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.
Check out more about the exploration of cooking and the primary sources of energy and heat transfer at the link below.
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.