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Jul 17, 2014 08:30 PM

Is it true that deep frying in bare cast iron is bad?

I've been reading that oils in contact with iron will degrade, oxidize and go rancid faster when deep frying in bare cast iron as opposed to non-stick or enameled cast iron. I always thought that the seasoning protects the food from coming in contact with bare iron. I have noticed that the oils I used when deep frying in my bare cast iron wok turns dark after 3 or 4 uses.

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  1. Um, ... don't you think it might be trying to get 3 or 4 deep frying uses that results in the oils going dark and bad?

    Its the heat that kills the oil. A wok can get pretty darn hot, and very likely too hot for the oils to last very long. If you are getting to four uses, I think you are doing very well.

    Are you thinking that you should get dozens of uses out of deep-fry oil because McD's and all those fast food places get so many uses out of their oils? Consider that they are using formulated oils in equipment with very precise temperature control that is supposed to never get the oil too hot. Of course, the oil still breaks down, gets dark and grungy and has to be replaced. Heck, I consider myself lucky to get two uses out of a batch of deep fry oil and I use non-stick and enameled and stainless pans (except for fried chicken, of course!).

    1 Reply
    1. re: JWVideo

      I deep fry with lard, so that's probably why I've been able to keep it going for a while...I only mentioned this because I wasn't sure if 3 or 4 uses was below average for number of uses before the fat must be thrown out, so I was wondering if the cast iron was shortening its lifespan.

    2. Hi, tadaki:

      I think you and your CI are OK. Heat and saponification take their toll on oils used for frying, regardless of the vessel. Without specially-formulated oils and institutional filtration, 3-4 uses is pretty good.

      As doughnut cookery proves, *some* acceleration in saponification actually is to the benefit of taste and texture. IF your Use #2 frying in CI is the equivalent of Use #4 in SS (and I'm not saying this is necessarily the case), you may be a better cook for it.


      PS: I'd be interested in reading what you've been reading on the subject. Any links?

      4 Replies
      1. re: kaleokahu

        Here's one link I was reading

        "When an unsaturated fat is heated to high temperatures, especially in the presence of a good catalyst like iron, it is broken down and oxidized, after which it polymerizes –joins into larger mega molecules the same way plastics do – and mixes with bits of carbon and other impurities. "

        Btw in your donut reference, are you referring to the practice of putting a bit of old cooking oil into fresh cooking to help it "stick" to the food and give it better texture?

        1. re: takadi

          Hi, tadaki:

          The article you cite is one of my favorites by Dave Arnold. At the point in the article you quote from, Dave is addressing how seasoning layers form, not health hazards from that reaction.

          Yes, I was referring to the phenomenon of totally fresh oil not being ideal for frying things, e.g., doughnuts. Having the oil *starting* on its way toward becoming soap can be a good thing. The trick you mention of adding back a little old oil into fresh makes for consistency--otherwise you have "meh" results on either side of some sweet spot.

          You will note that Dave also says the seasoning layer is "impermeable". That's an absolute term, and while it may be a good *generalization*, I don't think the claim is true in practice--scraping, flaking, charring, acidic foods, pebbly surface, etc., etc, are going to expose some iron to the new frying oil.

          Do you have any more cites? I'm not challenging you, I'm curious about these things.

          Considering all the other health hazards of frying, e.g., formation of potentially carcinogenic acrylamides, I'm not too worried about my oil going rancid by virtue of cooking in a catalyst like CI. Wahine's got a supertaster's nose for rancidity, and anything that flunks her smell test is tossed.


          1. re: kaleokahu

            The "articles" I've been reading are just random things I've found on the internet that all say the same thing, that the iron speeds up oxidation of oils. They are by no means credible sites, they were mostly those junky "wikihow" type sites...the site I posted is the most credible.

            There's not much out there on the chemistry of iron and fatty acid oxidation, but I'm also curious if it's a real thing or if it's just negligible

            1. re: takadi

              I would think that if it were an issue, it would have been documented with CI cookware a long, long time ago.

      2. <I always thought that the seasoning protects the food from coming in contact with bare iron.>

        Only partially, not remotely close to sealing off. That being said, I am not entirely convinced that cast iron is the main reason for oil oxidization. I would think your oil reacting with air/food under high heat is the main reason, but I can look into that.

        1. people have been using CI for frying for generations -- I can't imagine why it would be bad.

          Oil will turn dark after 3-4 uses if you're deep-frying in stainless -- it's the burned bits of the food, and the oil itself burning, not the vessel.

          1. I don't believe it! Traditional Japanese tempura pots are bare cast iron. Southern fried chicken is typically made in a bare cast iron skillet. OTOH, I don't think I would STORE oil in bare cast iron.