The albumen, the white part of the egg surrounding the yolk, is made up of four concentric layers. They are, listed from the yolk outward, the chalaziferous layer, the thick albumen, the internal thin albumen, and the external thin albumen. The external thin albumen, located nearest to the shell, encloses the other layers of albumen. In a high-quality egg, the thick layers stand higher and spread less than the thin layers when the egg is broken, while in lower-grade eggs the thick layers become indistinguishable from the thinnest layer. As the egg ages, the albumen tends to thin, as the egg protein changes in character over time. Together, the albumen layers account for about 67 percent of an egg’s liquid weight as well as half of its protein content and a good portion of its niacin, riboflavin, choline, magnesium, potassium, sodium, and sulfur. Albumen is actually more opalescent than pure white. Its cloudy appearance is linked to the carbon dioxide contained in the white. As an egg ages, carbon dioxide escapes, making the albumen of older eggs clearer and more transparent than those of fresh eggs. When vigorously beaten by hand or machine, albumen foams and can increase to six to eight times its original volume.