Tech + EngineeringTech & Engineering

    What happens when you season a cast iron pan

    Here is how oil and heat can form a durable coating.

    ByAttabey Rodríguez BenítezNOVA NextNOVA Next

    Image Credit: Jack Kennard, Flickr

    Ahh, cast iron pans. Love ‘em or hate ‘em, they’ve been around for hundreds of years. With proper maintenance, they can last long enough to become family heirlooms. A cornerstone of this pan’s upkeep is seasoning, the process of baking oil onto the pan. Contrary to the name, you won’t need salt and pepper, but fat. 

    To season, you heat the pan covered in fat to scalding temperatures in an oven. What is left behind is a somewhat nonstick surface that not only helps with cooking but protects the pan from rust.

    You can use the fat of your choice, but one thing to keep in mind is the smoke point, the temperature at which it starts to smoke. That temperature varies from oil to oil. For example, virgin avocado oil has a smoke point of 520 F, but coconut oil smokes at just 350 F. As soon as that temperature is reached, the seasoning process begins. 

    There are multiple hypotheses about what’s happening at the molecular level inside that hot oven. The predominant one is that the oil is polymerizing. Fats are made up of different fatty acids, like saturated and unsaturated. When heated, those acids break down into smaller molecules called monomers. And when monomers join together, they form larger molecules called polymers. When this joining happens over and over in an iron skillet, the polymers form a protective layer that keeps water at bay. 

    A group of researchers from Chongqing University studied this phenomenon in the context of another cast iron cooking tool: the wok. Woks, mainly used in Chinese-style cuisine, are usually round-bottomed. The team hypothesized that the slick surface formed during seasoning is due more to the iron than the oil. To test their hypothesis, the researchers coated a wok with beef tallow and heated it up. As the temperature increased, the fat started to reduce. And the scientists, using X-ray technology, started to see oxygen molecules sneaking into the places where iron was present. With this unexpected guest, iron atoms had to shuffle around to make space, creating small lumps along the surface. The final product is what the team called “iron nanoballs.” 

    They point out that these nanoballs didn’t make the wok completely nonstick, but rather made the surface conditionally hydrophobic. That means that it repels water when there isn’t much around, but gets wet when there’s a lot of it. The study authors claim this property comes in handy during cooking, because if the pans are completely nonstick, the fat will lump together, causing uneven cooking. If the surface can get wet, it creates an even layer, and the food can cook evenly as the water evaporates, creating less charring.

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