I am not sure, but a dedicated small freezer would be better I assume because they are usually not frost free and over the years of warming and cooling cycles of regular frost free fridge freezer it may take a toll on the nic. Not sure but just my thoughts.
Not sure the frost free function would matter all that much, but every time you open and close your freezer the temperature does fluctuate. At least in a dedicated box, it would only be opened when necessary for the nic, not for the frozen Brussels sprouts. If I were to go to that much trouble, I would get the chest freezer type. That way when you lift the top, the cold air stay in better than a side opening door (cold air settles).
Frost free function is literally a stove top heating element between the cooling exchange unit and the inside wall of the freezer.
In a regular size fridge, a good frost free unit will have an aluminum stamped piece that clamps onto the heating element and dangles into the drain tube to keep that from freezing too.
If not, you can strip a piece of wire from 12-2 household wire, wrap it around the element 4 or 5 times and hang the end into the drain tube.
Works wonders.
Frost free will create vastly more fluctuating temps than merely opening and closing the door.
BUT!
Even a frost free freezer doesn't thaw out and refreeze groceries.
Opening the door from time to time is miniscule in effects in comparison.
So pandering over chest style or front opening is trivial, really.
Going deep Temps theorhetically will do more to prolong the life. Such as with a medical freezer like
@Rossum did.
Having backup power if you have power outages in your area would also do well to help protect and prolong your investment.
And space in your other/food freezer in the event of an emergency or for periodic defrosting would be great to have.
Tapatyped