The Food Lab Fundamentals: The Science of Heat vs. Temperature

The Food Lab

Unraveling the mysteries of home cooking through science.

[Photographs: J. Kenji López-Alt]

This is an exclusive excerpt from my book, The Food Lab: Better Home Cooking Through Science, a grand exploration of the world of food science and how it can make everyday home cooking easier and more delicious. It's on sale now anywhere books are sold, or online.

What Is Cooking?

I know you're eager to jump right in and start cooking, but first answer this question: What is cooking?

If you're my wife, your answer will be, "It's that thing you do when that crazy look comes into your eyes." A great chef might tell you that cooking is life. My mom would probably say that it's a chore, while my wife's aunt would tell you that cooking is culture, family, tradition, and love. And, yes, cooking is all of those things, but here's a more technical way to think about it: Cooking is about transferring energy. It's about applying heat to change the structure of molecules. It's about encouraging chemical reactions to alter flavors and textures. It's about making delicious things happen with science. And before we can even begin to understand what happens when we grill a hamburger, or even what equipment we might want to stock our kitchen with, we have to get one very important concept into our heads first, as it'll affect everything we do in the kitchen, starting with which pots and pans we use. It's this: Heat and temperature are not the same thing.

At its most basic, cooking is the transfer of energy from a heat source to your food. That energy causes physical changes in the shape of proteins, fats, and carbohydrates, as well as hastens the rate at which chemical reactions take place. What's interesting is that most of the time, these physical and chemical changes are permanent. Once a protein's shape has been changed by adding energy to it, you can't change it back by subsequently removing that energy. In other words, you can't uncook a steak.

The distinction between heat and temperature can be one of the most confusing things in the kitchen, but grasping the concept is essential to helping you become a more rational cook. Through experience, we know that temperature is an odd measure. I mean, pretty much all of us have walked around comfortably in shorts in 60° weather but have felt the ridiculous chill of jumping into a 60° lake, right? Why does one but not the other make us cold, even though the temperature is the same? Let me try to explain.

Heat is energy. Third-grade physics tells us that everything from the air around us to the metal on the sides of an oven is composed of molecules: teeny-tiny things that are rapidly vibrating or, in the case of liquids and gases, rapidly bouncing around in a random manner. The more energy is added to a particular system of molecules, the more rapidly they vibrate or bounce, and the more quickly they transfer this movement to anything they are touching—whether it's the vibrating molecules in a metal pan transferring energy to a juicy rib-eye steak sizzling away or the bouncing molecules of air inside an oven transferring energy to the crusty loaf of bread that's baking.

Heat can be transferred from one system to another, usually from the more energetic (hotter) system to the less energetic (cooler). So when you place a steak in a hot pan to cook it, what you are really doing is transferring energy from the pan burner system to the steak system. Some of this added energy goes to raising the temperature of the steak, but much of it gets used for other reactions: It takes energy to make moisture evaporate, the chemical reactions that take place that cause browning require energy, and so on.

Temperature is a system of measurement that allows us to quantify how much energy is in a specific system. The temperature of the system is dependent not only on the total amount of energy in that body, but also on a couple of other characteristics: density and specific heat capacity.

Density is a measure of how many molecules of stuff there are in a given amount of space. The denser a medium, the more energy it will contain at a given temperature. As a rule, metals are denser than liquids,* which in turn are denser than air. So metals at, say, 60°F will contain more energy than liquids at 60°F, which will contain more energy than air at 60°F.

*All right, Mr. Smarty-Pants. Yes, at high enough temperatures, metals will melt into very dense liquids, and yes, Mr. Even Smartier-Pants, mercury is a very dense metal that is liquid even at room temperature. Got that out of your system? OK, let's move on.

Specific heat capacity is the amount of energy it takes to raise a given amount of a material to a certain temperature. For instance, it takes exactly one calorie of energy (yes, calories are energy!) to raise one gram of water by one degree Celsius. Because the specific heat capacity of water is higher than that of, say, iron, and lower than that of air, the same amount of energy will raise the temperature of a gram of iron by almost 10 times as much and a gram of air by only half as much. The higher the specific heat capacity of a given material, the more energy it takes to raise the temperature of that material by the same number of degrees.

Conversely, this means that given the same mass and temperature, water will contain about 10 times as much energy as iron and about half as much as air. Not only that, but remember that air is far less dense than water, which means that the amount of heat energy contained in a given volume of air at a given temperature will be only a small fraction of the amount of energy contained in the same volume of water at the same temperature. That's the reason why you'll get a bad burn by sticking your hand into a pot of 212°F boiling water, but you can stick your arm into a 212°F oven without a second thought (see "Experiment: Temperature Versus Energy in Action," below).

