We grow blueberries and freeze them. We pick them over and freeze them dry
in freezer bags filled full and tightly sealed. We sometimes suck out the
air with a drinking straw. They will last even two years!!!
The moisture is coming out of the berries and freezing on top, a form of
because the product is exposed to too much air.
I would avoid using tupperware; I don't think they are meant for freezing
stuff. You might use rubbermaid boxes or regular freezer boxes and put
some crushed waxed paper on top to fill in the air space. If there is a
lot of air in any container the frost is bound to collect.
The best bet is freezer bags. I also use bags that can be heat-sealed and
they are very good. Rival makes a heat-sealing gadget and special bags
that you can actually cook the food in later on.
Good luck Maggie