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:

probably because with one or 2 eggs, there needs to be more steam to fill the space ? — Max, Jun 13 '21 at 11:58
Maybe, but that doesn't explain 6 and 7 eggs requiring the same amount. — DavidPostill, Jun 13 '21 at 12:00
Does the boiler stay ON at a *constant power* for a *fixed amount* of time *irrespective of number of eggs* ? If that is the case, more water means, more energy input is spent heating the excess water to 100 deg C instead of heating a little water to hot steam????? — AJN, Jun 13 '21 at 13:37
@AJN I've no idea how the power is regulated. And I haven't timed how long it is on for. The whole point of buying this is so I don't have to worry about removing my eggs from boiling water at the right time :) — DavidPostill, Jun 13 '21 at 13:39
@AJN there is only an on/off switch. Nothing to indicate number of eggs. And it turns off by itslef. — DavidPostill, Jun 13 '21 at 13:41
So the boiler has no idea of the number of eggs or softness required. There may be some sensor, but we do not know. Can you post a photo of the cylinder? — AJN, Jun 13 '21 at 13:43
@AJN There is just a hotplate where you add the water. I don't see any sensors (and I'm not planning to dismantle it). The dome is just a lid that lifts off and which has some holes to release steam. The eggs sit on a removable tray that rests on the hotplate. — DavidPostill, Jun 13 '21 at 13:47
Near as I can tell, this device dumps a fixed amount of energy (heat) into the chamber. The energy is absorbed by the water and eggs. To cook an egg to the desired doneness requires some (approximately) fixed amount of energy, and heating one mL of water requires some fixed amount of energy. So, roughly speaking, you need water and eggs to, collectively, absorb the correct amount of heat to cook the eggs appropriately. The more eggs you have, the less energy you want the water absorbing, so you need less water. — Xander Henderson, Jun 13 '21 at 14:37
But this is just a guess, as I can't see how this device *actually* works. — Xander Henderson, Jun 13 '21 at 14:37
@XanderHenderson Maybe I should ask on [Physics Stack Exchange](https://physics.stackexchange.com/) as I don't either. — DavidPostill, Jun 13 '21 at 15:07
If you are already in possession of the device, measure the time it stays ON when different number of eggs are boiled. If the time remains same, then the above theory is likely the correct one. — AJN, Jun 13 '21 at 15:39
Similar question over on Physics SE: https://physics.stackexchange.com/questions/429363/why-does-an-egg-boiler-require-more-water-to-cook-fewer-eggs — BowlOfRed, Jun 13 '21 at 20:26
Wouldn't it be a simple matter of displacement? The more space taken up by eggs, the less room there is for water - if you used the same amount of water for 6 eggs as you used for 1, it would overflow... — Darrel Hoffman, Jun 14 '21 at 15:30
@AJN it might use a thermal switch of some sort to maintain temperature, not just like a rice cooker does, but cycling on and off like a hotplate, so time alone may not be enough — Chris H, Jun 14 '21 at 15:55
The currently posted answer mentions a temperature sensor and that answer looks quite reasonable. I agree about the presence of a temperature sensor @ChrisH — AJN, Jun 14 '21 at 16:00
A pot and a stopwatch is all you need. I can't imagine how cluttered kitchens become when you've got a special single-use appliance for cooking each ingredient - you don't need this junk, seriously. 95% of all kitchen gadgets are complete garbage that do nothing for you but coddle your insecurities about being in the kitchen. Boiling an egg is easy enough that a child can do it. Why waste money and precious counter space on a Rube Goldberg machine to do something that takes five minutes, zero brainpower, and nothing that isn't already in your kitchen? It boggles the mind... — J..., Jun 14 '21 at 17:09
@DavidPostill Don't make a career out of making wild bets. ;) — J..., Jun 14 '21 at 17:45
@J... because it makes life easier? Like pretty much everything in life. I have a toaster because it’s easier than lighting a fire and holding my toast on a fork. I have a microwave because it’s faster to cook ready-meals than the oven. I have a hot water dispenser because it controls the temperature for different drinks. Telling people what they should or shouldn’t spend money on, or use their counter space for, is probably not going to improve anyone’s insecurities about cooking (it certainly wouldn’t help mine!) — Tim, Jun 15 '21 at 09:20
@Tim But an egg cooker is not such a labour saving or convenience device. It takes up space, requires setup, cleaning, and putting away. Boiling water in a pot and putting an egg in it is already absurdly easy. This is not an improvement. It seems like more work, in fact. — J..., Jun 15 '21 at 09:59
@J... and this is easier than that. It is, objectively, an improvement in ease. You may think it’s not worth it overall (cost, space, etc) but plenty of people (myself included) disagree — Tim, Jun 15 '21 at 10:00

AMtwo · Accepted Answer · 2021-06-15T14:28:22.173

29

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.

edited Jun 15 '21 at 14:28

answered Jun 13 '21 at 15:39

AMtwo

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This looks like a likely explanation and it makes sense, thanks. I only eat boiled eggs once per week, but I will try some tests hopefully and come back with some results when I'm able. – DavidPostill Jun 13 '21 at 16:10
As it's very hot at the moment and I'm eating a lot of salads I may experiment with cooking different numbers of hard boiled eggs that I should be able to keep in the fridge for a few days and time the cooking. Normally I eat 3 soft boiled eggs once per week. – DavidPostill Jun 13 '21 at 16:14
Ah! This makes a lot more sense than my guess. I had not noticed that there were vents in the top of the device to allow steam to escape. – Xander Henderson Jun 13 '21 at 18:19
4

Your answer is confirmed by [thermodynamics - Why does an egg boiler require more water to cook fewer eggs? - Physics Stack Exchange](https://physics.stackexchange.com/questions/429363/why-does-an-egg-boiler-require-more-water-to-cook-fewer-eggs) – DavidPostill Jun 13 '21 at 21:27
I don't think you'll get the same result from plugging holes. Plugging the holes isn't going to contain the steam, there's nothing that can hold the pressure. Rather, if there's an egg there it will absorb energy from the steam and condense it back to water. – Loren Pechtel Jun 14 '21 at 02:58
"there's nothing that can hold the pressure" well, pressure will depend on the weight of the object. There are things that could apply similar pressure to an egg, but ping pong balls are so light that they won't do much of anything to block the steam. (The point about needing something to absorb the heat like an egg is very good.) – Gregor Thomas Jun 14 '21 at 03:29
9

Ping pong balls are clearly not what I would qualify as "heat-proof"! – Xi'an ні війні Jun 14 '21 at 08:18

Mark Johnson · Answer 2 · 2021-06-15T11:44:56.063

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).

Why does cooking fewer eggs require more water/steam?

2 Answers2

So why more water for fewer eggs?

A fun experiment