film on uncovered food?

Question

how come food (such as pudding) develop a film on the surface if they are left uncovered in the fridge? how come water condenses on the plastic wrap if it is covered?

Answer 1

Pudding develops a film on its surface because water evaporates from the surface of the pudding, leaving the surface drier than it was before. So the film is just dried out pudding.

If the pudding is covered, water still evaporates out of the pudding, and as the air above the pudding absorbs this water, some of it condenses onto the plastic wrap.

Answer 2

The water is evaporating out of the pudding, so that leaves the top dry - because the water has evaporated out. If you cover it with plastic wrap, that evaporating water is trapped for you to see.