Stability is measured by finding how much liquid drains from it on standing. This is usually done by transferring the foam to a funnel and measuring or weighing the liquid that drains from it. More the water drains means the foam is less stable.
Effect of beating time: As the time of beating egg white is increased their volume and stability increase at first and then decrease. The actual time required depends on the type and speed of the beater. Higher the speed, shorter the period of beating required. Maximum stability is reached before maximum volume.
Type of beater used: If the beater has thick blades or wires, they do not divide egg whites as easily as fine wires and resulting air cells are therefore larger. Egg whisks sometimes give a larger volume of beaten egg mass than do rotatory types of beaters but the cells are larger.
Type of container in which eggs are beaten: Bowls with small rounded bottoms and sloping are preferable to bowls with large flat bottoms because, in the former, the beater can pick up the egg mass more easily. The size of the bowl must obviously be adapted to the amount of egg to be beaten. If whisks are used for beating egg whites, a large plate is preferable to a bowl for holding the whites because of the over-and-over strokes that are used.
Temperature:egg white can be beaten more readily at room temperature than at refrigerated temperature possibly because of lowered surface tension at higher temperature.
Thick and thin whites : Thin whites can be beaten more readily and produce greater volume than thick white. Stored eggs may beat more quickly than those fresh eggs as in stored eggs the white is thin. Thick whites seem to produce more stable foam even though thin whites may initially beat to a larger volume. The volume of cooked products such as cakes and meringues is greater when thick whites are used rather than thin whites.
Addition of acid: Addition of acid to egg white makes the foam more stable but increases the time necessary for beating. Cream of tartar is more effective in increasing the stability than acetic or citric acid. Acid is best added during the first portion of beating period. The addition of acid to egg white also makes the foam more stable to heat. e.g., meringue.
Addition of fat: The presence of fat interferes with foam formation and decreases the foam. Similar effect is observed when yolk is added to white. This effect is the result of the fat probably the lipoproteins, in the egg yolk which may form a complex with proteins in the white.
Addition of salt: Addition of salt to egg white (1g to 40g egg white) or whole egg lowers the quality (volume and stability) of the foam as it becomes less elastic. The addition of salt to fresh egg white decreases the stability of foam unless the beating time is increased from 6-9 minutes. The addition of salt to whole eggs before beating results in a foam of small volume that would not form peaks. Sponge cakes made from such a foam has smaller volume and less tender when salt is added to the egg rather than adding salt to the flour.
Addition of water: Dilution of egg white with water upto 40 percent of the volume of egg increases the volume of foam but decreases the stability.
Addition of sugar: If sugar is added before beating is started, extensive beating is needed to produce a foam. Once formed the foam is stable and very fine although the volume may be less. The shine of egg white foam with added sugar is in part to the prevention of coagulation of proteins with accompanying opaqueness. Once sugar is added to egg white beaten to a foamy stage to the soft peak or to the stiff peak stage beating can continue longer without the foam being overbeaten. After sugar has been beaten into a foam it can stand for sometime without becoming coagulated and losing its elasticity. The foam can be manipulated and spread without rupturing the cells perhaps because sugar retards the denaturation of egg white.
Table: summaries the factors affecting volume, stability and time of beating of foams.
Participants