Hi, I’m Asad. You may remember me from such comments as “Unlike Jeffrey Steingarten, I know how to hold a spoon” and “This looks…NOM” as well as from Z’s link to my own blog about cheeseburgers. Now, I don’t just eat pre-made cheeseburgers and write about them. Sometimes I make my own food, and sometimes the food I make is an unholy-but-delicious intersection of my work life as a physicist and my food life as…a physicist. Such is the case with liquid nitrogen ice cream.

Liquid nitrogen is cold – very cold. Most everyone knows that steam, if you can cool it down, will condense into liquid water at 212° Fahrenheit (i.e. the opposite of boiling), and, if you can extract more energy from it, will freeze at 32° F. Carbon dioxide gas (CO2), if you can extract the heat from it, freezes at -109° F, forming what we call dry ice.

Fun Fact: CO2 can’t exist as a liquid at normal atmospheric pressures, so when dry ice “melts”, it actually turns directly into CO2 gas, a process called sublimation. This makes it extremely useful if you want to ship something frozen, because it won’t melt and waterlog the packaging, food, or illegally-harvested kidneys like regular ice would.

For reference, the coldest temperature ever recorded on Earth is -129° F, at a Russian base in Antarctica. At this temperature you would get dry ice frost deposited on surfaces directly from the air, but it’d be tough to see against the surely much more abundant regular ice frost.

Nitrogen gas (which actually has two nitrogen atoms, making it N2) condenses into a liquid at -320° F. In addition, nitrogen is extremely abundant, making up 78% of the atmosphere, and modern refrigeration systems can fairly easily cool it and condense N2 gas into its liquid form, liquid nitrogen (hereafter: LN2). Thus, LN2 is cheap if you can get it: our lab orders 160-liter containers which cost about $25 for the liquid alone, or about $0.60 per gallon. Naturally, I don’t feel that bad about, ahem, “liberating” a gallon or so for culinary adventures on occasion.

Ok, enough background — we want ice cream.

Making ice cream is, in theory, easy: a sweetened, dairy-based base, frozen while being continuously churned, gives you ice cream.

The problem with home ice cream makers is that this takes a lot of time. Home freezers aren’t all that cold (around 0° F, if I recall correctly), and the base, which is mostly water, takes a long time to freeze because of the large heat capacity in the water-to-ice phase transition. During this time, the little ice crystals that form as a part of the freezing process have a chance to grow into not-so-little ice crystals that make the end product something other than the smooth, creamy deliciousness that everyone loves about ice cream.

Not so with LN2. Due to its extremely low temperature, it can freeze things very quickly, so large ice crystals don’t have a chance to form. On the flip side, due to its extremely low temperature, it can freeze things very quickly, so if you’re not careful you end up with a big block of frozen stuff rather than ice cream. The way most people make LN2 ice cream is to take a serving’s worth of ice cream base in a styrofoam cup or bowl, add a spoonful of LN2, stir vigorously until all the LN2 evaporates, add a little more LN2, stir, etc. until the mixture reaches ice-cream consistency. I, being a gearhead, like to kick it up a notch*.

Enter: the KitchenAid.

The stand mixer allows hands-free stirring, leaving me free to drizzle in the LN2 slowly (very much like drizzling olive oil into a vinaigrette) while the mixture churns. I implemented this method over the weekend at a friend’s dinner party. She made the base (so, no recipe, sorry…), a standard french vanilla (custard base) while I supplied the LN2 and the KitchenAid. I put a few cups of the base into the mixer bowl, added a little bit of peppermint extract, and started the mixer on low speed. Adding the LN2 to the bowl results in an impressive billow of cold vapor as the LN2 evaporates, sucking heat out of the ice cream mixture in the process as you can see in the image at the top of this post (also, does anyone else think KitchenAids look a little bit like the alien from Aliens?).

After 5 minutes, it’s ice cream. Yep, that’s right. 5 minutes. And not just any ice cream, but likely the smoothest-textured ice cream you’ve ever had. And unlike regular home-made ice cream, you don’t need to let it set in the freezer to get the right texture — it’s ready to eat right away.

Towards the end of the churning process, we added some chocolate chunks to make it a mint chocolate chip. But I think an egg-free base would be better for mint ice cream, since the egginess of the custard base sort of overwhelmed the mint flavor. Live and learn.

For the next batch we made a peanut butter and chocolate chip ice cream, and the last batch was just a plain french vanilla. All made in under an hour even with us eating the various flavors in between each batch as they came out of the bowl. We also had some additional non-culinary fun with the leftover LN2 afterwards.

Science is delicious.

BAM!*

*I promise, from now on, no more Emeril references.