Cooking is one of those everyday words that everyone knows; but what does it really mean? At its most basic, cooking means applying heat to food. Cooking 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.
The proteins in food (like in meats, poultry, and eggs) become firmer. This is why the liquid interior of an egg gets hard when you boil it, and 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 heating them in certain ways, specifically through moist heat cooking methods. This is why tough cuts of meat like lamb shanks or oxtails can become so incredibly tender when braised slowly.
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.
Sugars & Starches
Carbohydrates like sugars and starches are also transformed by heating. Sugars turn brown, as we see 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 pasta noodles expand when we cook them.
Fats & 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.
Cooking food causes other, less obvious, changes, too. Nutrients like vitamins can be destroyed or leached out, literally cooked away. Anytime you boil vegetables, some nutrients naturally dissolve into the cooking water or into the air via steam. Flavors can be lost in this same way, too. When you smell the aroma of food cooking, what you're smelling are the flavor compounds evaporating into the air.