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By Prof. Sally E. Solomon, senior research fellow, University of Glasgow, Scotland
As a source of essential nutrients, the egg is unrivalled. It is a complex of protein, carbohydrates, fat, vitamins (A, D, B1, B12 and riboflavin) and has a mineral profile which includes iron, phosphorus, calcium, copper, iodine, magnesium, manganese, potassium sodium, zinc, selenium and sulphur.As might be expected of a chamber designed primarily to nourish and protect the developing embryo, many of the proteins display antibacterial properties. The table egg is likewise invested with the ability to withstand bacterial ingress.The mineral component on the other hand affects its role in processes as diverse as the maintenance of albumen quality, the elasticity of the paired shell membranes and shell formation. Defects in the formation of any of these components will have a profound effect on the functional capabilities of the table egg.Liquid egg coagulates when heated (cakes, bread), when emulsified the phospholipids and lipoproteins in egg yolk will form the basis of dressings for salads etc., when beaten, uncontaminated egg white will foam (meringues, mousse) and when boiled the egg will give up its shell and membranes to reveal its intact, solid contents.
So much for theory – in practice from my own experience, sponge cakes rarely rise, yolk always contaminates the egg white during separation and the shell of the hard-boiled egg rarely separates from the egg white without removing with it a plug of the latter. These failures in function can be explained in part by considering the role played by various fractions of the egg and the effects of mineral deficiencies in their formation.
Surrounded by membrane
Much has been written about the foaming properties of egg white and the changes that occur in the molecular configuration of the proteins during the process. Essential to the latter is the assiduous separation of yolk from egg white since the presence of even a trace of the triglyceride fraction of the yolk has been reported to reduce the volume of the foam by up to two thirds. The yolk is surrounded by the vitelline membrane. This two-layered proteinaceous structure is easily ruptured with handling and with prolonged storage it stretches and disintegrates as the yolk absorbs water from the egg white. This disintegration then permits the otherwise physically separated fractions of the egg contents to mix.
Much has been written about the foaming properties of egg white and the changes that occur in the molecular configuration of the proteins during the process. Essential to the latter is the assiduous separation of yolk from egg white since the presence of even a trace of the triglyceride fraction of the yolk has been reported to reduce the volume of the foam by up to two thirds. The yolk is surrounded by the vitelline membrane. This two-layered proteinaceous structure is easily ruptured with handling and with prolonged storage it stretches and disintegrates as the yolk absorbs water from the egg white. This disintegration then permits the otherwise physically separated fractions of the egg contents to mix.
With the exception of the phosphorus rich yolk which develops in a hierarchical manner within the ovary, all other components of the egg are synthesised within the oviduct. Thus the multi-layered albumen secreted by the magnum is known to contain an array of protein types (>100) in addition to its high water content (90%). During the beating process, changes occur in the molecular configuration of many of the proteins with the precipitation of some of the latter at the liquid-air interface.
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Source: World Poultry, volume 27, no. 1, 2011
