You may have thought you left physics and chemistry behind when you left school, but if you want to cook and eat well, understanding the physics and chemistry of cooking will help immensely. Here are some foundational concepts every outdoor cook needs to know.
Foods are composed mainly of water, protein, fat, and carbohydrates, with trace amounts of minerals and other compounds. Cooking is the process of changing the chemistry of food—usually by transferring energy in the form of heat—so that the food becomes safer to eat and more digestible, and to improve its flavor, texture, juiciness, appearance, and nutrition.
When you cook outdoors, heat is transferred to food in three different ways: conduction, convection, and radiation.
Conduction heat is when your lover’s body is pressed against yours.
Conduction heat is when your lover’s body is pressed against yours. This is when energy gets transferred to food by direct contact with the heat source. Think of cooking a hot dog in a frying pan. Heat from the burner is transferred to the pan. The molecules in the pan vibrate and pass the heat on to the wiener where it makes contact with the pan. As the surface of the meat gets hotter, the heat transfers to the center through the moisture and fats in the meat. That’s also conduction. On a grill, the grill grates transfer energy by conduction.
Convection heat is when your lover blows in your ear.
Convection heat is when your lover blows in your ear. This is when energy is carried to food by air, water, or oil. If you boil a hot dog, you are cooking with convection heat. If you cook the hot dog in your kitchen oven, where it is surrounded by hot air, that’s also convection cooking. A convection oven comes equipped with a fan to speed up the natural airflow, increasing the heat transfer and cooking many foods 25 to 30 percent faster than it would cook without the fan. If you put your hot dog on one side of your grill but only heat up the other side of the grill, that, too, is convection cooking, as the natural airflow inside the grill conveys heat to the wiener.
Radiant heat is when you feel the heat of your lover’s body under the covers without touching.
You know the feeling of radiant heat. Radiant heat is when you feel the heat of your lover’s body under the covers without touching. It’s the heat on your skin from the sun or from a space heater. Put a hot dog on a stick and hold it to the side of a campfire, and you are cooking by radiation.
The Magic of Infrared (IR)
Infrared (IR) radiant heat delivers more energy more quickly than convection heat. Let’s make believe we have two charcoal grills side by side. On one grill, the charcoal is pushed all to the right side. The air temperature on the left side is 325°F, as the convection flow of air from the right side circulates over the left side. Let’s put a big turkey on the left side. In a couple of hours it will cook perfectly and absorb a lovely smoky flavor.
On the second grill, we have charcoal spread evenly across the bottom and the air temperature on both sides is also 325°F. Let’s put a turkey on this grill, too. By the time the turkey is cooked to the proper temperature, it will be blacker than a mourning hat.
The air temperature of both ovens (remember, a grill is really just another kind of oven) was 325°F, but the IR radiant heat from below on the second grill, which can be over 1400°F, burned the bird.
Infrared waves are a part of the continuum of energy waves that surround us at all times, just up the road from visible light and down the road from radio waves.
Infrared waves pack a lot of heat energy, and they excel at creating the dark brown surfaces we crave on food. IR is the best way to deliver high heat to food. IR energy is delivered faster than convection but not as fast as conduction. In the past few years, gas grill manufacturers have added special burners that emit concentrated infrared. They are sometimes called infrared burners, IR burners, sear burners, or sizzle zones, and they are great for getting a good dark sear on steaks and crisping poultry skin. But as with the turkey, sometimes you don’t want IR.
The Difference Between Heat and Temperature
Heat, in the form of energy, not temperature, cooks food. Convection, radiation, and conduction all deliver energy, but in different quantities. Fire up your grill to 225°F. Open the lid, stick your hand in the warm air, and count how long you can hold your hand in there. Most people can handle more than a minute. Place your hand on the cooking grate. After you get back from the hospital, contemplate the fact that even though the air and the grate were both 225°F, not everything that is the same temperature transfers energy at the same rate.
That’s because steel has more molecules per cubic inch than air and stores 8,000 times more energy. That’s what causes grill marks.
Temperature measures the average energy of each atom, while heat is the total energy for all atoms. It’s like money. If the average income in the United States is $50,000 per family, the total income is in the trillions.
The Impact of Distance
The distance from a radiant energy source is another important factor. Energy dissipates and spreads out as it moves away from the source. In an $800 kamado grill, the charcoal may be 18 inches from the cooking surface, while on a $100 Weber Kettle, the charcoal is 4 inches away, and on a $30 hibachi, the coals may be 1 inch away. A steak on a kamado’s cooking surface will not brown as well as one on a Weber or a hibachi because the coals emitting IR heat are farther away.
How Heat Moves Within Meat
When we subject food to heat, energy is transferred to the exterior of the food. Once the energy excites the molecules on the surface of the food, they then transfer heat to the molecules inside by conduction, slowly passing the energy toward the center. In other words, on a grill, hot air cooks the outside of the meat, but the outside of the meat cooks the inside.
