Who doesn’t love ice cream? It comes in a nearly infinite number of flavours, and nothing beats a cold, creamy afternoon treat on a hot summer day. I may be biased, but ice cream is quite possibly mankind’s greatest invention…
But who actually invented ice cream?
Frozen desserts made with milk products have been around for thousands of years. Many credit the Chinese as having eaten the first ice creams around 200 BC. Even earlier than them, however, several other civilizations , including the Persians, consumed desserts made of flavored snow or shaved ice with toppings.
Ice cream as we know it today, however, was invented in Italy around the 16th century (some say that Marco Polo brought the idea of ice cream back with him to Italy from China, just as he was said to have done with pasta, though this has never been proven).
Modern ice cream is characterized by the use of cream, milk, and eggs. It is made by cooling a mixture of these ingredients (plus sugar, flavorings, and other add-ins) and mixing it continuously as it freezes.
In a traditional ice cream churner the mixture is poured into a container covered with ice. Then, the person making the ice cream has to continuously churn the mixture as it cools.
The mixing is very important because it makes sure the final product is smooth. When the water in the mixture freezes, it will want to form crystals. By churning the mixture, the water crystals are constantly being broken up so they don’t get too big and the ice cream doesn’t have a grainy, icy texture.
Well, that’s all the chemistry there is. Goodbye!
Hah, just kidding. We’ve got more to tell you!
You probably have heard of liquid nitrogen and how it’s really, really cold. But did you know you can use it to make ice cream? In fact, it’s done quite often! AND, liquid nitrogen makes ice cream better and faster than the traditional methods! Why? Chemistry!
But first – some information about liquid nitrogen
One of the first mentions of liquid nitrogen being used in food was in an 1890 recipe book called Fancy Ices by Mrs. Agnes Marshall. However, it wasn’t until pretty recently that liquid nitrogen became really popular in the culinary world. It has many advantages over traditional cooling methods. It boils at -197 ºC, so it is quite cold and can cool things very quickly. Later, you’ll see why this is especially good for making ice cream. It is also quite convenient – you can drop liquid into liquid nitrogen to freeze it into drops (think of Dippin’ Dots!) or pour it over things to cool them down. And of course, it looks super cool, which is a big reason why it has become very popular.
Dippin’ Dots ice cream, which is made with liquid nitrogen, and liquid nitrogen being poured over a drink.
By now, you may be wondering, where is all this liquid nitrogen coming from?
Well, you might remember that nitrogen makes up a lot (about 78%) of the air we breathe. The process of making liquid nitrogen is called fractional distillation.
It starts with boiling air (which is really cold). Then, the gas from the boiling air goes through a tube where it will expand and cool down. This effect is known as the Joule-Thomson effect and is related to the combined gas law: P1V1/T1 = P2V2/T2. P stands for pressure, V for volume, and T for temperature. The law states the relationship between these three properties as a gas changes. Because the pressure is lower the further down the tube the gas goes, the temperature of the vapor decreases. As the gases cool, those with higher boiling points will condense out of the gas phase, while those with lower boiling points will continue to move down the tube.
The boiling point actually depends on intermolecular forces in the liquid, that is, the forces between the molecules that make up the liquid. The stronger these forces are, the more energy it takes to overcome them, and thus the higher the boiling point. The two major components of our atmosphere are nitrogen and oxygen (all the other gases combined only make up about 1%). If we compare the two, we’ll see that oxygen has stronger intermolecular forces because it is more electronegative – that is, it attracts electrons more strongly. This creates stronger forces between the oxygen molecules because they’re all trying to grab onto each other’s electrons. Thus, it takes more energy to overcome these forces, resulting in a higher boiling point for oxygen. In the context of making liquid nitrogen, this means the oxygen will condense before the nitrogen as the vapors cool down. Nitrogen actually has the lowest boiling point of all the major gases in the atmosphere (these include argon and carbon dioxide), so it can easily be collected at the very end of the tube after all the other gases have condensed out. If you’ve ever wondered why we use liquid nitrogen and not, say, liquid oxygen, this is the main reason why – it’s the easiest to produce and it’s the most abundant in our atmosphere.
Now, let’s get back to that ice cream we mentioned before! As we said, liquid nitrogen is great for making ice cream. Can you guess why? It has to do with what we mentioned about the ice crystals! Mixing ice cream as it is being made is a difficult and labor-intensive process. With liquid nitrogen, the liquid ice cream mixture actually freezes so fast that the ice crystals don’t have time to grow large in the first place! This means you can have smooth, delicious ice cream in minutes with very little work! A common method of preparing liquid nitrogen ice cream is by pouring the mixture into an electric mixer and then adding liquid nitrogen while it is mixing. You can’t do this with traditional methods (try fitting an electric mixer into your freezer!).
Check out this video of ice cream being made with liquid nitrogen:
Of course, safety’s always important, so if you’re handling liquid nitrogen, make sure to be careful – it really is super cold!