Egg shells are covered with a natural coating that seals its pores. This helps to prevent bacteria from getting inside the shell and reduces moisture loss. Wiping or washing eggs removes the protective coating and reduces the lifetime of the egg.
The yolk is anchored to the white by tiny strands of egg white called chalazae. The more prominent the chalazae, the fresher the egg.
Egg shell and yolk colour will vary depending on the diet of the hen. Colour variations have no effect on the quality, flavour, or nutritional value of the egg.
The breed of hen determines the colour of the shell. Hens with white feathers and ear lobes lay white eggs; hens with red feathers and ear lobes lay brown eggs. Egg white (albumen) is actually clear when raw and only becomes white in colour when beaten or cooked. Using fresh eggs and cooling them quickly after cooking helps prevent darkening of the egg white.
The egg yolk
The yolk or yellow portion makes up about 33% of the liquid weight of the egg. It contains all of the fat in the egg and a little less than half of the protein. The yolk contains all this goodness because in fertilised eggs the yolk is the site of embryo formation.
With the exception of riboflavin and niacin, the yolk contains a higher proportion of the egg’s vitamins than the white. All of the egg’s vitamins A, D and E are contained in the yolk. Egg yolks are one of the few foods naturally containing vitamin D (the sunshine vitamin). The yolk also contains more phosphorus, manganese, iron, iodine, copper, and calcium than the white, and it contains all of the zinc. The yolk of a large egg contains about 59 calories. It is the yolk which is responsible for the egg’s emulsifying properties in cooking.
*For more detailed nutritional information click this link
Yolk colour – raw
Yolk colour depends solely on the diet of the hen. Artificial additives to enhance yolk colour are not permitted. If a hen gets plenty of yellow-orange plant pigments (xanthophylls), these will be deposited into the yolk. Hens fed mashes containing yellow corn lay eggs with medium yellow yolks. Those eating wheat or barley lay lighter-colour yolks. Golden yolks are preferred by most New Zealanders.
Yolk colour – cooked
A greenish ring around a hard-cooked egg yolk may be the result of sulphur and iron compounds in the egg reacting at the surface of the yolk, overcooking or a high proportion of iron in the cooking water. The eggs are still wholesome and nutritious, and their flavour is unaffected.
Greenish yolks can best be avoided by using the proper cooking time and temperature, and by rapidly cooling the cooked eggs in cold water.
Double-yolkers are created when a hen releases two yolks at the same time. This is quite common in young hens whose cycles may not be perfectly synchronised. Older hens will sometimes produce a double-yolker in an extra-large egg. Occasionally a hen will produce double-yolker eggs throughout her egg-laying career. It is rare, but not unusual, for a young hen to produce an egg with no yolk at all.
Occasionally small blood spots may be found on an egg yolk. These are caused by the rupture of a blood vessel on the yolk surface during egg formation. Less than 1% of all eggs produced have blood spots as they are normally removed during the candling, grading and selection process. The egg can be eaten as normal or the spot can be removed with the tip of a knife. The tiny red spots do not indicate a fertilised egg.