Why does cooking fewer eggs require more water/steam?

Question

I have purchased a Nutri-Q 34360 Healthy Eating Egg Boiler:

How does it work? - The eggs are cooked through hot steam. The instructions will guide you on the amount of water that is required depending on the quantity of eggs.

It comes with a little measuring cylinder for the amount of water to add which has gradations that look something like:

Hard boiled
- 1
- 2
- 3
- 4
- 5
- 6/7

Medium
- 1
- 2
- 3
- 4
- 5
- 6/7

Soft
- 1
- 2
- 3
- 4
- 5
- 6/7

I understand why more water is required to hard boil eggs compared to medium cooked eggs (they need to be cooked longer).

However, I don't understand the gradations within each range (the number of eggs to be cooked).

Why does, for example, cooking 1 medium egg require more steam than cooking 3 medium eggs? Surely it should be the other way around?

Can someone enlighten me?

Photo of the parts:

Answer 1

These egg cookers work by simply heating the water until it all evaporates. Most cookers sense when all the water is evaporated, and automatically turn off and/or alerting with a beep or noise. They have a sensor under the hot plate that detects temperature. When water is still in the pan, it keeps the pan cool, and when water evaporates completely the pan bottom will start to get hotter.

The water is heated in the pan in the bottom, works it way past the eggs, and eventually out the holes in the lid.

So why more water for fewer eggs?

The tray that holds the eggs has a bunch of holes in it. Each egg cup has a hole in the bottom so that the steam has direct contact with the bottom of the egg. When the tray is full, it reduces the number of outlets and traps more steam under the tray. Similar to putting a lid on a pot, this reduces the speed of evaporation.

With only one egg blocking one hole, the steam can escape more readily out from the bottom of the tray to the top, and steam will escape out into the room more quickly.

To compensate for the fact that steam is escaping the egg cooker slightly faster when there are fewer eggs, you use slightly more water when there are fewer eggs.

A fun experiment

You could use eggs (or a heat proof substitute... Maybe something ping pong ball or golf ball sized (but not ping pong balls--they melt at 80°C)) to block off the "egg holes" and collect a series of timings for how long it takes for all the water to evaporate as you vary the number of eggs & water.

You should find that when you follow package directions, the timing of the cycle is approximately the same.

Similarly, if you use only one egg, and the "6/7 egg" water amount, you'll find the cycle (and complete water evaporation) faster than the full "1 egg" water amount. Again, this is just because there's an easier path for the steam to escape.

Answer 2

The volume of space within the container (that must be filled with steam) is less with 6 eggs (258 ml) than with 1 egg (43 ml).

Therefore you must produce more steam (thus need more water) for 1 egg than for 6 eggs to have the same effect.

---

Sample:

The container, after the water and egg holder, has a volume of 500ml.

Each egg has a volume of 50 ml.

For 1 egg, 450ml of steam is needed (50+450=500ml)
For 3 eggs, 350ml of steam is needed (150+350=500ml)
For 6 eggs, 200ml of steam is needed (300+200=500ml)

The water needed for 200ml of steam (for 6 eggs) is less than the water needed for 450ml of steam (for 1 egg).

Answer 3

It is true that the more eggs there are, the longer it takes for all the steam to escape, but (as noted in @LorenPechtel's comment) this is not because more eggs block more holes in the tray, but because more eggs recycle steam back into water faster.

An egg steamer turns water into steam at a constant rate and stops when all the water has escaped out the vents at the top. The eggs are primarily heated at a constant rate by steam condensing on the surface of the eggs, and this condensation then drips back down to the bottom where it is reheated and turned back into steam. This steam recycling rate is proportional to the total surface area of all the eggs in the steamer. The more eggs there are, the larger the total surface area and the faster steam is condensed back into water before it can escape out the vents. So the more eggs there are, the longer it takes for the all the water to escape. If the same amount of water is used for more eggs, the egg steamer will take longer to shut off, overcooking the eggs.

This is discussed more in the accepted answer to "Why does an egg boiler require more water to cook fewer eggs?" over on Physics Stack Exchange and in this Physics Education journal article on "The egg steamer paradox".