[HN Gopher] Why do we salt the ice when making ice cream?
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Why do we salt the ice when making ice cream?
Author : iamkroot
Score : 114 points
Date : 2022-09-21 18:23 UTC (4 hours ago)
(HTM) web link (www.timothyrice.org)
(TXT) w3m dump (www.timothyrice.org)
| ortusdux wrote:
| 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 degC). 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/
| rootw0rm wrote:
| 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
| steve_john wrote:
| 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.
| nvr219 wrote:
| Great post about ice from Rice.
| nielsbot wrote:
| Would this work better with calcium chloride instead?
| quijoteuniv wrote:
| I think I finally almost understood it! :)
| OJFord wrote:
| 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'.)
| ndiddy wrote:
| 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
| OJFord wrote:
| 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.
| dmd wrote:
| The salt being discussed here does not end up in the ice cream.
| OJFord wrote:
| No, but 'alternative' uses of salt can.
| distortedsignal wrote:
| 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...
| OJFord wrote:
| 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.
| lend000 wrote:
| > 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 wrote:
| >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
| iamkroot wrote:
| 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 :)
| an1sotropy wrote:
| 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.
| Karliss wrote:
| 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.
| an1sotropy wrote:
| 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.
| majikandy wrote:
| 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?
| CliffStoll wrote:
| 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)
| mensetmanusman wrote:
| 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...
| lesuorac wrote:
| 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.
| majikandy wrote:
| A bit of alcohol would achieve that too right? Eg rum&raisin
| ice cream
| zwieback wrote:
| 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.
| majikandy wrote:
| Yep I hear that, but also a frappucino without Xanthan gum
| is just a floaty slushy mess on sugary water.
| zwieback wrote:
| Ah, I had no idea that's what they put in there. Xanthan
| gum is pretty amazing
| henryfjordan wrote:
| 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.
| iamkroot wrote:
| 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.
| ender341341 wrote:
| That matches my experience too 3 tbsp of vanilla/mint
| extract makes a much creamier result
| majikandy wrote:
| So just under a double shot of 40% abv spirit here in the
| uk per litre. You're right, not that much is it.
| Shadowed_ wrote:
| I heard explanation few times before but yours is the most clear
| and simple of them all.
| Taniwha wrote:
| 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
| topaz0 wrote:
| 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.
| iamkroot wrote:
| 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.
| Taniwha wrote:
| That's true of the surface of the ice, but the core is colder
| iamkroot wrote:
| For a little while, yes. Ice has middling thermal
| conductivity, it'll eventually homogenously warm to the
| melting point.
| topaz0 wrote:
| 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).
| fsckboy wrote:
| 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)
| dcow wrote:
| I understand Fahrenheit now.
| Taniwha wrote:
| 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
| itronitron wrote:
| Thank you, your explanation is much clearer than the article.
| foobarian wrote:
| 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.
| floren wrote:
| 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).
| martyvis wrote:
| 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.
| annoyingnoob wrote:
| You could use propylene glycol that is Generally Recognized
| As Safe.
| foobarian wrote:
| 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.
| bobthepanda wrote:
| 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)
| alliao wrote:
| antifreeze is really really really toxic for mammals so
| there's that, while ingesting bit of salt never hurt anyone..
| maybe diabetics
| iamkroot wrote:
| 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.
| aaron695 wrote:
| majikandy wrote:
| 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.
| AdamH12113 wrote:
| This is indeed a great explanation, and all the better for
| being both clear and concise.
| wombatpm wrote:
| If you play around with methonal and ice, you can get even
| lower temperatures
| iamkroot wrote:
| 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
| majikandy wrote:
| 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...
| iamkroot wrote:
| 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.
| lesuorac wrote:
| > 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 wrote:
| 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.
| tylerhou wrote:
| 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.
| ljf wrote:
| 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!
| aaron695 wrote:
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