Heavy Metal: the Science of Cast Iron Cooking

5 Dec

February 16th, 2010

posted by Dave Arnold

I originally wrote this piece for a print publication, but they said the tone was too dry and axed it. They said they wanted something more like the blog. Here it is on the blog.

Heavy Metal 

Cast Iron Intro:

While cast-iron cookware has been available for centuries, the advent of industrialized factory production in the mid 1800’s allowed cast iron to become widely available. The cast-iron skillet quickly became an icon of American cooking. Cast iron could be cheaply produced with minimum tooling in a wide variety of shapes –waffle irons, corn-shaped muffin pans, dutch ovens (dutch meaning “fake”, not “from Holland”), and skillets of every size. While many of these manufacturing advantages have since been supplanted, cast iron’s characteristic properties make it an excellent cookware choice in the modern kitchen. Corn bread made the classic way, in a pre-heated cast iron skillet, highlights cast iron’s cooking advantages: its temperature delivery power generates a good crust, and its temperature-regulating power provides even, constant heat –leveling out the temperature variations of your oven. The science of cast iron shows how these advantages work.

Cast Iron as a Pan Material:

The popular wisdom that cast iron cookware provides even heat is misleading. A cast iron skillet placed on a gas burner will develop distinct hot spots where the flame touches the pan. If you heat the center of a cast iron pan you will find that the heat travels slowly towards the pan’s edge, with a significant temperature gradient between the center and the edge. The pan will heat very unevenly, because cast iron is a relatively poor heat conductor compared to materials like aluminum and copper. An aluminum pan will heat more evenly because heat travels quickly across aluminum. Because of poor heat conduction, undersized burners are incompatible with cast iron cooking. The edges of a large cast iron pan will never get hot on a tiny burner. On properly sized burners you can minimize hot spots by heating slowly, but the best way to evenly heat cast iron is in the oven.

Sprinkling flour onto pans allows us to check their heating patterns. Just sprinkle with flour and heat. This is a variation on the technique Harold McGee uses. He puts paper in the bottom of a pan, covers the paper with beans, turns on the heat and makes a permanent print of the pan’s heating pattern.
A flour-dusted cast-iron pan being heated on a high output burner. Notice the intensity of the hot spot and how uneven the browning is.
The mainly aluminum All-Clad has a much more even heat pattern on the same burner.
The cast-iron pan also shows some serious un-evenness on our induction burner.
The aluminum is more even than the cast-iron, but still not great. The induction burner’s element is too small. Even a good conductor can’t make up for a burner that is too small.

Cast iron has a higher heat capacity than copper, so it takes  more energy to heat a pound of cast iron to a given temperature than a pound of copper. More energy is stored in each pound of the cast iron.  Aluminum has a higher heat capacity than iron (it stores more heat per pound) but is much less dense than iron. For a given volume, therefore, cast iron stores more heat than aluminum.  Because cast iron pans typically weigh much more and are thicker than the same size pan in another material, they tend to store more energy when heated. This combination of high heat capacity and weight means that cast iron takes a long time to get hot. Once hot, however, a cast iron pan usually contains more thermal energy than other pans at the same temperature — a significant cooking advantage. Cast iron has unparalleled searing power because it has a lot of available thermal energy – and unlike almost any other type of pan, cast iron pans won’t warp when left dry on a burner to heat up. Thick and heavy cast iron will remain flat and true.

Cast iron is slow to heat up, so it’s also slow to cool down. It is a good regulator. It retains its temperature longer than other materials and won’t produce temperature spikes. This behavior can be disconcerting to the uninitiated. Cooking with cast iron is more akin to driving a boat than a car: the pan doesn’t respond instantly to changes in the applied heat.

