By Harold McGee
Of all the major food molecules, proteins are the most challenging and mercurial. The others, water and fats and carbohydrates, are pretty stable and staid. But expose proteins to a little heat, or acid, or salt, or air, and their behavior changes drastically. This changeability reflects their biological mission. Carbohydrates and fats are mainly passive forms of stored energy, or structural materials. But proteins are the active machinery of life. They assemble all the molecules that make a cell, themselves included, and tear them down as well; they move molecules from one place in the cell to another; in the form of muscle fibers, they move whole animals. They’re at the heart of all organic activity, growth, and movement. So it’s the nature of proteins to be active and sensitive. When we cook foods that contain them, we take advantage of their dynamic nature to make new structures and consistencies.