Confused? Let's try an analogy.

Imagine the object being heated is a chicken coop housing a dozen potentially unruly chickens. The temperature of this system can be gauged by watching how fast each individual chicken is running. On a normal day, the chickens might be casually walking around, pecking, scratching, pooping, and generally doing whatever chickens do. Now let's add a bit of energy to the equation by mixing a couple cans of Red Bull in with their feed. Properly pepped up, the chickens begin to run around twice as fast. Since each individual chicken is running around at a faster pace, the temperature of the system has gone up, as has the total amount of energy in it.

Now let's say we have another coop of the same size but with double the number of chickens, thereby giving it double the density. Since there are twice as many chickens, it will take double the amount of Red Bull to get them all running at an accelerated pace. However, even though the final temperature will be the same (each individual chicken is running at the same final rate as the first ones), the total amount of energy within the second coop is double that of the first. So, energy and temperature are not the same thing.

Now what if we set up a third coop, this time with a dozen turkeys instead of chickens? Turkeys are much larger than chickens, and it would take twice as much Red Bull to get one to run around at the same speed as a chicken. So the specific heat capacity of the turkey coop is twice as great as the specific heat capacity of the first chicken coop. What this means is that given a dozen chickens running around at a certain speed and a dozen turkeys running around at the same speed, the turkeys will have twice as much energy in them as the chickens.

To sum up:

  • At a given temperature, denser materials generally contain more energy, and so heavier pans will cook food faster. (Conversely, it takes more energy to raise denser materials to a certain temperature.)
  • At a given temperature, materials with a higher specific heat capacity will contain more energy. (Conversely, the higher the specific heat capacity of a material, the more energy it takes to bring it to a certain temperature.)

In this book, most recipes call for cooking foods to specific temperatures. That's because for most food, the temperature it's raised to is the primary factor determining its final structure and texture. Some key temperatures that show up again and again include:

  • 32°F (0°C): The freezing point of water (or the melting point of ice).
  • 130°F (52°C): Medium-rare steak. Also the temperature at which most bacteria begin to die, though it can take upward of 2 hours to safely sterilize food at this temperature.
  • 150°F (64°C): Medium-well steak. Egg yolks begin to harden, egg whites are opaque but still jelly-like. Fish proteins will tighten to the point that white albumin will be forced out, giving fish like salmon an unappealing layer of congealed proteins. After about 3 minutes at this temperature, bacteria experience a 7 log reduction—which means that only 1 bacterium will remain for every million that were initially there.
  • 160° to 180°F (71° to 82°C): Well-done steak. Egg proteins fully coagulate (this is the temperature to which most custard or egg-based batters are cooked to set them fully). Bacteria experience a 7 log reduction within 1 second.
  • 212°F (100°C): The boiling point of water (or the condensation point of steam).
  • 300°F (153°C) and above: The temperature at which the Maillard browning reactions—the reactions that produce deep brown, delicious crusts on steaks or loaves of bread—begin to occur at a very rapid pace.The hotter the temperature, the faster these reactions take place. Since these ranges are well above the boiling point of water, the crusts will be crisp and dehydrated.

Sources of Energy and Heat Transfer

Now that we know exactly what energy is, there's a second layer of information to consider: the means by which that energy gets transferred to your food.

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Steaks searing—heat through conduction.

Conduction is the direct transfer of energy from one solid body to another. It is what happens when you burn your hand by grabbing a hot pan (hint: don't do that). Vibrating molecules from one surface will strike the relatively still molecules on another surface, thereby transferring their energy. This is by far the most efficient method of heat transfer. Here are some examples of heat transfer through conduction:

  • Searing a steak
  • Crisping the bottom of a pizza
  • Cooking scrambled eggs
  • Making grill marks on a burger
  • Sautéing onions
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Dumplings boiling—heat through convection.

Convection is the transfer of energy from one solid body to another through the intermediary of a fluid—that is, a liquid or a gas. This is a moderately efficient method of heat transfer, though in cooking its efficiency depends greatly on the way the fluid flows around the food. The motion of the fluid is referred to as convection patterns.

As a general rule, the faster air travels over a given surface, the more energy it can transfer. Still air will rapidly give up its energy, but with moving air, the energy supply is constantly being replenished by new air being cycled over a substance such as food. Convection ovens, for instance, have fans that are designed to keep the air inside moving around at a good clip to promote faster, more even cooking. Similarly, agitating the oil when deep-frying can lead to foods that crisp and brown more efficiently.

