Usually that thin, white film is jelled foam. If your fruit was foamy in the sauce pan and you didn’t skim the foam off, or if your mixture had a lot of air in it and you didn’t do air releasing before putting the jam in the jars, the foam or tiny air bubbles rise to the top of the jar and form that white film.
It is always a good idea to inspect a jar of jam that has been stored on the shelf before eating it. Do you see mold? Does the jam smell moldy or fermented (have a smell of alcohol)? If yes, throw it away. If not, taste it and if it tastes okay, it is fine to eat it.
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