How come ice cream is soft when frozen but after you melt it and freeze it, it becomes ice.
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Ice cream is churned while being cooled down. This works air into the mix so it’s light and fluffy. Once you melt it the air is lost so it freezes more solid. You could churn your melted ice cream again though.
When ice cream is made, the churning process adds air into the mix, which mixes with the liquid to make tiny ice crystals. Once it melts, the air escapes, and refreezing the liquid without churning it again will allow those tiny crystals to grow into bigger ones. It’s the same as with snow, once it melts and refreezes it turns to ice on the road because all the little snowflakes melt into a slush and turn to ice. You can do an experiment at home to see this in action by making a granita: tale some juice and throw it in the freezer in a shallow bowl. Every hour or so scrape it with a fork and then put it back in the freezer – you’ll get a slushie after a few hours.
Ice cream is made by mixing (churning) it as it freezes, so it’s full of little air bubbles. It’s frozen foam, more or less (it roughly doubles in volume during the process).
When you melt it, most of the bubbles collapse, and if you freeze it again without the air bubbles to make it soft, it freezes hard.
Great comments here already! I would add that different manufacturers have different ideas of how much air is ideal for ice cream. It has to do with culture and taste but there is a grading system from “frozen dairy dessert” up to “premium” and “super premium” ice cream.
It has nothing to do with bubbles or air.
When water freezes it forms ice crystals and if undisturbed these crystals can get long and interconnected. When ice cream gets made it is continually stirred and this stops the crystals from getting too long and sticking together. If you stir your ice cream while it freezes it goes back to the soft texture it had when you bought it.
It has to do with the size of ice crystals.
Large crystals are uniform, and very hard.
Small crystals are just as hard, but not really connected to their neighbors. since there are so many cracks and seperate crystals as a whole the ice cream is soft.
So manufacturing wants to control the size of the crystals and prevent then from binding to their neighbors. This is fine with two methods.
1) as mentioned in other posts, stirring the batch. It introduced air, breaks up forming crystals, and prevents forming bonds to neighbors.
2) rapid freezing. If you freeze the mix rapidly then the molecules don’t have time to organize into perfect patterns. And the crystals start to grow everywhere rather than just from the cold spot.
The refrozen Ice cream had neither of these, and so forms large uniform crystals with no weak spots.
Edit:. Boobs changed to bonds. There is always an autocorrect you miss. This was a weird one.
The term the others seem to have been unable to grasp so far is called *overrun*, the amount of air folded into the ice cream during the manufacturing process. The more overrun, the softer the ice cream is (e.g. soft serve ice cream). The ice cream you buy in stores also have some overrun, and when that ice cream melts and refreezes, that overrun is gone. That I would say is the main cause of refrozen ice cream being harder; the “longer ice crystals” thing may play a role, but not as much as the air.
Had a close friend work at Coldstone Creamery (a boutique ice cream shop at some shopping centers; customers can order ice cream and select the “mix-ins” like cookie dough or chocolate chips or whatnot, that the worker will then knead into the ice cream using tools on a, well, cold stone platform to keep the ice cream frozen). They would make the ice cream themselves. I’d usually pick her up after work and take her home, which often meant being around and in the back of the store shortly before and after closing time, if she was closing. I’d usually come home smelling like waffle cones.
Ice cream has a lot of air in it. The churning action of the princess used to make it ensures its more of an emulsion of air and flavored frozen dairy stuff. When it melts, the air is able to come to the surface and the remainder is just the flavored dairy stuff. When you refreeze that, it’s a more or less homogenous block of liquid frozen solid, there aren’t any voids of air to make it soft.
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