There are so many different ways to make ice cream. You can simply whip up some cream and mix it with a sugary liquid, you can make it with a custard and introduce air with the ice cream machine. Or you can do a combination of the two: make a custard and introduce the air using whipped cream, making this semifreddo.
The common aspect between those different preparation methods: the introduction of air. Unless you’re making popsicles the introduction of air in your ice cream is essential to make a light, soft and fluffy ice cream.
Air in ice cream?
Ever noticed that most ice cream packs do not give the weight of the ice cream? Instead, they will only mention the volume of the ice cream. The weight might be hidden away somewhere in a smaller corner. This is done because ice cream contains a lot of air. Therefore, where a pack of 1 liter of milk will weigh about 1kg, 1 liter of ice cream though might well weigh only 500g, that’s half! That difference is mostly because of the large volume of air in the ice cream.
The air in ice cream strongly contributes to the sensory properties of the ice cream. The air makes the ice cream light and fluffy and easy to scoop. When you try to scoop a ball of ice cream you will pass through ice cream, solid fat and air bubbles. Since it’s easy to scoop through air bubbles it’s these air bubbles that help you cut through that ice and fat. Also, thanks to the air flavour molecules from the ice cream are released more easily.
Incorporating air in ice cream
In homemade ice cream there are two main ways to add air to your ice cream and commercial production works quite similarly.
Whipping up cream
A way to incorporate air is to introduce it to one of the components that is good in holding onto air. Whisking water or milk won’t help you to hold on to air. Instead, most of the air will disappear before you even have the time to add other ingredients.
Cream on the other hand is very good in holding onto air. Whipped cream will remain airy and bubbly for quite some time. So simply whip the cream, fold in the other ingredients and you’ve got a solid base for an ice cream. This method is used in the semifreddo at the bottom of this post.
Freezing & mixing
Instead of using an ingredient that can hold on to the air at room temperature, you can use the lowering temperatures during freezing to whip up ice cream. Once the ice cream starts to freeze, whisk it/blend it to incorporate air. You can do this in an ice cream machine which will do the cooling and whipping simultaneously. However, you can also do this by placing the to-be ice cream in the freezer and taking it out every 30 minutes in the first few hours and mix it. This way the set fats and ice crystals will be able to hold on to the air!
Holding on to the air
After incorporation of the air into your ice cream you have to make sure the air actually stays in the ice cream. There are several ways to do this when making ice cream. The first is to freeze quickly. Once the ice cream has frozen and cooled down the molecules will stay put more easily. As a result, the air bubbles won’t be able to escape.
Another method is to increase the viscosity of your ice cream. A more viscous liquid is thicker and it is harder to travel through for an air bubble (read more on viscosity & rheology here). In a lot of ice creams some sort of a custard is made (e.g. in this corn starch ice cream). This makes the whole consistency slightly thicker, making it harder for air to escape once it’s incorporated. This method is also used in the semifreddo at the bottom of this post.
Fat and proteins can also stabilize air bubbles by sitting around these air bubbles. This way air bubbles cannot travel to one another through the watery phase. Instead, they’re kept apart from each other.
Semifreddo is Italian for ‘somewhat’ cold. And that’s just what this semifreddo is. It’s a cold (because frozen) dessert, however, it doesn’t taste that cold at all (partly thanks to the air!). The recipe itself isn’t hard to make and comes from ‘De Zilveren Lepel‘, an Italian cookbook.
This semifreddo uses a custard to prevent air from escaping, whipped cream to introduce air and fast cooling to hold on to the air.
- 3 eggs
- 120g sugar
- 120g blue berries (thaw first if using frozen)
- 375 ml of whipping cream
- Prepare a mould, for example a cake tray and line with plastic foil.
- Whisk the eggs and sugar together in a metal bowl, no need to incorporate a lot of air, just make sure the sugar is all dissolved.
- Place the metal bowl above a pan of boiling water (au bain marie). Keep the water at a low boil and continue whisking until the mixture has thickened. The temperature will be around 70C at this point but might also be slightly lower. This is your custard.
- Remove the mixture from the heat, keep on whisking for some time to prevent it from cooking further on the sides.
- Take a stand mixer or another electric mixer with the whisk attachment to whip up the cream. The cream should become thick and fluffy.
- Now gently fold the custard through the whipped cream. Be gentle and don't work too fast to keep in as much air as possible.
- Mix in fruit gently.
- Pour into the prepared mould and cover off with plastic foil. This plastic foil will protect the ice cream from drying out further.
- Place in the freezer to fully set. This will take at least 8-12 hours.
- Take out of the freezer. You should be able to take it out of the mould easily thanks to the plastic. Take a warm knife and slice off a slice per person.
- If refreezing, take care to wrap it up in foil again.
- Leave the slices at room temperature for a few minutes before eating.
Other ice cream science
Apart from incorporating plenty air, an ice cream will also need some additions which prevent the water from becoming rock solid ice. This phenomenon is called freezing point depression and can be achieved by adding sugar, salt or alcohols.
Sources & Notes
Do note, when not heating/pasteurizing eggs properly, you could run a risk for a Salmonella infection. So keep this in mind when making homemade ice cream, don’t take the risk if you don’t trust it.
Want to read more about ice cream? Serious Eats wrote a great post about it!
ACS also wrote about ice cream through the lense of a chemist.