Acetic anhydride had been used previously in order to extract citric acid from food samples. Presently acetic anhydride is no longer in use, so can anyone suggest a feasible method of extracting citric acid from solid foods?
As you know, extraction of citric acid from solid foods (including whole wheat flour) requires finding out a suitable selective "safe" solvent for citric acid. This is not an easy task as I have observed upon giving "synthetic citric acid" as an unknown to 2 of my good students in an advanced organic chemistry practical course long time ago. They experimented a lot but did not find an answer.
Recently, I found an interesting article on this subject but it needs patience in reading. It can give guidelines which may benefit your research,hopefully.