What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (46 page)
Read What Einstein Kept Under His Hat: Secrets of Science in the Kitchen Online
Authors: Robert L. Wolke
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.
•
The color of the pan:
A relatively shiny aluminum or stainless-steel cake pan obviously reflects visible light more than a dark-colored anodized one or a nonstick-coated one. Because all the light falling on an object must be either reflected or absorbed, that means that the dark surface is absorbing more light than the shiny one is. That extra absorbed light energy makes the dark-colored pan slightly warmer than the shiny one, even in a same-temperature oven. (To manage our body heat, we wear lighter-colored clothes in the summer and darker ones in the winter.)
But what light is there inside a dark oven, you ask? Infrared radiation, which many people refer to as infrared “light” even though it’s invisible to the human eye. A dark surface absorbs more of this radiation than a light-colored or shiny surface does. That’s particularly important because when an object absorbs infrared radiation it becomes warmer—significantly warmer than if it had absorbed visible light. Thus, a cake should cook faster in a dark pan than in a light one, and we are often advised to lower the oven temperature to compensate.
•
The material of which the pan is made:
A thin metal cake pan of any color conducts the oven’s heat efficiently into the batter. But compared with metal, a glass pan is a very poor conductor of heat and is quite sluggish at transmitting its oven-given heat into its contents. Using a glass pan and given the choice of fast baking at a high temperature or slower baking at a lower temperature, we would choose the latter, because the oven’s heat needs a longer time to penetrate through the glass to the batter. It’s not a big effect, so a relatively small decrease in oven temperature or a small increase in baking time (which some cake-mix instructions specify) suffices.
And now, as promised, I repeat that none of this really matters. Home ovens don’t work like the carefully calibrated equipment in the test kitchens of Betty Crocker or Duncan Hines. There, armies of lab-coated technicians painstakingly work out the best possible conditions for baking their mixes to ensure that the home cook garners accolades from his or her family and runs back to the store to purchase more. But in real life, home ovens may vary from their set temperatures by plus or minus 25°F (14°C ) or even more, and the issue of 350 versus 325 degrees is in most cases moot.
So use whatever kind of pan you have, and by all means, turn your oven dial to the recommended number. But don’t bet your cookies on it. After all the directives about pan material, oven temperature, and baking time, every cake-mix box that I’ve seen winds up admitting that the cake is done when, and only when, it
looks
done and a toothpick inserted into it comes out clean.
And that’s the truth.
Sidebar Science:
How ovens cook
WE SPEAK
of oven temperature as the main variable that determines how fast a cake will bake or any food will cook. But although the temperature is of primary importance, it is only one factor. Even at exactly the same oven temperature, the amount of heat energy a food actually receives and absorbs may not be the same.
By “oven temperature,” we mean the temperature of the air inside the enclosure, and that’s what the temperature control device regulates. But once the air is heated to a certain temperature, there are still three ways in which the air’s heat can be transmitted into the food: by conduction, by convection, and by radiation.
•
Conduction:
When two substances at different temperatures are in contact, such as hot oven air in contact with a food’s surface, heat will flow from the higher-temperature air into the lower-temperature food by the process of conduction. Just as water always flows downhill if it can, heat will always try to flow “down-temperature” from high to low. The heat energy is conducted from the air to the food across their interface by direct molecular collisions. That is, the hot air molecules are moving faster than the cooler food molecules (that’s actually the definition of temperature: it’s the average motion energy—
kinetic
energy—of the molecules), and when they collide with the food’s molecules they kick them up to a faster (hotter) speed, like a cue ball scattering a rack of billiard balls.
But conduction is very inefficient. Air molecules are separated from one another by interplanetary distances, relatively speaking, so the chances that a hot air molecule will collide with the surface of a cake pan or a roast are small. Heat conduction can be quite efficient between two solids in contact, such as your hand on a hot frying-pan handle, but not between hot air and anything else. You can put your hand in a 200°F (93°C) oven for several seconds without fear, because the rate of conduction of heat from the air into your skin is so extraordinarily slow. But don’t try dipping your hand into 200°F water. Water is a much better conductor of heat because its molecules are much closer together than air’s are.
And why are metals the best heat conductors of all?
In almost all other materials, the atomic electrons are parts of individual molecules. But in metals, the electrons belong, in effect, to all the atoms simultaneously. We can think of metal atoms as being embedded in a swarm or sea of shared electrons, like raisins in an electron pudding. When a metal comes in contact with the agitated molecules of a hot substance, it’s the electron swarm that transfers the agitation—the heat—rapidly to all other parts of the metal. That’s heat conduction.
In an oven, however, the other two heat transmission mechanisms, convection and radiation, are more important than conduction.
•
Convection:
Variable conditions inside the oven, such as inevitably uneven temperatures between one spot and another, make the air move, because hotter “pieces” of air rise, while cooler “pieces” fall, creating a kind of circulation that’s called convection, or convection currents. This circulation boosts the efficiency of heat transfer between the air and the food, because it increases the amount of contact between the food and the hot air molecules in the enclosure. A convection oven enhances this effect by means of a fan that circulates the oven’s internal air or some externally heated and blown-in hot air, leading to more efficient heat transfer and faster cooking. That’s why it’s good practice to lower the temperature by about 25°F (14°C) when using a convection oven rather than a standard one.
