Flash Freeze

Making ice cream is an example of a physical change, which occurs when a substance changes from one state of matter to another (see States of Matter). Traditional ice cream starts out as a liquid mixture of milk or cream, sugar, flavorings like vanilla, and emulsifiers. Emulsifiers are additives that keep the ice cream’s ingredients from separating and help maintain the creamy texture of the final product.

Normally, the liquid mixture is slowly chilled while being whipped to add in air bubbles. After about 15 minutes, the frozen mixture we know as ice cream is ready. During the slow freezing process, water molecules in the liquid form ice crystals that gradually grow larger and larger. That can result in “crunchy, gritty ice cream,” says Matt Hartings, a chemist who teaches the science of cooking at American University in Washington, D.C.

Making ice cream is an example of a physical change. That’s what happens when a substance changes from one state of matter to another (see States of Matter). Regular ice cream starts out as a liquid mixture. It contains milk or cream, sugar, flavorings like vanilla, and emulsifiers. Emulsifiers are added to keep the ingredients from separating. They also help the final product keep its creamy texture.

The liquid mixture is slowly whipped to add in air bubbles. And it’s chilled. After about 15 minutes, the mixture has frozen and the ice cream is ready. During the slow freezing process, water molecules in the liquid form ice crystals. The crystals grow larger and larger. That can result in “crunchy, gritty ice cream,” says Matt Hartings. He’s a chemist who teaches the science of cooking at American University in Washington, D.C.

Using liquid nitrogen to make ice cream causes the mixture to freeze so quickly that ice crystals don’t have time to grow very large. The result: extra-creamy ice cream. And since the ice cream is made to order and gets eaten right away, there’s no need to add emulsifiers. 

When liquid nitrogen is used to make ice cream, the mixture freezes very quickly. Ice crystals don’t have time to grow very large. The result is extra-creamy ice cream. And emulsifiers aren’t needed, because the ice cream is made to order and eaten right away.

Watching liquid-nitrogen ice cream being made is almost as fun as eating it. Behind the counter of the ice cream shop I visited, mist billowed from mixing bowls as the server poured in the liquid nitrogen.

When I visited the ice cream shop, I watched liquid-nitrogen ice cream being made. That was almost as fun as eating it. Behind the counter, the server poured in the liquid nitrogen. Mist flowed from the mixing bowls.

When the cold liquid nitrogen hits the warmer surrounding air and the cream mixture, it immediately boils away. But the mist I see isn’t nitrogen gas, which is invisible. It’s actually clouds of water droplets. They form as the liquid nitrogen and escaping nitrogen gas cool water vapor in the surrounding air, condensing it into tiny liquid droplets. The more humid the air is—that is, the more water it contains—the more mist forms.

Hartings of American University says liquid nitrogen is his favorite way to demonstrate how to make ice cream with his students. “It’s quick, it’s easy to make different flavors, and it looks awesome,” he says. “Using science to make great ice cream is one of the coolest things we can do.”

The cold liquid nitrogen boils away as soon as it hits the warmer cream mixture and air around it. But nitrogen gas is invisible. The mist I see isn’t nitrogen. It’s clouds of water droplets. The liquid nitrogen and nitrogen gas cool water vapor in the air around them. The water vapor condenses into tiny liquid droplets. If the air is more humid (holding more water), more mist forms.

Hartings of American University is a fan of liquid nitrogen. It’s his favorite way to show his students how to make ice cream. “It’s quick, it’s easy to make different flavors, and it looks awesome,” he says. “Using science to make great ice cream is one of the coolest things we can do.”