This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The extensive use of eggs in cookery is made possible by their protein content. The protein coagulates during heating, thus bringing about thickening as in custards or the binding of pieces of food together as in croquettes. The proteins of the egg are good emulsifying agents. The proteins form elastic films when beaten, thus incorporating air, which is used as leavening in such products as angel cakes and souffles. The elasticity of the egg protein is also important in products such as popovers where the egg stretches with expansion of steam, and later coagulates to aid in forming the framework of the popover.
The proteins of the egg. The proteins of the white are ovoalbumin, ovoglobulin, and ovomucin. There may be small amounts of other proteins and it is also possible that each protein is made up of component fractions. Hughes and Scott give the relative proportions of the proteins in the three portions of the white as shown in Table 41.
The principal protein of the yolk is ovovitellin. Sell, Olsen, and Kremers separated salted egg yolk into a soluble lipoid fraction and an insoluble residue. The latter consisted of sodium chloride and the protein-like material of the yolk. This residue they called lecitho-protein. It composed about 32.5 per cent of the yolk. This protein fraction contained nearly one-half the total lecithin of the yolk.
Outer thin layer of egg white
Thick layer of white
Inner layer of thin white
Solubility of the proteins. The albumin of egg forms a sol with water and dilute salt solutions. The globulin forms a sol in dilute salt solutions, but not in pure water. The globulin composes about 6.5 per cent of the total proteins of the egg.
Egg-white proteins belong to the group of hydrophilic colloids. Egg white and water are mutually soluble. Usually the addition of 1 tablespoon of water to an egg white, unless it is very watery, increases its extensibility, and when the egg white is whipped a larger volume is obtained. But with increasing quantities of water a stage is reached at which the egg white loses too much of its rigidity and will no longer retain air in small bubbles, the bubbles being large and floating on the more liquid part.
The ovovitellin of the egg yolk is combined with phosphorus and belongs to the phosphoprotein group. It is insoluble in water but is soluble in dilute salt solutions and in dilute alkalies.
Isoelectric point of egg proteins. Loeb has reported the isoelectric point of egg albumin as pH 4.8. Some investigators give pH 4.7 as the isoelectric point. Above the isoelectric point the albumin combines with bases to form salts like sodium albuminate; below the isoelectric point it combines with acids to form salts like albumin acetate, citrate, or tartrate. Above the isoelectric point the protein is negatively charged; below, it is positively charged. Since the reaction of the egg white is about pH 7.6 to 9, there will probably be few combinations of egg white with alkalies or alkaline salts in food preparation that will increase its alkalinity. Many combinations are made that increase its acidity. For example, the addition of a teaspoon of cream of tartar, a salt with an acid reaction, to a cup of egg whites, proportions often used in angel food cakes, increases the acidity and lowers the pH, often to about 7.5 or 7.0. As the proportion of cream of tartar is increased, the pH is lowered to a greater extent. The addition of fruit juices and fruit pulp to egg whites to make fruit whip, souffles, or similar desserts, increases the acidity. When 1 to 2 teaspoons of lemon juice are added to an egg white the pH is lower than 4.8.
No record could be found in the literature of the isoelectric point of ovovitellin. When lemon juice is added to egg yolk, the mixture is thickest at a pH between 4 and 5, as if the greatest tendency to curdle is at this point. This might indicate that the isoelectric point of the egg yolk proteins is between pH 4 and 5. This greatest thickening occurs with about 5 cc. of lemon juice to an egg yolk.
The addition of an acid like vinegar or fruit juice to the white and yolk beaten together tends to curdle the mixture. This occurs when the acidity is in the vicinity of the isoelectric point. When sufficient acid is added to lower the pH below the isolectric point of the egg proteins, and if the salt formed, such as protein citrate, is soluble, the coagulum dissolves and the mixture becomes smooth. With the exception of salad dressings and a few sauces, there are probably not many instances in which enough acid is added to lower the pH of the food mixture below the isoelectric point of the egg protein.
Peptization of egg proteins. Peptization of egg proteins increases the tenderness of some products. Freundlich states that peptization of proteins is frequently brought about by low concentrations of electrolytes, though to accomplish this the electrolyte must be intimately mixed with the substance to be peptized. The hydroxyl, citrate, acetate, and tartrate ions are effective for peptizing egg proteins. For example, when tomato or lemon juice is added to egg in amounts to bring the pH of the egg slightly above or about the isoelectric point of egg albumin, the tenderness of omelets is definitely increased. In some instances peptization of the egg proteins is detrimental. An example of this is the thinning of salad dressings, thickened with only egg yolk, when heated above the temperature at which optimum coagulation occurs.
Sugars (sucrose, dextrose, and levulose) through peptization tend to prevent coagulation of egg protein.
Denaturation. Denaturation, by which soluble proteins are rendered insoluble, of egg proteins is brought about in a variety of ways, including the action of acids, salts, heat, mechanical agitation, and radiation. Mechanical agitation or beating of egg white, as well as the tendency of proteins in surfaces to form films, causes partial denaturation of the egg proteins. Sugar tends to prevent this denaturation.