I believe it's more common to say that the food "is off" or "has gone off" in the U.K. and other Commonwealth countries such as Canada and Australia. It is less common to use those phrases in the U.S. Also, I believe that when food is "off", it's mostly started to "go bad" and might still be edible if you can stomach the "off" taste. If the food has "gone bad", it's really not edible without risking food poisoning. You can also say that the food is "rotten", especially if it's meat, fruit or vegetables. One is less likely to say "rotten" of grains, cereals, rice or bread. "Stale" is more appropriate for those foods. Most food is still edible when it's only a little past its expiration date. That date usually represents a "sell by" date or a "best if used by" date. The amount of time past the expiration date that one can safely consume a food depends on the food, your tolerance for food that is "off" and your risk tolerance.

I say, the food has gone bad, so that seems like the best answer to me

You can say the food is rotten or has gone bad and reached it's expiration date.

"inedible" is one word that will suffice.

This food is not eatable. That's enough!