Thursday, April 1, 2010

The “Coolest” Way to Make Ice Cream

What is the secret to delicious and quick homemade ice cream? Liquid nitrogen.

Don’t believe me? Stop by the Cambridge Public Library between noon and 4pm on Saturday April 24--the Science Carnival is hosting an event called “Liquid Nitrogen Ice Cream Making!” where YOU not only get to witness the amazing spectacle of making liquid nitrogen ice cream, but also get to consume the delicious final product.

It’s a pretty cool looking process. Here’s a photo from the first time I made liquid nitrogen ice cream:


Nitrogen is readily found in our atmosphere, but only in its gaseous state (incidentally, nitrogen gas makes up 70% of our atmosphere). Liquid nitrogen, on the other hands, does not occur naturally on Earth. Liquid nitrogen only exists under super-cold conditions. I’m talking -321°F cold, way colder than any place on Earth. By comparison, room temperature is around 70°F, and the coldest recorded air temperature on Earth was “only” -129°F (that honor went to Russia in 1983).

If liquid nitrogen hits any temperature above -321°F, it boils immediately into nitrogen gas. That’s why the ice cream looks like it is steaming in the above picture. Liquid nitrogen “steams” into gaseous nitrogen as it boils, just like how water steams into water vapor when it boils. Same concept and same process, except liquid nitrogen boils at a much lower temperature, and thus its “steam” is correspondingly much cooler. Colder things are denser than warm things, so while steam from water rises, the “steam” from liquid nitrogen sinks. (You can see this in the photo and at the festival!)

Due to liquid nitrogen’s coldness, you must handle it carefully with proper equipment, like gloves and specialized cold storage containers, and such matters will be properly taken care of at the Science Carnival.

But why use liquid nitrogen for making ice cream? It’s not necessary to have liquid nitrogen to make ice cream, but it certainly makes the task much easier (provided that you don’t have trouble acquiring liquid nitrogen).

Making ice cream sans liquid nitrogen is a slow process, one where you must churn the ice cream a lot while it is being cooled. This is the usual approach of ice cream making machines you find at factories and in home kitchens. Why the churning? For texture! We love ice cream not only for its flavor but also for its texture. Churning ice cream while it cools prevents it from solidifying into solid blocks--after all, eating rock-hard ice cream would be no fun. Churning also whips the ice cream, aerating it to the fluffy and smooth consistency we love. Like any recipe that involves a lot of aeration (ever tried making whipped cream or meringues, for instance?), this takes a while, but liquid nitrogen turns ice cream making into a snap.

The secret lies in the extreme coldness of liquid nitrogen. Boiling at about 400°F below room temperature, the transformation of nitrogen from liquid to gas form is incredibly violent. Think about a pot of boiling water on a stove. If you turn up the temperature on the stove, the water boils more violently. Same concept applies for liquid nitrogen (just at much cooler temperatures), thus liquid nitrogen boils with extreme ferocity: it fizzles and sizzles and immediately turns into vapor, like water splashed onto a very hot pan. This intense bubbling action serves as a whipping and aerating mechanism. All you need to do is create an ice cream base (a combination of milk, cream, sugar, and flavorings) and pour liquid nitrogen into the base while stirring, cutting down on the amount of work you need to do churning. Moreover, all of the liquid nitrogen evaporates, leaving you only with delicious ice cream.

Without a doubt, the “coolest” way to make ice cream is with liquid nitrogen.

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