Experimental cookery

Isoelectric point of egg proteins. Loeb has reported the isoelectric point of egg albumin as ^H 4.8. Some investigators give />H 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 /»H 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 332 EGG COOKERY 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 pro- teins 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 tar- trate 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 tender- ness 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.