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Why do we salt the ice when making ice cream?

timothyrice.org

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

#31

Post by fsckboy »

Taniwha wrote: post #27

Surely part of the issue is that the ice at If you add ice you reduce the equilibrium temp and as a result the < 0C ice temp can be passed to the liquid phase and as a result on to the inner con tain er where you're making the ice cream

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 because it was the coldest thing Dr. Farenheit knew how to produce in the lab)

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

#32

Post by itronitron »

Taniwha wrote: post #27

Surely part of the issue is that the ice at If you add ice you reduce the equilibrium temp and as a result the < 0C ice temp can be passed to the liquid phase and as a result on to the inner con tain er where you're making the ice cream

Thank you, your explanation is much clearer than the article.

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

#33

Post by foobarian »

Taniwha wrote: post #27

Surely part of the issue is that the ice at If you add ice you reduce the equilibrium temp and as a result the < 0C ice temp can be passed to the liquid phase and as a result on to the inner con tain er where you're making the ice cream

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.

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

#34

Post by henryfjordan »

zwieback wrote: post #22
majikandy wrote: post #19

Earlier quoted context omitted.

Yep I hear that, but also a frappucino without Xanthan gum is just a floaty slushy mess on sugary water.

Ah, I had no idea that's what they put in there. Xanthan gum is pretty amazing

It's amazing how potent that stuff is. You only need 1/8th of a teaspoon of the powder per drink. I tried using a whole teaspoon once but it was so thick that I couldn't get my coffee through a straw.

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

#35

Post by floren »

foobarian wrote: post #33
Taniwha wrote: post #27

Surely part of the issue is that the ice at If you add ice you reduce the equilibrium temp and as a result the < 0C ice temp can be passed to the liquid phase and as a result on to the inner con tain er where you're making the ice cream

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…

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).

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

#36

Post by an1sotropy »

Karliss wrote: post #29
an1sotropy wrote: post #18

Earlier quoted context omitted.

But which has a greater effect on removing heat from the cream-containing vessel: the decrease in temperature from the dissolution of salt, or the more efficient thermal coupling to the vessel provided by the salt/ice slurry (versus the original solid ice chunks)? The goal is to remove heat from the cream faster than the system as a whole warms up due to room temperature. I thought the value of salt was to help the c…

If the primary benefit of adding salt was improving thermal coupling through liquid by melting some of ice then you could achieve the same effect by adding some tap water. Which in my opinion would be a lot simpler and less messy than getting salt involved. Some energy would be lost to cool down tap water, but as mentioned in the article phase transition takes a lot more energy than changing temperature of water.

If you had really cold ice cubes, already tightly packed, then the water you add would freeze, making a solid ice sheath around the cream-containing vessel, and yes, that would work great.

But with too much space around the ice cubes, or ice cubes that aren't cold enough, adding water will just give you more cold (but not freezing) water.

I think people have converged on adding salt to ice because it's so forgiving (for a variety of ice cube temperatures and geometries), and the salt itself doesn't appreciably heat anything (unlike your added water). Other comments here quantify this better than I can.

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

#37

Post by foobarian »

floren wrote: post #35
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…

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.

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

#38

Post by iamkroot »

Taniwha wrote: post #27

Surely part of the issue is that the ice at If you add ice you reduce the equilibrium temp and as a result the < 0C ice temp can be passed to the liquid phase and as a result on to the inner con tain er where you're making the ice cream

The ice actually comes up to the temperature of the water while it's melting. That's what the equilibrium temperature is: the temperature of the entire ice / water system until it's been converted to all liquid or all solid.

Naturally there's some small local variations, but if you let the system come up to steady state, that's what will occur.

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

#39

Post by CliffStoll »

Now check out Eutectic mixtures ... old-timers may remember soldering with 63-37 tin/lead solder.

The reason? With any other mixture of lead/tin, the liquid solder freezes over a temperature range, often resulting in what very-old-timers called a "cold solder joint". For example, 50-50 tin/lead mixture starts melting at 183C and is fully melted at 214C.

Using Eutectic Solder, the phase transition happens at exacctly 183 C ... the lump is solid at 182C and liquid at 184C.

Geologists take advantage of this: when non-eutectic mixtures of lava freeze (say, a basalt flow in Hawaii or on the moon), different minerals will be found in the rocks. Analyzing the minerals, and assuming equilibrium, you can understand temperatures and pressures in the origination magma.

(ps - yep, new ROHS rules have largely eliminated lead based solder)

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

#40

Post by timrice »

foobarian wrote: post #33
Taniwha wrote: post #27

Surely part of the issue is that the ice at If you add ice you reduce the equilibrium temp and as a result the < 0C ice temp can be passed to the liquid phase and as a result on to the inner con tain er where you're making the ice cream

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 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.

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