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I am relatively new to deep-frying. Having seen the questions about oil reuse and conservation, I'm still at a loss about what's going on with my oil...

My sunflower oil foams like crazy. I've been reusing this oil for a while now and was going to discard it, however a friend of mine told me this oil was still looking good. Relatively clean (I filter it regularly), no off smell and it doesn't smoke (in my fryer, that is). I only fry potatoes in this batch of oil to avoid flavor transfer. These are cut into French fries, chips or straw potatoes. Specially the chips and straws retain a lot of water, and I guess that has something to do with it.

Also, I noticed the frying takes longer with the foaming oil.

Edit: I've tried poaching(?) the French fries, but they foam during poaching. Then when frying, they really foam. I have to keep an eye on it or the oil spills over the top. Anything I can do about it, or should I just go ahead and discard the oil altogether?

Edit: Serious Eats has an article about frying oil.

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up vote 6 down vote accepted

You are right that the moisture in the potatoes are what's causing the oil to 'foam'. If you don't dry your potatoes sufficiently they can quite easily cause the oil to 'boil' explosively out of the frier; this is why you never pour water on a deep-fat pan fire.

There are various ways to dry out your chips/straws, but one of the most effective ways to is to lay them out on a tray or plate and put them, uncovered, in the fridge for a couple of hours, making sure there are no strong odours in there first. Not only will this prevent too much foaming, it will result in really crisp chips!

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I'll do that. I've been keeping the potatoes in water right up until I was going to fry them. Then I'd dip them 'dry' with kitchen paper. Not dry enough, it seems. – BaffledCook Jun 5 '11 at 14:57
A salad spinner works wonders drying potatoes, as does a few hours (bare) in a freezer. – Bruce Alderson Jun 5 '11 at 16:01
Rinse in water, then wrap them in a tea towel (dish drying cloth) to dry them off. Don't use fabric softener when washing your tea towel - it reduces the water absorption! – James Barrie Jun 6 '11 at 1:19

The foaming is the moisture in your fries boiling off, with the starch in the fries making it foamier. You can solve this problem by blanching them briefly in boiling water to remove some excess starch, then pre-frying them at a much lower temperature to remove some moisture (draining off excess oil).

The pre-frying keeps the fries from getting soggy as fast (less core moisture), lets them fry faster, reduces foaming (less moisture to remove), AND (bonus) helps achieve better browning. It's one of the secret tricks that restaurants use (including the one I work for).

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I wasn't aware pre-frying was a secret. I thought it was standard procedure, but mainly for the purpose of cooking the fries through (and then using the hotter oil to quickly brown and crisp the outside). – derobert Jun 6 '11 at 20:54

I discovered after lots of experience in deep frying that after using peanut oil a few times for deep fried chicken strips or turkeys, with perfect results, the oil will begin to breakdown and start to foam. Time for new oil.

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Thanks for your feedback, Splenda. I haven't had this problem in ages. – BaffledCook Jul 2 '13 at 14:18

I have found this question to be one of the most difficult to get an answer to. I share your dilemma. The clue in your question, often ignored, is that you have been using your oil "for some time now". I agree it does not occur with new oil. Replacing it will clearly solve your problem but does not explain... why! The best I can offer is that the molecular structure of the oil must alter with use, perhaps combining with water. Oil is hydrophobic and works by driving water out of the food. This is observed by foaming even with new oil (but to a lesser extent). Why used oil should foam more readily and take longer to cook food remains a mystery to me, But it does! Like you, my much used oil appears clean and odorless as I never use it for frying crumbed, breaded or flour-dredged food. I am not easily persuaded that it may be harmful to use either.

share|improve this answer… – BaffledCook May 20 '14 at 10:25

It is happening to me as we speak with brand new oil. a mix of about 60 / 40 sunflower and coconut. I figure it must be the starch causing the foam. I dried my french-cut potatoes in the salad spinner beforehand.

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Maybe try a higher temperature? – BaffledCook Jul 14 '14 at 20:49

I came here looking for an answer. I have been using pure olive oil for frying potato chips without any problem. It cooked well, left no unpleasant odor or taste, handled the high temperature and never foamed. But, I had coconut oil and wanted to switch over to it so I added it to the olive oil and got a surprise. It swelled up from a couple inches of oil to over ten inches of foam. I haven't found any information concerning the foaming of just coconut oil so it must be the mixing of these oils that caused the foaming. I believe the two oils are different enough that their molecular structure won't allow the moisture in the potatoes to purge quickly enough to prevent the retention of bubbles causing the foaming.

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