Boiling eggs may sound like the simplest thing you could possibly cook, but if you've ever boiled an egg you've probably run into problems at one time or another. The challenges with boiling an egg generally fall into two categories: cooking the egg, and peeling the egg. Despite following a set of directions precisely you might have found your eggs under/over cooked, or perhaps when you tried to peel the egg it ended up looking like the surface of the moon.
The good news, is that both of these problems are easily resolved with a little understanding behind the science of eggs. Read on and you'll be boiling perfect oval eggs whether you prefer them hard-boiled or soft in the center.
How to Make a Perfect Boiled Egg
The first thing you have to understand is that the egg yolk sets at a much lower temperature than the egg white (70 degrees C vs 80 degrees C). Since the heat source (boiling water) is outside the egg, the egg cooks from the outside in. In theory this means that by the time yolk is set, the white has also reached it's higher setting temperature.
The problem is that since the boiling water is significantly hotter than the setting temperature of the egg, it's very easy to zoom past the desired temperature. Because the temperature is rising so fast, it's difficult to pinpoint exactly when to stop the cooking to get the egg exactly how you like it. The problem with most boiled egg instructions is that they create a formula (put eggs in cold water, bring to boil, boil for X minutes) assuming you put the exact same size and temperature of egg into the same amount of water in a the same pan on the same stove... Well you get the idea.
The best way to figure out what works for your setup is to boil a dozen eggs and start pulling the eggs out of the water at thirty second increments after about 3 minutes. But who want's waste that many eggs? Well, luckily I have a friend that produces eggs and I've been busy experimenting in my lab to come up with an (almost) foolproof method of boiling eggs.
Since the main problem with boiling an egg is the narrow window of time during which the egg is perfect, I asked myself how I could slow the cooking down to expand that window of perfection. I found my answer in the way I cook my chicken for chicken soup
This is how I adapted the technique for eggs. Put refrigerated eggs in a heavy bottomed pot and cover with cold tap water so they're covered by about 1" (2.5cm) of water. Bring the water to a full boil (100 degrees C) over high heat, and then remove the pot from the heat. Let the eggs cook the rest of the way using the residual heat in the water. As the temperature of the egg rises, the temperature of the water will fall, which will give you a much wider window when your egg is perfectly cooked.
Before I give you cooking times for the eggs though, you need to know that there are many other factors that will effect the cooking time of the egg and I've outlined some of the major ones below along with what I did in my kitchen.
- Initial egg temperature - An egg right out of the fridge will take longer to cook than an egg at room temperature. But since room temperature varies by season, and most people have their fridges set to around the same temperature. I decided to develop my method using eggs straight out of the fridge.
- Egg size - The bigger the egg, the longer it will take to cook. I did all my experiments with large eggs (not extra large or jumbo)
- Egg to water ratio - The more water you use relative to the number of eggs, the longer it will take to boil and the longer it will retain heat. Too much water and your eggs will cook too fast, too little and the temperature will fall too fast, resulting in uncooked eggs. Ideally you'd measure out a certain amount of water for each egg you're boiling. But that seems a little extreme for something so simple. I typically use a pot that comfortably houses the number of eggs I'm going to cook (not crowded, not too spaced out), and then cover the eggs with enough water so there's about 1" of water above the top of the eggs.
- Heat retention of the pot you use - Thicker pots, made of denser materials (iron vs aluminum) tend to retain heat better than thinner pots. I used a heavy bottomed stainless steel pot to boil my eggs. If you're using an electric stove (coil or plate type), you should remove the pot from the stove and put it on a trivet as these types of stoves tend to retain heat long after you've turned them off.
- Heat output of your stove - Some stoves are able to boil a pot of water much faster than others. IH tends to be the most efficient, while electric plate type stoves tend to be the least efficient. If your water take too long to boil, your eggs are going to be sitting in hot water for much longer, which will effect the amount of time they need to sit in the water after you turn the stove off. I used an IH stove for these experiments.
- Altitude - The boiling temperature of water falls as your altitude rises. If you live in the mountains, this will certainly effect your cooking times, so you'll need to adjust accordingly. I boiled my eggs at roughly sea level.
- Room temperature - If you're in a very cold room, your water will cool faster than if you're in a warm room. I boiled my eggs in a 24 degree C (75 F) room.
With all that in mind, here are the cooking times for various types of boiled eggs. The times start after the water has come to a boil and you've turned off the heat.
- 2 minutes - The white isn't fully set and the yolk is totally raw.
- 4 minutes - The white is fully set, but the yolk is thick and runny.
- 6 minutes - The white is fully set, and the yolk is mostly set, but still a little runny in the middle.
- 8 minutes - The white is fully set, and the yolk is set, but tender.
- 10 minutes - The white is fully set, and the yolk is fully set.
Remember to transfer your eggs to cold water as soon as you take them out of the pot to stop the cooking immediately. Otherwise, your eggs will continue cooking even after you've taken them out of the water.
How to Peel an Egg Perfectly
If you do a search on google for "peel an egg" there are literally dozens of purported ways to make peeling an egg easy. Most of them don't work with fresh eggs, and while aging an egg for 2 weeks works pretty well, who wants to wait that long to make a boiled egg?
The good news is, after some experimentation, I've come up with a method that works every time, no matter how fresh the egg is. The secret is to put a small crack in the bottom of the egg BEFORE you boil it. The crack needs to extend all the way through the hard shell, but it must not rupture the membrane (otherwise you'll end up with egg white spewing out of the crack as it boils). I use a small curved object (the end of a wooden pestle) to crack the egg on, because it creates a more predictable circular crack rather than a linear crack that could spread and rupture the membrane.
So why does this work? To understand this, it would help to understand why older eggs are easier to peel than fresh eggs. Unfortunately there is no scientific consensus on why an aged egg is easier to peel than a fresh egg. One thing we do know is that the albumen in a fresh egg contains more carbon dioxide, which means it has a lower pH (more acidic). This leads to one popular theory: that the acidity somehow makes the albumen adhere to the membrane more than an older egg with a higher pH.
As an egg ages, the moisture in the albumen seeps through the membrane and evaporates through small pores in the shell. This is what makes the air pocket inside an egg, and why it grows larger as the egg ages. Here's where another theory comes into play: that the larger air pocket somehow makes the albumen adhere less to the membrane.
Personally I don't buy either one of these theories because they don't fit with my observations. While I don't have any evidence to prove this (beyond my personal observations), my theory is that the higher moisture content in the membrane of an older egg prevents albumen from sticking to the membrane, making it easier to peel.
By putting a small crack in the shell, it allows water to enter the egg and saturate the membrane, mimicking the membrane of an older egg. If any scientists out there want to compare some cross sections of the albumen-membrane interface under a scanning electron microscope it would be awesome to finally put this mystery to rest.
Update:_ For very fresh eggs, you might find that the bottom of the egg (where the crack is) is easy to peel but the top is still problematic. For these eggs, just crack them all over using a blunt object such as the back of a spoon after they are boiled, and then soak them in cold water for 30 minutes. This should make the shell practically fall off.
- Take the eggs from the refrigerator, then tap the bottom of each egg on a curved surface to make a small circular crack through the shell, but do not rupture the inner membrane.
- Add cold tap water to the pot until you have 1" of water above the top of the eggs.
- Put the pot over high-heat until the water comes to a boil.
- Set a timer for your desired doneness (see times above), and turn off the heat.
- Remove the boiled eggs from the water when they are done and put them in a bowl of cold water to stop the cooking.
- Crack every bit of shell by tapping firmly with the back of a spoon and then soak the egg in cold water for 30 minutes before peeling.