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We here at Popular Mechanics respect the creative use of liquid nitrogen. We also know that at a chilly minus 320 F, liquid nitrogen will shatter glass, crack metal and explosively freeze anything containing water molecules. In other words, it's not a substance you can use to keep your beer cold. So when I recently came across a video of Sub Zero, a Utah scoop shop that freezes its ice cream using liquid nitrogen, I was intrigued. Place ingredients in bowl, add liquefied gas, and produce foggy special effects as well as tasty summer treat—what could be more sensible?

But as a responsible journalist, I couldn't resist investigating further using the skills I learned at ice cream school. Yes, my love for both science and full-fat frozen dairy products runs so deep that I once went to Penn State for Ice Cream 101, a weekend course packed with classes such as "Frozen Dessert Composition," "Flavors and Inclusions," and (an easy favorite) "Sensory Evaluation." But nowhere in the fat three-ring binder that I still keep within easy scooping distance of my desk could I find a tab for "Bad-Ass Freezing Methods." So I called up a few instructors and did my best to extrapolate as to whether or not freezing ice cream in liquid nitrogen is, culinarily speaking, a good idea.

First, a quick primer on how ice cream is typically made: An ice cream mix—which generally includes dairy products, sweeteners, flavors and, often, stabilizers and emulsifiers to improve the texture—is put in a batch freezer (which relies on mechanical refrigeration) or a continuous freezer (which uses a refrigerant like ammonia). The temperature of the mix drops to about 22 F, during which time about half of the water freezes. Air is forced into the mix through either mechanical means or high pressure, fat destabilizes and clings to the air cells forming a foam, and the sugars, salts and fat proteins become concentrated in the unfrozen water. The still-soft ice cream is then extracted from the machine, at which point mix-ins are added and it is packaged and placed in a hardening room to freeze further.

This is what happens when you pour minus 320 F liquid nitrogen into an ice cream mix: It freezes, instantaneously.



The faster you freeze an ice cream mix, the smaller and more copious the ice crystals—and the smoother the product. If you have ever eaten ice cream that's been around for a while, you've probably noticed that it tastes noticeably chunky with ice. That's because the ice crystals migrated together as the temperature of the storage freezer cycled up a few degrees and then refroze in clumps when the temperature dropped again.

But there are other factors that affect the quality of ice cream. When you agitate ice cream as it's freezing (and one has to agitate the mix for it to be legally considered ice cream), a certain amount of air, called overrun, gets whipped into the final product. As the fat molecules destabilize they structure themselves around the air cells and—along with the remaining unfrozen water—make the ice cream taste velvety. With liquid nitrogen, you're getting a different kind of agitation—a few quick hand stirs and the effect of the liquid nitrogen bubbling off as it vaporizes—so there's less, and less consistent, overrun.

And while super cooling ice cream may be good for ice formation in some respects, it can turn otherwise harmless bits of fruit and candy into rock-hard dental hazards. Ice cream makers typically stew fruit in sugar to lower the freezing point—strawberries are 91 percent water and blueberries, 80 percent—and add it to the ice cream as it comes out of the freezer. That becomes irrelevant when the fruit starts out in the mix and is then brought down to a temperature closer to that of liquid nitrogen. If you're storing the ice cream at minus 20 F the fruit will eventually equilibrate—but proceed carefully if you try to delve in as soon as the fog of condensation clears.

People who want to add more than a splash of liquor to their ice cream, on the other hand, should be delighted (the freezing point of 100-proof vodka is minus 40.78 F). But if you bring the ice cream up to a temperature that's comfortable to eat, it'll be too soft.

If there's one lesson I took home with my ice cream school certificate (which, yes, I consider an advanced degree), it's that the perfect ice cream is a balancing act. Too much sugar over-depresses the freezing point and the ice cream gets sloppy; too much fat and it tastes greasy, too little and it won't freeze; too much air makes the finished product frothy. The ideal ice cream depends in large part on the mix—but it's hard to control for any other factors when the freezing process occurs in less time than it takes to say "Baskin Robbins." Then again, that's what makes it a lot more fun. We'll be making our own liquid nitrogen ice cream here in the next few weeks—replete with the appropriate scientific sampling of mix-ins—and we'll report back on whether it's up to snuff.

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