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 How to prepare the perfect boiled egg

Many cook books suggest the following for boiling eggs: 3-6 min for a soft yolk, 6-8 min for a medium soft yolk and 8-10 min for a hard yolk. If you are satisfied with this, there is no need for you to continue reading. However, if you are in search of the ultimate boiled egg, you've come to the right place!

From a scientific view point, the above directions are not very precise. A number of important parameters remain unanswered: What is the size of the eggs? Where they taken directly from the fridge or were they room tempered? Where they put into cold or boiling water? If using cold water - do you start the timer when you turn on the heat or when the water starts to boil (and in that case, how much water do you use)?

There is in fact a formula for calculating the boiling time for eggs which takes a lot of these parameters into account. The formula was derived by Dr. Charles D. H. Williams, a lecturer in physics at University of Exeter. He has also set up a nice page about the science of boiling eggs. Given the starting temperature of the egg T0, the temperature of the water Twater, the desired temperature of the yolk Tyolk and the circumference of the egg c, the cooking time t of the eggs in minutes is given by:



As is evident from the formula, the boiling time depends on the size of the egg. For a larger egg, a longer cooking time is needed. The circumference of the egg is easily measured around the thick end using a piece of string which, after marking of the circumference of the egg, can be measured using a ruler. I have prepared a piece of string which I keep in the kitchen with three knots at 13, 14 and 15 cm respectively to make it even simpler.

The boiling time also depends on the starting temperature of the egg, T0. This also makes sense as it will take longer to heat up a cold than a warm egg to the desired temperature. In fact, the temperature difference between a room tempered and a refridgerated egg amount to a difference in boiling time of 30 seconds.

At sea level, the temperature of boiling water is 100 °C. At higher altitudes, the boiling is lowered. As a rule of thumb,  the boiling temperature of water is lowered 0.3 °C for each additional 100 m above sea level. For an accurate calculation, check out his calculator. As we shall see later, the formula can of course also be used prepare eggs at sea level, using water kept at temperatures less than 100 °C.

Lastly, we must decide how we like our eggs done. The doneness of the egg, depends on the temperature of the white and the yolk. Egg white start to coagulate in the range 62-65 °C. At these temperatures it is the most heat sensitive protein, the ovotransferrin, which constitutes 12% of the egg white, which coagulates. The major protein of egg white, ovalbumin, makes up 54% of the white and doesn't coagulate until the temperature reaches 80 °C. The yolk begins to thicken around 65 °C and sets around 70 °C. Further heating to around 80-90 °C produces the crumbly texture typical of hard boiled eggs. So to sum it all up:

Temperature / °C Egg white Egg yolk
65 Begins to set Begins to set
70 Tender solid Soft solid
80 Firm, Solid
> 90 Firm, rubbery texture Hard, crumbly texture

Now it's really up to you to decide how you like yor boiled eggs. And if you use boiling water, you have to decide how you like the yolk. The reason for this is that by the time yolk reaches the desired temperature, the egg white has reached an even higher temperature. Scientists typically refer to this as a temperature gradient. So when I like my egg yolks somewhere in the range 65-70 °C, the eggwhite will most likely be somwhere around 80-100 °C.

So now everything is set and we can figure out how long the eggs should be boiled. Knowing the circumference of the egg and the desired yolk temperature, you can use the chart below to figure out the boiling time in minutes. You can download a high resolution pdf with the chart here.




But is this the perfect egg? 

No actually not... keep reading! The problem with using boiling water is that while you do heat the yolk to the desired temperature, you have virtually no control with the temperature of the white. If your water holds 95-100 °C, so will the white (or at least the outer most part of the white). This gives it a firm, rubbery texture. So the problem is, to put it differently, that we want to heat the yolk to somewhere above 65 °C, but we do not want to heat the white above 80 °C. The solution to this problem is to "boil" the egg at a temperature lower than 100 °C, which means not to boil it at all.

And this is where things get complicated, because the formula given above doesn't work quite as well any more because of several approximations. Let's say we want to "boil" a egg at 65 °C. For a room tempered egg with a circumference of 14.5 cm, the formula gives an approximate "boiling" time of just over 21 min to reach a yolk temperature of 65 °C (calculated using Twater = 65.1 °C). But if you try this, both the white and the yolk will still be runny. The reason for this is that the formula does not take into account that the different proteins coagulate at different temperatures. If you keep the egg at 65 °C for an hour, you might notice something very strange: when you open the egg, you will have a runny white and semi-solid yolk. However, if you keep the egg at 65 °C for 6 hours or more, you'll get what I would call a perfect egg having a tender white with a semi-solid yolk. Erik Fooladi has posted pictures of such eggs on his blog, Erik's Sience- and food-ucation.

