The stability of foams depends on the arrangement of the proteins at the interface between the water and air.
FOAMS—LOW IN FAT BECAUSE they are essentially composed of air—first came to prominence with the rise of Nouvelle Cuisine in France in the 1960s and then gained broader popularity as a consequence of the growing interest in lighter foods on both sides of the Atlantic. Today, with the advent of molecular gastronomy and, in particular, the fame of the Spanish chef Ferran Adrià, they are very fashionable among gourmets. In the early days foams were obtained by vigorously beating egg whites, but the variety of eggs combined with ignorance of the optimal conditions for making foams led to irregular results, a fatal handicap from the point of view of the food processing industry. Physico-chemical analysis of protein foams has yielded a better understanding of the relationship between the composition of proteins and their foaming properties.
Composed of air bubbles separated by liquid films, foams retain their form only if the liquid forming the walls of the bubbles does not subside or if these walls are able to support themselves despite the draining away of the liquid. Beating an egg white reveals one of the conditions of stability, namely, that the air bubbles must be sufficiently small that the surface forces are stronger than the forces of gravity, which cause the water to fall and the air to rise.
By comparing the layers formed by the various proteins along the boundary where water and air meet, Roger Douillard and Jacques Lefebvre at the Institut National de la Recherche Agronomique station in Nantes and Justin Tessié in Toulouse showed that the stability results both from the interactions of the proteins present in the walls of the liquid films that separate the bubbles and from the viscosity of these films.
A key parameter in the study of foams is the interfacial tension of the protein films at the air–water boundary. The physical chemists measured this tension in terms of the force needed to extract a very clean platinum blade immersed in a solution covered with a protein film: The greater the amount of liquid that coats the blade, the greater the force needed to extract it. The proteins modify interfacial tension because they consist of chains of amino acids, with hydrophilic (water-soluble) parts and hydrophobic (insoluble) parts. Arranged at the water–air interface in such a way that their hydrophilic parts are in contact with the water and their hydrophobic parts with the air, they favor an increase in the surface area common to both air and water and facilitate the formation of foams.
A foam is stabilized by increasing the viscosity of its liquid phase (for example, by adding sugar and glycerol) and, above all, by modifying the drainage properties of the absorbent films. In protein foams these films are rigidified by intramolecular and intermolecular bonds, such as disulfide bridges between the cysteine groups of proteins, and weak bonds (van der Waals forces and hydrogen bonds).
The physical chemists from Nantes and Toulouse were particularly interested in the role of proteins in the formation of foams and sought to analyze the scale of interfacial tension as a function of protein concentration. They knew that very soluble proteins, which are adsorbed to only a small degree on the air–water interface, do little to reduce interfacial tension when their concentration rises. But the behavior of almost all other proteins was difficult to analyze because of their molecular complexity: Not only are proteins polymers, which is to say long chains of amino acids capable of being folded into a ball, but they are also molecules carrying electrically charged groups that interact within proteins and with charged groups of other molecules.
The current theoretical description of polymers, developed by Pierre-Gilles de Gennes and Mohamed Daoud at the Commissariat de l’Énergie Atomique in Saclay, predicts that the maximum lowering of interfacial tension ought to be obtained in the case of the molecules that make up the densest adsorbed layers. The presence of electrical charges complicates this prediction, for the greater the charge of a protein the greater its solubility and the less dense the adsorbed layer. By contrast, counterions, which surround the charged protein groups, are supposed to weaken intramolecular and intermolecular attractions and repulsions.
To test these predictions, the physical chemists from Nantes and Toulouse limited their attention to caseins, which, because they do not form a ball, cannot be unfolded (or denatured) during the formation of foams—one less mechanism to take into account. Comparison of various proteins revealed that for globular and nonglobular proteins alike, interfacial tension increases with the concentration of foaming proteins. Two populations of proteins were found to successively appear at the water–air interface, where they constitute distinct molecular layers. The first layer, a weak protein concentration, establishes itself on the surface, with the addition of supplementary proteins triggering the formation of a second layer.
The principal difference between globular and nonglobular proteins involves the structure of the molecules in the layer that is in contact with the air: Nonglobular proteins exhibit a single conformation, whereas globular proteins seem to divide into two subpopulations that differ in respect of the number of amino acids adsorbed at the interface.
What lesson can chefs draw from all of this? Perhaps that they should change their ways. There is no reason, apart from tradition, to make foams from the full protein content of the egg. Why not use specific proteins with superior properties? If they were to combine gelatin and water and beat the mixture for a very long time, for example, they would find that they can obtain quarts of a specific kind of foam from only a few sheets of gelatin. Of course, they would have to make sure that the water they use is flavorful.