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Why do we salt the ice when making ice cream?
- majikandy 4y 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 4y 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
- majikandy 4y 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 4y 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.
- lapetitejort 4y agoMy 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.
- simondotau 4y agoSurely there must be at least some flavours which could be compatible with carbonated ice cream. Cola ice cream, perhaps?
- iamkroot 4y 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.
- ljf 4y 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!
- tylerhou 4y 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.
- fuzzfactor 4y 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.
- AdamH12113 4y agoThis is indeed a great explanation, and all the better for being both clear and concise.
- wombatpm 4y agoIf you play around with methonal and ice, you can get even lower temperatures
- AngryData 4y 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 4y agoAlmost. For the salted cube, remember to pour the salt on your hand and then put the ice cube on top.
- quijoteuniv 4y agoI think I finally almost understood it! :)
- lend000 4y 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.
- majikandy 4y 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 4y 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 :)
- an1sotropy 4y 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 4y 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.
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- lesuorac 4y 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 4y agoA bit of alcohol would achieve that too right? Eg rum&raisin ice cream
- zwieback 4y 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.
- majikandy 4y agoYep I hear that, but also a frappucino without Xanthan gum is just a floaty slushy mess on sugary water.
- zwieback 4y agoAh, I had no idea that's what they put in there. Xanthan gum is pretty amazing
- henryfjordan 4y agoIt'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.
- nvr219 4y agoGreat post about ice from Rice.
- aaron695 4y ago
- ortusdux 4y 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_Browse_Laboratory_Analytics/Thermal_Values/melting-point-determination.html 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/ https://www.mcmaster.com/temperature-indicating-stickers/
- Taniwha 4y 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
- itronitron 4y agoThank you, your explanation is much clearer than the article.
- fsckboy 4y 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)
- CliffStoll 4y 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 4y 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 4y 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 4y 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.
- OJFord 4y 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'.)
- ndiddy 4y 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 4y 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.
- dmd 4y agoThe salt being discussed here does not end up in the ice cream.
- OJFord 4y agoNo, but 'alternative' uses of salt can.
- distortedsignal 4y 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-Minutes/dp/B07VYMWG4C https://www.amazon.com/Nostalgia-WICM4L-Electric-4-Quarts-Mi...
- steve_john 4y 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.
- Shadowed_ 4y agoI heard explanation few times before but yours is the most clear and simple of them all.
- nielsbot 4y agoWould this work better with calcium chloride instead?
- 0xbadcafebee 4y 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-4017627 https://www.thoughtco.com/how-cold-does-ice-get-with-salt-40...)
- ricardobeat 4y 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.
- bouncycastle 4y 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."
- refurb 4y 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).
- fortran77 4y 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.)
- shultays 4y 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 4y agoYou don't add the salt to the ice cream, you add it to the ice that surrounds the ice cream-making container.
- shultays 4y 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
- deleted 4y ago[deleted]
- gjs278 4y ago
- greenbit 4y agoBecause we can't get hold of dry ice to do a proper job of it? This seems to vary by state.