The article emphasizes that in order to create a stable foam from egg whites there must be a balance between protein, air and water. This stability is the result of interactions on the partof the side chainsof the amino acids that make up the egg white protein. Based on the nature ofthese side chains, some amino acids are attracted to water and some are not. That is, some arehydrophilic (attracted to water) and some are hydrophobic (not attracted to water). The proteins act like a surfactant, creating an interface between the water in the protein and the air bubbles that have been incorporated into the egg white. Hydrophilic amino acids have side chains that are either charged or polar, which means that they easily interact with the polar water molecule. The side chains in hydrophobic amino acid side chains are non-polar and neutral and do not interact with water, but do interact with air. The protein also serves as a framework that encapsulates the air bubbles.Now, most protein molecules in their normal configuration have a hydrophobic core and a polar hydrophilic surface in contact with the environment. While hydrophobic amino acid residues make up the core, polar and charged amino acids cover the surface of the folded molecule. When the egg white is whipped to create foam, the whipping process causes the protein molecules to unfold (the process is called denaturing) exposing the hydrophobic parts of the molecule as well as the hydrophilic parts. The whipping process also causes the molecules 64()Tryptophan()Tyrosine
to collide at a faster rate and the result is that intermolecular bonds that give the molecule its folded shape are broken. The hydrophilic sections of the protein are attracted to water and the hydrophobic sections, especially in the protein ovomucin, are attracted to the air bubbles created by the whipping process. New bonds are formed between protein molecules,especially the protein ovalbumin which coagulates during the baking process to stabilize the foam to create the light and airy angel food which may have a volume six to eight times that of the original liquid albumin.However, as the article notes, these side chain bonds are relatively weak and any disruptions may cause the protein framework to collapse. What are the bonds that hold the foamin place? Some bonds are, in fact, ionic in nature. The charged amino acid side chains do form ionic bonds, which in this context are called salt bridges. All of the polar side chains form hydrogen bonds with water. The non-polar side chains interact with each other and the air bubbles via van der Waals forces. And in the side chains that contain sulfur the sulfur-sulfur bonding creates covalent sulfur bridges, mentioned earlier. The polar side chains form hydrogenbonds with the water present in the foam as well. See below for more on these bonds.