Learn the science behind:
You might have seen it on a tv cooking show. Or maybe there’s an ice cream shop you’ve been to that uses it. Using liquid nitrogen sure is a spectacular way to make ice cream. But, it’s not just that. Liquid nitrogen also makes a very smooth ice cream. Let’s have a look at why.
- What is liquid nitrogen?
- You need to freeze ice cream
- Why liquid nitrogen makes a smoother ice cream
- Does liquid nitrogen make better ice cream?
What is liquid nitrogen?
Liquid nitrogen is exactly what it says it is: a liquid version of nitrogen (N2). Nitrogen is a molecule that’s all around us. As a matter of fact, about 80% of the air surrounding us is made up of liquid nitrogen.
At room temperature, nitrogen is a gas. However, if you cool down nitrogen enough it turns liquid. Much how hot steam turns back into water when it cools down. This phase transition from gas to liquid happens at around -196°C (-321°F). As such, liquid nitrogen is very, very cold! It is a lot colder than frozen water, aka ice or your freezer at home, which tends to be at -18°C (0°F).
If you pour liquid nitrogen out of its well-insulated container you’ll see a lot of vapors. This is boiling and evaporating nitrogen!
Don’t confuse it with dry ice
Liquid nitrogen is a liquid version of nitrogen. Dry ice on the other hand is the solid version of carbon dioxide (CO2). It isn’t as cold as liquid nitrogen, it turns solid at -78°C (109.3°F). But, it’s still very cold so some recipes may call for dry ice instead of liquid nitrogen. The principle is very similar, with the only added challenge that it’s a lot easier to mix in liquid nitrogen as opposed to solid dry ice.
You need to freeze ice cream
So liquid nitrogen is cold, very cold. And that comes in handy when making ice cream.
A crucial step when making ice cream is freezing the ice cream base. While freezing, water turns into ice crystals and liquid fats turn solid. As a result, your ice cream turns solid and firm.
To create a light and airy ice cream though, you don’t just want to freeze the ice cream. You also want to aerate the ice cream base, to incorporate air.
Using a regular freezer
You can incorporate air first and then freeze your ice cream mix. However, this only works if your ice cream base can actually hold onto air. If not, the air will escape before the ice cream has time to turn solid.
We use this method when making our 2-ingredient ice cream.
The disadvantage of this method is that it takes long for the ice cream to freeze. The cold needs to penetrate the whole ice cream, which is a slow process. It will take at least a couple of hours to freeze the ice cream in a regular 1-liter container. The bigger the container, the longer it takes!
Using an ice cream machine
If you’re making an ice cream from a base that can’t hold onto air when it’s warm, the first method won’t work well. In that case, many homemade ice cream recipes call for an ice cream machine. An ice cream machine cools and stirs the ice cream simultaneously. The stirring action helps to incorporate air and freeze the ice cream more homogeneously. All parts of the ice cream will touch the cold surfaces, freezing a lot faster.
Nevertheless, making ice cream this way can still take a while. Most ice cream machines need at least 30 minutes using temperatures down to -20 to -30°C (-4 to -22°C). Also, the ice cream will need to continue to harden out in the freezer afterwards.
Of course, factories use very efficient continuously running ice cream machines. These are also way more powerful than homestyle equipment, which is why it’s hard to replicate factory made ice cream at home.
The fast method: liquid nitrogen
None of these methods gives you ice cream within 5 minutes. That’s where liquid nitrogen comes in.
Remember that liquid nitrogen is very cold, as low as -196℃. Compared to regular freezers, which go down to -18℃ (0°F) or industrial freezers, which go down to -40℃ (-40°F), this is significantly cooler. These very low temperature allow the water in ice cream to freeze very fast. In a matter of a few minutes.
By stirring and adding liquid nitrogen simultaneously, you can make a light and airy ice cream in a matter of minutes, instead of hours!
Why liquid nitrogen makes a smoother ice cream
So, using liquid nitrogen speeds up the ice cream-making process considerably. But, there’s more. Using liquid nitrogen can also make a smoother ice cream. To explain why, we need to have a closer look at what really happens while you’re freezing your ice cream.
While freezing ice cream, you’re turning water into ice. More specifically, into ice crystals. For water to form these ice crystals, individual water molecules need to meet and connect, starting a new crystal. The formation of these new crystals is called nucleation. At lower temperatures nucleation happens more quickly. So more new crystals are formed.
Once a crystal has formed, it can grow in size. This is easier than starting a new crystal. The backbone is already there. New water molecules just have to join the crystal.
When you’re making ice cream you want to form a lot of crystals. But, you don’t want them to grow too large. Large ice crystals can make an ice cream gritty and grainy. To control the size, there are two main levers you can pull (not taking into account recipe influences here):
- Decrease temperature: this will induce more nucleation, so more new crystals are formed more quickly
- Shearing/mixing: this works twofold: by mixing the mixture you make it easier for molecules to meet and nucleate while at the same time you’re breaking down larger crystals, keeping them small.
Low and slow = Big and grainy
If you don’t stir your ice cream mix and don’t cool it very well, crystallization will happen slowly. Also, it becomes even more attractive to just join a current crystal, than start a new one. As a result, you may end up with a small number of very large crystals, it’s what happens when making granita.
Cold and fast = Small and smooth
On the other hand, if you stir a lot, you’re continuously breaking down these ice crystals, keeping them small. So, if you stir vigorously and freeze very quickly, you’ll end up with a lot of small ice crystals.
Does liquid nitrogen make better ice cream?
Commercial ice cream making equipment is also very good at freezing fast while stirring. This can also make a perfectly smooth ice cream. Also, at large scales liquid nitrogen is a lot more challenging to use than conventional freezing techniques.
At home, you are limited by equipment with less power and strength. This is where liquid nitrogen can make a difference. But, it really mostly makes a difference when you’re in a hurry – when you’re on a tv cooking competition for instance! – or looking for some additional entertainment.
Does it impact flavor?
Liquid nitrogen makes ice cream more smooth. Texture can play a role in how you perceive ice cream as a whole. So in some cases, it may seem as if it affects flavor, simply by changing the texture.
However, nitrogen itself doesn’t add or change the flavor of ice cream. Nitrogen is all around us and pretty inert. As such, mixing some into your ice cream while freezing it doesn’t change or add any flavor. Keep in mind that once liquid nitrogen warms up, it simply evaporates again!
Does liquid nitrogen ice cream melt more slowly?
Even though liquid nitrogen is very cold, the ice cream made with liquid nitrogen isn’t necessarily very cold. Generally speaking, it’s just as cold as regularly made ice cream.
Also, keep in mind that ‘normal’ ice cream is hardened for some time in the freezer. During this time, the ice cream cools down as a whole and stabilizes. When making ‘fresh’ liquid nitrogen ice cream, you don’t have this time and the ice cream may still be something softer and thus melt more easily.
What ice cream styles can be made with liquid nitrogen?
Just about any ice cream that starts out as a liquid at room temperature can be frozen with liquid nitrogen. The only thing that changes is the freezing process itself, not the recipe. So whether it’s an eggless corn starch based ice cream, a custard base, or an American style ice cream, they can all work!
Klaus D. Timmerhaus, Thomas M. Flynn, Cryogenic Process Engineering, 2013, p. 15, link