Question

why are pickles,jams,jellies preserved in glass,porcelain and plastic container

Answer 1

there are keep in glass because to preserve the food items for long time...

Answer 2

They are preserved in glass as to keep it in tasty and harmless. The following things has some kind of acid in there and they don't react in glass jar but they can have a reaction in plastic and other jar. So it is safe for us