How would you exaggeratedly express food that tastes nothing to sound even worse than "It tastes like cardboard"? I want a word that sounds beyond a average man can imagine. "Shit" would have its unique flavour, so I wouldn't put that in the list.
Food that's particularly bland and tasteless is often described as pablum (or Pablum):
a brand of soft, bland cereal for infants
It's amusing to note that when Pablum was introduced in 1931,
media reports at the time said the cereal tasted like "boiled Kleenex" and "had the consistency of mucilage and smelled like the inside of an old cardboard box."