•
Radiation:
The third mechanism by which food becomes hot in an oven is by absorbing radiation. The oven’s heating element or flame and its walls and floor are hot—they are what make the air hot—and hot things radiate infrared radiation. In fact, all materials at all temperatures are emitting some of their energy as infrared radiation. (See “Heat capacity and emissivity” on p. 398.)
For a given object, the hotter it is, the more infrared radiation it is emitting. When the infrared radiation coming from the hot oven walls and the hot air hits the food, the food molecules absorb it and move with increased energy. That is, they become hotter.
Infrared radiation isn’t heat, as many books will tell you. It is electromagnetic radiation, like radio, radar, and microwaves, but of just the right wavelength to be absorbed by most kinds of molecules, which thereby become more energetic and hotter. I call infrared radiation “heat in transit,” because it is emitted by hot matter and travels through space, yet it isn’t transformed back into heat until it is absorbed by other matter.
| | OVEN RUBBER | |
More and more, I’m seeing kitchen gadgets such as spatulas and pastry brushes made of silicone. What amazes me is the baking pans and muffin “tins,” which look and feel like rubber, but can supposedly stand oven temperatures up to 500°F. What’s the secret?
....
T
he secret, as Julius Caesar might have said, is that all rubber is divided into three parts. Or, in somewhat more modern language, that which we call rubber by any other name would not bake as well.
I’ll try again. There are three basic kinds of rubber, coming from three different kinds of plants: natural rubber, which comes from latex, the sap of the tropical tree
Hevea brasiliensis
; synthetic rubber, which is made in a chemical plant; and silicone rubber, which comes from, well, a different chemical plant. The last two were dreamed up by chemists in attempts to duplicate some of natural rubber’s unique properties and improve upon others.
A synthetic rubber called neoprene was first marketed by DuPont in 1931, and a wide variety of silicone rubbers have been manufactured by General Electric and Dow Corning since the 1940s. These two man-made products inherited the silly name
rubber
from the natural material, which was so christened by the English chemist and clergyman Joseph Priestley in 1770, when he found that it would rub out pencil marks, if not erase sins.
Unfortunately, in recent times the word
silicone
has been implanted, so to speak, in the public’s mind in but a single context: cosmetic augmentation. But silicones are a remarkably versatile family of chemical compounds with hundreds of uses. In culinary applications, the French fiberglass-reinforced silicone baking-pan liner called Silpat has been used in professional kitchens since it was introduced in 1982. But silicones have only recently invaded the American home kitchen in many forms, all approved by the FDA for repeated contact with food. Today, the whole baking pan, not just its liner, is made of silicone.
Before I go any further, I must straighten out some terminology, because the words
silicone
and
silicon
are so often mistakenly interchanged.
Silicon
(no
e
) is a chemical element, the second most abundant element on Earth (after oxygen). It is a rock-hard, brittle gray material that would make the world’s worst cake pans, not to mention surgical implants. However, silicon the element is a semiconductor, and therefore immensely valuable in the form of “chips” or microprocessors in computers and hundreds of other electronic devices. That’s why the high-tech region around San Jose, California, is called Silicon Valley. (It is to be carefully distinguished from Los Angeles, which has been dubbed Silicone Valley for reasons I need not explain.)
Silicones
, on the other hand, are chemical compounds that, like the natural and synthetic rubbers, are polymers, meaning that their molecules consist of long chains of thousands of smaller molecules tied together. Silicone molecules have spines made of alternating atoms of silicon and oxygen, to which are attached various groups of carbon and hydrogen atoms. Depending on the lengths of the chains and the identities of the attached groups, silicones can range from liquids (used in brake fluids and water-repellent sprays) to gels (in breast implants) to greases (in lubricants and lipsticks) to elastomers, the rubber-like materials in Silly Putty, Superballs, refrigerator door gaskets, and, now, kitchenware.
Silicone bakeware has a remarkably useful set of properties. First of all, the material is inherently translucent, so a veritable kaleidoscope of bright colors can be incorporated into the products. (KitchenAid’s line of muffin pans, loaf pans, and cake pans comes in red or blue.) Silicone pans can withstand high temperatures without melting (without their molecules flowing apart from one another) because the molecules are very long and tightly intertwined, like a plate of cold, leftover Spaghetti with Glue Sauce. That’s also why you can take them directly from the oven to the freezer or vice versa without any fear of cracking; the molecules, while individually flexible, are so rigidly fixed in place that the material can’t expand or contract very much with changes in temperature.
Silicones don’t absorb microwaves, but like all microwave-safe utensils they can get hot in the microwave oven from contact with the heated food. Because silicones are chemically inert, the pans are dishwasher-safe; caustic detergents can’t touch them. Also because of their nonreactivity, they are more or less nonstick; cakes and muffins release easily—most of the time—since you can flex the pans to pop them out. But don’t try to use them as Jell-O or aspic molds; sitting the mold in a warm water bath won’t release the gelatin because the silicone is a heat insulator.
Any disadvantages? Being electrical insulators (one of the most important properties of silicone rubbers in other applications), they are subject to static electricity and may collect dust in the pantry between uses. And their floppiness can be disquieting, for example when you’re carrying a batter-filled pan to the oven. Carry the pan on a rimless baking sheet, using the sheet as a peel when inserting the pan into the oven.