How often can I re-use vegetable oil?
We have a countertop fryer — the kind with a basket that submerges into a deep well full of oil. It is designed to store the cooled oil in the well, between uses. How often can we re-use the same oil that was used for frying? How long can we safely store this oil? How do you know when vegetable oil is starting to turn rancid?
I reuse my oil a few times, but that is because I usually cook the same thing in the fryer (chicken wings, and I make alot of em in football season). You really should put new oil in if you are going to be cooking something drastically different (ie: fish to chicken to fries). Veg. Oil has no problem keeping for a week or two, but then again I don't really want to eat that much fried food...
Smell the oil to determine if it is rancid. You will know - it will not smell appetizing. To extend the number of times you can use it, you can strain the oil after each use (but if one goes to this effort one might as well throw it out). Three to four months sitting on the countertop is probably too long for decent or even edible quality; I'd advise refrigerating it if you wish to keep it that long.
If your kitchen is not too hot and you've skimmed off the bits of food from the oil, it should keep a couple of weeks, otherwise, I would refrigerate it. I find that after 3 uses, the heat has caused the oil to break down and too much water has leeched out of the food into the oil. This makes the food less crispy. One can tell when french fries take longer to brown than the previous time. I always add some fresh oil to the old batch before I reuse it. Food comes out crispier.
There is much confusion about this point, and much mis-information. The answer is, without question, "it depends." Mostly it depends on how hot you have heated the oil, how old it is, and whether you have taken the trouble to keep it clean.
Commercial (ie restaurant) oil is re-used many times. It is called "fry life" in the biz. But these oils are formulated with additives and stabilizers for commercial use which extend their life. A typical heavy duty decent quality commercial oil has a "fry ratio" of maybe 15 or 16, meaning that you can fry 15+ lbs. of stuff (typically french fries) per pound of oil. Thus, a 35 lb. box of oil will fry maybe 600 lbs. of potatoes. This will take many fryings possibly over many days. Many restaurants have a rule of thumb of changing their oil on the same day every week, but of course it depends..... Good restaurants are diligent about keeping their oil clean, by filtering to get the charred bits out. And of course commercial fryers are built to not overheat the oil. Hopefully home electric fryers similarly avoid overheating, but for those who fry on the stovetop this requires careful watching.
At home, with typical oils available to home cooks, you certainly can re-use the oil, but you have to be careful to not let it overheat and you should filter it before putting it away. I use a simple fine-mesh coffee filter (plastic and metal). Of course, I have to let the oil cool before I filter it. Basically the oil is OK so long as the food comes out tasting OK and so long as it isn't showing signs of smoking, (since the heating will lower the smoke point and eventually make it dangerous). Inevitably some oil will be lost each time, and I top my used oil up with new oil.
I think throwing oil away each time is not only wasteful, but how do you dispose of it? Too often it goes down the drain, and that is a bad idea for many reasons, especially IMO the impact it may have on the local sewer system and sewage treatment plant.
BTW, as to freshness, experts claim absolutely fresh oil isn't the best--it needs to be used to be optimum, and some chefs add a little old oil to achieve this effect from the getgo.
UPDATE----Wonder of wonders. About an hour after I typed the above, Alton Brown comes on the tube with a show about oil. He says you can reuse the deep fry oil about 7 times, assuming you strain it, and he says you should keep it in the freezer (I think that's what he said--I got distracted). I'm not so sure about the freezer part anyway, even if he really did say it..
He also confirmed the "add a little old oil to the new oil" trick. It helps the new oil better absorb the water from that which is being fried and you get a better result.
Add a brick of lard to your oil to stabilize it. It'll last longer.
Always strain your oil and don't bother trying to save the bottom of the barrel where all the sediment is after you've let it cool down enough to handle.
Keeping the oil in a sealed container and in the freezer between uses will allow you to maintain it much longer. Also, if breading, make sure to shake off any excess flour or bread crumbs before frying. The excess will fall off anyways and just contribute to turning your oil back since it'll stay in oil for a longer period of time thus burning away and turning into carbon.
If you fry often it is worth investing in a fine metal strainer (with a long handle) to sift the oil between frying batches. The time it takes you to strain it will also allow the oil to return to proper frying temperature for the next batch anyway and the less free floating carbonized particles you have in the oil, the 'cleaner' your food will look after frying.