Tuesday, June 24, 2014

The Science of Ice Cream

Even though it's been a warm couple of months already, it's officially summer. A delicious, science-filled way to beat the heat? Making homemade ice cream.

Image Credit: St0rmz via Flickr
Over at Physics@Home there's an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?

Does the milk you use make a difference? 

Most ice cream recipes call for heavy cream, whole milk, or half and half. Does it matter if you use skim milk instead of whole milk? Yes.

There are several different kinds of milk that you see at the grocery store. The defining difference is the amount of fat. Of the ‘regular’ types of milk, whole milk has the highest fat content (3.25%) with skim milk containing almost no fat. Half and half is a mixture of cream and milk, and heavy whipping cream has the most fat. Buttermilk is usually a low fat milk with a (completely safe) bacterial culture added. So if you have that around, you could use that as a substitute. The taste is pretty different, but there are plenty of buttermilk ice cream recipes out there, too.

When making ice cream, the higher fat content in the milk allows for a better texture in your finished dessert. Most milk you buy in a grocery store is homogenized, which means that the fat molecules are already distributed evenly with the other liquids. Fats and water don't naturally mix together smoothly, they normally want to separate. Homogenized milk has already taken care of the problem, and the fat molecules are already safely suspended in clusters among the other liquids.
(A) Two types of liquids like oil & water (B) The liquids start to mix together (C) An emulsion forms, allowing for even distribution like oil molecules in water. Image Credit: Modified from Wikipedia
When you mix the ice cream base together, the fat clusters start to break apart. As you mix, you also introduce air, and the recently disturbed fat clusters keep the air pockets in place. When the air pockets are more stable, you get a smoother ice cream. It’s part of why low-fat/fat-free ice cream in the store has such a different feel in your mouth than the expensive, high-fat kind.

Can you skip cooling the ice cream mix before freezing? 

This also has to do with making a better texture ice cream. For a creamy, smooth ice cream you want to have lots of small, evenly dispersed ice crystals. If the base is room temperature before you put it in your ice cream machine, it takes a long time for the ice cream to freeze, and you get large ice crystals. But if you refrigerate the ice cream base so that it’s cold from the get go, it will freeze quickly and you will get small ice crystals. Even though you may want to skip this step and get to ice cream as soon as possible, it’s worth it to chill your ice cream base before you make it.

This is yet another reason why liquid nitrogen ice cream tastes so great. To make it you mix up a regular ice cream base and just add liquid nitrogen. It freezes the ice cream incredibly fast, but it is possible to over-freeze the base and end up with slightly crunchy ice cream.

Making liquid nitrogen ice cream. Image Credit: H. Michael Miley via Flickr

Why do so many recipes call for salt on the ice? 

If you’re making your ice cream with a crank machine or a bag, almost every recipe calls for ice and rock salt. It's for the same reason cold parts of the country spread salt on wet roads when it's close to freezing. Adding salt lowers the freezing temperature of the water and for wintery roads, it means that the water won’t freeze as easily.

For our ice cream, it allows the temperature of the mixture around the ice cream to get colder. Since the ice cream isn’t just water, it needs to be a little below 32°F to freeze. When using a crank ice cream machine, you frequently have to add more salt and ice, so you can watch the process of melting ice more than once. (It's like watching paint dry, only there's ice cream at the end!)

An electric ice cream maker that rotates a container holding the liquid ice cream in an ice/salt mixture.
Image Credit: Wikipedia
A lot of recipes call for rock salt (including our Physics@Home one), but rock salt isn’t the same thing as table salt. Rock salt isn’t processed as much as table salt, and is much chunkier. It also contains a lot of impurities, which is why it isn’t used for eating. Could you use table salt to lower the the melting temperature of ice? Sure. But there's another reason to use rock salt: it's usually cheaper.

So the next time you go to make homemade ice cream, hopefully you have a sense of what you can swap out, and what steps you really should follow.


  1. But why do we get ice cream headaches?!?!?!?

    I really want to know. - Buzz

    1. When you eat something cold, it causes the blood vessels in your head to get larger. It may cause the nerve center above the roof of your mouth to over-react and try and 'heat' your brain.

    2. I thought the blood in the blood vessels cools while its passing up towards your brain and so we are acutally cooling our blood to our brain. IDK.. Though, its hard to know what is true anymore about anything. lol. it sucks. but at least were thinking.

  2. One problem I always have with this...it's a hot summer day, the coldest thing in the whole system is fresh water ice at 0 C. Although the brine stays liquid at 0 C, as soon as it drops below 0 C, it is now colder than the fresh water ice and thus can no longer melt it. The molecules in the brine are moving more slowly than those in the ice, so the ice no longer slows them down and thus the temperature should not drop further. I know this is wrong in practice but I am clearly missing something. I asked my college physics professor this and he could not give me a satisfactory reply.

    1. ice is in solid form. Therefore, its particles are actually moving more slowly (still locked in lattice position) than those in the liquid brine (sliding past). Any thermal energy absorbed from the cream will first be used to raise the temperature of the brine (which requires more energy to accomplish because of the solute). Once the average kinetic energy of the molecules (aka: temperature) in the brine has increased enough, then they will begin to transfer energy to the ice, and it will again begin to melt. The temperature of the brine will not increase above the melting point until all of the ice has melted (all energy is used to accomplish phase change before increasing temperature)

  3. I am doing a science experiment with homemade ice cream and I found this site quite helpful. I will probably use your easy recipe that you had a link for.

  4. I still dont get it. :( Given the fact ice(snow) melts faster with salt. And ice basically melts slower with ice on it when making ice cream is a total non sense to me.
    So basically, if I put salt on an ice cube it melts faster than the other one without salt. Correct? Then on the other side, how come when making ice cream with the same procedure, when I put salt on ice its temperature gets lower so it will melt slower. So wheres the sense? oh please, i getting frustrated i really dont get it.

    1. Yes, Water is a very strange thing. so the misconception here is that the salt affects the average temperature of the ice ether making it hotter or colder depending on who you ask. it really doesn't. Liquid water conducts the transfer of heat very well, but in its frozen form it cannot transfer heat nearly as efficiently. in fact it acts more like an insulator than a conductor. in order to make ice cream you need the water to be cold enough to freeze (it must be below 32 degrees) but it can't be frozen or it won't be able to transfer the heat out of your ingredients fast enough to freeze it. the salt allows this to happen simply by allowing water to be in its liquid form at temperatures much lower than 32 degrees. so the ice DOES melt, but it stays the same temperature it was when it was frozen with the added benefit of being able to conduct heat much better.

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  5. Snow doesn't melt faster put an ice cube with salt and one without and see what happens,

  6. I always see the cream/whole milk mixture in ingredients. Can one make ice cream using only heavy cream?

  7. Couldn't you just say that the ice cube with the salt melts because with the salt it can't stay in it's solid frozen form because it now needs to be colder than what it was?