"Where is the food?" should be accepted.
Food is generally an uncountable noun. There are cases where "foods" is sometimes used, but this isn't one of them.
"but this isn't one of them"
Without context we can't see if this is one of those times. Both should be accepted.
"Where is the food?" is now accepted, but I wonder, does "foods" really exist? According to my dictionary the word "food" has no plural.
Yes, foods exists. It's used to distinguish different types of food.
Here is just one example of a book with foods in the title https://www.amazon.com/Medical-Medium-Life-Changing-Foods-Vegetables/dp/1401948324/ref=sr_1_4?keywords=foods&qid=1567523107&s=gateway&sr=8-4
Does the Latin "cibus" work the same as the English "food". Singular as a non-count and plural as different types?
The Oxford English Dictionary includes the words "analysis of foods".
That was something we said and did in our organic chemistry class.
i feel like cibus should alternatively be translated with "dish" - that's what we did in German class at least ("Speise"). it would make more sense in this sentence. or does cibus not work like that?