Every food/ingredient would have a completely different answer, so it stands to reason that they be asked as separate questions.
Remember that if and when the site goes live, most visitors aren't going to be looking for a gigantic list of methodologies for assessing the quality of every ingredient imaginable. For that, they could buy a book. What they really want is to solve a specific problem - I need to buy an X, but I've never bought an X before, how do I know if a specific X is a good X?
Of course, very similar questions like these should probably share a tag. I see you've used
[buying] and I don't know if I'm wild about that particular tag. Something a little more... specific, like
ingredient-quality or similar, might do more to provide a natural "grouping" of such questions. Especially considering that it's the same question whether that pineapple is sitting in a store or has been ripening in your fruit bowl at home.
That way, if somebody really does come looking for a compendium of food quality indicators, they can just search by the tag.
Aside from tagging, I see no reason to worry about the proliferation of such questions. They are all separate questions and all well within the site's definition.