What does the egg consist of?
Typically, a hen’s egg weighs between 50 and 70 g. 10.5% is shell, 58.5% albumen and 31% yolk. Obviously, the proportions can vary depending on many factors: the age of the hen, the breed, the feed, the rearing method, and the environmental conditions.
The egg: what is it and how is it formed?
From a biological point of view, the egg is a female reproductive cell that, once fertilised, contains the complete genetic information for new life. In the case of birds, in addition to the genetic information, the egg contains all the necessary nutrients for the growth of the embryo and is also equipped with a hard protective shell that isolates and protects the embryo while still allowing gas exchanges with the outside world. By definition, the egg for food use is the unfertilised hen’s egg, i.e. of the species Gallus domesticus.
What does an egg consist of?
On average, an egg weighs 50-70 g and consists of 10.5% shell, 58.5% albumen and 31% yolk. The proportions of the different constituents may vary depending on many factors, e.g. age of the hen, breed, feed, rearing method, environmental conditions.
The albumen
It is also called egg white. The egg white consists of approximately 60% water, 10% protein and small amounts of sugars and minerals. It appears as a watery, semi-transparent substance surrounding the yolk. After being laid down, the egg white gradually ‘ages’ and turns from gelatinous to fluid. So if you are faced with an egg whose albumen is totally watery, this will be an indication of poor freshness. In the albumen, you will also notice two small white, gelatinous cords coiled in a spiral. They are called socks and serve to keep the yolk suspended in the centre of the egg!
The yolk
The yolk is the richest part in nutrients. 50% is water, 30% are lipids and the rest proteins. Together they look like a sphere whose yellow-orange colour is influenced in tone (as already explained) by the hen’s diet. We have already mentioned some membranes present in the egg and the yolk is also enveloped in one of these, called the vitelline membrane, which separates it from the albumen. This membrane also weakens as time passes after laying, until it spontaneously breaks, at which point you would be faced with an egg that is not at all fresh and in which the white and yolk have mixed.
The colour of the yolk in no way indicates the quality, but depends very much on the feed. Wheat-based feeds give light-coloured yolks. Maize-based feeds, on the other hand, give more yellow/orange yolks. When the colour is judged ‘insufficient’ because many people prefer eggs with very orange yolks (this is therefore only a commercial point of view) natural carotenoid dyes are added to feeds for laying hens.
The nutrients in eggs
The hen’s egg is known to be the food of greatest nutritional value due to its high protein content of essential amino acids (which cannot be synthesised by the body), phosphorated lipids (which form part of the structure of cells) and its energy intake (equivalent to that of 100 g of milk). An average egg contains 73% water, 13% protides, 12% lipids and 2% carbohydrates and minerals. The egg offers a top-notch nutritional intake due to the maximum biological value of its proteins. Its digestibility and chances of assimilation by our organism are superior to those of meat, although influenced by the way it is prepared.
Egg protein
Egg proteins are the most digestible and are used as a reference standard along with milk and meat proteins. In the albumen (the ‘white’ of the egg), ovalbumin predominates. The yolk (the ‘yolk’ of the egg) contains proteins such as ovolivetin (formed from complex proteins, the phosphoproteins) and is relatively rich in lipids, including cholesterol.
Egg cholesterol
An egg contains about 250 mg of cholesterol, towards which there is some aversion due to the widespread confusion between blood cholesterol and dietary cholesterol. Cholesterol is essential for the proper functioning of the human body for the synthesis of bile acids and other sterols, including sex hormones, and is indispensable for the proper functioning of cell membranes. When healthy, the body synthesises much of the necessary cholesterol and is able to regulate and limit the endogenous synthesis of that which is ingested from food.
Lipids in eggs
Hen’s egg lipids (fats) contain one third of the total saturated fatty acids and are rich in linoleic acid (an essential fatty acid for the human body). Compared to butter, egg lipids are lower in saturated fatty acids and contain significantly higher amounts of oleic acid and linoleic acid. They also provide lecithin, which protects against hypercholesterolaemia, which together with the presence of methionine (a sulphur amino acid) contributes to the control and prevention of cardiovascular diseases.
Vitamins from eggs
Eggs contain vitamins A, B1, B2, D, but not C. The yolk is rich in vitamin A (provitamin or carotenoid). The egg, with regard to vitamin C, has the same deficit as meat. The vitamins in the egg vary in quantity depending on the time from laying to consumption. An old laying hen produces eggs that are always rich in protein but poor in vitamins. Preservation methods (refrigeration, drying…) do not alter the caloric value of the egg, but deplete it of vitamins.
Mineral salts in the egg
The egg is relatively rich in minerals such as sulphur (of protein origin), phosphorus, iron, sodium, potassium, chlorine, calcium and magnesium.
The editorial staff of M.A.C.
