By Harold McGee
Though it’s less dense than liquid water and so makes less frequent contact with the food, steam compensates for this loss in efficiency with a gain in energy. It takes a large amount of energy to turn liquid water into a gas, and conversely gaseous water releases that same large amount of energy when it condenses onto a cooler object. So molecules of steam don’t just impart their energy of motion to the food; they impart their energy of vaporization also. This means that steaming does an especially quick job of bringing the surface of the food up to the boiling point, and an effective job of keeping it there.