What causes food to brown when cooked in a pan?

The primary cause is the Maillard reaction, a chemical process that occurs when amino acids and reducing sugars in food are exposed to heat, typically above 280 to 330 degrees Fahrenheit. This reaction produces hundreds of new flavor compounds and brown pigments called melanoidins, which give seared meat, toasted bread, and browned vegetables their distinctive color and complex flavor. Caramelization is a separate but related process that applies specifically to sugars breaking down under heat, producing nutty, sweet flavors and brown color. Both reactions require relatively dry surface conditions, which is why patting food dry before cooking and avoiding overcrowding the pan produces better browning, since excess moisture creates steam and keeps temperatures too low for these reactions to occur.