I think NASA puts some food in tubes, but a lot of it is packaged in packets and de-hydrated (they get all of the water out of it). I once ate NASA ice cream. It is like a really light airy biscuit. I prefer normal ice cream, to be honest.
Yeah I think all the food started off in tubes but now astronauts have a much wider range of foods that is not in tubes. I think they are more like microwave dinners (de-hydrated like Paul says), they just heat up (and rehydrate) on the space station. Apparently it doesn’t taste too bad.