The egg11 June 2011
A look at what makes up the egg, one of the ingredients most-used in pasta production.
by Cristina Alamprese
From a biological standpoint, the egg is the female reproductive cell (gamete) of birds that is fertilized by the male in order to create a new organism. Therefore, because it contains all the nutrients required for embryonic development, it offers a high-level nutritional composition that makes it a valuable food for man. By definition, the egg used for alimentary purposes is that of the hen (Gallus gallus). However, other eggs may also be sold for human consumption (for example, quail, duck or goose) as long as the species producing them is specified.
Today, egg production is a continuous and highly-specialized process. Selected stocks of laying hens are used that can lay over 300 eggs per year. The stocks utilized can produce eggs with either white or colored shells (ranging from pink to brown). The color of the shell depends exclusively on genetic factors and bears no relationship to the quality of the egg.
On average, an egg weighs between 50 and 70 g and its size depends on the genetic type, age and diet of the laying hen, as well as environmental factors (for example, the micro- and macro-climate of the farm). It is oval in shape, with one rounded end (obtuse pole) and one more pointed end (acute pole). Moving from the exterior towards its interior, the egg is comprised of a shell (approx. 9-11% in weight), albumen (approx. 60-65% in weight) and yolk (aprox. 25-30% in weight). The albumen is separated from the shell by two membranes which, near the obtuse pole, separate and form the air cell. The height of the air cell (and, as a result, its volume) increases as the egg ages and, therefore, for commercial purposes, is used as an index of egg freshness. The increase in the volume of the air cell is due to the loss of water which depends on storage duration, but also the temperature and humidity of the storage locations and the thickness and porosity of the shell.
The albumen, whose name derives from the Latin albus (white), is a watery, semi-transparent, colorless solution and is the largest component of the egg. It has a two-fold purpose: to protect the yolk from microbe attack and provide water, protein and other nutrients to the embryo. Its consistency provides a parameter for evaluating the freshness of the egg. The older the egg, the more watery the albumen becomes.
The name yolk comes from the Old English word for yellow. Its function is to provide fats and energy to the developing embryo and it is surrounded by the albumen from which it is separated by the vitelline membrane to which are attached the chalaza – protein strands whose function is to connect the yolk to the albumen. It is spherical in shape and yellow-orange in color, which varies on the basis of yellow carotenoid content (especially xantophylls such as zeaxanthin and lutein) and red pigments (in particular capsanthin) which derive, naturally, from some of the components of the hens’ diet or are specifically added to their feed. In fact, depending on consumer preference, eggs can be produced with a range of yolk colors by utilizing natural pigments extracted from vegetal or animal organisms, or synthetic pigments which provide more intense and stable coloring than natural ones. The color of the yolk can also be adjusted on the basis of what the egg is to be used for. For example, in pasta production, yellow pigments are preferred, whereas for baked goods and the foodservice industry, red pigments are preferred. In Italy, on the basis of current legislation, the maximum amount of total carotenoids permitted in the feed for laying hens must not exceed 80 ppm, and synthetic red pigments (cantaxanthin) must not exceed 8 ppm (directive 2003/7/EC).
Within the yolk, the fats are comprised of 65% of triglycerides, 28.3% of phospholipids (above all lecithin) and 5.2% of sterols (primarily cholesterol). They are easy to digest and, although of animal origin, they have a relatively low saturated fatty acid content and represent a good source of essential fatty acids. Fatty acid composition is influenced by the diet of the laying hen, while the overall fat content is quite stable and only marginally influenced by the fat content in chicken feed. Cholesterol content has been reduced significantly in recent years and is now around 200-220 mg/egg.
The proteins in eggs have a high nutritional value thanks to the balanced presence of essential amino acids and the quantity and composition of amino acids in them would seem to depend on genetic factors and the proteins found in hen diet. The low digestibility of proteins in raw albumen is significantly improved through cooking, including at low temperatures. On the contrary, proteins in the yolk are easily digested when raw and over-cooking worsens their digestibility.
In terms of their lesser components, eggs are particularly rich in assimilable phosphorous and iron and are a good source of vitamins, with the exception of vitamin C. By adjusting the diet of hens, the level of a number of minerals can be increased (e.g., iodine, manganese, selenium and fluorine) and some vitamins (e.g., A, E, thiamin, riboflavin and pantothenic acid).