Why does ice cream get icy in storage?
Smooth on day one, icy by day ten, same tub, same freezer setting -- this isn't the ice cream going bad, it's a slow crystal rearrangement with its own name and its own trigger.
The kitchen answer
Ice cream doesn't get icy just from sitting in the freezer for a while -- it gets icy from small temperature swings while it sits there. Every partial thaw-and-refreeze cycle nudges the ice crystals a little bigger.
What's actually happening
This is Ostwald ripening: whenever the mixture experiences even a slight temperature rise, the smallest ice crystals (which have a slightly higher surface energy and melt a touch more readily) partially melt first. When it re-freezes, that water tends to redeposit onto larger, already-existing crystals rather than forming new small ones. Repeat that cycle enough times and you end up with fewer, larger crystals -- which is what "icy" texture actually is at a structural level.
Fat and stabilizers slow this down by physically coating crystal surfaces, making it harder for water molecules to migrate from small crystals to large ones. They don't stop the process outright, they just raise the number of fluctuation cycles it takes to become noticeable.
What does that mean for your recipe
Where you store ice cream matters as much as how cold your freezer is set. Keep it in the back or bottom of the freezer (most stable temperature, least exposed to door-opening swings), in an airtight, low-headspace container -- a container with a lot of empty air space gives the surface more room to fluctuate in temperature every time the door opens.
What can go wrong
- Ice cream stored in the freezer door: the door shelf experiences the largest and most frequent temperature swings in the whole freezer.
- Large container with lots of empty headspace: more air space above the ice cream means more room for that air (and the ice cream surface) to warm and cool with every door opening.
- Fully melted, then refrozen: a full melt-and-refreeze is the extreme version of the same process, and reliably ruins texture in one step rather than gradually.
- Low-fat or low-stabilizer recipes: less material available to coat and protect small crystals, so ripening runs faster for the same amount of temperature fluctuation.
Try it yourself
Same tub, two storage locations:
- Split one batch of ice cream into two identical, sealed containers.
- Store one in the main body of the freezer, the other in the door shelf.
- Compare texture after a week of normal freezer use (door opened as usual).
Watch for: the door-stored tub noticeably icier despite being in the "same freezer" the whole time -- direct proof that fluctuation, not just average temperature, drives the effect.
Go deeper
Ostwald ripening isn't unique to ice cream -- it's a general phenomenon in any system with a population of small crystals or droplets sitting in equilibrium with a surrounding medium, driven by the same underlying thermodynamics as sugar crystallization in fudge: a less-ordered, higher-energy population of small crystals slowly converting into a more stable population of larger ones whenever it gets the chance.