An essential part of any holiday (in a warm place that is) is ice cream. It’s fun to find a nice ice cream shop and take plenty of time to choose your flavour (even though you’ll always choose the same anyway) before enjoying the refreshment.
In most cases you’ll find an ice cream shop that has put all flavours out there in front of you. Ice cream generally isn’t made fresh, even though it is served fresh. Since ice cream is frozen, there’s not really a need to make it fresh either is there? Well, while looking out for an ice cream shop on a recent trip to the US we ran into one that made ice cream fresh. Right in front of our nose, within a matter of a few minutes.
How they made the ice cream so fast? Using liquid nitrogen, couldn’t get more sciency could it? It was almost a shame the ice cream makers weren’t wearing lab coats & safety goggles. But why use liquid nitrogen to make ice cream?
Liquid nitrogen looks fancy
Let’s get this reason out of the way first: a good reason to make ice cream with liquid nitrogen is because it’s cool and it looks fancy. Just like you’d like to see someone fry your steak in front of your eyes, people like to see things being made, as you go. So aside from all other more scientific reasons: it looks interesting, admit it. That’s why there’s a line in front of the store, especially since the ice cream itself didn’t even taste that good at the place we were at.
Making ice cream
Ice cream making starts by making a liquid that can be transformed into an ice cream. This can be a custard (even one without eggs), or a lot of other styles of mixtures. They will tend to contain at least some sugar (or alcohol). This sugar is required to prevent all the water from freezing when making ice cream (freezing point depression). But apart from that, there’s quite a lot of flexibility here. Once the base is made, you have to transform it into an ice cream. This requires two things to happen:
1) the mixture has to be frozen
2) air has to be introduced.
Thus when making ice cream you’re making a very airy frozen mixture. We’ve discussed why incorporation of air is important. The air makes the ice cream softer and easier to eat and scoop. By freezing the ice cream you prevent the air from escaping. You first introduce the air, by using whipped cream for instance and then freeze it. You’ll have to freeze the ice cream for several hours at -18℃. The ice crystals need time to form.
You can also incorporate the air while freezing the mixture. When you make ice cream in an ice cream machine this is what you’re doing. An ice cream machine does take quite a while to freeze your ice cream. A home style ice cream make will easily take half an hour or longer to freeze & stir. And the ice cream isn’t finished yet at that point. You will have to freeze the ice cream to firm up. In the ice cream machine some ice crystals have been formed, but they have to continue to grow in the freezer.
The fast method – Liquid nitrogen
None of these methods gives you ice cream within 5 minutes. That’s where the liquid nitrogen comes in. Liquid nitrogen is super cold nitrogen of -196℃. Keep in mind that most home freezers only go down to -18℃, industrial freezers may go down to -40℃. This allows it to freeze the water in the ice cream very fast, in a matter of a few minutes.
In the mixers at the ice cream shop you can also see a smart stirring mechanism. The ice cream is made by simultaneously mixing the ice cream (introducing air) as well as liquid nitrogen (to freeze the water).
Apart from speed though, why would you use liquid nitrogen? For that we’d have to dive into the science of crystal growth.
How to grow ice crystals
In order to start growing ice crystals the temperature has to be well below zero degrees Celsius. Once the temperature is low enough crystals will start forming spontaneously, this can be helped along by stirring the mixture. The formation of these first crystals is called nucleation. These first crystals themselves are called nuclei. If there are places for the crystal to grow onto, it will nucleate more easily. In an ice cream mixture there tend to be enough particles onto which an ice crystal can form.
Once these nuclei are formed these will continue to grow into larger crystals, while at the same time new nuclei can also be formed. However, it tends to be easier for liquid water molecules to sit on an existing nuclei instead of forming a new one.
If you gently cool the ice cream mixture without any stirring the formation of new crystals will be slow. Instead, the growth of crystals will be higher. In a worst case scenario you could end up with one or a few very large hard crystals. This will not give a good quality ice cream. Instead you are looking for a smooth ice cream with a lot of smaller crystals.
How to grow a lot of (smaller) crystals
You can make a lot of crystals through two main mechanisms:
- Decreasing temperature: this will induce more nucleation, so more new crystals are formed
- Shearing/mixing the mixture: by mixing and moving the mixture you initiate more nucleation, so more crystals again, also, you might break up crystals that have already formed into smaller ones, preventing the formation of a few big ones.
If you combine the two, the effect is even stronger. The longer you continue to both cool and stir, the better the result, until all ice crystals have formed. If you’ve ever made ice cream without an ice cream machine but by directly freezing your base you might have noticed it. You have to take the base from the freezer quite often and whisk it through.
Why make ice cream with liquid nitrogen
Apart from the reason we mentioned at the start, it being very fast, liquid nitrogen has another advantage. Because of its low temperature (-196℃) new crystals are formed very fast, combine that with the stirring at the same time for an even stronger effect. As a result you end up with a lot of smaller crystals. These aren’t chunky in your mouth and will give a smooth ice cream.
In cooking shows you might have also seen them use liquid nitrogen. Instead of using a machine to mix the base liquid, they’ll be using a whisk, which will work perfectly fine as well, just take care of your hands, liquid nitrogen is very very cold.
Does liquid nitrogen make better ice cream?
Not necessarily. Commercial ice cream making equipment is also very good in freezing fast and stirring. This can also make a perfectly smooth ice cream. Liquid nitrogen isn’t that suitable for making large batches. You’ll need some additional safety measures and since freezing goes so fast, you’d need a different stirring mechanism.
It is a good way to make small batches pretty quickly though. However, it does remain more of a show. The ice cream we tasted wasn’t necessarily better than normal ice cream. The texture was good and smooth, but not exceptional. And, if we’re honest, we didn’t really like the flavour of the ice cream. Although that probably doesn’t have anything to do with the liquid nitrogen. But seeing the prices (a lot higher than normal), we certainly wouldn’t go back.
Aside from the fact that the ice cream isn’t necessarily better, it seems as if this method is a lot more energy consuming. You’re using a lot of super cold nitrogen in a not too well insulated mixer. Also, you’re making just one batch at a time instead of a more efficient larger batch scale. Cool it is, yes, but it seems to use more energy, a lot more labour and a lot more time to serve out an ice cream (remember that line in front of the shop and add the time you have to wait for your ice cream after ordering).
Does ice cream have to be made fresh? No, of course not. Ice cream is a way to keep ingredients good for a long time. At these low temperatures the ice cream won’t spoil. As long as you keep the ice cream at a constant temperature (varying temperatures are horrible for your ice crystals) and keep it airtight if stored for a longer period of time, it will be perfectly fine!
That said, we enjoyed the show, a holiday without ice cream simply wouldn’t have been a holiday ;-).