Pique the Geek: Cooking Utensil Chemistry I, 16 November 2008

Crossposted at Dailykos.com

Early edition for Politicook readers!

This topic is the result of several comments in last night’s What’s for Dinner. I was originally going to talk about egg chemistry, but just could not get excited over it.

Cooking utensils, for the purpose of this discussion, include pots, pans, skillets, and the like. I want to focus on those utensils because they are in contact with food longer than plates, cutlery, and the other items that have only short contact times with food. Storage utensils are included, because they are often in contact with food for extremely long periods of time.

With two notable exceptions, cooking utensils are either metal or glass. The exceptions are both polymers: silicone rubber baking ware and nonstick polytetrafluoroethene (Teflon) lined cookware.

Common metals include aluminum, stainless steel, copper, sheet steel, and cast iron. Each has advantages and disadvantages. For example, aluminum and sheet steel are relatively light but easily dented. Sheet steel also is prone to rust, and is usually coated with a porcelain glaze to prevent it. The glaze is fragile and prone to chipping.

All of the metals react more or less with many foods during cooking. Aluminum reacts with acidic foods, like tomato based materials or fruits (technically, tomatoes are fruits). Typically the reaction is with citric acid, but other fruit acids and vinegar will also react with aluminum. Uncoated aluminum should not be used for acid foods. Several decades ago there was a suggestion between aluminum and Alzheimer’s disease, but many studies have been conducted since and no link has been found. The reason not to cook acid foods in aluminum is that off flavors can develop, and the utensil can be stained and etched.

Unlike the other common metals, aluminum also reacts with alkaline foods. Not many foods are alkaline enough to cause much of a problem, but certain food preparation processes (such as making lutefisk or hominy) require the use of highly alkaline hydroxides or carbonates, and can actually corrode the aluminum away. One of the products is hydrogen, which forms explosive mixtures with air, a bad thing in a kitchen. Large amounts of heat are also produced. (If you ever take a close look at crystal Draino you will see flecks of aluminum in it to produce a higher temperature in the drain than hydroxide by itself would.

Thin aluminum is fine for boiling, especially where a large excess of water is used. It gets hot spots and so thicker foods tend to stick and scorch in thin aluminum pans and it is a challenge to fry in sheet aluminum skillets. Thick aluminum is a superior material for all kinds of cooking, except for the cases just mentioned. Aluminum is fairly light, conducts heat well, and is easy to clean. It is also relatively inexpensive.

Copper also reacts with acids in food. As a matter of fact, all copper cookware is coated with a layer of tin on the food contact surfaces to prevent that. Old pieces often have some or all of the tin worn away and should not be used unless they are retinned. Copper has excellent heat conduction, but it is heavy, expensive, and hard to clean. Without constant upkeep, copper discolors very quickly. When well maintained, it is very attractive, but unless you have hired help in the kitchen, I do not recommend it.

Stainless steel is one of the better materials, but it is expensive. It is so expensive that most stainless steel skillets and heavy saucepans have aluminum bases for heat conduction and thin sides. In some cases the aluminum is embedded between two layers of stainless steel. (Really expensive cookware has copper inserts). This is an advantage, because stainless steel is one of the easiest materials to clean.

Just what is stainless steel? For most kitchen applications, it is the “18-8″ alloy, meaning that it is mostly low carbon steel, with 18% chromium and 8% nickel in the alloy. The chromium forms a thin layer of extremely hard and insoluble chromium oxide, making the material nonrusting. (This is why the chrome trim on cars do not rust). The carbon and nickel make it harder and more durable.

Stainless steel is suitable for any type of cooking except in the microwave oven, of course. The surface is so inert that even caustic does not affect it.

While it is going out of favor, cast iron is an excellent cooking material except for very acidic materials, and, when properly seasoned, is OK for them as well. What is “properly seasoned”? Actually, cast iron was the first “coated” cookware. To season a cast iron utensil, you take some suet or pork fat and fry it out, making sure to contact the entire surface with the frying out fat. Some of the fat actually carbonizes in the utensil, forming an inert and slick surface. You can also season cast iron by coating it with a light film of fat or oil and putting in in a very hot oven (around 450 F). A well seasoned cast iron utensil is almost as nonstick as Teflon, and nonreactive since you are really cooking on a thin layer of carbon.

Cast iron, when handled properly, is pretty much rustproof because of the carbon layer. They must be washed carefully, and soon after each use to avoid damage to the coating. Some advocate using just hot water to clean cast iron, but I always use a little liquid detergent. Just don’t let water, with or without detergent, stay in it for any length of time. Dry well after washing.

Skillets used mainly for frying will pretty much stay seasoned forever. If you boil in them very much you have to reseason from time to time because the coating is gradually washed away.

Cast iron is fairly inexpensive, has excellent thermal properties, and, unless dropped on something like concrete with a lot of force, indestructible. It will not dent (cast iron cracks rather than bends), and is one of the few materials that will work on conventional induction cooktops. The major disadvantage is that it is heavy, which can be a real problem for disabled or older people.

The only other metal much used is silver, and that is usually for limited use, like silver plate tea and coffee services. There are two types of silver: sterling and silver plate. Sterling silver is solid all the way through, is 92.5% silver and 7.5% copper. It is fairly durable, but heavy and so expensive that it rarely used for anything but cutlery. Silver plate is a thin coating of pure silver over a base metal, such as brass. It is delicate, and since the pure silver is much softer then sterling, it is easy to polish off.

That brings up the major problem with silver, other than the cost: it tarnishes. Traces of sulfur in the air, in foods, and in other common materials blacken it. (if you have an old silver item, try wrapping a rubber band around it. The sulfur in the rubber band tarnishes it in a couple of days). To clean silver, you can polish it with a fine abrasive. That is not too bad for sterling, but eventually the plating wears away.

The better way to remove tarnish is to convert it back to silver. To do this, take an aluminum kettle, fill it with water, and add about a tablespoon of baking soda per quart of water. If you do not have an aluminum kettle, you can tear off a sheet of aluminum foil and put it in any other kettle. Bring to a simmer. While the water heats, wash your tarnished silver items in detergent and hot water to remove any grease or oil, because the process will not penetrate grease.

Take your silver item and put it in the kettle, making sure that it touches the aluminum in at least one place. You just made a battery. The electrolytic action causes the tarnish (silver sulfide) to revert to silver and sulfate ion. At the same time, the aluminum corrodes. Since there is not a lot of tarnish, damage to the aluminum kettle is minimal. This works equally well on silver plate and sterling, and will not remove silver like polishing does. After the tarnish is gone, rinse your silver with hot water and dry with a soft cloth.

The best way to prevent silver items from tarnishing is to go to your local coin shop and buy a couple of sheets of corrosion inhibitor that collectors use to keep their silver coins free of tarnish. Bag your silver in plastic bags (handle with a clean cloth so fingerprints are not left behind (latex gloves contain sulfur, so do not use them to handle silver) and put a sheet in the bag, then seal it. The sheets contain a material that neutralizes sulfur compounds in the air. To use the silver, just unbag and rinse with hot water.

That pretty much covers the basics of metal cooking utensils. I am sure that I left out a couple, so please feel free to point any out. Since this is getting so long, next time I will talk about glass (including porcelain coated metal), and another time plastic. As always, comments, questions, suggestions, and other science topics are welcome as comments. I always learn more than I teach when I makes these posts.

Warmest regards,

Doc

Tags: Cooking Utensils, Learning, Metals, Pique the Geek, Teaching