Some form of post-manufacturing treatment or end-user seasoning is mandatory on cast-iron, which rusts rapidly when heated in the presence of available oxygen, notably from water, even small quantities such as drippings from dry meat. Food tends to stick to unseasoned iron and carbon steel cookware, both of which are seasoned for this reason as well.
Other cookware surfaces such as stainless steel or cast aluminium do not require as much protection from corrosion but seasoning is still very often employed by professional chefs to avoid sticking.
Seasoning of other cookware surfaces is generally discouraged. Non-stick enamels often crack under heat stress, and non-stick polymers (such as Teflon) degrade at high heat so neither type of surface should be seasoned.
Methods of seasoning
Food sticks easily to a new bare metal pan; it must either be oiled before use, or seasoned. The natural coating known as seasoning is initially created by a process of layering a very thin coat of oil on the pan. Then, the oil is polymerized to the metal's surface with high heat for a time. The base coat will darken with use. This process is known as "seasoning"; the colour of the coating is commonly known as its "patina".
To season a pan (e.g., to season a new pan, or to replace damaged seasoning on an old pan), the following is a typical process:
- cleaning the cookware to remove residues from manufacturing or manufacturer-applied anti corrosion coating and expose the bare metal,
- applying a thin layer of animal fat or cooking oil (ranging from vegetable oil to lard, including many common food-grade oils), and
- heating the cookware to generate the seasoned coating.
If it is not pre-seasoned, a new cast iron skillet or dutch oven typically comes from the manufacturer with a protective coating of wax or shellac, otherwise it would rust. This needs to be removed before the item is used. An initial scouring with hot soapy water will usually remove the protective coating. Alternatively, for woks, it is common to burn off the coating over high heat (outside or under a vent hood) to expose the bare metal surface. For already-used pans that are to be re-seasoned, the cleaning process can be more complex, involving rust removal and deep cleaning (with strong soap or lye, or by burning in a campfire or self-cleaning oven to remove existing seasoning and build-up. Once the pan has been heated, dried, and thinly layered with oil or fat, it is placed in an oven, grill, or other heating enclosure for the oil to be polymerized onto the metal's surface. The process of polymerization is dependent on the oil, temperature of the enclosure, and the duration. The precise details of the seasoning process differ from one source to another, and there is much disagreement regarding the correct oil to use. There is also no clear consensus about the best temperature and duration. Lodge Manufacturing uses a proprietary soybean blend in their base coats as stated on their website. Others use lard, or animal fats. Some advocate the use of food-grade flaxseed oil (a drying oil). The temperature recommended for seasoning varies from high temperatures above 260 °C (500 °F) to temperatures below 150 °C (302 °F). Some say that a temperature around the smoke point of the oil or fat should be targeted since this will allow vaporization of the lighter hydrocarbons from the oil, leaving behind heavier molecules for optimal polymerization and carbonization to occur. And, there is also no consensus on the correct duration of heating: from half an hour to an hour is often recommended.
Seasoning a cast iron or carbon steel wok is a common process in Asia and Asian-American culture. While the vegetable oil method of seasoning is also used in Asia, a traditional process for seasoning includes the use of Chinese chives or scallions as part of the process.
The process of heating a pan to cause the oil to oxidize is analogous to the hardening of drying oil used in oil paints, or to varnish a painting. But whereas the curing of oils is the result of autoxidation at room temperature for a painting, for a pan, the thermoxidized oil undergoes a conversion into the hard surface of the seasoned pan at the high temperatures of cooking.
When oils or fats are heated in a pan, multiple degradation reactions occur, including: autoxidation, thermal oxidation, polymerization, and cyclization. Often seasoning is uneven in a pan, and over time the distribution will spread to a whole pan. Heating the cookware (such as in a hot oven or on a stovetop) facilitates the oxidation of the iron; the fats and/or oils protect the metal from contact with the air during the reaction, which would cause rust to form. Some cast iron users advocate heating the pan slightly before applying the fat or oil to ensure that the pan is completely dry.
The surface is hydrophobic, and oils or fats for cooking will spread evenly. The seasoned surface will deteriorate at the temperature where the polymers break down. This is not the same as the smoke point of the original oils and fats used to season the pan because those oils and fats are transformed into the plasticized surface. (This is analogous to how the smoke point for crude oil and plastic are different).
As with other cast iron vessels, a seasoned pan or dutch oven should not regularly be used to cook foods containing tomatoes, vinegar or other acidic ingredients, as these foods will eventually remove the protective layer created during the seasoning process. Cast iron ovens are best suited to cook food high in oil or fat, such as chicken, bacon, or sausage, or used for deep frying. Subsequent cleanings are usually accomplished without the use of soap. Detergent soaps and dishwashers can remove the seasoning on cast iron, so some cookbook authors recommend only wiping the pans clean after each use, or using other cleaning methods such as a salt scrub or boiling water. The protective layer itself is polymeric and not very susceptible to soaps, and many users do briefly use detergents and soaps. However, cast iron is very prone to rust, and the protective layer may have pinholes, so soaking for long periods is contraindicated as the layer may start to flake off.
Other surface types
In the process of bluing, an oxidizing chemical reaction on an iron surface selectively forms magnetite (Fe3O4), the black oxide of iron (as opposed to rust, the red oxide of iron (Fe2O3)). Black oxide provides some protection against corrosion if also treated with a water-displacing oil to reduce wetting and galvanic action.
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