Why does dark chocolate turn white (and not so tasty any more) after staying in cold for some time? Usually this effect can be seen after storing chocolate in fridge.
Its called chocolate blooming.
There are two types:
Fortunately, there are no safety problems with bloomed chocolate, and it can be remelted with no inherent loss of quality. So bloomed chocolate is perfectly useful in recipes where it will be melted; or it can be melted and re-tempered if desired.