Heat vs. Temperature
What Is Cooking?
I know you’re eager to jump right in and start cooking, but first answer this question: What is cooking?
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.
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 randomly.
The more energy 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 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.
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.
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.
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, and 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 at which most custard or egg-based batters are cooked to set them fully).
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: how that energy gets transferred to your food.
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.
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:
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 a convection pattern.
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 are crisp and brown more efficiently.
Radiation is the 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.
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.
You might notice that these three types of heat transfer heat only onto the surface of foods.
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 foods—you don’t need to wait for the relatively slow transfer of energy from the exterior to the center.
The difference between the definition of temperature and the definition of energy is subtle but extraordinarily important.
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, even though 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 your hand much more rapidly.
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.
