The Science of Ice Cream: Part One

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.

(We’ve since updated this article to include the science behind vegan ice cream. To learn more about ice cream science, check out The Science of Ice Cream, Redux)

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.

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