The cubes appear to be soft, but not as soft as a marshmallow, definately like fruit, and while searching I found this comment from Claudia Black on the food cubes in the episode 'DNA Mad Scientist':
"Now, these food cubes, I know this is a bit killing the mystery, but they're called 'Hawnalis'. It's a fruit. H-A-W... an Asian, um, some kind of like a sweet... biscuit."
Now I've seen the food cubes do look different from episode to episode, so I'm curious as to if they are always made from the same stuff or if they are made of different food in each episode.