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Why do we salt the ice when making ice cream?

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Re: Why do we salt the ice when making ice cream?

#71

Post by fsckboy »

Taniwha wrote: post #48
fsckboy wrote: post #31

Earlier quoted context omitted.

it's not just the coldness of the ice, salt dissolving in water actually decreases the temperature, it's an endothermic reaction. the salt dissolving into the water brings the water down to 0... omg time for Farenheit to shine... brings the water down to 0F without freezing it (because of the lower equilibrium temp), which is -17.8C (Farenheit uses this endothermic salted water temp as its definition of 0, I think be…

That's why I said it was "part of the issue" - but once the salt has dissolved the system is kept at -5 for a period of time because inside of the ice cubes are < 5C

yes but as long as there is more undissolved salt (and when making ice cream you use a lot of salt), the melting ice will continue adding pure water to be chilled/saturated with salt.

Re: Why do we salt the ice when making ice cream?

#72

Post by hammock »

martyvis wrote: post #60
floren wrote: post #35

Earlier quoted context omitted.

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).

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

Re: Why do we salt the ice when making ice cream?

#73

Post by hammock »

timrice wrote: post #40
foobarian wrote: post #33

Earlier quoted context omitted.

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 stor…

Author here 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 proportional…

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

Re: Why do we salt the ice when making ice cream?

#74

Post by eyelidlessness »

topaz0 wrote: post #70
Taniwha wrote: post #64

Earlier quoted context omitted.

yes - then it wont be ice any more - but while there's still ice the temp of the liquid will stay at the equilibrium point - after that it will start to warm

"then it won't be ice anymore". Not so: the freezing point is the temperature at which ice and water are in equilibrium. If you take a bowl of ice to its freezing point it will still be entirely frozen. Then if you continue adding energy, it will remain at its freezing point while progressively more of it melts, i.e. going from 100% ice to 50% ice/50% water to 100% water. All at one temperature. In other words, if yo…

> and none of it goes to increasing the temperature

You’re clearly more knowledgeable than I am, but this strikes me as probably wrong? The temperature at the surface has raised, which is how some of the surface ice melted. The temperature on average has to raise because thermodynamics. If I understand the article, the core temperature of the remaining ice can decrease and localities can decrease with them. But the energy is increasing temperature in every other area at a higher rate, because it has to (conservation of energy) and because it has to (more ice melts than freezes).

Re: Why do we salt the ice when making ice cream?

#75

Post by dpflug »

alliao wrote: post #44
foobarian wrote: post #33

Earlier quoted context omitted.

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 stor…

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.

Re: Why do we salt the ice when making ice cream?

#76

Post by nkurz »

timrice wrote: post #40
foobarian wrote: post #33

Earlier quoted context omitted.

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 stor…

Author here 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 proportional…

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?

Re: Why do we salt the ice when making ice cream?

#77

Post by Dylan16807 »

nkurz wrote: post #76
timrice wrote: post #40

Earlier quoted context omitted.

Author here 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 proportional…

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 prop…

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.

Re: Why do we salt the ice when making ice cream?

#78

Post by schrodinger »

hammock wrote: post #72
martyvis wrote: post #60

Earlier quoted context omitted.

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.

Re: Why do we salt the ice when making ice cream?

#79

Post by iamkroot »

nkurz wrote: post #76
timrice wrote: post #40

Earlier quoted context omitted.

Author here 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 proportional…

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 prop…

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

Re: Why do we salt the ice when making ice cream?

#80

Post by lupire »

eyelidlessness wrote: post #74
topaz0 wrote: post #70

Earlier quoted context omitted.

"then it won't be ice anymore". Not so: the freezing point is the temperature at which ice and water are in equilibrium. If you take a bowl of ice to its freezing point it will still be entirely frozen. Then if you continue adding energy, it will remain at its freezing point while progressively more of it melts, i.e. going from 100% ice to 50% ice/50% water to 100% water. All at one temperature. In other words, if yo…

> and none of it goes to increasing the temperature You’re clearly more knowledgeable than I am, but this strikes me as probably wrong? The temperature at the surface has raised, which is how some of the surface ice melted. The temperature on average has to raise because thermodynamics. If I understand the article, the core temperature of the remaining ice can decrease and localities can decrease with them. But the e…

At melting, the energy is going into the state change.
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