How to keep my (deep frying) oil usable as long as possible?

Question

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

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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.