Flax oils can oxidize at seasoning temperatures, but does the polymerization process make that concern redundant? Read on to find out, and share your thoughts in the comments below. Be sure to also check out our popular article: The Safest Cookware Choices for You and Your Family
If you are an owner of a cast iron pan, you surely know that they can be an emotional journey. We love our cast iron pans. We love the way they cook, their ability to go in the oven, how good they look as a serving dish and, if we treat them right, their chemical free, non-stick surface. However, neglecting to season cast iron can quickly turn feelings of affection into frustration. Instead of food gliding off the pan, what’s left of those scrambled eggs is now glued to the bottom. Disaster!
An effective pan seasoning is essential. Yet are all seasonings safe? What food components could we inadvertently be introducing into our cooking, and what concerns might they present?
Flax oil as a seasoning agent
Using flax oil as a seasoning for cast iron pans is widely recommended in the cooking blog world. See examples here, here and here. These articles suggest that using flax oil at high temperatures will create a very successful and resilient cure.
According to these articles, an appropriate seasoning process should harden the oil on the pan through a process called polymerization. Flax oil is particularly good at polymerization because its molecules have multiple double bonds that can be broken and reattached to other molecules. This chemically-hardened oil stays on the surface of the pan, even after washing.
The unique ability of flax oil to “dry”, or polymerize, is the reason that it is also used in oil painting (as linseed oil). An oil painting must sit for an extended period of time to allow the fatty acids to polymerize, but we can speed up the process using heat and a catalyst like iron.
It does sound tempting! But the assumption is also that the hardened product is non-toxic, will stay on the pan, and not get into the food. Why might that be a problem?
Concerns about heating flax oil
As nutritionists and Functional Medicine practitioners, we are concerned about the ability of polyunsaturated oils like flax oil to create reactive oxygen species when heated. In fact, we routinely recommend NEVER heating these oils.
Reactive oxygen species are highly reactive products, that when ingested (or created endogenously) are known to damage cells and are associated with cancer, dementia, diabetes, cardiovascular disease and many other health conditions. But how does this translate to flax oil seasoning? Let’s dive deeper.
Can the flax oil coating break down over time?
This is a key question: Even though a flax oil cure creates a reasonably durable coating on the surface of your cast iron, can those oxidized fat particles (flax oil’s version of reactive oxygen species) break off over time and leach into your food? Research is disappointingly scant to help answer this, but here are three reasons why we think it does:
- Linseed oil used in paintings does break down over time. And those paintings aren’t subjected to the daily grind of cooking and cleaning.
- Cast iron pans are known to contribute to dietary iron intake. As an example, 100 grams of spaghetti sauce cooked in cast iron contain 5.7 mg iron, compared with just 0.6 mg in the sauce not cooked in cast iron. For our iron-overload patients, actually, we recommend avoiding cast iron cookware for this very reason. So, if cast iron pans can leach iron, presumably they’re leaching part of their surface coating too.
- Even proponents of flax oil seasoning concede that cookware has to be re-seasoned over time.
Our nutrition resident, Julia Lammers, did her own ‘test’ seasoning using flax oil: “I found that the oil, when applied in thin layers, does make a hard surface to the touch. I did not put as many layers as suggested yet it certainly decreased the non-stick ability drastically. I did notice that after a few test washings and scrubbings, I saw the flax coating change. This signified to me that the flax cure could potentially come off and get into food.”
How much of a concern is this?
Without being able to quantify the amount of oxidized oil released into food, it’s hard to say definitively. However, we can try to put this into context:
For instance, the slow release of small amounts of oxidized fat from cast iron is likely minute compared with eating a browned steak, whose surface molecules have clearly undergone that characteristic Maillard reaction on the order of millions of times. Not that we’re suggesting you eat highly-browned steak…! For comparison only, folks.
There isn’t any way to completely avoid our exposure to reactive oxygen species. We’re all exposed to airborne particulate matter and ozone that have the ability to create these free radicals. Toxic metal exposure can lead to reactive oxygen species formation. Our cells even produce them as part of normal metabolism. While we should all be working to reduce our exposure levels, small amounts of exposure from cast iron seasoning probably isn’t the end of the world, especially if you are taking care of yourself in other ways.
Ways to reduce your risk if you decide to use flax oil
It is a good idea, always, to minimize our exposure to reactive oxygen species. Here are our top recommendations:
- During the process of seasoning, use very thin layers of oil only. Be sure to remove as much excess oil as possible before each layer has been heated.
- Avoid metal brushes and abrasive cleaning processes that may loosen your cure prematurely.
- Avoid or minimize cooking acidic foods in cast iron, such as tomatoes, lemon juice or wine.
- Consider using a saturated fat like coconut oil or clarified animal fats. While this isn’t a true ‘cure’ since saturated fats don’t polymerize in the same way, it’s still possible to build up the layers to achieve a similar nonstick effect.
- Don’t add polyunsaturated oils such as flax during the cooking process. Same reasons as above. Instead use a stable oil, and keep your temperatures low-to-medium. Good oil choices for cooking are coconut, avocado, olive, ghee, or lard from pasture-raised animals.
- Eat antioxidant foods! Colorful plant foods, especially herbs and spices, vitamin C foods, vitamin E foods, and sulfur-containing foods can also increase our levels of internal antioxidants to combat reactive oxygen species.
