Why do we salt the ice when making ice cream? | Hacker News Reader

Why do we salt the ice when making ice cream?

timothyrice.org

298 pointstimrice106 comments

Testing for melting point depression is a common diagnostic method used in chemistry to check for impurities. Pure compounds melt at known temperatures, and typically do so within a narrow range (+/- 0.5 °C). Impurities almost always lower the melting point and widen this band. I spent a lot of my undergrad chem courses packing my products into capillary sized test-tubes and watching them slowly melt.

https://www.mt.com/us/en/home/applications/Application_Brows...

Some companies leverage this effect to make non-reversible temperature indicators that change color at specific temperatures.

https://www.mcmaster.com/temperature-indicating-stickers/

that's funny, i literally just grabbed a melting point apparatus out of storage to get rid of, because I have too much stuff laying around I probably won't use again

Boy did I hate staring at a mel-temp apparatus...

Surely part of the issue is that the ice at < 0C while the liquid portion is at 0C (because of the equilibrium thing) - but it's the liquid portion, not the ice, that's most physically connected to the inner container you're trying to freeze (this is the important point).

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)

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.

I understand Fahrenheit now.

I thought I did, but just thinking about what temperature is, i.e. the degree of hotness or coldness something is, so it relates to heat, which is the energy transferred from something hot to something cold, so it relates to energy, the more energy something has the hotter it is, the less energy the colder. So I am utterly confused now why wind cools or even can be cold, and wind chill should be a contradiction, and still stuffy air should be cold. Wind has more energy than still air, so wind should be hotter and still air cooler. Wind turbines deployed on a global scale should solve Global Warming even if no energy is generated, because they should slow down the energetic and hot wind, cooling the atmosphere and the planet with it.

The air around you is (usually) colder than your body. That means your body is transferring energy to the air. When the wind blows, convection causes you to transfer more energy to the air than you do when the wind doesn't blow. That increased transfer of heat away from you makes the wind seem colder/creates wind chill.

Unfortunately for this theory, brine freezes around -6f

You need something colder than brine to freeze brine. Fahrenheit didn't have that so the theory/story is still plausible.

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

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.

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.

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.

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

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.

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.

That's true of the surface of the ice, but the core is colder

For a little while, yes. Ice has middling thermal conductivity, it'll eventually homogenously warm to the melting point.

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 you are at the freezing point, then all of the energy you add goes to melting the ice (and none of it goes to increasing the temperature), until all of the ice is gone. That is the case even if you wait to true equilibrium, e.g. for all temperature gradients to go away.

> 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).

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

And the state change occurs when the portion of solid ice becoming liquid water warms however slightly, even if there’s a cooling side effect on the remaining ice as more energy is dissipated during that warming. Or am I still misunderstanding?

Yes. It is literally an isotherm. Energy is added without changing the temperature.

>> 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 [...] if you are at the freezing point, then all of the energy you add goes to melting the ice (and none of it goes to increasing the temperature), until all of the ice is gone.

> 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.

What do yo you think is wrong in the quoted passage?

The "temperature on average" of the system described is the temperature of every part of it: the temperature where the liquid and solid phases co-exist. When you heat that system (i.e. add energy) the temperature doesn't have to increase "because of thermodynamics". The ice melts and everything remains at the same temperature. Only when the ice is gone the water starts to warm up.

Right on. It’s the same for boiling, isn’t it? The water “can’t” get above 100 degrees C - added energy just goes into making steam. Right?

That's right (well... assuming constant pressure lower than the critical pressure.)

To be fair to GP, it does take some energy to heat the ice from freezer temp to 0C (or the new, depressed freezing point), part of which will come out of the ice cream. It's just that that amount of energy is very small compared to the other energies we're interested in here (as you pointed out elsewhere).

This effect should be negligible. The whole point here is that it takes way more energy to take a chunk of ice and turn it into water (at fixed temp -- namely the freezing point) than it does to heat that chunk of ice a few degrees (below the freezing point). And remember that you don't just have to cool the cream to its freezing point, you also have to remove enough energy to overcome its latent heat of fusion. If you were doing this just with the heat capacity of ice from like -20 to -5 C, you would need many times more ice than you could make ice cream. Like tens to hundreds of times. The blog discusses some related facts a bit near the end.

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)

Without lead you get tin whiskers. I wonder how the math works out in terms of what’s better for the environment if electronics break a lot more often…

A giant pile of scrap electronics is probably easier to deal with than a very large pile of scrap electronics that's leaching toxic heavy metals into the ground and runoff water.

Most electronics these days seem to get upgraded before tin whiskers can cause problems anyway.

Given the explanation above, this now makes a lot more sense: if the (non-eutectic) solder has cooled gradually, some the higher melting component will freeze first at the coolest location, and things will move around, leaving less of the "low-melting" components in the remaining molten solder. Once it's all solidified, some areas will have more tin and other areas will have more of the other elements. This will induce some weirdness, maybe the tin will want to diffuse to other parts where there is less (is this possible?) or maybe just there will be areas of relatively pure tin, perhaps more susceptible to whisker formation?

