Is there a single word which means "turned out to be''?

Answering the general question, the word is proved. In fact one definition for prove over at both Wiktionary and Merriam-Webster is literally "to turn out to be". Quoting the former,

3. (copulative) To turn out to be.
Have an exit strategy should your calculations prove incorrect.

In your specific example sentence, however, I'd prefer actually saying "turned out to be". Otherwise the register seems off.

'was' fits in the sentence you give. "Turned out to be" implies a change that never actually happened.

I thought the food was real, but it was plastic.

I thought the food was real, but discovered it was plastic.

Not exactly a match for "turned out to be" since "discovered" is about you and not the food, but for this sentence it seems to work as needed.

'Transpired' might fit the bill, but it requires a slightly different structure.

(of a secret or something unknown) come to be known; be revealed:

I thought the food was real, but it transpired it was plastic.