Cast Iron – the OG Non-Stick Material:

Cast iron is naturally non-stick when seasoned properly. New cast iron is anything but non-stick, and it can easily rust. Seasoning — rubbing oil or fat into the cast iron and subsequently heating it — fixes both problems. Unsaturated fats work best (unsaturated means that some of the carbons in the fatty acid chains contain reactive double bonds). Nineteenth century American cooks typically used lard because it was readily available and unsaturated enough to polymerize well, but almost any oil will work. 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. This tough, impermeable surface adheres to the pores and crevices in the cast iron as it is forming. The surface is non-stick because it is hydrophobic – it hates water. Water soluble proteins make foods stick to their pan; a hydrophobic surface prevents sticking. The bits of carbon in the seasoning may also act as an additional release agent.

Cast iron isn’t the only cookware with a burnt-oil based non-stick surface. This is a Korean dolsot –hot stone bowl. I routinely heat this thing to 615 F. I love dolsots. I have 8. I did not say anything about them in this post, but I could not resist putting in a picture. Maybe I’ll do a post.

There is no quick way to fully season a cast iron pan; the surface of cast iron becomes slicker and blacker the more it is used. Though most cast iron today is sold “pre-seasoned,” this cursory seasoning protects against rust, but not against sticking. A good non-stick surface forms over time, with use. The oil polymer on a well-used piece of cast iron is built of many thin layers deposited over time. Thick layers can flake off in large pieces. Thin layers will remain adhered to the pan and will slough off microscopically. A true seasoned surface will only form properly at temperatures well in excess of the 350-375 degree F temperature that some manufacturers recommend for seasoning cast iron. Low temperatures do not completely polymerize and break down oil and will leave a brown, somewhat sticky pan instead of a black, non-stick one. 400-500 degrees F is the effective range for seasoning.

Good seasoning is good protection. Both these pans received the same amount of abuse and neglect. The one on the left was newly seasoned, the one on the right is 50 years old.

Early cast iron was sold either polished or unpolished. Polished cast iron isn’t polished the way silver is, it merely has a surface that was sanded or machined to make it smoother. The polishing process reveals more of the internal pore structure of the iron, and these pores make the seasoning adhere better to the pan. Polished cast iron is slick like glass when properly seasoned. Most modern cast iron is unpolished, meaning its surface has a pebbly appearance from the grain of the mold in which it was cast. Eventually, through years of seasoning, unpolished cast iron can become extremely smooth, but never as smooth as polished cast iron. New, unpolished pans can be sanded with rough sandpaper to approximate polishing.

The bumpy, non-polished surface on the left is now standard for cast iron, older pans also came polished, like the one on the right –a much better surface.

Caring For Cast Iron:

Many cooks are unnecessarily worried about maintaining their cast iron cookware. The seasoning on a good piece of cast iron is very durable. Modern soap will not harm seasoned cast iron. Old, lye based cleaners will hurt seasoned cast iron because lye dissolves the oil-polymer. Seasoned cast iron can also tolerate gentle scrubbing with non-metallic abrasives. Vigorous washing is not recommended on new, weakly seasoned pans.

Sometimes, the surface of a cast iron pan can become damaged through abuse or neglect. In this case the pan has to be stripped down to metal and re-seasoned. The best way to remove an old or bad seasoning job is to use a fireplace or the self-clean cycle of your oven to reduce the seasoning layer to ashes. This happens around 800 degrees F.

Another good maintenance technique with cast iron is to use metal cooking implements. The gentle scraping of metal along the bottom of the pan while cooking helps to even out the surface of the seasoning and make it more durable, not less.

Cast Iron Nutrition:

Studies show that cooking in cast iron can leach iron into food. Foods that are high in moisture, very acidic, or are long-cooked leach the most. For many people the extra iron is beneficial, but for a small minority of people who are sensitive to iron it can be harmful. The most quoted study on the effects of cast iron cookware on iron levels is the July 1986 study in the Journal of the American Dietetic Association. The pan used in that study had only been seasoned by daily usage for a couple of weeks prior to the study. As the study pointed out, better seasoned pans leach less iron. There are no data on iron leaching in decades-old pans.

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(¸.•´ (¸.•` ¤ This is a reblog. Visit the original post here 


One Response to “Heavy Metal: the Science of Cast Iron Cooking”

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