I refer, for example, to, "In which foods does one find carageenan?" vs. "In what foods...?" Thank you!
That depends on context. If it is understood that one is selecting from a specific list of foods, it would be "which". If the question is open-ended, it would be "what".
These two options make the choice clearer, because "of these" makes it obvious that one is selecting from a specific list:
- In which of these foods does one find carrageenan?
- In what foods does one find carrageenan?