Who doesn’t like ice cream? A soft, slowly melting soft serve, some Italian gelato, a milkshake or a popsicle? Do you prefer chocolate (I do!), fruit, nut, bubble gum flavour? Do you prefer large or small crystals, a cold or a warm ice cream? Starting to get harder to choose? That would be great, because then I have loads to tell you in this post.
I really enjoy ice cream, but also find it highly fascinating. How come some kinds of ice cream can be scooped? How come ice cream is not a big solid rock of ice? Why does ice cream taste so cold sometimes? Why do some sorts of ice cream contain big ice crystals?
I’ve made a start answering these in my post about semifreddo, which is a very easy to make at home ice cream recipe, not requiring an ice cream machine. This post will be more sciency, but will greatly help you in developing your own ice cream recipes!
Freezing point depression
Essential for making ice cream is the concept of freezing point depression. This is a physical phenomena that you will also see in winter time, when salt is sprayed onto roads when it freezes. By throwing salt on the road the water on the road will not freeze. However, the effect is limited, if it gets cold enough (pretty cold in the case of salt) water will freeze. The same happens in ice cream, by adding sugar or alcohol, the freezing point of the water in ice cream lowers.
You might find this surprising, since ice cream is still frozen, it doesn’t just flow away like water or cream would. And that’s correct, however, the trick is that not all water is frozen, only part of it.
Let’s dive into thermodynamics to explain this in a little more detail without using all sorts of formulas. When you dissolve sugar in water the entropy of this solution will be higher than that of pure water. What this means is that there is more ‘disorder’, which makes sense, instead of only having water molecules, all of a sudden sugar molecules float around as well. This will result in a so called lower ‘chemical potential’. This again results in the fact that it needs to be colder to actually freeze.
Sounds complicated? It’s enough to understand that by adding sugar or alcohol to your water, the freezing point goes down. The more sugar you add, the lower the freezing point will become (although you have to make sure that you’re still able to dissolve all this sugar).
Amount of ice in ice cream
When ice cream is cooled to sub zero temperatures, it is the water in the ice cream that will make the ice cream freeze. As we just discussed, the addition of sugar lowers the freezing point of ice cream. However, that does not mean that there won’t be any ice anymore. Thermodynamics is all about equilibria, so also in this case there will be an equilibrium, here between the ice crystals and the liquid water.
When a sugar solution is cooled down part of the water will freeze and form ice crystals. These ice crystals leave the sugar solution. You will be left with a sugar solution + ice crystals. They will still be homogeneously mixed. Because some of the water has formed ice crystals the remaining sugar solution will become more concentrated. If it’s too concentrated, no more ice crystals will be formed. There will be an equilibrium.
The point at which no more water crystallizes depends on the amount of sugar that has been added. You can do something similar as well by adding alcohol or salt to your ice cream. Fat (milk fat for example) doesn’t have this effect. The fat and water don’t dissolve in one another.
If you like you can even calculate how much ice crystals have formed and you can calculate what the new freezing temperature of your sugar solution is. Let’s get back to that later though. You’ve now learned why it’s important to add something like sugar or alcohol to your ice cream.
I’ve used several resources for writing this post, these might also be interesting if you’d like to understand the thermodynamical concept of freezing point depression somewhat more: UC Davis, Bristol University