It is this ability to attract both lipids and water that allow phospholipids such as lecithin to act as emulsifiers." The proteins from both the egg yolk and the egg whites contribute to the structure of the dough through a process called coagulation. When heat is applied to the dough, the egg proteins will begin to unfold, or denature, and then form new bonds with one another, thus creating a gel-like network that can hold water and gas. Shortening is responsible for providing tenderness and aerating the dough. In terms of its molecular structure, "a typical shortening that appears solid [at room temperature] contains 15-20% solids and, hence, 80-85% liquid oil ... this small amount of solids can be made to hold all of the liquid in a matrix of very small, stable, needlelike crystals (beta-prime crystals)." This crystalline structure is considered highly stable due to how tightly its molecules are packed. The sugar used in baking is essentially sucrose, and besides imparting sweetness in the doughnut, sugar also functions in the color and tenderness of the final product. Sucrose is a simple carbohydrate whose structure is made up of a glucose molecule bound to a fructose molecule. Milk is utilized in the making of doughnuts, but in large scale bakeries, one form of milk used is nonfat dry milk solids. These solids are obtained by removing most of the water from skim milk with heat, and this heat additionally denatures the whey proteins and increases the absorption properties of the remaining proteins. The ability of the casein and whey proteins to absorb excess water is essential to prolonging the doughnut's freshness. The major whey protein in the nonfat milk solids is known as beta-lactoglobulin, and a crucial feature of its structure is that there exists a single sulfhydryl group that is protected by the alpha helix, and when heating of the milk solids occurs, these groups participate in disulfide exchanges with other molecules. This interchange prevents the renaturation of the whey proteins. If the crosslinking of the sulfide groups does not occur, the whey proteins can rebond and weaken the gluten network. Water is a necessary ingredient in the production of doughnuts because it activates the other ingredients, allowing them to perform their functions in building the doughnut's structure. For example, sugar and salt crystals must be dissolved in order for them to act in the dough, whereas larger molecules, such as the starches or proteins, must be hydrated in order for them to absorb moisture.