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Name: Todd
Status: student
Grade: 9-12
Location: WI
Date: Fall 2013


Question:
I was assigned an article in cooking class, an article about cast iron cookware. It did not make sense. I asked the chemistry teacher who told me about specific heats of materials, mass, heat capacity, energy transfer and a few other things. He said the article was not exactly correct. I understood it at the time, but I confused myself> and I do not want to go back to him. The article appeared in Experience Life November 2013. Volume 15 Issue 9. The article is called Iron Chef, pages 46 - 50. It says, "Holds Heat Well: Cast iron takes a few minutes longer to heat than most other pans but once it heats up, it holds that heat very well. The material's relative superiority to stainless steel, copper, and aluminum pans is due to it> weight and thickness. When a piece of refrigerate> meat is added to hot cast iron, for example, i> starts cooking immediately, because cast iron> unlike other materials, doesn't react to the food's cooler temperature. Cast iron also manages high heat particularly well, making it especially effective for sauteing and browning meats, poultry, or fish." How can it not react to the food's cooler temperature? What is it about iron that is not true of stainless? I would think that all of the metal> would handle the heat well, otherwise, why make a pan out of it?



Replies:
Hi Todd,

You are right to question some of the statements in the article you quote! Much of it is meaningless, and some statements are flat-out wrong! For example, the statement "Cast iron also manages high heat particularly well, making it especially effective for sauteing and browning meats, poultry, or fish" is scientifically meaningless.

Similarly, the statement "When a piece of refrigerated meat is added to hot cast iron, for example, it starts cooking immediately, because cast iron, unlike other materials, doesn't react to the food's cooler temperature." is utter nonsense!

Thermally, cast iron as a frying pan material has, in fact, several disadvantages. Among them, it must be made thicker (and heavier) to offset its rather poor specific heat value compared to aluminum. Specific heat is the ability of a material to store heat.

Unlike cast iron, which, being cast, can be made any desired thickness, stainless steel frying pans are pressed out of flat sheet, and therefore cannot be made thick to offset the poor specific heat of stainless steel.

Thermal conductivity is another factor. Aluminum or copper in fact have FAR better thermal conductivity than cast iron or stainless steel, so they transmit heat much better from the underside to the top.

Ideally, a good frying pan should have a thick bottom and good thermal conductivity, in order to spread the heat, and to effectively conduct it from the flame under it, to the food.

Since aluminum and copper should not used in a frying pan in contact with food, some of the best frying pans are made of relatively thin stainless steel, with a very thick layer of copper or aluminum bonded to the underside. This gives excellent thermal conduction, and good heat storage capacity, yet ensures that the food is safely in contact with stainless steel. The thick layer of highly conductive copper or aluminum also ensures excellent spreading of heat from bottom to top, resulting in even heating across the surface of the pan.

But one reason some chefs like cast iron has nothing to do with its thermal qualities (or lack thereof!). In use, cast iron frying pans develop a microscopic layer of "cooked" oils on the surface that resists foods from sticking. This is called "seasoning" of the frying pan. Stainless steel does not develop this "seasoning" and therefore foods tend to stick to a stainless pan.

Hope this helps to clear things up. Regards, Bob Wilson


Hi Todd,

Thanks for the questions. First, do not be afraid of going back to teacher. You are developing into a man and a leader. You need to develop the skills necessary to confront people in a professional and respectful manner.

When a material has a large heat capacity, it will take some time for it to heat up. That is why it takes a pot of water a long time to boil. The heat capacity of a material depends on its mass as well as its intrinsic ability to hold heat. This intrinsic ability to hold heat is called the specific heat capacity.

The sentence "When a piece of refrigerated meat is added to hot cast iron, for example, it starts cooking immediately, because cast iron, unlike other materials, doesn't react to the food's cooler temperature." is not clear to me. I suspect that the author is trying to convey that the idea that cast iron does not cool off when a piece of cold meat is added to it. It does not cool off much since there is so much heated stored in the cast iron pan.

I hope this helps. Please let me know if you have more questions. Thanks Jeff Grell


Todd,

First, we need to discuss what specific heat means. Let's say we have object A and B, with A having a higher specific heat than B. If I take 1 gram of each object and heat them so that they both go up in temperature by 1 degree (centigrade, Celsius, or Kelvin), I will find that object A - having a higher specific heat - will require more heat, more energy. Let us further suppose that the specific heat of A is 10x higher than that of B, this means that for the same mass (1 gram) and same rise in temperature (1 degree), 10x more heat is required for A than for B to go up by that temperature.

This means that (a) the amount of heat taken in by object A is 10x higher, (b) we can think of A as storing 10x more heat than B for every 1 degree change in temperature, (c) if the heat supplied is at the same steady rate for both A and B, it will take longer for A to reach the 1 degree rise in temperature, and (d) if the two objects were exposed to something that might take the heat away (like the same mass, size, and shape of cold meat) then A will not only take longer to lose the heat to make a 1 degree (lower) change, it will also lose a lot more heat to the meat in the process of cooling down - the meat will absorb more heat from A than from B.

If we look at the specific heat (in j/g-K) data for cast iron, aluminum, and copper, we find: 0.46, 0.91, and 0.39 respectively. This means that aluminum, having the highest specific heat, will have similar properties to the object A properties described above, and copper would be more like object B.

One more thing: specific heat is a number that compares objects with the same mass (in our examples and the data above, 1 gram). Manufacturers of cast iron pans that try to achieve the same effects from aluminum pans will make their pans thicker so that there is a lot more mass - and therefore have a lot more heat storing capability.

Greg (Roberto Gregorius) Canisius College


http://www.cookingissues.com/2010/02/16/heavy-metal-the-science-of-cast-iron-cooking/

http://chowhound.chow.com/topics/738175

http://blog.khymos.org/2007/03/01/staying-warm-cast-iron-vs-stainless-steel/

http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html

http://www.engineersedge.com/properties_of_metals.htm

Above are some references that you may find useful in answering some of your questions. First, “all metals DO NOT handle (whatever you mean by that) heat well. You can find the numbers in the references above. I am just getting that point “off the table”. The data for aluminum and cast iron I will summarize below; however, there are many types of both aluminum and cast iron, so a particular number may not apply to the particular cookware you are asking about. The data for stainless steel are also tabulated but I want to keep it simple see the issues in general. The specific heat capacity of Al is twice that of cast iron on a per weight basis.

That is the opposite of what “intuition” might suggest. However, Al being much lighter than cast iron, means that it takes more Al than cast iron on a mole basis or a volume basis, so a direct comparison is not really easy to make. In addition, the thermal conductivity (the rate that heat transfers per foot per hour of Al is about three times that of cast iron so heat moves “faster” in Al than it does in cast iron. Here also the units are expressed on a weight basis, but in a kilogram of Al there is more matter than in a kilogram of cast iron. So a direct comparison becomes foggy. Lastly, these two factors tend to move the numbers in the opposite direction!!

There is a difference in the amount of radiation the two materials emit, but I do not have any numbers of that. Hopefully, that is not a big effect, but I do not have any data. Another factor is the statement cast iron “holds heat well”. It is not clear what that means in terms of the properties of the two metals.

Your “easy” question is really very complicated to analyze quantitatively in a scientific way. There many conflicting parameters at work.

Vince Calder


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