Gelatinizing emulsions produces a new kind of chocolate cake.
EMULSIONS ARE AN INEXHAUSTIBLE SOURCE of culinary discoveries. Here we will use them only as a point of departure for investigating more complex physicochemical systems that anyone can use in cooking.
In the preceding chapters we have considered several types of emulsion, foremost among them mayonnaise, the prototype described by all textbooks dealing with the physics of soft matter. Mayonnaise is a dispersion in water of oil droplets stabilized by the proteins of the egg yolk. A great many variations on this theme are possible. We have already looked at two of them: one made without a yolk, the other without any egg at all.
A yolkless mayonnaise is made in the same way as a classic mayonnaise, but in place of the yolk one uses the white of the egg. Albumen is made up of 90% water and 10% proteins, which have the same type of tensioactive properties as the proteins of the yolk. In a bowl one adds oil to an egg white, drop by drop, while whisking. At first the albumen foams, but gradually the air in the bubbles is replaced by the oil, which comes to be divided into smaller and smaller droplets by the shearing action of the whisk. The oil droplets, like the bubbles, are stabilized by the proteins of the egg white, for the proteins are unfolded in the course of whisking, so that their hydrophobic parts come into contact with the oil while the hydrophilic parts remain immersed in the water.
An eggless mayonnaise may be obtained by dissolving a half-sheet of gelatin in a small amount of heated water (or, for example, stock made from shellfish) and then whisking oil into the liquid just as one does in the case of a mayonnaise. At first a white emulsion appears as the proteins contributed by the gelatin attach themselves to the oil droplets at the interface of the water and oil. Once this emulsion has settled and cooled, it is transformed into a gel as the gelatin molecules become linked together at their extremities, forming a network within which the emulsified liquid is trapped.
Gelatinized Emulsion
The process of gelatinization can be viewed indirectly under the microscope. The oil droplets partially coalesce because the gelatin molecules coating them migrate away from the water–oil interface, forming a network (or gel) that traps the enlarged droplets. Once the gel has formed, by the way, whisking it again causes it to break down (in a classic mayonnaise, by contrast, the gel is stabilized further by whisking). Beating the gel dissociates the bonds of the network, with the result that the oil droplets are released.
The principle of a gelatin-based mayonnaise, then, is that an emulsion is created and then trapped in a physical gel, which is to say a gel that breaks up on being heated and reforms on being cooled. Can we devise additional variations on this theme by changing certain ingredients? Let’s go back to the mayonnaise made with egg whites and look for a way to chemically gelatinize it—in other words, to cook it. What we want to end up with is a chemical gel that, as in the case of gelatin itself, is permanent rather than physical.
Cooking Mayonnaise
Let’s begin by cooking the egg white mayonnaise in a microwave oven for about a minute so that the proteins covering the oil droplets are fused. What we get is a coagulated mass in which the oil is trapped, which is to say a physical system similar to a gelatinized mayonnaise. But is the oil securely retained by this body? If you squeeze the gel you will see that the oil comes out.
By manipulating the texture of the mayonnaise we seem only to have created a sponge for soaking up oil. Let’s try replacing the oil with chocolate (which is composed mostly of cocoa butter) and melting it in a pan with a bit of liquid that contains water (whether from rum, coffee, orange juice, or something else). Then, when the temperature of the resulting chocolate emulsion is still lower than the temperature at which albumen coagulates (62°c [144° F]), whisk the chocolate emulsion into an egg white. Finally, put this mixture in the microwave. What will happen? The proteins of the egg white will gelatinize and imprison the chocolate emulsion.
If you try this experiment yourself you will end up with a delicious chocolate cake whose flavor is much more powerful than that of an ordinary chocolate cake, probably because the chocolate is in a dispersed state, producing a novel texture (which you can vary as you like by modifying the proportions of water and chocolate). I suggest calling this dessert a chocolate dispersion. It remains to study how gelatinization disturbs the emulsion in the three systems we have considered.