The only thermostat most people have in their kitchen, is the oven. So a more practical way to do this is to to use a slightly higher temperature to save some time: Preheat your oven to 70 °C. Then heat 1 L of water to 70 °C, put the eggs in, cover with a lid and leave the pan in the oven for one hour. Unfortunately, the white is a little to soft and the yolk a little to hard, according to my taste. So I guess a higher temperature and less time will give perfect eggs. But of course - if you're able to keep eggs at 65 °C for 6 hours or more, that's the best.


Boiling eggs on a comet...

 Jules Verne

 In his book "Off on a comet", science fiction author Jules Verne shows that he was actually aware of the possibility of "boiling" eggs at a temperature lower than 100 °C. Here is a short excerpt from the Gutenberg e-text version:

The skillet was duly set upon the stove, and Ben Zoof was prepared to wait awhile for the water to boil. Taking up the eggs, he was surprised to notice that they hardly weighed more than they would if they had been mere shells; but he was still more surprised when he saw that before the water had been two minutes over the fire it was at full boil.

"By jingo!" he exclaimed, "a precious hot fire!"

Servadac reflected.  "It cannot be that the fire is hotter," he said, "the peculiarity must be in the water."  And taking down a centigrade thermometer, which hung upon the wall, he plunged it into the skillet.  Instead of 100 degrees, the instrument registered only 66 degrees.

"Take my advice, Ben Zoof," he said; "leave your eggs in the saucepan a good quarter of an hour."

"Boil them hard!  That will never do," objected the orderly.

"You will not find them hard, my good fellow.  Trust me, we shall be able to dip our sippets into the yolks easily enough."

The captain was quite right in his conjecture, that this new phenomenon was caused by a diminution in the pressure of the atmosphere. Water boiling at a temperature of 66 degrees was itself an evidence that the column of air above the earth's surface had become reduced by one-third of its altitude. The identical phenomenon would have occurred at the summit of a mountain 35,000 feet high; and had Servadac been in possession of a barometer, he would have immediately discovered the fact that only now for the first time,
as the result of experiment, revealed itself to him--a fact, moreover, which accounted for the compression of the blood-vessels which both he and Ben Zoof had experienced, as well as for the attenuation of their voices and their accelerated breathing. "And yet," he argued with himself, "if our encampment has been projected to so great an elevation, how is it that the sea remains at its proper level?"

Once again Hector Servadac, though capable of tracing consequences, felt himself totally at a loss to comprehend their cause; hence his agitation and bewilderment!

After their prolonged immersion in the boiling water, the eggs were found to be only just sufficiently cooked;
the couscous was very much in the same condition; and Ben Zoof came to the conclusion that in future he must be careful to commence his culinary operations an hour earlier. He was rejoiced at last to help his master, who, in spite of his perplexed preoccupation, seemed to have a very fair appetite for breakfast.
Amazing how much science Jules Verne manages to incorporate in his novels! He has correctly observed that water boils at lower temperature at higher altitudes, and that on a fictional comet of appropriate mass, water will boil at 66 °C. The temperature is wisely chosen, because by keeping them at 66 °C, you really can't do anything wrong. Now from the last paragraph it seems that the eggs were not fully cooked after "a good quarter of an hour". Of course, there is also no mention about the size of the eggs, so any further speculations end here. 


Other aspects to consider when boiling eggs

An egg has somewhere between 7000 and 17000 pores, meaning that water slowly evaporates. This is also why eggs age faster at room temperature than in a refridgerator. Beacause of the pores, eggs should not be stored next to foods with a strong smell such as onions (unless of course, you want onion flavored eggs).

If (carefully) dropped into boiling water, eggs typically crack. The reason is that the air pocket in the blunt end of the egg expands more quickly than the air can escape through the pores. To avoid the cracking, pinch a small hole into the blunt end to let the expanding air escape. Another possibility to reduce the damage, is to add salt or vinegar to the water. This will help the white coagulate faster and thus plug the crack.


(picture of egg pore from http://science.exeter.edu/jekstrom/SEM/SEM.html)

If boiled for a prolonged period of time, the surface of the yolk turns greenish. This is caused by a rection between iron in the yolk and hydrogensulfide which is formed by decomposition of sulfur containing proteins in the white.


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