If you were transported in time back, say 5000 years ago, what modern food item could you show people of that era that would be most unrecognizable to them as being food? It could either be by reason of its’ appearance or its composition or possibly some other criteria.
The floor is open for nominations. Radium orange colored flaming hot Cheetos anyone?
Freeze-dried ice cream. Not only would the natives not know what ice cream is, or cookies for that matter (or chocolate for that matter…), but the closest comparison they could make for this stuff is chalk.
5000 years is a long ways back. I don’t even know if leavened bread was around yet.
So my pick: red velvet cake
But you don’t need to go back in time at all: just take a trip to Japan ! I was served stuff that I could not even recognize as being from this planet !