Introduction

Eggs are products rich in nutrients and therefore essential in a balanced diet.

When we speak of eggs, we generally refer to those from hens (ovum), although eggs from other domesticated animals (ducks, geese, turkeys, quails…) are also consumed in much smaller quantities.

There is EU legislation that governs the different methods of rearing: free-range, outdoor, barn, and caged systems.

In free-range farming, the minimum space allotted per hen is 10 m², whereas in caged systems, it is only 0.4 m².

Eggs intended for consumption are unfertilised, without an embryo.

Hens lay around 300 eggs per year without the presence of males.

Properties of the Egg

From a legal standpoint, only hen eggs can be called “eggs”, although other types are also sold. Duck eggs, slightly larger than hen eggs, have a smooth, white shell. Goose eggs are large, strongly flavoured, and contain more fat in the yolk.

Quail eggs are also available, small, speckled, and generally sold as a delicacy.

Parts of the Egg

An egg weighs around 55-60 g and consists primarily of shell (10%), yolk (30%), and white (60%).

The shell is the inedible part of the egg and is composed of calcium and magnesium carbonate, protein fibres, and phosphates. It includes the following layers from outside in:

• Cuticle: the outermost layer, which protects the egg from external contaminants.
• Shell: porous, with the pores covered by fibrous proteins that hinder the entry of microorganisms.
• Membrane: a skin-like layer beneath the shell.
• Air cell: enlarges at the wider end of the egg.
• Inner membrane: separates the air cell from the egg white.

The white is the semi-solid, translucent part surrounding the yolk and is made up of albumins, water, sodium, small amounts of vitamin B₂, and antinutrients such as avidin and ovomucoid, which are inactivated by cooking.

The white includes the following parts:

• Structures that maintain the outer shape of the white.
• Two twisted spiral cords that hold the yolk in the centre. When the egg is cracked, these surround the yolk.
• Chalaziferous layer, which covers the chalazae.
• The white is divided into three layers: outer thin layer, thick layer, inner thin layer (the fluidity depends on protein content).

The yolk is located in the centre of the white and is made up of lipids (phospholipids and cholesterol), proteins (albumins, vitellin, lutein, phosphoproteins…), minerals (S, Fe, P), vitamins, and water (less than in the white). It includes the following parts:

• Vitelline membrane: the outer layer or film.
• Blastodisc: found at one end of the yolk. Visible as a white spot. If fertilised, this is where the embryo begins to form.
• Yellow yolk: arranged in concentric layers.
• White yolk: arranged in concentric layers.
• Latebra: the central part of the yolk.

Nutritional Value of the Egg

Eggs are highly nutritious due to their ideal combination of protein, fat, minerals, and vitamins. The proteins and lipids in eggs also have very useful properties: emulsifying, foaming, texturising…

Each part has a different chemical composition and is therefore studied separately.

Chemical Composition and Nutritional Value of Eggs

Chemical Composition of the Egg White

The egg white is mainly composed of water (90%), proteins (10%), and small amounts of sugars and minerals.

Fresh eggs have a pH of 7.7, but this increases over time to 9.7 due to the release of CO₂.

The proteins in the white are glycoproteins with varying sugar content:

• Ovalbumin: a phosphoglycoprotein that makes up approximately 60% of the total protein in the white and contains phosphorus and 3.2% carbohydrates.
• Ovotransferrin: accounts for 12% of the egg white protein. It inhibits microbial growth and binds ions such as Fe, Cu, Zn, and Mn.
• Ovomucoid: comprises 10% of the egg white protein and contains 23% carbohydrates.
• Globulins G1, G2, G3: make up 12% of the egg white.
• Ovomucin: 1.5% of albumin protein; in raw eggs, it acts as an antinutrient by inhibiting protease activity. It gives thickness to parts of the white.
• Flavoproteins: 0.8%, strongly bind riboflavin.
• Ovoglycoprotein: 0.5%, contains 30% carbohydrates.
• Ovoinhibitor: a protease inhibitor.
• Avidin: 0.05%, binds biotin.