This means that cooked meat is not uniform in temperature. The surface may register up to 212°F (evaporation of moisture keeps it from getting a lot hotter), but the temperature will gradually decrease toward the center. While a hotter, well-seared exterior surface is often desirable, the trick is to get the interior to be close to the ideal temperature from top to bottom.
This takes time, because meat is about 70 percent water, and water is a good insulator and heat absorber, especially when trapped within muscle fibers and mixed with fat, an even better insulator. Physics dictates that the meat seek equilibrium in an effort to make the temperature the same from edge to edge, and so the heat moves inward.
On a grill, hot air cooks the outside of the meat, but the outside of the meat cooks the inside.
The points and corners of the meat also cook faster because heat can attack on multiple fronts. Bones heat at a different rate than the muscle tissue in a cut of meat because they are filled with air or fat, not water. In most cases, the bones warm more slowly than the rest of the meat.
Carryover Cooking
After you take food off the heat, it may continue cooking for 20 minutes or more, even at room temperature, taking a perfect medium-rare roast to medium-well and ruining it. This phenomenon is called carryover cooking.
Myth Plan on a 5 to 10°F carryover.
BUSTED! There is no easy rule of thumb for calculating carryover. The thickness of the meat is a major factor in determining how much its temperature will rise in the carryover phase. Thick cuts hold more energy than thin cuts. High cooking temperatures pump more energy into the outer layer of the meat than low temperatures do, so cooking over high heat produces more carryover.
When we remove the meat from the heat, it goes on cooking because the energy stored in the outer layers of the meat continues to move toward the center.
1. On the grill or smoker. In the left image above, we see a cross-section of a beef roast cooking at 325°F in convection air, absorbing heat from hot air on all sides. When the center hits 130°F, medium-rare, we remove it from the heat. The exterior has a nice dark brown crust and beneath it a band of brown meat, then tan, then pink, and finally a beautiful rosy cylinder.
2. 10-minute rest. In the center image above, the meat has been removed from the heat and rested for 10 minutes. Energy from the hot surface continues to be passed toward the center, slowly cooking the meat even though it is sitting at room temperature. The surrounding air is now cooler than the meat, so some of the heat escapes into the room and the exterior cools as energy moves away. The exterior remains dark brown and crusty on most sides, but gets soft on the bottom where it rests on the platter. The cylinder of meat in the center has now moved past medium-rare.
3. 20-minute rest. In the right image above, the meat has rested for 20 minutes. It has come close to an even temperature throughout, and now more heat is escaping than moving inward. The crust has cooled, the center has warmed, and the two are pretty much the same temperature, medium-well-done. Meanwhile, moisture from the inner layers has moved into the drier outer layers, softening the crust. The roast has approached equilibrium and is almost at the point at which you have to start apologizing to your guests for the overcooked meat.
Myth Meat needs to rest after cooking.
BUSTED! Many recipes tell you to let steaks and chops “rest” for 10 to 15 minutes—and roasts for up to 30 minutes—after cooking. We are told that if we rest meat it will be more juicy.
People who preach the importance of letting the meat rest say that if you cut into the meat when it is fresh off the heat, the juices pour out of the muscle fibers, which they think are like skinny water balloons. If you let meat rest and cool, they say, the pressure drops, the fibers relax, and fewer juices escape.
The pressure theory is a myth, says meat scientist Antonio Mata, Ph.D., because fibers are not like balloons. Water is not trapped in the fibers or the spaces between them, so the pressure equalizes quickly. And at relatively low meat temperatures, water does not expand much.
To test this theory, my colleague Professor Greg Blonder cooked two 13½-ounce ribeye steaks to 125°F. He cut one into strips immediately, rested the other for 30 minutes, and then cut it into strips. He collected the juices from the steaks and measured them. The steak that had not rested expelled about 6 teaspoons. The steak that had rested gave up 5 teaspoons—not much of a difference. Also, the meat temperature on the rested steak rose to 145°F from carryover cooking, well past medium-rare to medium-well. Naturally, the careful scientist repeated the experiment several times. Keep in mind, when we eat a steak, most of us cut into it one piece at a time; we don’t slice it into strips. And that juice isn’t lost. We mop it up with the meat on our fork.
Professor Blonder then turned his attention to pork loin roasts. He cooked two large 33-ounce roasts, removing them when their internal temperature had reached 140°F. He let one sit for 3 minutes and then cut it into slices, collecting the juices released by the meat. He rested the other for 20 minutes before slicing, waited 5 minutes, collected the juices, and weighed them. The unrested meat released 3 ounces of juices, compared to 2 ounces from the rested meat, a difference of only 1 ounce.
