The best all-purpose word is really edible, like @Vaelus noted on @Rob's answer. You don't use it to describe desirable food, just stuff that's barely passable. Speaking of which,

#passable

is one possible word that hasn't been mentioned yet.

2. Of adequate or acceptable quality; sufficient; satisfactory. In later use: just good enough to be acceptable.

 

1893 June 12, The Times, p. 4:

Potatoes appear in eight departments... very good, 13 good, 17 satisfactory, 18 passable, six mediocre, and three bad. [sic]

The best all-purpose word is really edible, like @Vaelus noted on @Rob's answer. You don't use it to describe desirable food, just stuff that's barely passable. Speaking of which,

passable

is one possible word that hasn't been mentioned yet.

2. Of adequate or acceptable quality; sufficient; satisfactory. In later use: just good enough to be acceptable.

1893 June 12, The Times, p. 4:

Potatoes appear in eight departments... very good, 13 good, 17 satisfactory, 18 passable, six mediocre, and three bad. [sic]