A few questions to that effect have come in the last couple of days. The answer is that it can be frozen, it just isn’t as functional afterward. It can’t be whipped up very high, for instance. It also tends to separate a bit and often needs to be shaken up to re-establish the butterfat emulsion. But what exactly happens to cream in the freezer?
If you’ve been browsing the comment fields the last few days you probably know already that the fat in cream occurs in the form of little blobs or globules, each of which is surrounded by a membrane of protein. Those membranes are pretty tough, at least where temperature is concerned. They’re surprisingly resistant to heat, which is why you can boil milk or cream without causing the fat to congeal in a greasy pool.
They don’t hold up well in the face of shear forces though, which is why a little churning or shaking (or over-whipping) will cause the membranes to rupture and spill their fat into the surrounding liquid. If that liquid is chilly, the fat molecules will start stacking up on each other. The result is large crystals, which go by the name of “butter.”
However globule membranes don’t necessarily have to be ruptured for butterfat crystals to form. If the temperature is cold enough for long enough, butterfat crystals can form inside the globules, in which case their pointy ends start poking through the membranes from the inside like sticks through a lawn and garden bag.
Later when the cream warms up the butterfat starts to leak out and pool, which is why frozen cream can appear greasy relative to fresh, unfrozen cream. It’s that pooled “free” butterfat that also inhibits whipping. All of which is not to say that frozen cream is useless. It’s still great for pretty much every other application, especially things like sauces.