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?
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.
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.
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.
One of the fast food restaurants I work at changes their (canola) oil and cleans out the fryers twice a week, regardless of the colour of the oil. The Fryers have a built-in filtration system that needs to be done. This process is done by hand taking out the particles and giving the vats a quick brush, while using appropriate levers. This is done 3-4 times a day.
Also when "extending" your oil, keep in mind the 6 enemies of oil: Salt Water Air/ Oxygen Heat (wrong temperature) Carbon (food particles) Soap
As for the actual "timeline" of oil, Your best bet for at home frying is when the oil becomes a dark yellow/ almost brown.
Source: Fast food Worker and basic food safety course
At our home we leave the oil in the fryer for about a two month time frame. We never take the oil out of the fryer until time to change it and clean the fryer out. We've never had a problem with the oil and never had any problems with leaving the oil in the fryer for that time frame. We use the fryer practically every day for either French fries, hash browns, chicken nuggets, etc.
I do change the oil sooner (in about a month) if frying fish sticks often as I figure it'll change the taste of the other products.
One of the very popular fast food restaurants showed me their "color chart" they use for determining their deep fryer oil changes. There is no time limit, only color limit as to when to change the oil. I'm sure they must filter the oil daily as they do quite a bit of frying.
In Asia we have oil cans. They have a top part that sets in the can with a screen as fine as those found in lawn mowers in America. Pour the warm to hot oil in them & filter after each use we do. This remove most particals in the oil. What is left will settle for most to the bottom of the can overnight. Once a week check the cooking oil. Store in cool place. Remove filter to pour out oil. Do not pour the very bottom dark oil out. Clean once a week then put used good oil back in. To reuse. If not burnt oil can last for months this way. Can should hold 1 bottle of oil. So if filtered oil does not spoil fast. Pouring hot to warm oil back in your oil can is best. It flows faster into the can. Clean filter as needed. As it will stop up. Heep lid on oil can when not in use. Use only 1 type of oil in each can. But some do add some heavy lard to bottom of can As a additional settling trap as the oil settles. You can refilter the oil threw the filter as you pour it out with these. By removing filter & pouring oil thew them for a second filtering.
I can't imagine only using oil with an in home fryer 2-4 times - such an exorbitant cost if you're using a good oil for frying like 100% peanut oil! You can't beat 100% peanut oil for french fries, and it's usually around $4-$5 per 900ml container (or whatever the size is, just under 1L at any rate)
I will fry up beer-battered onion rings (homemade), chicken burgers, chicken wings and french fries, there isn't much else I use the fryer for (if anything). I will change the oil about 3x year, every four months or so. I won't filter it either, once a week usage for 4 months results in around 12 times. Oil holds up well with these timelines.
I've had a fryer for years, never had issues doing it this way.
Also as a tip for emptying, pour the fryer oil into a green bin/compostable bag that's sitting in a metal mixing bowl, put this in the freezer until it's green bin pick up day (garbage day) and then put it out, very easy, not messy at all and easy to dispose of.
A couple of points to add. Choose an oil that has a high smoke point. Do not ever let your oil go over the smoke point. Also be aware that each use lowers the smoke point a bit. That being said. Reuse for sure! You should be able to get a good number of uses out of your oil. Use your senses though! If it smells off etc do not reuse!
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.
We have been using a counter deep fryer for many years and normally change the oil every 2 to 2.5 months. The oil sits in the deep fryer for all this time on the counter. The deep fryer is used every day and we try to not get the food crumbs into the fryer as much as possible. If we would see any degradation in the oil we would change it sooner, but that is usually not the case. If we would cook any amount of fish, it gets changed at a sooner date. We use Canola oil for the fryer.
Serious Eats describes a method of using gelatin to clarify used deep-fry oil. To summarize the article by J. Kenji López-Alt,
The clarified oil is ready to use. We find that small batches of oil treated this way can be re-used in our home kitchen at least five times. We reserve one batch of oil to cook proteins and another to cook vegetables and legumes to reduce flavor mixing.
The problem with deep frying is not the oil...it is with the deep fryer...Heck of a job to clean. I threw it away after using it 3 times a week for 10 months. Try induction. Super fast.
