What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (45 page)
Read What Einstein Kept Under His Hat: Secrets of Science in the Kitchen Online
Authors: Robert L. Wolke
And while we’re picking nits, what we usually call grilling—that is, placing food above red-hot charcoal or gas flames—can also be called broiling because it’s not the rising hot air that cooks the food so much as the infrared radiation. Even so, largely because of the charcoal’s smoke and the juices that drip down onto hot surfaces and vaporize, this kind of broiling imparts very different flavors to the food than “indoor” broiling does.
Broiling is a good cooking method for tender meats, poultry, and fish, because it’s a dry, high-temperature, short-time method. Less tender meats generally need long, moist cooking to break down the collagen in their connective tissue. Beef steaks and other red meats are a natural for broiling, while pork, chicken, and fish have to be watched carefully to prevent their drying out.
The biggest question in broiling is how close the meat should be to the heating element or gas flames, because a small difference in distance can make a big difference in temperature. The right distance will depend on the type and thickness of the meat, on its fat content, and especially on the idiosyncrasies of the broiler itself. As you’ve noticed, your broiler isn’t your mother’s or your neighbor’s broiler. They’re all different. In general, though, the top surface of the meat should be 3 to 6 inches from the heat source, thin meat relatively closer and thick meat farther away so it can cook through before its surfaces char.
Should you leave the door open? Usually it’s left open in electric ovens to prevent hot-air baking and to let the smoke out. In stove-bottom gas broilers, the drawer is kept closed because the flames consume the smoke, and leaving it open could make a greasy mess on your kitchen floor.
Should you preheat the oven? It’s generally not necessary, although I’ve seen an almost equal number of “always preheat” and “never preheat” admonitions. The best advice—in fact the only good advice—is to follow carefully the directions in the broiling chart in the instruction manual that came with your oven. The manufacturers have spent a lot of time and effort to determine the best conditions for broiling various kinds of meat in their equipment. If you’re one of those people who throw away instruction manuals, or if you can’t find yours (have you looked in your kitchen’s “everything else” drawer?), you can usually order a free replacement from the manufacturer.
Remember that the fat-catching broiler pan that came with your oven is an important part of the picture, so don’t expect to get the same results with any old pan of perhaps a different size. When I use the recommended broiler pan, shelf height, lack of preheating, door ajar, and cooking time for broiling chicken in my electric oven, it comes out perfectly, even though it looks to me as if the chicken is much too close to the heating unit and the door is open too wide. It doesn’t pay to second-guess manufacturers. They know their stuff best.
| | TWO TIMES ONE EQUALS 1.8 | |
The instructions for my microwave oven tell me how long to reheat one serving of this or that. But sometimes I want to reheat two or more servings at the same time. To reheat x servings, should I set the timer for x times the number of minutes recommended for a single serving?
....
N
o. Heating two servings of something takes less than twice the amount of time required for heating one.
Different foods absorb microwaves to different degrees. Water and fats absorb microwaves efficiently, while proteins and carbohydrates don’t absorb much at all. That’s why different foods require different amounts of time to heat or cook. Furthermore, the microwave generator (the
magnetron
) varies its output according to how big a “load” of absorbing material (a.k.a. food) is in the oven.
Here’s a very rough way of looking at the problem. Let’s say that your particular food absorbs—and turns into heat—a certain percentage of the magnetron’s microwave output. But when there are two servings in the oven, neither one is being exposed to the magnetron’s full output of microwaves; each gets only the un-absorbed “leftovers” from the other. So naturally it will take more time to heat two of them than to heat one. But how much more?
I’ll spare you the arithmetic, but the way it works out is that if one of your servings absorbs, say, 40 percent of the microwaves that it is exposed to, then it will take only 25 percent more time to heat two portions than to heat one. This time increase won’t always be 25 percent; it will be different for different foods that have different appetites for absorbing microwaves.
I tested these ideas with my own “smart” microwave oven, which has pre-programmed cycles for various common heating and cooking chores. For “Heating a Beverage,” for example, the oven first asks me to press a button to tell it how much liquid I want to heat. It then begins its pre-programmed heating cycle for that amount. I timed the heating periods, and here’s how long they lasted: for 0.5 cup, 30 seconds; for 1.0 cup, 50 seconds; for 1.5 cups, 70 seconds; for 2.0 cups, 90 seconds. You can see that the first half-cup requires 30 seconds but that each additional half-cup requires only 20 additional seconds. Two cups took only 1.8 times as much time (90 ÷ 50) as a single cup.
Another example: For “Baked Potatoes” (they’re actually not being baked, but I’ll let that go), the oven cooks one potato in 4
1
/
2
minutes, and adds 3 minutes and 10 seconds for each additional potato. Putting it another way, two potatoes take only 1.7 times as long as a single potato; three potatoes take 2.4 times as long, and four potatoes take 3.1 times as long.
Lacking an omniscient oven that has pre-programmed cycles for every conceivable type and amount of food, all we can do is make an educated guess. For two servings, your first guess should be about one and three-quarters times the time required for a single serving. Doubling the time would be likely to overheat your food, perhaps making it splatter or dry out. It’s best to be conservative, because you can always zap it a little longer.
Sidebar Science:
Say what?
MICROWAVE OVENS
are very complicated devices, truly understood only by their electrical-engineer designers. The analysis above, based on a constant percentage of microwave energy being absorbed by each portion, is oversimplified. But I didn’t think you wanted to get involved in load impedances, cavity resonances, and loss constants. And neither did I, for the plain reason that I don’t understand them.
| | NOTHING BEATS-A PIZZ-A | |
A new pizza restaurant opened recently in my area, with everything imported from Italy, from the furniture to the brick oven to Roberto, the owner, who actually earned a diploma in pizza-making in Naples. He attributes much of the superb quality of his pizzas to the brick oven. Also, some home cooks I know swear by their pizza stones for baking pizzas and crusty breads. Is there really something to these claims, and if so, what’s so special about brick and stone?
....
I
t’s true. Dough, whether for pizza or for bread, baked on a stone surface such as the floor of a brick oven or one of those flat stoneware oven accessories called pizza stones, really does come out crisper and browner than if it were baked on a metal baking sheet or pan. If the oven walls are also stone or brick, so much the better. Early bakers
had
to build their ovens out of available natural materials such as stone and bricks made of clay. Today we bake our bread in “improved,” technologically sophisticated ovens made of steel. And ironically, they don’t do nearly as good a job.
Brick and stone have two properties that make them work so well: high heat capacity and high emissivity.
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?
....
N
ot 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.