For something so familiar, cooking is incredibly mysterious. Ask a home cook what happens to a steak in a pan—hell, ask most chefs. It gets hot. It gets brown. It gets juicy. How do you like yours, again?
We burn, sear, boil, broil, grill and fry our food to make it appetizing, and, with any luck, delicious. This is how we think and talk about food, and it obviously serves us pretty well. But it says nothing of what cooking actually is: Chemistry. Here's what really happens to your steak.
"Well, a lot of things," says Dennis Miller, Professor of Food Science and Nutrition at Cornell University. "The iron in the myoglobin—a reddish pigment, similar to hemoglobin—on the surface of the steak, gets oxidized, which results in a change in color, from reddish to brown." The char, or the sear, or whatever you want to call the first changes in a sizzling steak are unappetizingly explicable by hard chemistry. The chef has oxidized myoglobin into metmyoglobin. He is a meat chemist.
And that's just the beginning. "When you heat most any food that contains protein and carbohydrate—and there is some carbohydrate in meat—the two can work together to form flavor and aroma compounds." This process, called Maillard browning, happens when cooking a tremendous number of foods: bread crust, whiskey, coffee, all get their colors from this reaction. Weirdly, so do Snooki and Sitch: Some heat-activated self-tanning creams exploit the Maillard reaction.
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The Maillard reaction actually strikes at the heart of what most think of as cooking: Browning. Maillard browning is one type. Another, that you've probably heard of, is called caramelization, which is caused by the decomposition of sugary carbohydrates. When sugars hit a certain temperature, they break down into a complex mixture of chemicals that tend to taste and look nice. (Think Coke, or caramelized onions.)
The third type of browning, and this encompasses a lot of processes, is called enzymatic browning. This, in scientific terms, is browning caused (or catalyzed) by enzymes. An obvious example of enzymatic browning is the transformation of grapes into raisins. Drying a water-rich grape obviously reduces its size, but it also damages the basic structure of the tissue in the grape, which speeds along an interaction catalyzed by enzymes, creating a brown compound. This is why the raisin is a different color than its parent grape.
And although enzymatic browning can be used in cooking, you probably know it best for its negative effects: the browning of newly cut apples, or the greying of seafood.
So anyway, those are the three main types of browning. But our steak's not done yet.
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The color has changed, and so have elements of the taste and smell. Next up: texture! Paging Dr. Miller: "Proteins are generally very large molecules, composed of long chains of amino acids, and they're folded up." This property of proteins is partly why muscles tissue is able to contract, and why raw meat is so chewy and tough. "When you heat them, they tend to get denatured, or in other words, unfolded. This has tenderizing effect."
Think about that for a second. Coiled, intricately structured proteins, which are some of the most fundamental building blocks of life, basically give up on being proteins just because you made them hot. This causes a profound change in the essence of the food. Tough, squishy meat becomes tender and chewable. Clear, viscous egg albumen becomes solid and white. In the hands of an amateur, an inedible food becomes edible; under the care of a culinary expert, raw food matter becomes sublime.
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Browning, protein denaturing, and the oxidization of flesh can tell the story of a steak, and indeed, most living—or rather, ex-living—tissues. But something is curiously absent from this account of one steak in one pan, something that's as vital to the cooking process as perhaps anything but heat: water.
"Water is basically a medium for conducting heat," says Professor Miller, who isn't ready to forget about our steak just yet. "When you're cooking a steak, water can actually serve as a heat conductor," bringing the intense surface heat from the pan or grill or oven into the center of the steak, which helps ensure that that pink center isn't too pink.
Water is a medium for chemical reactions as well as heat, and it can dramatically affect how vitamins and oxygens interact. Duties as a medium aside, though, water's star role in cooking is a disappearing act: Evaporation. Loss of water can dramatically alter the texture of a food, as can the addition of water. Bread crust is awesome because it lacks water; dried pasta becomes edible when it gets wet again. Freeze-dried products last years because they lack water—and all but cease to be food until that H20 is added again.
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Understanding how cooking works doesn't help explain the primary weirdness of cooking, which is that we do it at all. No other animal scorches their food like we do, and that only started a couple million years ago at the very most.
Tons of fruits and veggies are obviously delicious raw. If left uncooked, Professor Miller tells me, even our steak would edible and perhaps even a bit more nutritious than if it were cooked. But in most preparations it would be unpleasant to our palates, which are adapted to cooked foods, and unless sterilized by some other method like exposure to radiation, possibly unsafe. A few foods, like beans, only give up their nutrition to humans after they've been cooked.
As for widely and furiously voiced concerns "killing" your food, or somehow cooking the vitamins out of it, well, don't, you know, change your lifestyle over it. Some enzymes are inactivated during cooking, but this has minimal effect on our digestion or nutrition. Boiling vegetables like spinach can cause some water-soluble vitamins to leach into the pot. This can be avoided, however, by steaming (yay!) or microwaving (ugh.) the veggies.
Whether (or what) you choose cook, the takeaway is the same. Every time you prepare yourself a meal, you're not playing chef. You're a scientist.
Raw steak from Wikimedia Commons, cooked meat courtesy of Flickr user Adactio.