On the other hand an evenly-frozen mix of lead and tin (eutectic) wouldn't have these non-uniformities as there would be no driving force for it. One location freezing before the other wouldn't change the composition of the remaining melted solder.

I would also imagine the very quick solidification also prevents larger crystal formations. Crystal growth itself can also lead to separation of the metals and the extra few seconds or more of a half-liquid state solder could represent decades or more worth of crystal growth in ambient temperatures.

> It turns out, yes! What happens is that when the salt is added some of the ice melts – pulling heat from the system – until the temperature has reached the new, lower equilibrium point.

Correction, or addendum here: the actual dissolution of the salt is an endothermic process, so even if there was no ice, the temperature of water decreases when salt is dissolved.

Presumably that is less significant a drop than the equilibrium melting freezing point being 5 degrees lower as even if endothermic it won’t be much will it and will just return back up when you add the warmer mixture bowl?

Author here.

Hah, that's true, but I didn't want to mention it as it's not entirely in the aim of the essay :)

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 cream win that race by making a better heat sink.

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.

>Correction, or addendum here: the actual dissolution of the salt is an endothermic process, so even if there was no ice, the temperature of water decreases when salt is dissolved.

That's technically true, but it's a rather negligible amount.

Salt has an enthalpy of dissolution of +3.9kJ/mol (1) and a molar mass of 58.44g/mol (2), for roughly 67J/g.

For comparison, water=ice has an enthalpy of fusion of 334J/g (3), and you'll be adding at least three times more ice than salt (as max salt concentration is around 25% g/g (4) ). When you take this into account, it's a whole order of magnitude of difference, so for practical purposes you can outright ignore the heat being consumed by the dissolution of the salt.

Sources:

1. https://chem.libretexts.org/Bookshelves/Physical_and_Theoret...

2. https://en.wikipedia.org/wiki/Sodium_chloride

3. https://en.wikipedia.org/wiki/Enthalpy_of_fusion

4. https://en.wikipedia.org/wiki/Saline_water

Always heard about this but never tried it. Sounds like fun. Great description, very clear and much better than just saying it lowers the temperature! Nice writing.

Author here, thank you!

It's been bouncing around in the back of my brain for a long time.

I couldn't find any clear and concise explanations about what really happens when salt is added to ice, so I did some research and wrote it out myself :D

More importantly, how was the ice cream you made? Apparently liquid nitrogen ice cream makes smaller crystals or something and tastes better? That could be the sequel…

> Apparently liquid nitrogen ice cream makes smaller crystals or something and tastes better?

It's about how fast you freeze the ice cream so the crystals don't grow.

You can get N2 from a local gas supplier but a lot of grocery stores stock CO2 (Dry Ice) that can also be used for ice cream.

CO2 has the disadvantage of if you get it in the ice cream it makes it carbonated but the smoke looks like a witches cauldron so it looks cool imo.

My favorite use of dry ice is making Boo Bubbles:

1. Create a mixture of soap, water, and glycerin. This will make bubbles harder to pop.

2. Get a sealable container, poke a hole in the lid, the glue a tube in the hole.

3. Fill the container with water, then drop in pieces of dry ice. The CO2 should escape through the tube.

4. Stick the other end of the tube into the soap mixture.

5. Large bubbles will form with a cloudy gas of CO2.

6. Use towels to carry the bubbles, throw in the air, and combine with other bubbles.

I do this every Halloween. Kids get a kick out of it. A lucky few will stick around and learn about CO2, tensile strength, etc.

Surely there must be at least some flavours which could be compatible with carbonated ice cream. Cola ice cream, perhaps?

Delicious, of course!

And yes, the other commenter is correct. LNO2 works so well because it freezes the ice cream so fast that the crystals don't have time to grow very large, which produces a nice and smooth texture in the final product.

Thanks for this - I was watching a video of ice cream making with my son the other day, and the guy making the ice-cream said how it lowered the temp, and I totally didn't believe it was correct and started to explain my theory before realising I had no idea. Great to see it laid out so clearly!

Here is another way that I thought of it.

1. To make ice cream, we need to cool milk/cream below the freezing point of water (because milk/cream contains water).

2. To cool things, liquids have good thermal conductivity properties, so we would prefer to use a liquid.

3. We need some substance which is still a liquid at slightly below freezing.

4. It happens that salted water has this property and is relatively cheap.

Remember for point 4 it needs something in that salted water that can keep it at the lower temperature.

Also, we would like a phase transition slightly below freezing so that it's easier to keep a stable temperature.

When you do the math or work it at the bench, the difference is quite remarkable in the number of kilos of ice needed (per kilo of ice cream) when starting with 0C ice versus -20C ice.

Hint: start with rock salt at -20C also.

Edit: and the prepared cream premixed and chilled to 0C.

This is indeed a great explanation, and all the better for being both clear and concise.

If you play around with methonal and ice, you can get even lower temperatures

You can quickly experience it yourself by holding an ice cube in each hand but pouring salt on one of them. While both cubes contain approximately the same deficit of energy, the salted one with the lower equilibrium temperature will pull heat out of your hand much faster and feel much colder. I guess you could always put it in two cups too.

