When air is incorporated into a liquid or viscous solution, the solution entraps the air bubbles, forming a foam. If the foam is stabilized by proteins, it leavens a food, increasing its height and reducing its density. The viscosity of all egg products is ideal for incorporating air cells during the whipping or beating process. As whipping or beating progresses, air bubbles decrease in size and increase in number, all the time surrounded by egg proteins. Liquid egg products have low air-liquid interfacial tension; thus, when eggs are beaten or whipped, the proteins denature, or simply, they unfold. This exposes two oppositely charged ends of the protein molecule: the hydrophobic, or water-hating end, and the hydrophilic, or water-loving ends. The proteins align themselves between the air and water, securing the air bubbles with their hydrophilic chains pointing into the water and dangling their hydrophobic chains in the air. During baking, these proteins bond with each other, forming a delicate, yet reinforced network.
Egg whites do this much better than yolks because of the unique proteins found in whites. In fact, even though the term foam technically refers to any system where there are entrapped air bubbles, in the food industry, when discussing egg white products, the term tends to be exclusive to egg whites foams. This is because egg whites, unlike any other natural food ingredient, are able to create the largest possible food foam, a foam six to eight times greater in volume than unwhipped, non-aerated liquid egg white.
Whole eggs and egg yolks can also increase the volume of foods, including certain baked goods and dairy desserts such as ice cream and custard, but just not as much as egg whites alone. Visually, whipped yolks may double or triple in volume, while whipped whole eggs produce less volume than either yolks or whites whipped separately. They are also less thick than yolks alone.
There are four powerhouse proteins in egg whites that are barely present in yolks. Ovalbumin, which is about 54% of the white’s protein content, coagulates when heated, forming a solid framework around entrapped air that enables the wall structure to resist collapse. Ovalbumin does not unfold much when the egg is beaten, but without it, the foam would collapse when baked. Simply, ovalbumin allows a liquid foam to solidify. The highly elastic conalbumin, which is about 12% of the white’s protein content, together with two other proteins — ovomucin and globulins — both present in quite small amounts, stabilizes the foam at room temperature.
A number of variables can impact the stability of egg white foams. For starters, it is possible to overbeat or over whip egg white foam. When this happens, the foam dries out and sets, which prevents the air bubbles from further expanding during the baking process. If egg white foam stands for more than five minutes, air starts to escape and it quickly returns to its liquid state. A little bit of a recipe’s sugar content can be added during foam preparation to prevent air from escaping. If added properly, the result is a smooth, stable foam that does not collapse or drain as quickly as a foam without added sugar. However, sugar can also retard foaming if too much is added, or added too fast.
Historically bakers and chefs used copper bowls to stabilize egg white foams. The copper in the bowl combines with the sulfur component of conalbumin. The bond created is extremely tight, as the sulfur molecules are prevented from reacting with any other material. Today, a more common approach to stabilizing egg white foam is to add cream of tartar, known chemically as potassium acid tartrate. This acidic salt lowers the pH of egg white, which in turns increases the number of free-floating hydrogen ions and helps stabilize the foam, much like copper.
There are a number of other variables that impact egg white foam stability. For example, salt can decrease the foam’s stability by weakening the matrix of the protein bonds. Water can increase the volume and lightness of a foam, though there is a greater likelihood that some liquid will drain off due to the reduction by dilution of proteins. In fact, egg white diluted by 40 percent or more of its volume in water cannot produce a stable foam.
Temperature also impacts the development of egg white foam, which reaches its greatest volume if beaten closer to room temperature than refrigerated temperature.
There is no ingredient as detrimental to egg white foam as fat…even a trace amount. Because fat molecules have hydrophobic and hydrophilic ends similar to protein, fat competes with protein for special alignment with gas bubbles. However, unlike proteins, fats don’t bond with each other. They won’t create any cross-linked bonds to reinforce the network of gas bubbles.
Functional Properties & Uses