At its most basic, cooking means applying heat to food. Whether the food is baked, fried, sautéed, boiled, or grilled, it's all cooking. Evidence suggests our ancestors began cooking over an open fire over 2 million years ago. People still cook some foods over an open flame, in addition to using tools like microwaves, toasters, and stovetops.
In scientific terms, cooking is transferring energy from a heat source to the food. It is as much about the ways heat changes the food as it is about the heat itself. That's because heating food does more than just make it hotter.
When you cook an egg, the interior turns from liquid to solid. This is because the proteins in food (like in meats, poultry, and eggs) become firmer when heat is applied. This also why a well-done steak is tougher than one cooked medium-rare.
Interestingly, other proteins, namely the collagens that make up cartilage and other connective tissues in meats, can be made to break down by cooking. Tough meats like oxtails and lamb shanks become incredibly tender when cooked slowly using moist heat cooking methods like a long braise.
Cooking also causes proteins to lose moisture, typically via evaporation in the form of steam. This loss of moisture then causes protein-rich food to shrink, as we see with burgers that shrink when cooked on the grill.
Fruits and Vegetables
Cooking food causes other, less obvious, changes, too. Nutrients like vitamins can be destroyed or leached out, literally cooking away. Any time you boil vegetables, some nutrients naturally dissolve into the cooking water or into the air via steam. Conversely, certain vitamins are made available to the body for absorption during cooking, such as thiamin and folate.
Flavors can be lost in this same way. When you smell the aroma of food cooking, you're smelling the flavor compounds evaporating into the air.
Cooking often affects the color of foods as well. Green vegetables (like green beans) first brighten when cooked, but they eventually take on a drab olive hue if they're cooked for too long.
Carbohydrates like sugars and starches are also transformed by heating. Sugars turn brown, as seen when we caramelize the tops of a crème brûlée. The browning of bread when we bake it is caused by the caramelization of the carbohydrates. Starches tend to act like sponges, soaking up water and expanding in size, as when rice or pasta noodles expand when we cook them.
Fats and Fiber
Fats, such as butter and oils, liquefy, and eventually start to smoke when they get too hot. The fibers in vegetables and fruits soften and break down, which is why a cooked carrot is softer than a raw one.
Some foods are not safe to eat without first being cooked. Cooking not only heats the food, but it can also help kill harmful bacteria. Raw meats are especially prone to carrying bacteria and should be cooked to specific temperatures to ensure they are safe to consume.