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Comment by foobarian | Hacker News Reader

This was a bit hard to spot in the writeup, as I have no clue about how ice cream making and machines work. Otherwise we could just use ice, which will be as cold as the refrigerator can get it. (I doubt the endothermic reaction of dissolving the salt contributes very much to the cooling).

Now that I think about it, if I were doing this I would use antifreeze for the coolant instead of wasting salt. Bonus, I can store the antifreeze when done, but the salt water is wasted unless I'm going to use it to make some kind of soup or similar.

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Surface contact is one reason you want an ice/water slurry instead of just ice, but the real reason is that ice melting consume a lot more energy than just ice being warmed up to it's melting point.

The ice will quickly come up to it's melting (equilibrium!) point, without cooling the ice cream mixture very much. Remember, we're trying to freeze the ice cream (not just cool it down), which is proportionally just as thermodynamically expensive as melting ice. Bringing the ice up to it's melting point alone won't suck enough heat out of the ice cream mixture to freeze it.

This right here is the explanation that clicks for me. It’s not enough to say “salt makes the ice colder than 32”. Which might cause one to wrongly assume you do it so the ice cream freezes “faster.”

What you say here is the reason WHY that is needed in the first place, and you say it very clearly

Remember, we're trying to freeze the ice cream (not just cool it down), which is proportionally just as thermodynamically expensive as melting ice.

Is it just proportional, or is it actually pretty close to 1:1? That is, how accurate is the view that if you want to freeze 1L (or kg) of ice cream you need to melt 1L (or kg) of ice? Although I guess ice cream is not just frozen water, so perhaps that forms a fixed proportion. Alternatively stated, how much ice do you need to start with to freeze a given quantify of ice cream?

According to some random post on google, ice cream needs about 2/3 as much energy per gram to melt or freeze.

Of course you also have to worry about losses, extra margin, and the heat of stirring.

You're right, I should have said equivalently :D

Salt's way cheaper than antifreeze, and I'd be a lot happier about getting a little stray salt in my ice cream than getting a little stray ethylene glycol (with bittering agents, since 2010).

I suppose I could store the salt water solution instead of throwing it away. Assuming I made ice cream often this would be acceptably frugal for me.

Also, if you really want to recover the salt, you’d just have to boil away the water.

Alternatively, pickling is a perfectly good use of brine used for frugal purposes (extending the shelf life of produce, eggs, and what have you)

> Also, if you really want to recover the salt, you’d just have to boil away the water.

This is probably the point the poster was making: this requires a lot of energy. There's a lot of NaCl in the world, but getting it out of solution is expensive.

If that’s a major concern, you can literally leave it in the sun like the other person suggested.

Boiling is quick and convenient, but just evaporating the water out with ambient air is all but free.

You could use propylene glycol that is Generally Recognized As Safe.

What's in the blue liquid in those ice cream making bowls that is normally sealed but sometimes people report it leaking. The manufacturers say it is nontoxic.

It’s propylene glycol. The same stuff they fill “ice packs” with. And it’s also the thickening agent in coffee syrups. Supposedly food safe. I wouldn’t season my food with it though

It’s also what’s used in vape cartridges.

antifreeze is really really really toxic for mammals so there's that, while ingesting bit of salt never hurt anyone.. maybe diabetics

I think the ice cream would be a bigger concern for diabetics.

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