Best practice metal detection on pouch tuna

what is the best way to use the metal detector before sealing or after sealing the pouch tuna?

I'd say after sealing, as long as you can prove the MD successfully detects your test pieces in the sealed bag with product.

If you MD prior to sealing, you'll need a step to verify there's no chance of contamination between MD and final sealing.

John is correct. The product should always be checked after sealing, if that is possible.

However if the pouch is foil (metalised film) then this may be difficult. If foil is used then the closest point to the sealed product as possible.

When validating the MD, place the test piece on or in the pouch (as appropriate) as close to the centre of the MD opening as possible, as this is the weakest point of the MD.

Important to note that the center of the aperature rule only applies to normal metal detectors. It's my understanding that X-ray units are weakest at the bottom.