Metal detection of frozen meat blocks, false positive

I’ve recently started working as a supplier quality technician at a frozen raw dog food manufacturer, and I’m encountering issues with one of our raw materials. The product arrives as large frozen blocks (40 × 60 × 15 cm). At intake, we carry out a visual inspection for colour changes or off‑odours, check the temperature, then temper the blocks and run them through a metal detector. This step is not a CCP, as our CCP metal detection is performed on the finished product. The detector sensitivity is set to ferrous 6 mm, non‑ferrous 6.5 mm, and stainless steel 8 mm, and the machine was calibrated specifically for this raw material by metal detector engineers.

Because the material is beef or lamb tripe, it naturally contains traces of hay and other environmental debris. Recently, we’ve been experiencing a high number of metal detector rejections. When the quality team defrosts and inspects some of these rejected blocks, they occasionally find small wires, grit, or bolus. However, most rejected blocks are not inspected — they are simply returned to the supplier. This has created tension, as we are rejecting product without physical evidence and relying solely on metal detector alarms. The supplier maintains that their tripe is thoroughly cleaned and metal‑detected three times, and they argue that the high iron content of tripe can lead to false positives.

I recognise that we need to implement visual verification on our side and provide evidence with each rejection. The difficulty is that the quality team is pushing back, as defrosting every rejected block is time‑consuming and sometimes involves 30 or more blocks in a single run. If no foreign body is found, the rejection is likely a false positive.

I’m looking for ideas on how to reduce false positives with this type of raw material and how to improve our metal rejection process so we can maintain a good working relationship with the supplier. Any suggestions would be greatly appreciated.

Because the material is beef or lamb tripe, it naturally contains traces of hay and other environmental debris. Recently, we’ve been experiencing a high number of metal detector rejections. When the quality team defrosts and inspects some of these rejected blocks, they occasionally find small wires, grit, or bolus. However, most rejected blocks are not inspected — they are simply returned to the supplier. This has created tension, as we are rejecting product without physical evidence and relying solely on metal detector alarms. The supplier maintains that their tripe is thoroughly cleaned and metal‑detected three times, and they argue that the high iron content of tripe can lead to false positives.

If you sometimes find small wires, your supplier's metal detector either isn't working as intended, or the wires are too small to be detected with the required sensitivity. Are they using the same limits you are? 

Since you're also finding grit and bolus, I question their cleaning methods. Have you (or anyone else from your company) seen their procedures or visited for an in-person audit?

"When the quality team defrosts and inspects some of these rejected blocks, they occasionally find small wires, grit, or bolus"

I guess I'd be curious how often they find foreign material in defrosted blocks.   "Some" and "Occasionally" doesn't sound like a lot, so it would be interesting to see how many.   Although I know that it can be difficult at times to locate whatever caused a kickout as well.

I agree with SHQ that a meeting and visit may be in order, and this may be an issue that can be rectified together with your supplier with some discussion and overview of the processes of each location, and just tell them you don't want to accuse them of anything, but just work together to get to the bottom of it.   It's a pita for both parties no doubt, so working together to figure out root cause would benefit you both, make safer food, and save money.