When water freezes it creates crystals that are hard. An ice cube is like 1 solid crystal. Ice cream is a mix of water and fats and gets churned as it freezes which means those ice crystals don’t grow very large so you have a creamy, fatty mix with millions of tiny ice crystals rather than 1 big ice crystal like an ice cube.
There are also a lot of air trapped in there to make it even softer.
High quality ice cream usually has less air and is way harder.
EDIT: I just wanna add that the sugar content also helps.
2 things. Ice crystal size and sugar.
Ice cream is constantly stirred (read churn) while freezing. This causes the ice crystals to break up while freezing. You have tiny little ice crystals suspended in a sugar solution (more on that in a bit) that will never freeze, making a scoopable consistency. If you don’t churn it while freezing, you end up with a single block of solid ice.
For sugar, it’s function in ice cream is not only sweetness. Adding sugar (or salt) to a liquid decreases it’s freezing temp. As the water itself freezes, the remaining sugar solution keeps on getting concentrated until it can’t freeze anymore. Combine the sugar water and tiny ice crystals and you have…. Sorbet.
Ice cream has of course milk and fat which make things even creamier but the idea is the same. Air is also added while churning which makes things even softer. Primarily, it’s the tiny crystals and sugar water though.
A lot of modern ice cream products have additives that are designed to prevent crystallization. Basically, much of what we call ice cream today is a far cry from the traditional “old fashioned home-made” ice cream, which would actually often crystallize if kept frozen for long. things like xanthum gum, say, or agar. These additives interfere with the organization of the water into networks, and crystals are what you get when the networks grow big. The big companies put in a lot of effort figuring out what additives can enhance “smoothness” in the product as well as what will prevent crystallization.
Ice cream from many big-name companies tend to turn into a gooey plastic type of thing as it ages. Some have so much additive and substitution that they can’t call it ice cream, but instead call it desert product or something.
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