Ask Dr. Universe: Why do we keep things like ice cream and popsicles in the freezer?

Washington State University

Dr Universe: Why do we need to keep things like ice cream and popsicles in the freezer? –Asia, 9 years old, Seattle

Dear Asia,

You may have noticed that ice cream and popsicles melt when left out of the freezer for too long. To find out exactly why this is happening, I went to the Washington State University Creamery.

My friend John Haugen, the manager of the creamery, was happy to answer your question. He said a lot of the answer has to do with what’s called matter. All things in our universe are made of matter, even ice cream and popsicles.

Matter is made up of tiny particles called atoms. There are also three main states of matter: solid, liquid and gas. Temperature is one thing that has a great effect on the state of matter of a frozen treat.

Haugen reminded me that ice cream starts out like liquid milk. At the WSU dairy, workers add a few different ingredients to the milk, including fats and sugars. They keep the liquid ice cream mixture at exactly 40 degrees, which is about as cold as the inside of your refrigerator.

At this temperature, all the atoms that make up the liquid mixture are able to spread out and move. They can travel freely in their container. But when the mixture goes through a cooling process, things start to change.

First, the creamery workers put the liquid into a machine with a blade that agitates the ice cream mixture to help it freeze into ice cream as it moves through a tube. The ice cream mixture comes out much like a soft-serve. It’s thicker than a liquid, but not quite solid.

Then this ice cream goes in a -20 degree freezer. In these very cold conditions, atoms slow down a lot. They go into some sort of organized or ordered pattern, and they don’t move as much as they do in a liquid state.

Ice cream that’s in the -20 degree freezer becomes way too solid for anyone to pick it up. It must go in a regular freezer at 0 degrees for a whole day before it is ready to serve.

If you eat ice cream in hot weather, the atoms start to absorb some of this heat energy. The energy causes atoms to start moving faster again. The solid turns liquid and you might end up with a mess melting on your hands.

Another reason we keep ice creams and popsicles in the freezer has to do with food safety. If the ice cream has been taken out of the freezer for too long, it could invite bacteria to eat it. These bacteria could potentially make us sick. Keeping food at the right temperature is important for our health.

The next time you go out for an ice cream, or maybe even make it yourself at home, think about all the atoms that make up your frozen treat. Now that’s soft science!

Truly,

Dr Universe

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