Eggs: The Hard-Boiled Truth
Q: What are the worst parts about being an egg?
A: You only get laid once and it takes 8 minutes to get hard.
Eggs are culinary champions. From sweet to savory; baked, boiled, or fried, many a solid recipe stands squarely on the shoulders of an egg. As ingredients, they are used as structural components (cakes, meatloaf), emulsifying agents (mayonnaise), foaming agents (meringue), coloring components (brown or yellow), and flavor additives (for things that you want to taste like egg). Taken alone, eggs pair well with toast and hash browns (hold the bacon, please) to form the archetypal American breakfast. Nutritionally, eggs are a source of high quality, complete protein. They contain all nine of the essential amino acids required by humans and our bodies are able to process egg protein ultra-efficiently; so efficiently, in fact, that egg protein is the industry standard for the measurement of the amount of protein from a food that is utilized for growth. In addition to all this wonderful protein, eggs provide a good source of other vitamins and minerals as well as cholesterol and fat. To round out this list of egg-complishments the egg can also, under the proper conditions, usher forth life in the form of a baby chicken…assuming it was a chicken egg, of course.
We are all probably familiar with the anatomy of an egg, but maybe not its composition. On the extreme outside is the characteristic shell made up almost exclusively of calcium carbonate, the only substance used in both cement and antacids. It protects the contents of the egg while also allowing gas exchange to occur through the shell’s thousands of tiny pores. This is the reason why you should store your eggs away from strong smelling foods (i.e onions) in your refrigerator, as they have a tendency to take on flavors from their neighbors. Directly underneath the shell is the air sac, the size of which is inversely proportionate to the quality of the egg. One indicator of good, high quality eggs is that they have small air sacs, or so says the USDA. Under the air sac is where you get to the business parts of the egg, the albumen and the yolk. The average egg has a 2:1 ratio of albumen to yolk. About 10% of the albumen is composed of various proteins and the rest is plain old water. It contains no or very little fat and is mainly in charge of protecting the yolk. The yolk is about 60% water, 15% protein and 25% fat; with some fat-soluble vitamins thrown in there for good measure, it’s the perfect diet for a growing chick embryo.
But how does this chicken-fetus Happy Meal transform from the liquid that spills out of the broken egg shell into the delicious opaque gel sitting on my breakfast plate? To answer this question let’s consider that oft-attempted but rarely-perfected kitchen staple, the hard-boiled egg. On the surface, this may seem to be the most basic of culinary feats – egg meets hot water and sits there until done. However, if you were to ask a hundred people how to accomplish this task, you will probably get a hundred different variations on the standard answer of “boil the egg in water for 10 minutes.” Most likely, people will provide you with whatever recipe their mother used for preparing hard-boiled eggs, and will then go on to defend their recipe and their methods practically to the death; if you’ve watched Top Chef, you know the kind of temper a cook can have. Should you use an old egg or a new one? Should you prick the shell to release some air? Should you start it in hot water or cold? Even trickier is the question, how long should you leave it on the heat? As you can see, all these variables cause a seemingly simple procedure to increase in complexity very quickly. This is all compounded by the fact that you only get one shot at it. You can’t peek inside the egg without opening up the shell and afterwards, well, you can ask all the kings men about trying to put one back together. And, of course, the final mitigating variable is the fact that everyone has a different idea of the perfectly done hard boiled egg.
To the food scientist, the process that takes an egg from liquid to semi-solid is known as gelation. At room temperature, the egg proteins exist in their native form, folded into globular conformations and held there by intra-molecular bonds. As heat is applied to the egg these bonds break and the proteins unfold into long chains. In the second step of gelation, the protein strands form strong intermolecular bonds and aggregate into a matrix. In this step, the viscosity of the egg increases rapidly and the liquid white or yolk becomes a solid gel. Both of these transformations occur at rates that are dependent on the amount of heat, the type and concentration of protein in the egg, and the pH and salt concentration of the surrounding fluid.
Within the albumen there are a multitude of proteins, each with its own gelation properties. The majority of these proteins coagulate and set between 60°C and 65°C, but there are a few proteins in the mix that can remain fluid up to 80°C. Yolk proteins have a slightly higher temperature for gelation, setting in the range of 65°C – 70°C. When an egg is gently heated into the gelation temperature range, the protein matrix will trap a significant amount of moisture, resulting in a hard boiled egg with a soft and creamy texture. If, however, the egg is heated too aggressively and is allowed to overcook, moisture is kicked out of the matrix. The whites become increasingly rubbery and the yolk dry and powdery. Another side effect of overcooking an egg is a green ring begins to form at the white-yolk interface. This unappetizing, but harmless, side effect is caused by hydrogen sulfide gas (produced by overcooking the egg whites) reacting with iron in the yolk to produce iron sulfide.
