M.P.
Does she closely examine anything else besides food? If so, this may be a sensory or obsessive compulsive disorder.
Does she handle food this way at school or with friends in their homes?
When you ask her why she does this what does she tell you? If she expresses concern about the safety of her food or if she says she can't stop doing it even tho she wants to, I'd get a psychological evaluation.
If she doesn't know why she does it or just shrugs, my inclination would be to tell her that this is not acceptable behavior and send her away from the table and the food when she starts doing this. She can return when she's ready to eat, using utensils and without the excessive examination of food. If she's still not able to comply after a few days of enforcing good behavior, I'd talk with her pediatrician or get a psychological evaluation.
I'd tell her ahead of time what you plan to do. I'd be light handed about doing it. No anger, using a light calm voice. I would not allow snacks if she's unable to eat at the table in a reasonable manner. I'd be patient, not expecting her to completely stop at first.
I guess I'm thinking that this could just be a bad habit while my instincts are telling me that this is most likely an emotional issue. Is she showing anxiety in any other ways? Is she sleeping well, getting along with others, worried about anything?