EGG WHITE FOAM SCIENCE
Egg whites have this special property of being amenable to making of a foam. Egg white foams can be used in several types of dishes, from savory souffles, to a delicious chocolate mousse, or a simple meringue. Usually, beating the egg white into a foam is not very difficult, but I've seen and heard people telling me that they have had troubles with just that. It is common that the egg foam, once beaten and formed, can start to disintegrate into two fractions - one runny watery fraction on the bottom of the bowl, and the dry, coarse foam on top. The glossiness and nice texture are gone. What can possibly cause this, and how can we become better at beating egg whites properly? To determine this, we need to discuss some things on egg white proteins and how they help to make a foam.
All proteins have a certain kind of three-dimensional structure, which is important for their proper function. In many proteins, this structure keeps the proteins water-soluble. What happens when we are beating the egg white that contains a lot of proteins? Some of them lose their three-dimensional structure and become unfolded; this is accomplished by the very mechanical force that we apply by whisking the egg white. Also the mixing in of air helps to unfold the proteins. The nice thing about this phenomenon is that the proteins, once water-soluuble and separated from each other, now become very much sticky to each other. They simply aggregate into a network of proteins. What more, they aggregate so that they can capture small air pockets within the network and eventually create the foam. The air pockets are thus covered by the thin film of water in which the proteins have formed a fairly stable "wall" that hinders the water from collapsing and break down the foam.
Of course, the foam will not live forever! If left to rest, it will become coarse and the water will start to slip out of it. That's why we usually use flour in egg foam dishes - it helps to stabilize the foam.
What many people don't consider however is that overbeating the egg foam can also make it coarse and separated. The reason is...the proteins! As we beat them more and more, more of them will become unfolded and more of them will start to aggregate with each other. This aggregation can be overdone though! If too many proteins bind to each other too tight, the air pockets will shrink as the air gets squeezed out. There are some tricks to prevent this from happening however - use something acidic or use a copper bowl. I guess the simplest thing is the first alternative: adding half a teaspoon of lemon juice to one egg white will prevent the proteins from disaster.
Some people also add salt to egg white and think it will aid in the process of foam making. Actually the opposite is true! Salt will effectively prevent the proteins from bonding to each other, and foam making will be much harder. The same thing goes for sugar if it's added before we start beating the egg white. Strangely enough (?) it can be added to the foam after it is formed and actually make it more stable.