The Cold Hard Science Behind Ice Cream

People in the United States love ice cream. Each American eats about 20 pounds (9kg) of it every year! Even George Washington served it to his guests. Ice cream has three main ingredients: Milk, cream, and sugar. How do three simple things become a tasty frozen treat?

It all starts with atoms. Atoms are the tiny building blocks that form everything around us. When two or more atoms are stuck together it is called a molecule. An object's atoms and molecules are always moving. (Atoms are much too small to see, so we don’t notice the movement.) The hotter something is, the faster its molecules jiggle around. Take water, for example. When water molecules move their fastest, they make steam. Water molecules with less energy form the liquid we call water. Take away more energy, and the water molecules freeze into a solid. That’s where ice cream begins.

Milk and cream both contain water. The water inside them is what freezes to make ice cream. How? Ice cream factories put the ingredients into a big machine and surround them with cold. The cold slows down the molecules in the mixture’s water. Ice crystals form.

Fats in the milk and cream keep the ice crystals from sticking together. The sugar in ice cream thickens some of the water to slow down the freezing process. That’s partly why it’s ice cream and not an ice cube. But freezing ice cream as it sits makes large, rough ice crystals. Making smooth ice cream takes both mixing and air.

An ice cream machine turns and mixes the ice cream as it freezes. Moving water doesn’t have time to form large ice crystals as it freezes, so it makes small ones. Mixing also adds air. The air makes the mixture light and fluffy. Together, small ice crystals and air create smooth, creamy ice cream. (Ice cream makers add other thickeners to help the ice cream stay smooth. And of course they also add plenty of flavors.)

Ice cream doesn’t refreeze well because of its ice crystals. They lose the air that was frozen into them as they melt. Unless new air is mixed in, ice cream refreezes with larger crystals. That’s why the melted ice cream you put back into the freezer comes out grainy and rough.

So, what’s the difference between ice cream, soft serve, and frozen custard? In the United States, ice cream is required to have at least 10% milk fat. (Ice milk only has to have 2.5% milk fat. It’s often sold as low-fat ice cream.) Soft serve is made with more air and less fat than regular ice cream. It’s also served at a warmer temperature to keep it soft. Frozen custard has egg yolks in it as well as less air. Together they make frozen custard thicker than ice cream.

Whether you love ice cream, soft serve, or frozen custard, they all start the same way — with some yummy ingredients and a lot of science. What’s your favorite kind?