Concepts in Heat

I've said a lot about heat in my collection of experiments, and I thought it would be worth talking about a few concepts related to heat because it would help a reader to better understand what I say at times. I learned about these as early as high school chemistry class, but there are lots of people who never take chemistry at all. It's interesting how something from high school chemistry actually relates to baking cookies!

Heat is a form of kinetic energy at the molecular level. The hotter a material is, whether solid, liquid, or gas, the more kinetic energy its molecules have. When the molecules have no kinetic energy at all, that's as cold as anything will ever get. It doesn't matter if you're talking about cookie dough, a baking pan, or a rock on the moon - if its molecules have lost all their kinetic energy, the material will be at the temperature referred to as "absolute zero."

If you put two materials at different temperatures into contact with each other, the kinetic energy of molecules from the warmer material is transferred, in part, to the molecules of the cooler material. It's a little like balls on a pool table - when the cue ball, which has high kinetic energy, hits another ball with no kinetic energy, the cue ball slows down and the kinetic energy it lost is transferred to the other ball. The same thing happens with molecules, and the measurable effect is a transfer of heat. Because of the way heat works, I always think of heat being transferred to a cooler object, rather than cold being transferred to a warmer object. The quantifiable commodity that is transferred is kinetic energy, which goes from the warm object to the cooler one.

In baking, the materials (cookie dough, baking sheets, oven racks, oven sides, mixer bowls and beaters) all have two characteristics that have big effects on the heating. One is called "heat capacity," and the other is the rate of heat transfer.

Heat capacity refers to how much heat it takes to raise the temperature of the object by a certain amount. Heat is kinetic energy, so another way of putting it is, heat capacity refers to how much kinetic energy a material will take into itself during the process of having its temperature raised by a certain amount. It can be phrased in reverse, as well - it's the amount of kinetic energy a material loses when its temperature drops by a certain amount.

It's easy to find out about the heat capacity of a few of the materials you have in your kitchen, but it's not the sort of thing we usually pay attention to. If I wanted to find out the heat capacity of the metal in one of my baking sheets, for example, I'd first put the baking sheet in the oven with no cookies on it for a while, long enough to be sure that the temperature of the baking sheet had reached the temperature the oven was set to. I'd place a known, small quantity of water in the bathtub (or large sink), and measure its temperature accurately. I'd take the hot baking sheet from the oven and immediately submerge it in the bath water. A tiny amount of the bath water would instantly boil away, and the water vapor would carry away some of the kinetic energy and that amount of energy would be lost to the measured results - we can't easily measure how much water vapor is boiling away. But you can imagine that the baking sheet will almost immediately cool to the temperature of the water in the tub. The bath water would also rise in temperature, with the temperature rise being larger when there is less water in the tub. I would measure the new temperature of water in the tub. Say the water's temperature went up by 3 degrees Centigrade. Remember that a Calorie (large C) is, technically, the amount of energy required to raise one kilogram of water by one degree Celsius. I measure how much water I added to the tub. Heat capacity is calculated on the basis of weight or volume. We can easily get the weight of the cookie pan. So having all that information, I would calculate the heat capacity of the metal in the cookie pan with these calculations:

Calories lost = ((end temperature bath water) - (start temperature bath water)) / kg bath water

That's the amount of heat the water gained, which I am saying is the same as the amount of heat the baking sheet lost.

Degrees lost = (oven temperature) - (end temperature bath water)

That's how many degrees the pan dropped. We can't measure the temperature of solid metal in our house easily, so we figure out the temperature drop by assuming the temperature of the metal is the same as the temperature of the material with which it is surrounded.

Heat Capacity of the Pan = (Calories lost) / (Degrees lost)

That's on the "whole pan" basis. To compare two different cookie pans, you have to get the heat capacity on a "per pound" basis. So the technical heat capacity of the metal in the baking sheet would be

Heat Capacity = (heat capacity of the pan) / (weight of the pan)

Weights have to be converted to kilograms and temperatures to degrees Celsius to get numbers to talk to people about, but that's easy to do.

