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It could be due to boredom that I notice this, but it seems that only on airplanes ice cube have holes. Why is that?

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I have seen those in bars and restaurants too, I believe some machines are designed to produce these holes, apparently they have some advantages. Usually they are also cylindrical rather than cube-shaped. –  Relaxed May 19 at 16:05
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I bet it's a choking thing. If I accidentally swallow a cube like the kind you are speaking off, I can still breath through the passage easier then a whole brick. –  John Riselvato May 19 at 16:10
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The reason is probably most trivial: greater surface area allows for a greater rate of cooling. Holes are a neat way to increase surface area without increasing overall dimensions of the ice cube. –  mindcorrosive May 19 at 16:11
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This question would get more detailed answers on Physics SE, but I wouldn't go so far as to say it's off-topic here. –  R.. May 19 at 17:14
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I doubt it is to prevent choking. There is a small angle across 2 axis of rotation that would allow you to breath thru an ice cube with a hole. The odds of it getting lodged at precisely that orientation is pretty small. –  Joshua Dance May 19 at 18:16

4 Answers 4

up vote 46 down vote accepted

They have the holes because of the machines that made them (check DavidG's answer), anyway this ice cubes with holes are better for planes for a few reasons:

  • Because they are lighter (believe it or not, every gram in the aviation business counts)
  • They cool things faster due to larger area of contact with the liquid
  • They also will allow more liquid to be filled in the already small airplane cups.
  • Last thing, they do not tend to stick together in the ice bucket, so cabin crew won't need to smash it to break it down.
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It also takes less energy to produce them (less volume/mass of water) for the same cooling power (surface area). On the other hand, it means you end up giving customers more beverage/less ice, which might have a cost in itself. ;-) –  R.. May 19 at 17:13
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they can also be made clear because the freeze to the rod from the inside out so the ice has room to expand out rather than cracking when freezing from the outside in –  ratchet freak May 19 at 17:58
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@R.., On many flights, I receive the entire can (355mL?) It's not true of every airline, but it's common enough. So more beverage/less ice is not necessarily an issue. –  Brian S May 19 at 18:15
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Can you explain briefly why they do not stick together? –  Daniel Wagner May 19 at 22:16
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This is incorrect - your answer merely lists side effects. DavidG has the correct answer. –  Rory Alsop May 20 at 13:22

In the retail drinks trade (pubs, bars etc), machines that make these hollow ice cubes are often* used where there is not much room to store large quantities of ready-produced ice (such as in a much larger machine). The larger surface area to volume ratio means fresh ice cubes can be made more efficiently and quickly to meet demand - by the time one batch is used, the next is ready. The ice machine only needs a small storage capacity at a time to keep up.

*Of course, in warmer climates where ice is used in larger quantities, or in establishments where drinks depend on more ice (cocktail bar versus English pub), they will usually have larger capacity machines, but even then the hollow cube is common.

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It's because the machine that creates the cubes has metal prongs that the ice 'grows' around.

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This is the correct answer. The most efficient ice makers (and the majority of industrial ice makers) are not those that cool the surface, but those that use refrigerated metal prongs that stick down into the water pool. As the water freezes around the prongs, they rotate out of the pool and deposit the ice into a vessel. –  Rory Alsop May 20 at 13:21
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+1 for detail. This is part of the point I was making without discussing the mechanics of the machine - the "surface" in this case is not the "outside" of the cube, but the inside. It comes down to the least distance between the cooling part (the "prong") and the outside of the cube. A thin wall will be more efficient to cool than a solid lump –  AdamV May 21 at 11:36
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But why do airlines use this machine whereas ice cubes we encounter in many other places seems not to be made with a similar machine? –  hippietrail May 21 at 15:23
    
@hippietrail Most likely because other places haven't bought a machine for making ice. Or perhaps there are others types that make other shapes and they are more expensive. –  DavidG May 21 at 15:31
    
@DavidG: Which, in turn, lead to more questions: Why do airlines buy ice making machines that most other places serving ice do without? Why do airlines choose the less expensive of several available ice making machines which result in different shapes. In any case your answer makes it seem like a total coincidence in which the airlines definitely play no conscious decision making part whatsoever. Which is fine if it's true, but you don't show that it is actually true. –  hippietrail May 21 at 15:40

Typically those are used to identify ice cubes made from drinkable water. Restaurants use them in most parts of the world and those are not limited to planes. Actually, we were at a steakhouse just yesterday and my smallest daughter asked exactly the same question. She probably was bored too.

The comments to your question are interesting because they outline several advantages of these. Risk of choking being reduced is a great one. Obviously cost saving and efficiency are important too.

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"from pure water"? What else would ice cubes be made from? –  Greg Hewgill May 19 at 20:17
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It could be not drinkable water — or less enjoyable (i guess that's what he meant) –  Jay Claiton May 19 at 20:27
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Yes, sorry, the term is not universal. In many parts of the world, where tap water is not drinkable, they call the one that is pure water or the equivalent expression in the local language. –  Itai May 20 at 1:11
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Oh, so you might have one kind of ice cubes that you put in a bucket to keep (unopened) drinks cool, and another kind of ice cubes that are made from drinkable water that you can put in drinks. Makes sense, thanks. –  Greg Hewgill May 20 at 2:30
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@GregHewgill: From tap water of course. Some tap water tastes like pure water. Just the other day I was served some very yucky water that would've tasted just as bad if it had been made into ice. –  hippietrail May 21 at 15:26

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