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. Another important consideration of water is its degree of hardness, which measures the amount of impurities in the water source. Pure water consists of two parts hydrogen and one part oxygen, but water used in baking often is not pure. Baker's salt (NaCl) is usually used as an ingredient due to its high purity, whereas the salts in water are derived from varying minerals. As an ingredient, "salt is added to enhance the flavour of cakes and breads and to ‘toughen up’ the soft mixture of fat and sugar." If relatively soft water is being used, more salt should be added in order to strengthen the gluten network of the dough, but if not enough salt is added during the baking process, the flavor of the bread will not be appealing to consumers. Health effects Doughnuts are unhealthy, though some are less so than others. According to Prevention Magazine, doughnuts made from enriched flour provide some thiamine, riboflavin, and niacin, along with some fiber, but they are high in sugar and calories. Steps to improve the healthiness of doughnuts include removing trans fats. Dough rheology An important property of the dough that affects the final product is the dough's rheology. This property measures the ability of the dough to flow. It can be represented by the power law equation: where is the tangentic stress, is the viscosity coefficient, is the shear rate, and is the flow index. Many factors affect dough rheology including the type of ingredients, the amount of the ingredients, or the force applied during mixing. Dough is usually described as a viscoelastic material, meaning that its rheology depends on both the viscosity and the elasticity. The viscosity coefficient and the flow index are unique to the type of dough