The Food Lab Fundamentals: The Science of Heat vs. Temperature

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, and hastens the rate at which chemical reactions take place. What's interesting is that 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.