Egg formation and quality

Unlike any other bird, chickens lay eggs even if they are not pregnant. Chickens will lay approximately one 50-60 g egg each day. Layers are never inseminated, not artificially or naturally. Their eggs will thus not develop into chicks even if they were incubated. Hens lay eggs for 12-15 months, after which their production decreases and they are often culled.

Eggs are developed in the oviduct and uterus. 2-3 hours prior to ovulation, the hormone levels of progesterone and luteinizing hormone increase for a short time. The first ovulation of a chicken happens in the morning right after sunrise. Each subsequent ovulation occurs ~15 minutes later. After the delay has increased to several hours, the hen will not lay an egg for a day or two. Then she ovulates at sunrise again, and a new egg-laying period begins.

The formation of an egg. (c) Unknown

Ovulation is the release of an ovum. Nearly 500 000 immature ova exist in the ovaries at the time of hatching. 300-500 of them ever mature. Maturation of each ovum takes months, during which the ovum grows from 8 to 37 mm and 0,08 g to 15-18 g. Each day one ova fully matures into a follicle. The follicle develops into a yolk, which consists of darker and lighter layers, a latebra and a germinal disc. Yolk is mostly water, protein (16 %) and fats (33 %), and it is surrounded by a yolk sac. Originally the contents of the yolk were produced in the liver of the hen, and transported into the ovaries by blood. The formation of egg yolk is called vitellogenesis.

During ovulation the yolk is released and dropped into the funnel-shaped beginning of the oviduct. The funnel is called infundibulum. Infundibulum is the site of fertilization, if the chicken has been artificially or naturally inseminated. The yolk remains approximately 18 minutes in the infundibulum, before it moves forward in the oviduct.

The next phase of the oviduct is the magnum, a site where the egg-white is developed around the yolk. Egg-white consists of two thick and two thin layers. Unlike yolk, egg-white is mostly water and proteins. The thick layers form two chalazae, a pair of twisted "strings" which keep the yolk in the center of the egg. This protects the yolk from touching the egg shells and thus from being contaminated by bacteria and microbes. Both thick and thin layers of egg-white protect the yolk, which, if fertilized, would develop into a chick.

After magnum the yolk and egg-white move on to isthmus. In the isthmus two protective, thin layers are created on top of the egg-white. The inner shell membrane is attached to the egg-white, and the outer will be attached to the egg shell. An air bubble forms between the two membranes. As the egg is laid and stored for consumption, it will dry, and the contents shrink. The air bubble can expand, so the shrinking membranes do not break the egg shell.

So far the egg formation has lasted approximately 5 hours. The last phase is forming the shell. The shell is formed in the uterus. It is uncertain whether the egg shell is pigmented in the uterus or already in the isthmus, but generally brown chicken breeds lay brown eggs. Formation of the hard shell takes nearly 20 hours. When the egg is finished, it is moved from the uterus to the vagina. Oviposition, or laying the egg, occurs in 10 minutes. Only an hour after the oviposition a new ovum matures and the next yolk is released into the infundibulum.

Structure of the egg

(c) Encyclopaedia Britannica

The yolk consists of dark and light layers. 50 % of yolk is water, and one third of the dry matter is fat. The latebra is the "primal yolk", the part around which the rest of the layers are formed in the oviduct. Should the egg have been fertilized, the fetus would start developing from the germinal disc. Rest of the yolk would provide all the needed nutrients for the developing chick. Hence it is rich in vitamins (A, D, E and K),  cholesterol, B-vitamins, iodine, selenium, zinc, proteins and fats. One egg contains 20 % of the recommended daily amount of phosphorus for humans and 31 % of selenium. The yolk is surrounded by yolk membrane. As the egg is stored (in the shop or fridge), the yolk membrane gradually breaks. When the egg is broken, the yolk in old eggs spreads on the egg-white, while fresh yolks retain their shape.

The yellow color is caused by carotenoids received solely from chicken feed. The pigments in the feed can be either natural (paprika, pumpkin, shellfishes, etc) or synthetic. The color of the yolk is simply an esthetic quality factor, and has no impact on the nutritional value of the egg.

Contents of yolk and egg-white.

The egg-white or albumen consists 66 % of the mass of the entire egg. It is mostly water, but has some proteins, minerals and B-vitamins as well. One albumen has 41-51 kcal / 100 g, while a yolk has 348-378 kcal / 100 g. Considering a developing fetus, the albumen is only meant to serve as a protection. It has some antimicrobial features, and it protects the yolk from shaking and bouncing. The membranes around the albumen and inside the shell are 70 µm thick in total. Like yolk, egg-white dries as the egg is stored. In freshly laid eggs the albumen stays as a thick, round layer when the egg is broken. In older eggs it spreads into a thin layer.

The outermost layer of an egg is the hard shell, which is covered by a waxy layer called a cuticle. Cuticle protects the air ducts, which are formed between hard calcite shards. The cuticle is only 10 µm thick and very sensitive. If an egg is washed, polished or otherwise handled, the cuticle will rub off and the egg will spoil. All eggs are collected and sold to consumers as they are, without any washing. Otherwise the shelf-life of eggs could be reduced to less than a day. If the cuticle is intact, eggs can be stored in cool temperatures for months without any spoilage. The egg shell allows oxygen inside the egg, and carbon dioxide and water out from the egg.

Quality of eggs

The quality of eggs is controlled in many ways. The components of quality can be divided into three sections:

1. Inner quality: blood or meat spots, size of the air bubble, quality of the yolk, firmness of the albumen (Haugh-value)
2. Outer quality: egg size, cleanliness, qualities of the shell, form of the egg
3. Other quality factors: microbiological quality, taste, odor, nutritional content

Inner quality factors can be measured by candling, checking the egg against a bright light. This is done in an egg packing facility. Spots of blood, any develoment of a fetus and the position of the yolk can be observed in candling. The size of the air bubble is also examined. If the egg as any faults, it will be rejected. Good quality eggs are stamped, classified by size and packed. Grade A eggs are then sold to stores and to consumers. Grade B eggs, which are the wrong size or have some minor cosmetic faults, are sold to the food industry as raw material (as intact eggs or egg mass). Any egg which is unedible and/or dirty is entirely rejected by the packer and not used in any kind of processing.

Blood spot. (c) The poultry site

All eggs may have blood spots of tiny brown "meat" spots. Neither are dangerous or signs of a fertilized egg. Blood spots may occur if there has been minor haemorrhage during the ovulation. Haemorrhage can be caused by an unsuitable light program, lack of vitamins, unsuitable feed, fright during ovulation or a disease. Brown spots are tiny pieces of the hen's tissue, or most commonly a broken down blood spot. Older hens and some breeds lay more eggs with brown spots.

Physical methods for determining the quality of an egg include measuring the thickness of the albumen in an broken egg and measuring the egg's corrected weight by sinking the egg into salt solutions of different concentrations. The shape of the egg is described as the ratio of its length (from the blunt end to the sharp end) and height.

Sandpaper egg (c) The poultry site

The quality of the egg shell is measured as the weight and thickness of the shell, and the deformation of the shell. "Sandpaper eggs" have a very hard and uneven shell, because the egg has been too long in the uterus. It can also be caused by lack of water, disease or an unsuitable light program. Mottled or transparent eggs are caused by too high a humidity in the poultry house. The egg shell dries very slowly, causing the color errors. IB disease can also cause both sandpaper and mottled eggs. "Leather eggs" are eggs without the hard shell. They are caused by lack of calcium and some diseases. Leather eggs are more common for young layers than adults.


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