There are several reasons not to actually perform that experiment. One is that it's kind of difficult to look at a number that represents a kitchen utensil's heat capacity and figure out how that number is going to affect the cookies you bake. Another is that plunging an extremely hot baking sheet into room-temperature water might not be good for it and the steam that is generated might conceivably burn you. Another is that there are several indeterminate sources of lost energy in the whole process, which will cause the final calculation not to represent the real heat capacity of the object. To get the actual heat capacity, the experiment would have to be done in a far more controlled environment which isn't practical to construct in your house.

Still, understanding the concept of heat capacity can sometimes help understand some things. For example, the heat capacities of the metal in different baking sheets that otherwise look the same affect how much heat goes into the cookie dough. You can figure that the oven is a nearly inexhaustible source of heat, and the baking pan, if it is a good one, will come to the temperature of the oven and stay there. The amount of heat the cookies get from the pans will depend on the heat capacities of the two metals. The pan will give some heat to the cookies, drop in temperature, and then get more heat again from the oven. The greater the heat capacity of the pan, the more heat goes into the cookies as the pan drops a bit of a degree.

Even cookies have heat capacity. You can assume butter and sugar have totally different heat capacities. Their heat capacities affect how rapidly their temperatures rise, which in turn affects other things such as, for example, how much the cookie spreads before it becomes too "baked" to spread further. If two cookies have different proportions of butter and sugar, then it will take different amounts of time in the oven for them to reach a given temperature.

All of this is perfectly true, but sometimes the differences are too small to make a practical difference. We could investigate how heat capacity affects baking times, but honestly, I don't foresee myself pursuing that. I'm more interested in practical - and easier! - experiments that will help me and others learn how to make better cookies. Heat capacity experiments seem out on the fringe of usefulness in my kitchen. But still, the heat capacity of everything I am using is affecting every baking result I get.

The other property of materials that affects baking is the rate of heat transfer. Different materials give up or take in heat at different rates. This is totally different from how many degrees their temperature goes up when they take in a certain amount of heat. Imagine two sticks of butter, for example, one wrapped in foil and one wrapped in treated paper. Metal is notorious for having quick heat transfer rates. If you put something metallic into water, its temperature drops very quickly. If you put melted wax into water, its temperature will take longer to drop. The wax that touches the water would cool quickly, but the wax inside the glob of hot wax will take longer to cool, because wax doesn't transfer heat as quickly as metal. When you take sticks of butter out of the refrigerator, their wrappers stand between the room temperature air and the butter. The rate at which heat penetrates to the stick of butter wrapped in foil will be greater than the rate for the stick wrapped in waxy paper - and the butter can't warm up any faster than it can take on kinetic energy from the air. The best plan in warming butter in a hurry is to remove the wrappers so it is exposed directly to the air and plate.

Yet here, a lack of factual knowledge implants a little doubt about whether removing the wrappers actually helps butter warm faster. If the rate of heat transfer for butter is slower than that of waxed paper or foil, then helping heat to be transferred more readily to the interface between the butter and its surroundings won't actually help the butter to warm more quickly. I know foil has a high rate of heat transfer, so removing a foil wrapper would probably not actually make any observable difference. But I don't know how the rate of heat transfer through typical wrapper paper for sticks of butter compares to the rate of heat transfer for butter; therefore I don't know if it's worthwhile to remove wrapper paper while warming it. That would be a good experiment to do sometime - measure the rate of temperature increase in a stick of wrapped butter versus an unwrapped stick.

Similarly, the "butter boat" I described yesterday is affected by the rate of heat transfer. The mixing bowl stands between the water in the sink and the butter. Whether the bowl is of glass or metal and how thick that material is will have an effect on how rapidly the butter warms. For the most rapid warming of butter to the temperature of the water, I'd submerge the butter directly in the water. But in this case, I don't think the result would be very easy to deal with - somehow I'd have to dry off all those slices of butter afterwards! No, I don't think I'll be doing that. :-)