Professor Blonder poured the 3 ounces of liquid from the unrested pork on top of the sliced meat. The meat drank up about 1 ounce of the juices, precisely the difference between the rested and unrested meat.
Resting meat has other disadvantages: making the crust or skin soft and wet, making the fat waxy, and causing overcooking. I say, serve meat hot. It will “rest” while we eat.
How Boiling Temperatures Impact Cooking
When liquid is heated, its temperature will increase until it hits the boiling point and not go any higher. So no matter how high we turn the burners under a pot of water, the water will not get hotter than 212°F (water boils at lower temperatures as you go up in altitude because the weight or “pressure” of the column of air on top of the water is lower, and it boils at slightly higher temperature if you add impurities like salt).
Steam can form at a lower temperature than 212°F, as molecules of water get hot and escape the surface of the warming water. That’s why we see vapor escaping a pot of water before the water actually boils. As meat heats on a grill, some of the water on its surface escapes as steam. Even though the grill may be a lot hotter than 212°F, the meat’s surface will idle along at about 212°F as water keeps steaming away.
While the hotter molecules escape, the cooler ones are left behind, so the temperature of the meat plateaus as the surface dries out and forms a crust, or bark. At low cooking temperatures such as 225°F (the temperature I recommend for a lot of my recipes), the rate of evaporation can be so great that the meat cools as fast as it heats. In this case, the temperature of the meat in its center can get stuck—usually in the 150 to 165°F range—and remain pretty much unchanged for hours, driving the novice cook nuts. This phenomenon, called the stall, does not happen if the cooking temperature is higher, say, 325°F (another temperature I recommend frequently).
The Two-Zone Setup and Indirect Cooking
Temperature control is the most important skill you can learn. That’s why I recommend a two-zone setup in almost every situation. The most common grilling mistake is spreading coals across the bottom of the entire grill or turning on all our gas burners. That forces us to work quickly, flipping burgers and losing track of which went on first, rolling blackened hot dogs around, trying to tame flare-ups with a squirt gun, and sheepishly serving charred hockey pucks that are raw in the center.
A two-zone setup gives us better temperature control on both charcoal and gas grills. One side of the grill is hot and produces direct radiant heat, while the other side produces no heat. Food placed on that side cooks by indirect convection heat wafting over from the hot side. We’ll call the hot side the direct-radiant-heat zone and the other the indirect-convection-heat zone.
Using a two-zone setup, we can . . .
Control the heat. We can move food to the indirect zone, where it is bathed in gentle convection heat, to warm it slowly and evenly inside. We can also sear the heck out of it for a minute or two in the direct zone when we want a golden brown and delicious crust. That’s how we win the day on the Fourth of July.
Gently smoke a big turkey in the indirect zone, evenly cooking all 18 pounds to juicy, tender perfection, and be the heroes of Thanksgiving.
Slowly bring a prime rib to medium-rare with no gray meat, get a perfectly crunchy crust, and become our mom’s favorite on Mother’s Day.
Start chickens over the indirect zone at a low temperature, cook them evenly throughout until they are almost done, then move them to the direct zone to crisp the skin, and bask in the glory at the church picnic.
Manage several foods at once when the thickness and water content of each is significantly different, causing them to cook at different rates. Put baking potatoes in the indirect zone for an hour, add lobster for the last 20 minutes, and then, 10 minutes before dinner, sear asparagus over the direct zone for an incredible picnic on the beach.
Prevent sweet foods from burning. We can cook the most tender ribs with a sweet dry rub in the indirect zone and never burn a grain of sugar, and then move the ribs to the direct zone to caramelize the sauce to finger- licking goodness and prove to Dad you turned out all right.
When to Put a Lid on It
Most grills come with lids, thankfully. A lid is essential for most outdoor cooking with a few notable exceptions, chief among them searing meats. You can cook on a lidless grill, but you will be severely handicapped. It’s like doing all your cooking on a stove top. On a grill, most of the heat and smoke comes from below, but much of it goes right past the food. The lid captures heat and smoke so your grill becomes a smoky oven that can cook foods with heat from all sides. In short, a lid gives you much more versatility.
You can cook on a lidless grill, but you will be severely handicapped.
As a rule of thumb, whether meat or vegetable, you want a dark, well-cooked crust and a tender, juicy center. If the food is ¾ inch thick or less, forgo the lid. If you were to close the lid, heat would attack from above and below, and the center would be done before you could get good color and flavor to both sides of the crust. For thin foods, crank up the heat, leave the lid off, and flip the meat every minute or so to prevent heat buildup on either side.
But if the food is thicker than ¾ inch, put a lid on it. The lid helps thick foods cook evenly and reduces your chances of an undercooked interior.
There is a middle ground: Sometimes you might want to wedge the lid open an inch or two to allow hot air to escape if you are having trouble getting the heat down to a target such as 225°F.