Marlene Parrish - What Einstein Told His Cook 2

Heat capacity is a technical term meaning, well, the capacity to hold heat. If a substance has a high heat capacity, it can absorb a lot of heat without its temperature going up very much. That resistance to having its temperature changed cuts both ways: during heating and during cooling. Once the substance has had its temperature raised, it doesn’t want to cool down any more than it wanted to heat up, so it retains its temperature for a relatively long time.

Stone and brick have higher heat capacities than metals. For the same thickness, an oven floor made of fire clay has twice the heat capacity of iron and two and a half times the heat capacity of copper. So once heated to the desired temperature (and that may take a long time), a clay floor holds its heat well, staying uniformly at that temperature and resisting temperature changes, such as when relatively cold dough is placed on it. Note also that the larger the mass of a material, the higher its capacity to hold heat, just as a bigger pitcher can hold more water. That’s why massive brick ovens with thick floors and walls have always been valued for their baking prowess. On a smaller scale, that’s also why a heavy frying pan “holds its heat” (that is, stays at a constant temperature) better than a thin one.

Brick, clay, and stone have a second, even more powerful advantage over metallic oven materials: their vastly superior emissivities .

Infrared (loosely called “heat”) radiation in a hot oven is absorbed by the molecules of the materials it strikes, which then re-emit much of the radiation almost instantly. In some substances, notably metals, most of the absorbed radiation is dissipated before it can be re-emitted. Only a fraction of the absorbed radiation (16 percent in the case of a stainless-steel oven wall) is returned promptly to its environment: the air in the oven. (In techie talk, the emissivity of a stainless-steel surface is 0.16.) The rest of its heat stays in the oven wall and is wasted, as far as the food is concerned, except that it can slowly and inefficiently work its way back into the air.

Even at the same temperature, then, stone emits more infrared radiation than metal does. And because infrared radiation doesn’t penetrate beyond the surfaces of materials, more infrared radiation striking the dough results in better browning and crisping of its surface.

So whether you’re reheating a delivered pizza, making one from scratch, or baking a free-form loaf of bread, place it on a preheated pizza stone. If the stone is unglazed and therefore porous, it will have the additional advantage of absorbing the steam emitted from the bottom surface of the dough, keeping it dry for even more effective crisping.

Sidebar Science: Heat capacity and emissivity

• Heat Capacity:Let’s take water as the most familiar example of a material that has a relatively high heat capacity.

When we heat water, we’re pumping calories of heat into it; its temperature will therefore rise. Temperature is a measure of how fast the molecules are moving. Because water molecules stick quite tenaciously to one another (by dipole-dipole attraction and hydrogen bonding ), it’s relatively difficult to goose them into moving faster. We have to add a whole (nutritional) calorie of heat in order to raise the temperature of a kilogram (a liter) of water by a single degree Celsius. (That is, the specific heat of water is one kilocalorie per kilogram per degree C.) Conversely, when water cools, it has to lose a lot of heat—that same one nutritional calorie per kilogram—for its temperature to be reduced by a single Celsius degree.

A couple of consequences of these facts are that (1) it takes “forever” for a heated pot of water to come to a boil, and (2) a body of water, such as a large lake or an ocean, moderates the surrounding climate by refusing to heat up or cool down as easily as the land does.

• Emissivity:In any environment above absolute zero in temperature—and that includes all environments—there is infrared radiation flying through the space. When such radiation strikes a surface, the molecules in that surface absorb some of it. They exhibit the fact that they now contain more energy by moving more agitatedly: twisting, rotating, and tumbling like a hyperactive kindergarten class during a Ritalin shortage. Each kind of molecule has its own unique ways of rotating and tumbling, corresponding to the unique, characteristic sets of energies that it is capable of absorbing. (That is, different molecules have different infrared absorption spectra .)

After absorbing the radiant energy, the excited molecules “calm down” by re-emitting some of it. Some kinds of molecules re-emit virtually all the energy they had absorbed, while others retain some, converting it into different forms of energy. A substance that re-emits 100 percent of the energy it absorbs is said to have an emissivity of 1.00. (In Techspeak, it behaves like a black-body radiator .)

In general, metals have very low emissivities because their loose electrons can soak up the energy like a sponge. Aluminum, for example, re-emits only 5 percent of the infrared radiation that strikes it; copper, only 2 percent. In contrast, materials such as stone and brick re-emit virtually all of the radiation they absorb: 90 percent for dark brick, 93 percent for marble, 97 percent for tile; that is, their emissivities are 0.90, 0.93, and 0.97, respectively. That’s because the molecules in these substances are fixed rigidly in place, and can’t retain the energy by oscillating and tumbling. In these materials, very little infrared energy is wasted; almost all of the infrared radiation that strikes these stonelike surfaces is re-emitted toward the food.

BAKING BY TOOTHPICK

Why do the directions on cake-mix boxes tell us to lower the oven temperature by 25°F if we’re using a glass cake pan or dish instead of a metal one?

Not all of the cake-mix boxes tell us that. In a perusal of the acres of cake-mix boxes on the shelves of my supermarket (in space consumption probably second only to breakfast cereals), I found, as expected, a wide variety of baking instructions, specifying a wide variety of baking times and temperatures for different pan sizes, shapes, and materials. And that’s not even considering the plight of those unfortunates who live at high altitudes, who are exhorted to modify almost everything from the time and temperature to the amounts of flour and water.

The necessity of changing the time and temperature for various pan shapes and sizes is easy to explain. It’s a matter of surface-to-volume ratio. That is, if the same volume of batter is spread out into a wide pan, exposing a large surface area to the oven’s heat (a large ratio of surface to volume), such as in a sheet cake, it will cook faster than if it were poured into a bundt pan, which exposes relatively little surface area to the hot air.

Then there’s the question of what the pan is made of. In my supermarket survey I found that for standard, shiny aluminum pans, almost all the mixes specify a preheated oven temperature of 350°F (177°C). For dark-colored pans, many of the boxes specify a lower temperature of 325°F (163°C). Several boxes specify 325°F for glass baking pans, but several also say 325°F for glass or metal pans, without mentioning dark-colored pans at all. And one devil-may-care box, bless its heart, says “350°F (any type pan).”

So what’s a guy to do?

I am now going to violate the most fundamental principle of expository writing, if not of teaching, by admitting at the start that none of the recommendations matter in the end, and then asking you to bear with me while I explain the scientific reasons behind the recommendations.