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I have a new deep fryer. I am pleased with its results so far. I am not a 'heavy user' though - say once every 2-3 weeks. I want to re-use the oil a couple of times. The deep fryer is covered but not airtight, however.

The new deep fryer has a cold zone, which means the oil won't become dirty due to particles over-frying. I don't overheat the oil either. So, in principle, I'm frying correctly, I think.

So, how long can my oil safely sit in the deep fryer, if I ensure it's thouroughly heated next time I fry? Should I be allowing the oil to cool and store the fat in (sealed) containers instead, if there's going to be this long a gap between uses?

Edit: yes, I've seen this question and answers: http://cooking.stackexchange.com/questions/3014/frying-oil-reuse

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My family has always reused the oil until it either looks wrong (too viscous or a funny colour) or smells wrong (burnt, of food, or rancid). We keep the oil in the fryer and take it out to filer it after it's cool. I'm not sure if this is wrong but it doesn't appear to have done any damage so far. I'd say we use the fryer once a month and change the oil a couple of times a year. –  vwiggins May 23 '12 at 15:12

6 Answers 6

up vote 6 down vote accepted

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.

Source:

http://www.heatandcontrol.com/technical%20articles/Maximizing%20Cooking%20Oil%20Life.pdf

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3  
Actually, almost all oil is left in the fryers exposed to air and whatever else comes along in fast food restuarants. You'd be lucky if they even throw a pan over the top. Also, ruse is a matter of days to weeks rather than hours to days, but that is because the industry has filters that run at least twice daily and filter powder that removes the harmfull stuff out. –  sarge_smith Aug 31 '10 at 23:13
    
@sarge: Really? I don't have any fast food experience. I assumed they at least covered it. –  hobodave Aug 31 '10 at 23:33
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naw, why cover something that absolutly nothing can live in once its at temp? but like I said, there is an entire sub industry devoted to extending the life of your oil in the fast food world. I wish I could post some of the oil management training materials, they are pretty brilliant. –  sarge_smith Sep 1 '10 at 0:24
    
@sarge: has nothing to do with what can live there, but the quality of the oil –  hobodave Sep 1 '10 at 1:42
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I know, but for fast food, oil quality is destroyed through use long before it can degrade any other way. Its a special case, unlike anything else that normal restuarants have to do to mantain quality. –  sarge_smith Sep 1 '10 at 2:01

One important note is to use a more saturated oil to avoid breakdowns that form the toxic compound HNE. See Science Daily for a summary of the research. One of the authors of the quoted research has written many more papers on the issue. If you use a highly unsaturated oil you can only use it for half an hour at frying temperature before HNE begins to form.

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This seems to run counter to common practise (as also mentioned in the other linked question) in both fast-food joints / restaurants with fries. Is it common to be able to buy the 'wrong' type of oil (in a Western European country)? –  Tobias Op Den Brouw Aug 31 '10 at 17:36
    
@Tobiasopdenbrouw - are you sure that they use vegetable or another highly unsaturated oil for at least 30 minutes at a frying temperature? There appears to be a growing body of research on HNE and oil out there - springerlink.com/content/l016112242257293. –  justkt Aug 31 '10 at 18:03
    
no, I'm not sure about this. –  Tobias Op Den Brouw Sep 1 '10 at 6:11

First thing, a cold zone doesn't prevent particulate build up, it just cuts down on the amount that will remain suspended in the oil. You should still filter if you want to keep your oil as long as possible. You can detect bad oil (meaning rancid) by a few ways, like smoke point or smell.
You should store your oil someplace dark and cool, which could be the inside of your fryer as long as you filter and clean the sides before you stick the oil back in. Another thing you can do to extend the life of your oil is switch to tallow (beef fat) or rended pigs fat (lard) as saturated fats do last a lot longer than the poly-unsaturated.

Once again, you should lose the oil when the smoke point drops, or if it is to dark, or if it develops any foul odor. And as I said before, once you use the oil once, you should NOT use it past six months under any circumstances, if you do, you are taking your gastro-intestinal tract into your own hands.

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I keep my old plastic coffee cans for storing the oil in after use. They are large enough and seal tight. I also store it in the refrigerator. It does get a little cloudy but once heated it clears up. Probably use 4 or 5 times in 3 to 4 months unless I fry fish and then it gets thrown away. It tends to get a slight odor after that even if it isn't dark. I always strain it before putting back into the fryer.

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if you fry a potato in the oil after you cook fish it will absorb the "Fishy" taste> –  user17927 Apr 19 '13 at 1:23

FROM

http://www.fsis.usda.gov/wps/wcm/connect/65f762d0-e4d0-4278-b5cb-2836854a3eda/Deep_Fat_Frying.pdf?MOD=AJPERES

If you are planning to reuse the oil, strain it through a cheese cloth or sieve. Store the used oil in a sealed and light-proof container for up to 3 months. For best quality, refrigerate used frying oil that you want to use again. If the oil is clouded or if the oil starts to foam or has a foul odor, taste, or smell, discard it.

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I would think that fresh oil would be best used, as even though the oil may be strained and cooled in fridge, it will still contain some of the cooked substance, and recooking stuff over and over is not healthy.

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