The proteins in the white coagulate when heated as their folded structure is denatured and chains bond together.

Other components of the white:

• 0.3% lipids.
• 1% carbohydrates (proteins and monosaccharides).
• Vitamins: 0.3% riboflavin, 0.15% pantothenic acid, 0.1% niacin.
• Minerals: S (0.2%), Na (0.16%), K (0.15%), etc.

Chemical Composition of the Yolk

• 50% water
• 30% lipids
• 16% proteins
• 1% carbohydrates
• 2% minerals

The pH of fresh yolk is slightly acidic, approximately 6, and increases to around 7 over time. Nearly all of the egg’s fat is found in the yolk in the form of lipoprotein complexes.

Of the yolk’s lipids:

• 64% are triglycerides
• 30% phospholipids
• 6% cholesterol

It is the food with the highest content of phospholipids and cholesterol, with lecithin (phosphatidylcholine) being the most abundant phospholipid.

The densest proteins found in the yolk include:

• Lipovitellin: (35% of the yolk’s lipid content). These are high-density lipoproteins (HDL) composed of phospholipids, triglycerides, and cholesterol.
• Livetins: globular proteins that make up about a third of the yolk’s proteins.
• Phosvitin: a phosphoglycoprotein rich in serine units bonded to phosphate (10% phosphorus content), capable of binding magnesium, iron, and calcium ions.
• Lipovitellins: low-density lipoproteins (LDL) with high lipid content (85%). They form globules with a fatty centre and a surface composed of proteins and phosphatides. This lipoprotein structure resembles milk fat colloidal particles, stabilised by their protein coating.

The yellow colour of the yolk’s fat content is due to carotenoids such as zeaxanthin, lutein, and beta-carotene.

The carbohydrate content is low (1%), partly bound to proteins and partly present as monosaccharides.

Vitamins:

• 7% pantothenic acid
• 1% vitamin A
• 3% sterols
• Small amounts of other vitamins

Minerals: phosphorus (0.7%), calcium (0.2%), magnesium (0.1%), potassium (0.2%), sulphur (0.02%), etc.

Chemical Composition of the Shell

The shell of eggs, calcareous in nature, is composed of calcium carbonate crystals integrated into a fibrous network of polysaccharides and proteins.

Egg Nutrients

Due to their high protein and fat content, eggs have high biological value. Their proteins are very well balanced in essential amino acids and are used as a reference by the FAO.

They should not be eaten raw, as raw eggs are less digestible than cooked ones.

Raw egg contains avidin, an antivitamin that reduces biotin absorption, and proteolytic enzyme inhibitors that decrease protein utilisation.

When cooked, eggs lose vitamins like thiamine and riboflavin, but they become more digestible despite losing some nutrients.

The lipid fraction, located in the yolk, is very rich in phospholipids—around 30% of total lipids—and contains a high amount of cholesterol (1.2 g per 100 g of fresh yolk).

As for minerals, the yolk is particularly rich in phosphorus due to the abundance of phosphoproteins and phospholipids. It also has high levels of calcium, potassium, and iron.

Vitamin contributions are also significant, especially in tocopherol, riboflavin, pantothenic acid, folic acid, and vitamin B12.

Industrial Properties of the Egg

Coagulating and binding power of eggs: proteins in the white and yolk denature when heated, a property widely used in baking.

Foaming power of the white: lysozymes and globulins denature through mechanical action (whisking), forming foams by incorporating air (meringues, soufflés, etc.)

Anti-crystallising power of the white: very useful in confectionery.

Emulsifying power of the yolk: phospholipids allow the preparation of sauces…

Colouring, flavouring, and antioxidant power of the yolk: useful in pasta…

Myths Regarding the Nutritional Value of Eggs

It is popularly believed that brown eggs are more nutritious than white ones — this is false.

The colour of the yolk, the more yellow the better. This colour is influenced by the hens’ diet. Interestingly, organic eggs (code 0) tend to have lighter yellow yolks, while caged hens’ eggs (code 3) have a deeper orange yolk.

A double yolk being due to a special diet is also false. It results from hormonal imbalances in very young or old hens.