Here are some examples of heat transfer through convection:

  • Steaming asparagus stalks
  • Boiling dumplings in stock
  • Deep-frying onion rings
  • Barbecuing a pork shoulder
  • The top of a pizza baking in an oven
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The top of a pizza cooks via convection and radiation.

Radiation is transfer of energy through space via electromagnetic waves. Don't worry, that's not as scary as it sounds. It doesn't require any medium to transfer it. It is the heat you're feeling when you sit close to a fire or hold your hand above a preheated pan. The sun's energy travels to the earth through the vacuum of space. Without radiation, our planet (and indeed, the universe) would be in a lot of trouble!

An important fact to remember about radiant energy is that it decays (that is, gets weaker) by the inverse square law—the energy that reaches an object from a radiant energy source is proportional to the inverse of the square of its distance. For example, try holding your hand 1 foot away from a fire, then move it 2 feet away. Even though you've only doubled the distance, the fire will feel only about one-quarter as hot.

Here are some examples of radiant heat transfer:

  • Roasting a pig on a spit next to hot coals
  • Toasting garlic bread under the broiler
  • Getting a tan from the sun
  • Broiling some marinated salmon

Most of the time, in cooking, all three methods of heat transfer are used to varying extents. Take a burger on the grill, for example. The grill grate heats the patty directly where it is in contact with it through conduction, rapidly browning it at those spots. The rest of the underside of the patty is cooked via radiation from the coals underneath. Place a piece of cheese on the burger and pop the lid down for a bit, and convection currents will form, carrying the hot air from directly above the coals up and over the top of the burger, melting the cheese.

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Grilled burgers cook through all three forms of energy transfer.

You might notice that these three types of heat transfer heat only onto the surface of foods. In order for food to cook through to the center, the outer layer must transfer its heat to the next layer, and so on, until the very center of the food begins to warm up. Because of that, the outside of most cooked foods will almost always be more well done than the center (there are tricks to minimizing the gradient, which we'll get to in time).

Microwaves are the only other standard method of energy transfer we commonly use in the kitchen, and they have the unique ability to penetrate through the exterior of food when heating it. Just like light or heat, microwaves are a form of electromagnetic radiation. When microwaves are aimed at an object with magnetically charged particles (like, say, the water in a piece of food), those particles rapidly flip back and forth, creating friction, which, in turn, creates heat. Microwaves can pass through most solid objects to a depth of at least a few centimeters or so. This is why microwaves are a particularly fast way to heat up foods—you don't need to wait for the relatively slow transfer of energy from the exterior to the center.

Phew! Enough with the science lesson already, right? Bear with me. Things are about to get a lot more fun!

Experiment: Temperature Versus Energy in Action

The difference between the definition of temperature and the definition of energy is subtle but extraordinarily important. This experiment will demonstrate how understanding the difference can help shape your cooking.

Materials

  • 1 properly calibrated oven
  • 1 able-bodied subject with external sensory apparatus in full working order
  • One 3-quart saucier or saucepan filled with water
  • 1 accurate instant-read thermometer

Procedure

Turn your oven on to 200°F and let it preheat. Now open the oven door, stick your hand inside, and keep your hand in the oven until it gets too hot to withstand. A tough guy like you could probably leave it in there for at least 15 seconds, right? 30 seconds? Indefinitely?

Now place a pan of cold water on the stovetop and stick your hand in it. Turn the burner to medium-high heat and let the water start to heat up. Stir it around with your hand as it heats, but be careful not to touch the bottom of the pan (the bottom of the pan will heat much faster than the water). Keep your hand in there until it becomes too hot to withstand, remove your hand, and take the temperature.

Results

Most people can hold their hand in a 200°F oven for at least 30 seconds or so before it becomes uncomfortably hot. But let it go much above 135°F, and a pan of water is painful to touch. Water at 180°F is hot enough to scald you, and 212°F (boiling) water will blister and scar you if you submerge your hand in it. Why is this?

Water is much denser than air—there are many times more molecules in a cup of water than there are in a cup of air. So, despite the fact that the water is at a lower temperature than the air in the oven, the hot water contains far more energy than the hot air and consequently heats up your hand much more rapidly. In fact, boiling water has more energy than the air in an oven at a normal roasting temperature, say 350° to 400°F. In practice, this means that boiled foods cook faster than foods that are baked or roasted. Similarly, foods baked in a moist environment cook faster than those in a dry environment, since moist air is denser than dry air.

- Excerpted with permission from The Food Lab: Better Home Cooking Through Science, available wherever books are sold from W. W. Norton.