Readit NewsNow 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.
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.
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
Naturally there's some small local variations, but if you let the system come up to steady state, that's what will occur.
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.
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.
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.
Hah, that's true, but I didn't want to mention it as it's not entirely in the aim of the essay :)
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
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?