In November 2006, Yvonne went to Europe for some teaching and left me with 18 fresh eggs in the fridge, so I've had a boiled egg for breakfast every day. It's easy to boil an egg, right? That's why none of my myriad cookbooks have details. Somewhere in the back of my head I had this concept of a “3 minute egg”. The first time (not recently) that I tried this, the egg came out almost raw: only some of the white had started to coagulate.

What went wrong? Details, as usual. There are two basic ways to boil an egg:

1. Put an egg in cold water in a saucepan, bring to the boil, and boil for three minutes. This will give you an egg where (roughly) the white is solid and the yolk is still liquid.
2. Bring some water to the boil in a saucepan. Add the egg and boil for six minutes. Again, this will give you an egg where (roughly) the white is solid and the yolk is still liquid.

I went looking on the web and found references to both methods: eHow.com gives only the first method, while Delia online gives both, the second with a twist: take the pot off the boil after one minute and leave another 6 minutes (7 in total).

Somehow both of these are missing a number of important points:

• The boiling time for an egg is dependent on its weight, and more importantly, its diameter. The eggs I have here range in weight from 44 g to 71 g, and from 40 mm to 47 mm. The eggs in these photos both came in the same box.

  

  No recipes I know address the diameter of an egg, and almost none address the weight. The EU regulations divide eggs into 7 classes. Class 7 is anything less than 45g (like my second example), class 1 is 70 g and more, and the others are in 5 g steps in between. My old copy of the American book “The Joy of Cooking” states that the recipes are for 2 oz (i.e. 57 g) eggs, probably rather smaller than most eggs nowadays.

  The cooking time depends on the following factors:

  1. The temperature at the surface between the water and the egg. This is easy to specify for the second alternative: it's 100°.
  2. The surface area itself. For a first approximation, we can assume that the heat flow into the egg is proportional to the surface area.
  3. The diameter of the egg. The further the heat has to go, the longer it will take.
  4. The mass (“weight”) of the egg. The cooking time is roughly proportional to the heat flow divided by the mass, and thus the surface area divided by the mass. For eggs of the same shape, the mass is proportional to the cube of the diameter, and the surface area to the square of the diameter, so this ratio is proportional to the 1.5th power of the diameter.
  5. The previous point would be more exact if the heating were very gradual (i.e. the temperature differences small). That's not the case here: in the second case, we have an egg at about 20° being plunged into water at 100°. The first case is more complicated, but even there the temperature of the water rises much faster than the temperature of the egg.

     At a first approximation, I'd assume that the cooking time in hot water is proportional to the radius (and thus the diameter) of the egg: in other words, the time taken to get the heat to the middle of the egg depends on how far it has to travel. Combined with the previous factor, this would suggest a cooking time in proportion to the 2.5th power of the diameter.

  6. The previous calculations make a number of implicit assumptions. in particular that the thermal resistance of the shell is the same as the inside of the egg (also assumed to be uniform). But it's a basis for thought.

  Even if you assume that the cooking time is roughly proportional to the diameter, the egg with 47 mm diameter would take nearly 20% longer to cook than the egg with 40 mm diameter—that's over a minute difference for the second method. It's difficult to guess what the difference is for the first method.

• Eggs are relatively fragile. If you put a cold egg into boiling water, the pressure inside will build up suddenly, and the egg may crack or burst. The larger the temperature difference, the more likely this is to happen.

  There's a simple solution for this, but I haven't seen it in English-speaking countries: an egg piercer:

  

  You put the big end of the egg onto this device and press down; the spike in the middle makes a small hole in the shell, which allows the pressure to adjust.

• The cooking time depends on the initial temperature of the egg. Many warn against taking the egg directly out of the fridge: Delia writes:

  Don't ever boil eggs that have come straight from the refrigerator, because very cold eggs plunged straight into hot water are likely to crack.

  I've already addressed that issue. But what difference does the initial temperature make in the boiling time?

• If you put the egg in cold water and bring it to the boil, you have at least two sources of inaccuracy:

  1. The time it takes for the water to come to the boil depends on the strength of the flame and the amount of water. During this time about half of the cooking takes place, as you can see by the comparison of the cooking times for the two alternatives.
  2. The time it takes for the water to come to the boil is also dependent on the number of eggs you put in the water: the more, the longer it takes to boil.
  3. It's difficult to decide exactly when the water is boiling. Is it when the first large bubbles appear? Or when it's boiling vigorously? These two events can be 30 seconds apart.

• Delia's idea of taking the water off the boil after a minute also introduces a small inaccuracy: the remaining time depends on how quickly the water cools down. The fact that she adds a minute to the cooking time suggests that this is non-trivial (in fact, I wouldn't expect it to make that much difference), but she doesn't explain what use this twist is.

• When removed from the water, the surface of the egg is hotter than the middle. If you don't do anything, heat will continue to transfer to the middle, and it will continue to cook. You can slow this down by running cold water over the egg after removing from the saucepan. eHow mentions this, but Delia doesn't.

What does this mean in practice? I use method 2 because it's more reproducible. Ultimately you're going to have to find your own times, but if you have a box of eggs like the ones shown above, you'll have an idea how to proceed. I've been eating the big eggs so far; when I get to the small ones, I'd guess that they will only need 5 minutes instead of 6.