Almost. For the salted cube, remember to pour the salt on your hand and then put the ice cube on top.

I thought it was going to about adding salt to the ice cream but was not :/

I have a compressor so I have no use of a salted ice bath but I find that using salt in the mixture will make the ice cream not as hard when left overnight or longer in the freezer.

A bit of alcohol would achieve that too right? Eg rum&raisin ice cream

Right, alcohol, sugar and salt all change the hardness of the ice cream but you can only add so much until the flavor isn't what you want anymore. I don't think the ice cream texture is due to the melting point, though, that the post talks about, it's also whether large enough ice crystals can form.

It feels like cheating but adding stabilizers (gums, mostly) was really a game changer for our homemade ice cream.

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.

Hahaha I made the same mistake making a thc syrup. Ended up ‘one lump or two’!

Tiny amounts needed.

Which texture hacks have you played with besides xanthan gum?

In addition to the ones mentioned above we've also tried powdered milk (cribbed from Salt&Straw, a local favorite) and this thing called "Avacream Ice Cream Stabilizer". That product works great as well but not sure I can really tell the difference, maybe the xanthan/dried milk is a little less smooth.

OP author here.

Yes, though you have to be careful. If you add too much alcohol you'll prevent your mixture from properly freezing.

David Leibowitz, author of "The Perfect Scoop" recommends no more than 45ml of 80 proof liquor per 1 liter of ice cream mixture.

That matches my experience too 3 tbsp of vanilla/mint extract makes a much creamier result

So just under a double shot of 40% abv spirit here in the uk per litre. You’re right, not that much is it.

It's easier to think of a closed system and what the temperature of water vs. ice would be with just a phase change.

1 kg of ice turning into 1 kg of water requires 333,550 J.

1 kg of water require 4184 J to warm up 1 C.

So ignoring all the physical constraints, if you were to turn 1kg of ice "magically" into liquid water, keeping the total energy of the system the same, you'd end up with 1 kg of water at -80C (yes, I know I'm ignoring entropy).

Or put the simpler way: salt makes the icewater colder than 0C, and cream needs to be about 0F to freeze.

The ions from the salt get in the way of water molecules aligning to crystallize into ice. When salted ice melts, the water can't refreeze as readily because the saline isn't pure water anymore and because the freezing point is colder. As more ice melts, more heat is absorbed, bringing the temperature down even lower. (https://www.thoughtco.com/how-cold-does-ice-get-with-salt-40...)

Adding salt to an ice and water mix in a cooler is a known trick in hotter countries. Makes your beer cool down faster, and can get it down to -2C, the sweet spot for light Pilsner.

Creating a saltwater slush and packing this around our ice cream base allows us to cool the base enough so that it starts to thicken and freeze before the ice melts completely.

Great post about ice from Rice.

I heard explanation few times before but yours is the most clear and simple of them all.

I always wondered why not other things with lower freezing points, like alcohol/vodka? (Not to add in the ice-cream, but to immerse the container that the ice-cream mix is in.)

Spoiler: "Salt added to an ice / water slurry in an ice cream machine lowers the temperature of the mixture beyond the typical freezing point of water."

Would this work better with calcium chloride instead?

I think I finally almost understood it! :)

Because we can't get hold of dry ice to do a proper job of it? This seems to vary by state.

We add some salt to most deserts as well, to make them more tasty (I am no expert so I don't actually know how it makes them "more tasty"). I highly doubt the salt in ice cream is for melting point, and probably it is also for taste reasons. OP is probably overthinking this

You don't add the salt to the ice cream, you add it to the ice that surrounds the ice cream-making container.

Aha, sorry I should give it a proper read. Didn't notice it was talking about the ice

Also in the recipes I googled right now, salt is also an ingredient for icecream

Alternatively (edit - in the product itself): because it's delicious.

If you like 'salted caramel' ice cream, try sprinkling some salt on vanilla ice cream. (I bet you'll find it's the 'salted' you like more than the 'caramel'.)

Do you happen to know if MSG also works? I’m a big MSG fan, I would try this myself but I don’t have any vanilla ice cream at the moment

No, sorry, I've never used it (as an artificial/extracted additive I mean) - not opposed to it, I'm curious to experiment with it vs. 'natural'/more traditional ingredient sources.

I've seen ice cream served with parmesan crisp though, which is probably fairly salty too, but that's close.

The salt being discussed here does not end up in the ice cream.

No, but 'alternative' uses of salt can.

The article is specifically talking about salting the ice in a homemade ice cream maker like this[0]. The ice is used to reduce the temperature of the milk/sugar/etc. to "freeze" the ice cream. None of this salt gets into the ice cream.

I agree with you - people should try salting their ice cream. But the article is about a different part of the ice cream making process.

[0] https://www.amazon.com/Nostalgia-WICM4L-Electric-4-Quarts-Mi...

Yes, that's why I said 'alternatively'. I anticipated something more along the lines of reasons for salt in the product going in. TFA was much more interesting, frankly, just thought I'd offer the other use in comments.

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