Legends about eggs have abounded throughout the eons. Early Phoenicians thought that a primaeval egg split open to form heaven and earth; Egyptians believed that their god Ptah created the egg from the sun and the moon; and American Indians thought that the Great Spirit burst forth from a giant golden egg to create the world. In all of the early legends the chicken is never mentioned, making the answer to the question of which came first — the chicken or the egg — seem obvious. The most common egg used for food today is the hen's egg, though those from other fowl — including duck, goose and quail — are sold in many areas. Hens' eggs have long been bedevilled by their high cholesterol content (about 213 milligrammes for a large egg), which is contained entirely in the yolk. Since the American Heart Association recommends that adults limit their cholesterol consumption to no more than 300 milligrammes of cholesterol a day, strict cholesterol watchers generally either drastically reduce their egg consumption or eat the whites only. Most hens' eggs on the market today have been classified according to quality and size under USDA standards. In descending order, egg grades are aa, A and B, the classification being determined by both exterior and interior quality. The factors determining exterior quality include the soundness, cleanliness, shape and texture of the shell. Interior quality is judged by "candling," so named because in days gone by an egg was held up in front of a candle to see inside. Today, candling is more likely to be accomplished electrically, with the eggs moving and rotating on rollers over high-intensity lights. The interior quality is determined by the size of the air cell (the empty space between the white and shell at the large end of the egg — smaller in high-quality eggs), the proportion and density of the white, and whether or not the yolk is firm and free of defects. In high-quality eggs, both the white and yolk stand higher, and the white spreads less than in lower-grade eggs. Eggs come in the following sizes based on their minimum weight per dozen: jumbo (30 oz. per dozen), extra large (27 oz. ), large (24 oz. ), medium (21 oz. ), small (18 oz. ) and peewee (15 oz. ). Large eggs are those on which most recipes are based. An eggshell's colour — white or brown — is determined by the breed of hen that laid it and has nothing to do with either taste or nutritive value. The egg white is an excellent source of protein and riboflavin. Egg yolks contain all of the fat in an egg and are a good source of protein, iron, vitamins A and D, choline and phosphorus. The colour of the yolk depends entirely on the hen's diet. Hens fed on alfalfa, grass and yellow corn lay eggs with lighter yolks than wheat-fed hens. chalazae are the thick, cordlike strands of egg white attached to 2 sides of the yolk that serve to anchor it in the centre of the egg. The more prominent the chalazae, the fresher the egg. Blood spots on egg yolks are the result of a natural occurrence, such as a blood vessel rupturing on the surface. They do not indicate that the egg is fertile, nor do they affect flavor. Contrary to popular belief, fertile eggs — expensive because of high production costs — are no more nutritious than nonfertile ones. They do contain a small amount of male hormone and do not keep as well as other eggs. Storing eggs: Eggs must always be refrigerated. When stored at room temperature, they lose more quality in 1 day than in a week in the refrigerator. Eggs should be stored in the carton in which they came; transferring them to the egg container in the refrigerator door exposes them to odours and damage. They should always be stored large-end-up and should never be placed near odoriferous foods (such as onions) because they easily absorb odors. The best flavour and cooking quality will be realised in eggs used within a week. They can, however, be refrigerated up to a month, providing the shells are intact. Leftover yolks can be covered with cold water and refrigerated, tightly covered, for up to 3 days. They can be frozen only with the addition of 1/8 teaspoon salt or 1 1/2 teaspoons sugar or corn syrup per 1/4 cup egg yolks. Tightly covered egg whites can be refrigerated up to 4 days. They can be frozen as is up to 6 months. An easy way to freeze whites is to place one in each section of an ice cube tray. Freeze, then pop the egg-white cubes out into a freezer-weight plastic bag. Both frozen egg yolks and whites should be thawed overnight in the refrigerator before being used. Hard-cooked eggs should be refrigerated no more than a week. Eggs are available in other forms including powdered and frozen (whole or separated). Commercially frozen egg products are generally pasteurised and some contain stabilising ingredients. Another egg product available to consumers is table-ready pasteurised liquid eggs, which can be found in a supermarket's refrigerated section. This product mixes the white and yolks, then pasteurises them at a heat level that kills any bacteria without cooking the eggs. pasteurised eggs are sold in 8- and 16-ounce cartons (4 1/2 and 9 whole eggs respectively). They can be refrigerated unopened for up to 12 weeks from the pack date (see open dating). The multitalented egg is delicious not only as a food in its own right but has numerous other uses as a leavener in cakes, breads and soufflés; a base for dressings such as mayonnaise; a thickener in sauces and custards; a clarifying agent for stocks; and a coating for breaded or battered foods. See also egg substitutes.