The unsuspected nature of an egg yolk.
BEHOLD THE EGG, WHOLE AND RAW, in its shell. Where is the yolk? No doubt, the physicists will say, on a vertical axis—for reasons of symmetry. Yet there are other possibilities. The yolk could be in the upper part, in the center, or in the lower part. How can we determine its location? Put a yolk in a tall, narrow glass and cover it with several whites: The yolk rises to the top, which suggests that the same thing occurs in a whole egg.
But could it be that the membranes that surround the yolk and bind it to the rest of the egg prevent the yolk from rising in its shell? There are several ways to show that this is not the case. If you boil an egg standing on its end and examine the yolk you will see that it is lodged in the upper part of the shell. Coagulation may have disturbed the internal arrangement of the egg, you say? Place a whole fresh egg in its shell in vinegar. After two days or so, the shell will be dissolved by the vinegar, and you will see the yolk floating on top of the white (the egg retains its shape because it is held in place by membranes and because the external layers have been coagulated by the action of the vinegar). Or try an even simpler experiment: Remove the top of the shell of a fresh egg and look to see where the yolk is.
More elaborate procedures can be followed as well. Although radiography gives poor results (because the shell is opaque to X-rays), ultrasound yields surprising results. In images obtained by immersing an ultrasound probe in an egg through a hole in the top of the shell, the yolk can be seen to be composed of concentric layers similar to those of a tree.
Why is it that no one who has eaten a soft-boiled egg has ever suspected the existence of this structure? The yolk is an alternation of layers called deep yellow, 2 millimeters thick, and clear yellow, 0.25?–0.4 millimeters thick. These layers are produced by the hen during the day and during the night, respectively. The difference between the two types results from the rhythm of feeding, which produces a weaker concentration of yellow pigments during the night than during the day. Of course, these layers become mixed together when you pierce a yolk, so you cannot see them.
Granules and Plasma
If we continue our investigation under the microscope, we see that the two layers, light and deep yellow, are not homogeneous. Instead they are composed of granules dispersed in a continuous phrase called plasma. Marc Anton and his colleagues at the Institut National de la Recherche Agronomique station in Nantes separated the granules from the plasma by centrifugation and observed that roughly half of the yolk is made up by water, a third by lipids, and about 15% by proteins. Proteins and lipids often are associated in particles that are distinguished according to their density: low-density lipoproteins (LDLS) in the plasma and high-density lipoproteins (HDLS) in the granules. Isolating them makes it possible to test their properties. For example, it can be shown that the LDLS combine to form a gel when they are heated to a temperature of about 70°C (158°F). It is these structures—composed of proteins and lipids (notably cholesterol)—that are responsible for the setting of the yolk during cooking.
It has long been claimed that mayonnaise, which consists of droplets of oil dispersed in water (from either egg yolks or vinegar), is stabilized by lecithins and other phospholipids in the yolk. Anton and his colleagues sought to answer this question by determining whether the emulsifying properties of the yolk come from the plasma or the granules. Because the solubility of proteins depends on acidity, the Nantes biochemists began by studying their solubility in terms of pH (a measure of acidity) and salt concentration. They found that plasma proteins are completely soluble at all levels of pH and all degrees of salt concentration, whereas the solubility of the granular proteins varies: They have low solubility at a pH of 3—that of mayonnaise—but become more soluble at neutral pH in a low-salt environment (sodium ions replace calcium ions, which establish bridges between the granular proteins inside the granules, with the result that these proteins are released).
Protein solubility is not the only thing that must be taken into account in order to make a successful emulsion, however. The less upward movement there is by the oil droplets in the water phase, the more stable the emulsion. In the plasma this movement is minimal for a pH of 3, and salt concentration has no effect in an acid environment; emulsions obtained from granules, on the other hand, are sensitive to both acidity and salt concentration. Emulsions made with whole egg yolks behave like those obtained with plasma.
In sum, the component elements of plasma are responsible for the egg yolk’s emulsifying effect, and proteins do a better job than phospholipids of preventing the oil droplets from moving upward, thereby stabilizing the emulsion. Is this because the proteins in the LDLS of plasma act by electrostatic repulsion at the surface of the oil droplets, causing the droplets to repel one another? Or because they protrude from the surface of the droplets and act instead by steric repulsion? At a pH of 3, proteins are electrically charged and repel one another; at a pH of 7, however, it is the proteins’ steric properties that stabilize the droplets by blocking their tendency to fuse with one another. The exact mechanisms of this behavior have yet to be understood.
All this for an ordinary egg yolk.