Why Does Cast Iron Seasoning Go Rancid

Are you wondering, “Why Does Cast Iron Seasoning Go Rancid?”

You went to get out the dutch oven that you hadn’t used in a while and immediately noticed an unpleasant odor coming from the pan.

Or you’re cooking in the pan you use every day, but now it suddenly smells rancid, and the food you just cooked tastes terrible.

Although rancidity isn’t an everyday problem, it does happen.

Let’s look at several reasons why and what you can do about it.

Table of Contents

Why Does Cast Iron Seasoning Go Rancid?

Actual cast iron seasoning will not go rancid because the oil has been polymerized and bonded to the iron. However, if you notice your pan has an unpleasant smell, and your food tastes funny, it’s most likely due to rancidity. But it’s rancid from the unpolymerized fat left in your pan. Many people put on a thin layer of fat after cleaning their cast iron or leave the oil in their pan after cooking. This oil is not polymerized and can go rancid. Rancidity most often occurs in pans that are not used very often or using the same oil for too long.

Let’s Talk About Two Kinds of Rancidity

Rancidity is most often associated with words such as rotten, putrid, sour, bad, and foul. It’s when the fat alone or within food breaks down and produces an undesirable odor and taste.

There are mainly two types of rancidity that affect cast iron: oxidative and hydrolytic.

Oxidative rancidity is the most common, and it has to do with the breakdown of the oil by exposure to air.

Cooking oil is the purified fat from plants or animals and is always in a state of oxidation.

However, when the fats degrade enough, it leads to rancidity.

Hydrolytic rancidity is a second type, and it has to do with the presence of water.

It can happen from using the same oil or grease for too long or from cleaning your pan with salt too often.

The salt absorbs the moisture and forms deposits in the cast iron. These deposits attract water which causes hydrolysis.

The deposits are also challenging to remove through everyday cleaning.

For some, salt is their standard way of cleaning cast iron. If this is you, be aware that hydrolytic rancidity is real.

Is it harmful?

Well, you might experience abdominal discomfort, but it generally won’t make you sick.

Rancidity happens in stages, and you might first notice it when your food (containing fat) doesn’t seem as fresh. People don’t typically think of stale food as rancid, but it’s the beginning.

4 Ways to Fix a Rancid Pan

I have never had any rancid oil, so these five ways are based on my research. I don’t know if one way will work better than the other or if every method works. But here they are:

1. For those who never wash their cast iron with soap, start there. Soap is a degreaser, and if the rancid oil hasn’t baked in, you can try to wash it off. Occasional use of soap is not harmful to your cast iron.

2. Bring water to a boil in the cast iron pan, then add some kosher salt. Lower the temperature and let it “cook” for an hour, stirring occasionally.

3. Put 6 tablespoons of kosher salt in your pan. Add 3 tablespoons of high smoke-point oil (no lard) and heat the mixture over medium-high heat. Remove and scrub your pan until the rancid odor is gone.

4. Burn off the rancid oil in the oven. Or put it through an oven self-cleaning cycle. This method is a last resort. Basically, you are stripping the seasoning and starting over.

How to Finish

With the first three methods, your last steps will all be the same:

rinse your pan in hot water
dry it thoroughly (preferably with heat)
add a thin layer of fat that isn’t rancid, if desired.

I often skip the third step as I don’t see the point. If your pan has a good base layer of polymerized seasoning, the seasoning is already protecting it from rust and keeping your pan nonstick. Adding a thin layer seems like overkill.

However, if your seasoning is weak, the thin layer can potentially add to the seasoning the next time you cook.

In the fourth method, you stripped off the seasoning. To finish, you have to reseason the pan.

Watch the video below for a similar method to no. 2 above.

Curing a Rancid Dutch Oven

Keep Your Oil From Getting Rancid

Oils are always in a state of oxidation and will go rancid eventually.

So knowing ways to slow down the process isn’t a bad idea. Here are a few:

Don’t expose the oil to the light or air longer than necessary since both can speed up oxidative rancidity. In other words, keep the lid on your oil when storing it.
Store your oil in a cool, dark place such as a kitchen cabinet or refrigerator. Light can lead to rancidity quicker.
Avoid leaving the oil sitting in your iron (or metal) skillet after frying because metals can speed up rancidity.
Turn off the heat after cooking since prolonged heat can also accelerate rancidity.

To keep your pan from getting rancid when putting it in storage for a while, ideally, it won’t have any unpolymerized oil on it. However, if you are concerned about rusting, then covering it with a thin layer of oil is recommended.

Since both rust and rancidity can lead to having to strip and reseason your pan, use your best judgment.

Additionally, avoid daily cleaning with salt to keep your pan from hydrolytic rancidity. Salt (like soap) can be used occasionally, but too much can create salt deposits in the pan.

Final Thoughts

Why does cast iron seasoning go rancid?

Well, hopefully, you now understand it’s not the seasoning that goes rancid because the seasoning is polymerized oil. Only oil or fat that hasn’t been baked into the iron can become rancid.

There are two types of rancidity most often associated with cooking in cast iron. Both are due to the degradation of oils.

Oxidative rancidity has to do with the oil’s exposure to air, while hydrolytic rancidity happens in the presence of water.

Three primary factors that can increase the rate of rancidity: heat, light, and metal.

However, you can slow the speed down by storing your oil in a cool dark place and taking certain precautions when using oil in your cast iron.

I hope you have enjoyed learning about the rancidity of oil in connection to cooking with cast iron.

Feel free to share any thoughts on the subject in the comments below.

Why Does Cast Iron Seasoning Go Rancid – Or Does It?