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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.

            1. I'm not sure why anyone would want to do deep frying in a cast iron container.

              11 Replies
              1. re: kagemusha49

                <I'm not sure why anyone would want to do deep frying in a cast iron container.>

                Heat retention would be a reason. A heavy cast iron pot could strike a good balance between heat loss and recovery time. Although it's going to be slow to recover, it's heat retention ability should help minimize heat loss, sort of "smoothing out" the loss/recovery wave. Not saying other pans won't work just as well*, but that's a good reason to use cast iron.

                *I read recently, but can't find it now, an objective test that ranked, IIRC, a cast iron DO a close second behind another pot. The other pot may have been aluminum, but I can't be sure anymore. I'll keep looking.

                  1. re: kagemusha49

                    Heat retention?
                    It works?

                    1. re: kaleokahu

                      I would think that heat retention would be a negative when deep frying. If you start seeing blue smoke, you want the temperature to drop quickly.
                      I was also thinking that all that time and effort to "season" your ci pans would be academic once you fill up and deep fry in oil.
                      I'd also think that ci is more likely to shed iron and other stuff into the oil - or at least catalyze unwanted reactions.
                      Finally, I just think that deep frying is just one of those fairly simple things that you don't need fancy pans for - just keep an eye on the temperature &/or smoke.

                      1. re: kagemusha49

                        Hi kagemusha49,

                        < If you start seeing blue smoke, you want the temperature to drop quickly.>

                        If you start seeing blue smoke, you should probably run out of the house. Seriously, waiting for the oil to smoke is no way to deep fry. It should be done with a thermometer, or barring that, test bits should be fried as soon as you think the oil is getting close to temp. You should never walk away from a pot of oil on the cooktop, ever. In all my years cooking, I've never seen smoke when deep frying. Shallow frying, sure. But deep frying, never.

                        Deep frying in CI won't strip off any seasoning or cause the pot to shed anything, because oil isn't acidic. The worst that can happen, if the pot is WAY overheated, is that you'd ADD to the seasoning layers.

                        While I don't own a CI pot, I think it's pretty much a given that they're about as un-fancy as a pot can get. There's no reason to go out and purchase any kind of CI pot just for deep frying, but if you've got one, it's a great vessel for the task.

                        1. re: DuffyH

                          Hi, Duffy: "There's no reason to go out and purchase any kind of CI pot just for deep frying..."

                          I have a Le Creuset fondue pot that is the ideal height-to-width proportion for deep-frying small batches of things like pupus and short skewers. I might buy this expressly for that purpose.


                          1. re: kaleokahu

                            Hi Kaleo,

                            No one trick ponies allowed in my kitchen. And to purchase a small pot for deep frying only nibbles? That's half as useful and would never fly.

                            Now if it were also used for fondue? Well, that would be different.

                            1. re: DuffyH

                              Hi, Duffy:

                              I already had the LC fondue, and it works OK for that. So it's at least a 2-trick pony. My point was that, if I needed to buy a small vessel that minimized oil purchases for small batches, knowing what I know now, I might buy it only as a OTP.


                              1. re: kaleokahu

                                Hi Kaleo,

                                Wouldn't a wok work well for that, too? That's what I was thinking, anyway. A 12"-14" wok will have a pretty small well that I'd try for small batch frying.

                                Of course a carbon steel wok approaches the heat loss problem from the other side, fast recovery. So although heat loss will be greater than with CI, it recovers so quickly that it should work fine, yes?

                                1. re: DuffyH

                                  Hi, Duffy:

                                  No, a wok doesn't work as well for me for deep-frying small batches. The volume of oil required to attain a depth of 4+ inches in a small wok may be smaller than a similarly-sized saucepan, but IMO it's still nonetheless large and wasteful. And a flat-bottomed fry basket won't work in them without a lot MORE oil.

                                  Here's a photo of the model I have; I take it out of the stand, obviously. I have a small fry basket that fits very closely for loose morsels, and skewers can be put in just as the fondue forks are shown in the photo--so you just grasp them to remove when done. With a sufficiently powerful rechaud (think induction hotplate), you can do tabletop fondue-style deep frying, a la Chinese hotpot, Vietnamese fondue, Czech cislk, Merican fried Snickers bars, Twinkies, etc.

                                  IMO, it's the height-to-diameter of this that recommends, so a Russian saucepan or French milk pan would also be good. Again, the fondue is just what I had laying around.

                                  For larger batch frying (investment quantities of oil), I agree with you that a wok is a good thing. For me, the tippiness of a wok is the only major worry I have deep-frying large batches this way. I won't do it with a ring over an open gas grate--the woodstove with a cover removed is rock solid though, and less of a fire hazard.


                        2. re: kagemusha49

                          Hi, kagemusha:

                          Interesting points. You're correct that there can be a downside to heat retention--let's just say the CI boat is not going to answer the helm as fast as a vessel made of most other materials. *However*, I can attest to the advantage of small-batch frying in a pan that stores heat well--more total heat goes into the food without the oil temp dropping out of range. This isn't a problem with commercial fryers, because they usually use large reservoirs of oil and the temperature hardly varies because of thermostatic controls and powerful 220v elements.

                          Seasoning academic? Why? Deep frying certainly doesn't hurt seasoning, and might actually improve it.

                          Shedding iron into the oil? OK, maybe a little. That might be a concern for the tiny % of folks who should avoid dietary iron. Just as "shedding nickel" from SS might be for someone with an acute sensitivity to that element. I'm open to learning about what unwanted reactions are taking place in a CI fryer, so school me if you can.

                          Re: fancy pans, I agree. But I can tell you that hundreds of thousands of Southern families for around 200 years have been frying daily in what are admittedly very UN-fancy bare CI skillets, chicken fryers, Scotch bowls, spiders, Dutch ovens, rendering/soap tubs, etc., etc.

                          Is frying in CI necessary? No. If you have the right setup, it might not even be advantageous. But for folks who have underpowered hobs and deep fry in small vessels, CI can be an advantage. I'd still like to learn more about claims that it's not healthy to do...


                    2. you need to watch out cooking anything in cast iron. both of my grandmothers (cajuns) from louisiana each had about 6 of various sizes and cooked everything in them and it killed both of them. nanny eva was 96 and granny inez was 94 when they died. beware it may cut your life short.

                      3 Replies
                      1. re: jameslee

                        So true. If only they use the super healthy Green cookware, then they would have live to 97 and 95 respectively.

                        1. re: Chemicalkinetics

                          Ah, crap! My ceramic nonstick pan has blue coating, not green. I'm doomed. :-(

                          1. re: DuffyH

                            :( Apparently, the QC/QA was color-blinded. I hope you can still return it.