Day old chicks
Humans and other animals have used eggs as a food source for many centuries. Today, we associate the hen’s egg so much with its role as food that it is easy to overlook its fundamental biological role.
Cross Section of an Egg as Incubator
Nature’s role for the egg of the laying hen is as an incubation chamber for the developing chicken. For an egg to develop into a chicken, it needs to have been fertilised by a rooster (also called a cockerel). Once the fertilised egg is laid, it must be kept warm. This may be done naturally by the hen sitting on the egg, or artificially by placing the egg into an incubator.
Ovary and Oviduct
The egg contains all the nutrients that the developing chick needs during the three-week incubation period and also for the first couple of days after hatch. Water vapour and the gases oxygen and carbon dioxide are able to move across the egg shell through small openings called pores. The developing chick starts off as a single fertilised cell on the surface of the yolk and progressively grows, using up the yolk, some of the albumen and some calcium from the inside of the egg shell. In the final stages of development, the chick takes up the last bits of yolk into its own digestive system.
Cross Section through the egg
Eggs that are produced commercially for the purposes of eating have not been fertilised by a rooster. Therefore, they cannot develop into chickens. The production of the bird egg consists of a series of steps that occur as the egg enters and passes along the hen’s reproductive tract (oviduct). The yolk of the egg enters the top of the oviduct and passes into the infundibulum where it spends about 15 minutes. A membrane is added around the yolk and, if the hen has been inseminated, fertilisation occurs in this section of the oviduct.
The yolk then spends about three hours in the magnum where the egg white is formed and then one hour in the isthmus where the shell membrane is laid down. The main part of the egg shell is formed in the tubular shell gland and the shell gland pouch and this takes about 20 hours. The egg shell is sometimes referred to as a bio-ceramic because it is made up of calcium carbonate with an organic matrix running through it.
Diagrammatic side view of the egg shell
In Australia, people like a yolk that is bright yellow-orange. Europeans like their egg yolks even darker whereas, in North America, paler yolks are the norm. Fifty years ago, most people had a few chooks in the backyard, even if they lived in a city. These chooks would often be fed only on table scraps and what they could scavenge in the back yard. They would eat many types of plants which are a source of a group of pigments known as carotenoids. These get laid down in the egg yolk giving it a bright yellow-orange colour.
As far as we know, these pigments do not contribute to the nutritional value of the egg. When egg production became more intensive and laying hens were fed a complete diet, egg producers added carotenoids to the poultry feed to produce the bright-coloured yolks that people were accustomed to from the back-yard hen days. In the commercial egg industry in Australia, most egg producers use a synthetic form of carotenoid. It is possible to obtain natural pigments, which are produced from plant products such as marigold flower petals or red capsicums, but these are more expensive that the synthetic pigments. The grains fed to chooks contain some carotenoids particularly corn (maize) which is commonly used in poultry diets in North America. In commercial free range production, it is usual to add carotenoids to the feed also.
It is possible to enrich table eggs with minerals (iron and iodine), antioxidants (selenium, vitamin E), omega-3 fatty acids and vitamins. There are some enriched eggs available in Australia at the moment but this represents a "niche" market for eggs as only a small percentage of consumers purchases such eggs.