Julia Lammers MS, a resident in our nutrition team, contributed to this article. Julia holds a Master’s Degree in Human Nutrition from the University of Bridgeport, CT, and is a CNS candidate. She is a nutrition enthusiast and lover of good food, especially when shared with friends. She believes nutrition is foundational for building our best selves and that good nutrition can be exciting, flavorful, and satisfying.
Interesting article about the flax oil for seasoning. The traditional method uses lard, if you use safe lard from animals raised organically and the lard is processed correctly, do you think this is the best oil to reseason a cast iron pan?
There seems be be a plethora of info on seasoning cast iron. I just got one for Christmas. Many seem to recommend a high smoke point oil. I wound up using coconut for my first application and avocado oil for subsequent applications. Martha Stewart recommended Crisco. I don’t like tha t idea since it is full of heart damaging trans fats.
I agree, Sharon. No need for Crisco. Sounds like you made good choices. Enjoy your pan! DrKF
Do NOT use flax seed oil for seasoning. Mine flaked after 4months. I have since joined a cast iron and carbon steel group and all members say use grapeseed or Crisco. Stay far away from flax seed.
What fat would you recommend for seasoning then? Animal fat like lard or tallow?
Sure, a clean animal fast would work fine.
HOW can anyone read the article with the CONSTANT FLASHING/MOVEMENT on the right-hand half of the page???? I’ll be avoiding this site in the future.
So, as I approach the ‘seasoning’ of my skillet : shall I ‘paint’ a thin layer of flax oil on the pan, then heat til smoking … and thEN what? Do i then ‘paper towel’ off the residual flax oil after it’s experienced a ‘smoking session’ and do i then add another ‘dash’ of the oil for a ‘re-experiencing’ of the ‘smoking stage’ ? and how many times should we repeat this process, for maximal ‘non-stick’ and minimal health risks?
Ideally, remove excess flax oil before heating. Consider using a saturated fat like coconut oil or clarified animal fats instead.
This article is one of the most obtuse things I’ve ever read. First of all, let’s talk about the oxidation question. Of course the oil will oxidize! That’s pretty much the definition of a drying oil. And, in fact, that is the goal! You may as well be complaining that water is too wet.
Second, seasoning a pan is the same as cooking with that pan. Seasoning is, for all intents and purposes, a manufacturing process. This article makes it sound like the act of heating the oil above it’s smoke point is inherently problematic. In reality, all seasoning will require heating the fat (whatever you use) to be heated above it’s smoke point. So if that act, in and of itself, is your concern, then you should simply be telling people to avoid cast iron altogether–not objecting to a particular seasoning medium.
Third, the fact that free radicals form during the seasoning/curing process does not mean that they will contribute to dietary free radicals afterwards. It’s a series of chemical reactions that are being done, here. When seasoning cast iron, free radical formation is merely a middle phase of the overall polymerization process. A process, by the way, that involves those free radicals bonding with monomer molecules, creating stable long chain growth polymers! So, to once again use water as an analogy, the complaints in this article are akin to saying that melting ice is dangerous due to the drowning risks associated with liquid H2O.
Fourth, the example of pasta sauce leeching iron is an example of misuse. Acidic foods, like tomato sauces, are already known to damage cast iron cookware, to include the seasoning, and it is known that such acidic food should not be cooked with cast iron cookware. Nearly anything can pose risks when misused, including OTC medications.
Finally, this article relies on a tragic failure to understand a day-one principle of chemistry, by suggesting that the potential for free radical formation would apply to the resulting seasoning created by subjecting flax oil to a chemical change. Hydrogen is a substance, oxygen is a substance, but when they undergo a chemical reaction to bind together into a new molecule, the water that results is an entirely different substance. The same fact applies to seasoning a cast iron pan. The seasoning process causes the oil to go through the chemical reaction of polymerization, resulting in a distinct substance. Hydrogen can pose certain risks (it is a highly combustible fuel). Oxygen can pose certain risks (it is oxidative, and is a necessary component for combustion of an available fuel). In the presence of oxygen, hydrogen can burn or even explode. But water is not flammable. To the contrary, water is usually the preferred solution to extinguish fire. Likewise, the polymer created when seasoning cast iron cookware does not inherit the chemical properties of its components.
So, in summary, this article presents complaints against using flax seed oil that, at best, are universally applicable to any and all cast iron usage and seasoning agents. The closest this article comes to a seemingly reasonable concern is based on misuse. But mostly it dishonestly (or perhaps ignorantly) suggests dangers based on hypotheses that would reflect a profound lack of understanding about fundamentals of chemistry that would be taught in most high school level courses.
I appreciate your detailed response to this article. Are you a materials chemist yourself or, perhaps, a HS chemistry teacher? I think our nutrition director Romilly Hodges (along with our former resident Julia Lammers) make a compelling, cited argument against using flax oil, while also appreciating its capacity to work marvelously- at least at first. My best to you, DrKF
I questioned the wisdom of using Flaxseed oil for this very reason! Without understanding the science behind it, I knew it was a low heat oil and would become toxic at high heats. Glad to stumble upon you post to confirm my suspicions. I’ve been using avocado oil with some success. Any thoughts on plant-based vs animal based oils?