The boiled egg is one of the simplest dishes in the kitchen but to make a perfectly boiled egg is a skill - soft or hard. It's difficult to get the desired runnyness (is that a word?) or if you're doing hard boiled, you have to be careful not to over boil it. All cookbooks and websites offer different views on how it should be done from starting the egg in cold water to adding vinegar to the water to lower its pH or baking soda to raise it but few give their reasons why. Well I came across one blog, appropriately titled "The Food Lab", which provides a little more insight into the whole egg boiling process and the science behind it, in case you're interested.
According to the source, the age of the eggs only matters to a degree. As the old saying goes, "Old eggs are for boiling, fresh eggs are for frying." You may not have ever noticed but peeling a freshly laid boiled egg is actually more difficult—the inner membrane of the shell has a tendency to stick to the white. But this will only apply to those of you with your own chickens as eggs from the supermarket have likely been around for up to 30 days before they hit the shelf and then they're good until their expiration date. So this is actually pretty redundant.
Secondly, the pH of the Water is also a pretty negligible factor. Over time acidic liquid can dissolve the shell while alkaline levels enhance the ease of peeling but the egg is only boiling for such a short period of time that the pH of the water has little effect. In other words, don't bother adding vinegar or baking soda... despite what the cook books say.
It appears that the only factors that matter when boiling an egg are time and temperature.
The Temperature Timeline of Boiling an Egg
Now, here's what happens as an egg white cooks (i've always wondered why and how the egg white turns white):
From 30 -140 degrees: As it gets hot, the egg's proteins, which resemble coiled up balls of yarn, slowly start to uncoil.
At 140 degrees: Some of these uncoiled proteins—called ovotransferrin—begin to bond with each other, creating a matrix, and turning the egg white milky and jelly-like.
At 155 degrees: The ovotransferrin has formed an opaque solid, though it is still quite soft and moist
At 180 degrees: The main protein in egg whites—ovalbumen—will cross-link and solidify, giving you a totally firm egg white (after a boiling time of 7 to 9 minutes).
180 degrees-plus: The hotter you get the egg, the tighter these proteins bond, and the firmer, drier, and rubbier the egg white becomes (the 11-15 minute eggs). At this point, hydrogen Sulfide, or "rotten egg" smells start to appear.
Egg yolks, on the other hand, follow a different set of temperatures:
At 145 degrees: They begin to thicken and set up.
At 158 degrees: They become totally firm, but are still bright orange and shiny.
At 170 degrees: They become pale yellow and start to turn crumbly.
170 degrees-plus: They dry out and turn chalky. The sulfur in the whites rapidly reacts with the iron in the yolks, creating ferrous sulfide, and tinging the yolks.
Perfect Soft Boiled Eggs
The ideal soft-boiled egg should have a completely opaque white but not to the point of rubberiness (somewhere in the range of 155 to 180 degrees), and the yolk should be completely liquid (no hotter than 158 degrees).
So long as your water never goes above 180 degrees (the quivering stage just below a simmer) you have no chance of overcooking. For best results, heat water to 180 degrees as measured on a thermometer. After adding eggs, adjust heat to maintain temperature at 180 degrees for duration of cooking time.
Time wise, this guy suggests 6 whole minutes of cooking time for soft boiled. And use a timer to be precise.
Perfect Hard Boiled Eggs
If you drop the eggs directly into boiling water, the exterior heats up much faster than the interior and by the time the very center of the yolk reaches 170 degrees, the white and outer layers of yolk are hopelessly overcooked. On the other hand, if you start the eggs in cold water, as the water gently heats up the eggs gently heat up right along with it, greatly reducing the temperature differential between the interior and exterior.
The volume of water is equally important when hard boiling eggs. According to this guy's tried and tested methods, it should be exactly 1.5 quarts. And if you accidentally forget about your egg and leave it sitting in the water, there is no chance that it will overcook because by the time the egg is done (about ten minutes), the water temperature has dropped far enough that the egg will stop cooking.
I'm going to go boil an egg now.
Sources of Information
The post is made up of the author's original content, or is a compliation of material from various places.