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STANDARDS

NGSS: Core Idea: PS1.A    

CCSS: Literacy in Science: 3    

TEKS: 6.2E, 7.2E, 8.2E, C.2I, I.7A

Flash Freeze

Science World editor Jennifer Barone investigates the cool chemistry behind super-smooth ice cream

ANNE-MARIE JACKSON/TORONTO STAR VIA GETTY IMAGES

SPEEDY SWEETS: A server in a shop in Toronto, Canada, pours liquid nitrogen into a mixer, freezing the ice cream ingredients in seconds.

ESSENTIAL QUESTION: What role does science play in creating some of our favorite foods?

On a recent trip to Moab, Utah, I stopped into the Moab Garage ice cream shop and ordered a cone of chocolate chip cookie dough. My server didn’t scoop my dessert out of a freezer. Instead she put sweetened cream and chunks of cookie dough in a bowl attached to an electric mixer. As it whirled, she added a surprising ingredient: liquid nitrogen (N2).

Nitrogen (N) is an element that normally exists on Earth as a gas. In fact, nitrogen gas—containing molecules of two nitrogen atoms bonded together—makes up about 78 percent of our atmosphere. But when cooled below its condensation point, -195.8ºC (-320.4ºF), nitrogen gas turns into a liquid. When the server poured this ultracold liquid into the mixer, the concoction inside froze almost instantly. That rapid cooling created ice cream that was incredibly smooth.

A growing number of ice cream shops are whipping up super-creamy frozen treats in a flash using liquid nitrogen. Science World investigates how this element works its mouth-watering magic.

Not long ago, I took a trip to Moab, Utah. At an ice cream shop called the Moab Garage, I ordered a cone of chocolate chip cookie dough. My server didn’t scoop my dessert out of a tub in a freezer case. Instead, she put sweetened cream and chunks of cookie dough into a bowl. The bowl was attached to an electric mixer. As it spun, she added one last ingredient. It was liquid nitrogen (N2).

Nitrogen (N) is an element, and on Earth it’s usually a gas. In fact, nitrogen gas makes up about 78 percent of our atmosphere. The gas contains molecules of two nitrogen atoms bonded together. But nitrogen gas can turn into a liquid. That happens when it’s cooled below its condensation point, -195.8°C (-320.4°F). The server poured this extra-cold liquid into the mixer, and the ingredients inside froze almost instantly. The result of that fast cooling? Extremely smooth ice cream.

More and more ice cream shops are using liquid nitrogen. Now they can make super-creamy frozen treats in a flash. Science World looks at how this element works its mouth-watering magic.

SUPER CHILL

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.

FROZEN TREAT

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.

DAVID ALLEN/INLAND VALLEY DAILY BULLETIN/SCNG

LIQUID COLD: A massive tank holds liquid nitrogen at an ice cream shop in California.

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.”

ANNE-MARIE JACKSON/TORONTO STAR VIA GETTY IMAGES

CORE QUESTION: Describe three physical changes that take place as a server mixes up a batch of ice cream using liquid nitrogen.

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