Desiccant, now I got it! I think I like rice better because it can be regenerated more easily (possibly in the saucepan).

Regarding how paranoid is my worry, well, I read somewhere many years ago, on some Italian photography magazine, that films were to be put in the fridge inside their own canister exactly for the reason that the factory-filled canister do not contain a dangerous level of humidity. And they wrote that canisters are air tight for this specific reason. And in fact, if one thinks about it, no film - however cheap - is sold without a canister (roll film is sold in sealed bags) which makes me think there is some logic in all this waste of plastic.

From what you tell me, it is nonetheless safe to put the film in the fridge inside an already opened, but then well-closed canister, because the air content inside the canister is not enough to make a veil of frost. I am glad to learn that. I do suppose that it would be somehow dangerous, for instance, to put the cartridges in the fridge without canister. I see that it's the amount of air circulating inside the fridge, any time one opens it, that creates the problem. If I insulate the film from the air in the fridge (by using the plastic canisters, or the metal canisters for bulk film) it should be all right.