Thus, the creation of the perfect hard cooked egg (whatever that may be according to your tastes) is simply a matter of carefully controlling the temperature on the inside of the egg. The egg parts should be brought to the temperature of optimal preferred protein gelation and no further. Historically, the kitchen tool that has been used to accomplish this feat is the good old-fashioned pot of boiling water. The problem with this method is that boiling water offers very little in the area of temperature control, as it is offered at only one temperature, 100 ºC (give or take a few degrees in Colorado). As we’ve already learned, this is well above the gelation temperature of an egg. In order to control the temperature of the proteins on the inside of the egg, we must therefore exert our efforts on controlling the amount of time that the egg is exposed to the super-gelation temperatures.
When the egg is boiling in water it goes through a process of equilibrating its temperature with that of the water. This occurs with a rate that is dictated by the geometry and heat-transferring properties of the egg. As the heat penetrates the egg, a heat gradient is created such that the outer portion of the egg, the whites, reach equilibration temperature faster than the core, or yolk. Practically, this means that by the time the yolk reaches a temperature of “doneness” the whites will be on their way to being overheated.
The relationship of the temperature of the yolk and the temperature of the boiling water with respect to time can be modeled as a relatively simple heat transfer problem and shown as:
tyolks = 0.0015(egg diameter)2 loge [(2(Twater-Torig))/(Twater-Tyolk)]
This is actually a simpler form of the original equation worked out by Dr Charles Williams in 1996. The derivation of this equation makes some approximations and is based on an idealized spherical egg that you won’t find in any supermarket (although you can apparently make them into cubes), but it’s sufficient to elucidate the salient factors affecting the heating of the egg. The important features of this equation are that the time to reach the desired yolk temperature (tyolk) is proportionate to the original temperature of the egg and the square of the egg diameter.
Another technical hurdle of cooking an egg in boiling water is that with sudden heat changes and the constant rolling of the bubbling water, there is always the danger of injuring the shell and emptying the egg into the cooking water. This is obviously non-ideal and messy. Some of the tips that I have gleaned for keeping the shell intact include starting the egg in cold water, making a little pin prick one end of the egg, and including salt and/or vinegar to the water. If you just drop a cold egg into boiling water the air contained in the air sac under the shell will expand and need a way to escape. While some of it can diffuse out the pores of the shell, most times this escape route is not sufficient, and the buildup of pressure bursts through the egg shell. Pricking the shell is aimed at supplying an alternate means for air escape, but with it comes the danger of breaking through the inner membrane yourself and canceling out any good that you would have done. Adding salt or vinegar to the cooking water is intended more as post-catastrophe damage control than prevention: should the shell break and some of the white leak out into the water, the salt and acidity provide an environment that promotes quick coagulation, causing the proteins to coagulate and plug the hole, much the way a scab covers and plugs a cut. That’s nice and all, but not what I would call “cooking with confidence”.
An alternative to the boiling water method is to cook the egg in an environment that is a constant temperature (whatever temperature you determine produces the optimal gelation for your ideal egg) for enough time to allow the egg to completely equilibrate to that temperature. For most of us whose roster of kitchen appliances do not include thermostatically-controlled waterbaths, the easiest way to cook an egg in this manner is using a pot of water in an oven. Unfortunately, due to the approximations made in the derivation of the above equation, the model doesn’t do such a great job of describing the time required to cook an egg using this method. That is OK for our purposes though, because by fixing the temperature variable we essentially can disregard time. The egg just has to be left in the heat long enough to come to equilibrium with the environment. This usually happens in about an hour, but you could leave it there overnight and come out with the same level of doneness (check out this site for an actual experiment, with pictures). Once the egg reaches the desired temperature it will progress no further down the gelation chain and will therefore never cook past the desired point to become unwantedly rubbery -- unless that’s the consistency you were aiming for. This is a much longer time than any of the methods that use boiling water, but by sacrificing some time the end product is, according to many culinarily inclined professionals, a superiorly cooked egg.
In these days of the internet and microwave ovens, we generally don’t want to wait all day for our eggs to get hard, even if there is a slight bump in culinary quality. So until we find a chicken that craps out her eggs pre-cooked, the ordinary folks are going to remain committed to hard cooking their eggs with an 8 minute bath in boiling water, even if making the perfect “hard-boiled” egg should ideally not involved any boiling at all.
The Mad Gastronomist
It has been said that The Food Network is becoming the new MTV, and it is clear that there has been a surge in the popularity of all things food and cooking related. The casual weekend chef now has an almost unlimited amount of instructive information, and as their culinary acumen becomes more sophisticated, so too does their level of investigation. At this point, it is no longer satisfactory for cooks to ask “how do I bake a cake?”; the question has now become “how do these ingredients interact to result in a delicious cake?” To answer this question you have to turn away from the cookbook and bury your nose in a scientific text book. Or you could just read this column.