Why do we salt the ice when making ice cream?
298 points by timrice 2 years ago | 106 comments- ortusdux 2 years agoTesting 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.
- Taniwha 2 years agoSurely 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
- fsckboy 2 years agoit'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)
- dcow 2 years agoI understand Fahrenheit now.
- Maursault 2 years agoI 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.
- kimixa 2 years agoUnfortunately for this theory, brine freezes around -6f
- Maursault 2 years ago
- Taniwha 2 years agoThat'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
- fsckboy 2 years agoyes 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.
- fsckboy 2 years ago
- dcow 2 years ago
- iamkroot 2 years agoThe 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.
- topaz0 2 years agoTo 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).
- Taniwha 2 years agoThat's true of the surface of the ice, but the core is colder
- iamkroot 2 years agoFor a little while, yes. Ice has middling thermal conductivity, it'll eventually homogenously warm to the melting point.
- iamkroot 2 years ago
- topaz0 2 years ago
- topaz0 2 years agoThis 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.
- foobarian 2 years agoThis 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.
- timrice 2 years agoAuthor 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.
- hammock 2 years agoThis 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
- nkurz 2 years agoRemember, 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?
- aaron695 2 years ago
- hammock 2 years ago
- floren 2 years agoSalt'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).
- foobarian 2 years agoI 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.
- annoyingnoob 2 years agoYou could use propylene glycol that is Generally Recognized As Safe.
- martyvis 2 years agoWhat'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.
- foobarian 2 years ago
- alliao 2 years agoantifreeze is really really really toxic for mammals so there's that, while ingesting bit of salt never hurt anyone.. maybe diabetics
- dpflug 2 years agoI think the ice cream would be a bigger concern for diabetics.
- dpflug 2 years ago
- timrice 2 years ago
- itronitron 2 years agoThank you, your explanation is much clearer than the article.
- fsckboy 2 years ago
- CliffStoll 2 years agoNow 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)
- mensetmanusman 2 years agoWithout 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…
- marshray 2 years agoA 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.
- HPsquared 2 years agoGiven 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.
- AngryData 2 years agoI 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.
- AngryData 2 years ago
- marshray 2 years ago
- mensetmanusman 2 years ago
- lend000 2 years ago> 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.
- lvxferre 2 years ago>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
- an1sotropy 2 years agoBut 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.
- Karliss 2 years agoIf 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.
- an1sotropy 2 years agoIf 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.
- an1sotropy 2 years ago
- Karliss 2 years ago
- majikandy 2 years agoPresumably 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?
- iamkroot 2 years agoAuthor here.
Hah, that's true, but I didn't want to mention it as it's not entirely in the aim of the essay :)
- lvxferre 2 years ago
- majikandy 2 years agoAlways 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.
- timrice 2 years agoAuthor 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
- tylerhou 2 years agoHere 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.
- majikandy 2 years agoRemember for point 4 it needs something in that salted water that can keep it at the lower temperature.
- majikandy 2 years ago
- ljf 2 years agoThanks 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!
- majikandy 2 years agoMore 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…
- lesuorac 2 years ago> 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.
- iamkroot 2 years agoDelicious, 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.
- lesuorac 2 years ago
- fuzzfactor 2 years agoWhen 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.
- tylerhou 2 years ago
- AngryData 2 years agoYou 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.
- DIARRHEA_xd 2 years agoAlmost. For the salted cube, remember to pour the salt on your hand and then put the ice cube on top.
- DIARRHEA_xd 2 years ago
- AdamH12113 2 years agoThis is indeed a great explanation, and all the better for being both clear and concise.
- wombatpm 2 years agoIf you play around with methonal and ice, you can get even lower temperatures
- timrice 2 years ago
- lesuorac 2 years agoI 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.
- majikandy 2 years agoA bit of alcohol would achieve that too right? Eg rum&raisin ice cream
- iamkroot 2 years agoOP 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.
- majikandy 2 years agoSo just under a double shot of 40% abv spirit here in the uk per litre. You’re right, not that much is it.
- ender341341 2 years agoThat matches my experience too 3 tbsp of vanilla/mint extract makes a much creamier result
- majikandy 2 years ago
- zwieback 2 years agoRight, 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.
- iamkroot 2 years ago
- majikandy 2 years ago
- refurb 2 years agoIt'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).
- 0xbadcafebee 2 years agoOr 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...)
- ricardobeat 2 years agoAdding 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.
- steve_john 2 years agoCreating 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.
- nvr219 2 years agoGreat post about ice from Rice.
- Shadowed_ 2 years agoI heard explanation few times before but yours is the most clear and simple of them all.
- fortran77 2 years agoI 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.)
- bouncycastle 2 years agoSpoiler: "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."
- nielsbot 2 years agoWould this work better with calcium chloride instead?
- quijoteuniv 2 years agoI think I finally almost understood it! :)
- greenbit 2 years agoBecause we can't get hold of dry ice to do a proper job of it? This seems to vary by state.
- shultays 2 years agoWe 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
- ohwaitnvm 2 years agoYou don't add the salt to the ice cream, you add it to the ice that surrounds the ice cream-making container.
- shultays 2 years agoAha, 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
- shultays 2 years ago
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- gjs278 2 years ago
- ohwaitnvm 2 years ago
- OJFord 2 years agoAlternatively (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'.)
- dmd 2 years agoThe salt being discussed here does not end up in the ice cream.
- OJFord 2 years agoNo, but 'alternative' uses of salt can.
- OJFord 2 years ago
- distortedsignal 2 years agoThe 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...
- OJFord 2 years agoYes, 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.
- OJFord 2 years ago
- ndiddy 2 years agoDo 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
- OJFord 2 years agoNo, 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.
- OJFord 2 years ago
- dmd 2 years ago
- aaron695 2 years ago