Experiments with acids and bases.
EGGS USED TO BE PUT IN SAND, SAWDUST, OR WAX in order to be preserved. Asian peoples devised recipes that took advantage of aging, instead of compensating for its effects, in order to create what were variously known as hundred-year-old eggs, centenary eggs, and even thousand-year-old eggs—names that symbolized links with the past as well as longevity. How credible are these recipes from the chemical point of view? A few experiments reveal the unexpected behavior of eggs in acid and basic environments.
The origins of the Chinese art of preserving eggs are lost in the mists of time. Initially, eggs seem to have been immersed in juices extracted from a local tree. Then it was discovered that by putting them in a mixture of ashes and earth and keeping them in a dark, cool place one could obtain the same culinary result but in only ten to twelve weeks. What inspired these practices? Can others be imagined?
Recipes for hundred-year-old eggs vary from region to region in China. Some call instead for placing duck eggs in a plaster that contains various ingredients: lime, saltpeter, bicarbonate of soda, mud, fragrant herbs, tea, rice straw, and so on. The eggs are left to rest for at least three months, and their flavor is said progressively to improve. It is surprising to note that some of these ingredients are also used in parts of France; even more recent civilizations such as our own make use of lime and ashes, which contain potash (potassium hydroxide). Thus recipes for preserving eggs fall into two classes: ones that contain only eggs and others that put them in contact with an alkaline compound. What is the effect of these bases? The effect of acids? After all, at least one modern French recipe also advises placing quail eggs in vinegar.
An Egg in Vinegar
Let’s experiment by placing a whole egg, in its shell, in a large transparent container. When it is covered with white vinegar, bubbles soon escape from the shell. Why? Because the acetic acid of the vinegar is attacking the calcium carbonate? A lighted candle, placed in the container, eventually goes out, a sign that the acid gives off carbon dioxide, which, being denser than air, accumulates in the container, driving out the air (the same thing can be demonstrated more technically by collecting the gas in limewater, which becomes cloudy). Then, after half a day or so, a thin, red surface layer detaches itself from the shell. This is why the eggs have a pink shell: the white of the carbonate and the red of this layer combine to produce the final color (at least in the case of French eggs; I have heard that eggs in England remain white).
Let’s continue observing. After one or two days of slow gaseous emission, the egg seems to have gotten bigger. Is this merely an illusion? The sequence of events shows that the enlargement is real: The final volume can be more than twice the initial volume. The shell has been completely dissolved, but the contents of the egg have not spread into the vinegar, for the acidity causes the white to coagulate. Experiments with several eggs allowed to sit for different periods of time show that this coagulation, limited at first to a thin outer layer, extends to an increasing proportion of the albumen, reaching even as far as the yolk. On reflection, this effect is not entirely surprising, for one finds the same thing when one pours vinegar on an egg white in a bowl: The superficial layer of the white coagulates, among other reasons because the H+ ions contributed by the acid prevent the acid groups of the proteins from being ionized while triggering the ionization of the base groups, which thus become positively charged (bases have the opposite effect). Electrical repulsions between the charged groups of proteins thus unfold the proteins, which are then bound by forces called disulfide bridges that link sulfur atoms. Coagulation occurs because water is trapped by the resulting network of proteins.
Osmotic Expansion
If the dissolution of the shell and the coagulation of the albumen are simply explained, it may seem less clear why dilation occurs. Could it be that osmosis is responsible for the increase in size? Water molecules tend to go from areas where they are most concentrated to areas of least concentration. Whereas the water concentration is about 95% in the vinegar, it reaches only 90% in the egg white. Moreover, whereas the acetic acid migrates toward the interior of the white (this can be verified by measuring the acidity of an egg white that has sat for several weeks in vinegar), the protein molecules dissolved in the water of the whites are too large to pass through the coagulated membrane. In other words, the water of the vinegar enters into the albumen, increasing the water concentration inside the egg.
To show that this is what happens, one has only to leave the eggs in an acetic acid solution whose acid concentration is greater than 10%. Once again the shell is dissolved, but this time the egg ends up being smaller because the osmosis is reduced. How would an egg placed in a concentrated solution of chlorhydric acid turn out? One would obtain the same smaller egg produced by the acetic acid solution, but the coagulation would be more rapid and clearer.
The Floating Yolk
I invite you to conduct your own experiments; many other surprises await those who are patient enough to observe carefully. For example, when the shell is dissolved and the egg white is still translucent, you can actually see the yolk floating in the white.
As for bases, adding caustic soda (sodium hydroxide) to an egg white causes it initially to coagulate. A chemical reaction produces a nauseating sulfur gas, and the egg then turns clear again. Obviously the soda dissociates the proteins after having first precipitated them. If we put eggs in ashes or in lime, which have lower pH levels, we can wait—for a hundred years.