Eggs

Because it is so rich in nutrients that are indispensable for the body, the egg is a particularly valuable food in human nutrition. Besides its high biological value, the egg is also a very inexpensive food compared with meat and fish.

Hen's eggs are eaten most often, but the eggs of other bird species (duck, turkey, goose, quail, ostrich and so on) can also be used; these differ from the former in size and in the proportions of their various components.

A hen's egg generally weighs 35–70 g, a duck egg 67–70 g, a goose egg 150–200 g, a pigeon egg 15–20 g, a quail egg 10–11 g and an ostrich egg 1.2–1.8 kg.

The chemical composition of the egg depends on a range of factors, such as the species or breed of the bird, the composition of its food, the time at which the egg was laid, and the size and weight of the egg. The egg contains: proteins, lipids, carbohydrates, minerals and water.

Proteins play an important role and are represented, at the level of the egg white, by ovalbumin, ovomucoid (an inhibitor of the intestinal enzyme trypsin), lysozyme, ovoinhibitor (another trypsin inhibitor) and avidin (a biotin inactivator), while at the level of the yolk the more important one is lipovitellin, which is bound to lipids in what are known as lipoprotein complexes.

Egg proteins are notable for their high biological value and have a very stable and balanced composition of amino acid components, which is what led to their use as reference proteins for assessing the biological value of other kinds of protein.

Egg fats are concentrated in the yolk, where they are present in emulsified form. This makes them easy to digest. Among the lipids, we note the presence of cholesterol in large amounts, anyone watching their blood lipid levels is best advised to discuss a suitable amount with a health professional.

The egg is an excellent source of water-soluble (B2, B6) and fat-soluble vitamins (A, D), the latter being concentrated mainly in the yolk. Their proportion varies with the birds' feed and their living conditions.

Minerals are concentrated mainly in the yolk and are represented by sulphur, phosphorus, iron, potassium, calcium, iodine, manganese and copper.

Inside the yolk there is also a range of pigments that give it its colour and that are represented by beta-carotene and lutein.

Because it is so rich in high-quality nutrients, the egg is a complete food that allows the bird embryo to develop very quickly.

As a food it is particularly valuable for children, convalescents, anaemic people, undernourished people and so on.

Depending on how it is prepared, the egg takes a longer or shorter time to leave the stomach. While fresh yolk can be eaten raw, raw egg white can cause some digestive upsets (diarrhoea) and, on top of that, it contains a substance that inactivates vitamin H (biotin) and some digestive enzymes. This substance is destroyed by cooking the egg white, so it is recommended that it be eaten only after heat treatment.

Soft-boiled eggs or eggs prepared as a custard are the easiest to digest, while eggs prepared by frying or prolonged boiling (hard) are very hard to digest.