters I put over a funnel, putting it all right back in the container I bought it in.

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+1 for the filtering procedure, I was just about to ask a question on how to filter the oil after first using it this weekend ... –  takrl Oct 24 '11 at 6:43
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I know there are deep fryer OIL cleaners for commercial businesses, They actually clean the oil and ad life to the oil. They cost a couple of Thousand dollars - but I want one my home.

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They just make it easier to filter out oil and they get expensive depending on the amount of oil you plan to filter and how. You will get the same effect at home by using coffee filters or similar, using oil appropriate for your task at hand, and keeping different items to fry separated. There are also additives you can get to extend the life of your oil. Most of that advice is overkill for home use though. I don't even bother at my restaurant where we fry 100+lbs of fries alone daily - we clean the oil every 2-3 days and toss after 5. –  janeylicious Oct 30 '13 at 18:51
    
Agreed. At a restaurant I worked at that did a lot of frying, our "oil cleaner" was basically a frame that held a giant coffee filter (thicker paper than a regular coffee filter) over a pot so you could filter the oil. It's not magic. –  sourd'oh Oct 30 '13 at 20:41
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