The oxygen reacts with elements in the substance to change it. The color of food is often affected. Similar to blanching, but the cooking time is longer. The soft, white membrane that lies between the peel and the pulp of a citrus fruit. It has a bitter flavor.
Liquids can vary from broths, to water, to syrups.
In short, swelling. Yeast proofs when it swells and becomes bubbly. And a dough proofs when it swells and rises to twice its original size. Applied to cooking, this means to boil a liquid until its volume is reduced by evaporation. This thickens the liquid and intensifies the flavor.