Metal is considered a hazard and the FDA rarely accepts a factory with a high production capacity juice filling line without a validated metal detector somewhere after the bottle has been capped. It is fine to have a light checker for other particles such as rubber or cloudiness but for metal fragments light checkers are often not so effective and metal particles in a drink is not only a hazard but a potential liability problem. So manufacturers rely on a metal detector for safety. Often times the metal detector is connected to the control system of the line so that the line is stopped or somehow the bottle with the problem can be eliminated from the juice bottling line.