By Rob Stevens, Market Manager, Mettler-Toledo Safeline Metal Detection
Metal detection is a crucial technology for food safety, helping manufacturers maintain high quality standards while meeting regulatory and retailer requirements. However, to be truly effective, metal detectors must undergo routine testing to verify performance, confirm compliance and reduce contamination risks. Understanding the different testing methods and their role in quality control is key to preventing costly product recalls and achieving the utmost consumer protection.
Validation, Verification and Routine Performance Monitoring
Testing metal detectors is not a one-off task. Instead, it involves three distinct processes: validation, verification and routine performance monitoring.
- Validation is conducted at the time of installation to confirm that the metal detector is fit for its intended purpose. It assesses whether the equipment meets the specified objectives based on the products being inspected and their unique characteristics.
- Verification takes place at regular intervals (typically every 6 to 12 months) to confirm that the detector continues to operate within the required sensitivity levels. This process means it is optimised to identify non-ferrous and ferrous metals, plus stainless steel and reject contaminated products while adhering to safety standards.
- Routine performance monitoring occurs frequently during production, checking for any shifts in sensitivity that may result from machine setting adjustments, product variations or equipment degradation.
Testing Methods for Different Metal Detectors
Metal detectors can be used at various critical control points across the complete production line, from raw material inspection at the beginning of the line to packaged end-of-line and everywhere in between. Types of metal detectors range from metal detector heads which can be integrated with a conveyor, gravity-fed systems for bulk or free-falling products and pipeline systems for pumped products. Each application requires a specific testing approach for accurate detection.
For conveyor metal detection systems handling packaged products, test samples should be placed in different positions – leading edge, middle and trailing edge of the pack – to check if the detector can consistently identify and reject contaminants regardless of their location. Similarly, bulk product lines require evenly spaced test samples introduced into the product flow to verify rejection mechanisms are functioning correctly.
In free-fall (vertical packaging) systems, test pieces should be dropped into the product stream above the metal detector, ideally at the point where the product starts to fall, to confirm detection and rejection accuracy. For pipeline applications, which inspect liquids, pastes and slurries, test samples should be introduced upstream of the detector to be sure that contaminants are successfully diverted to the reject bin.
Each test should replicate real-world production conditions as closely as possible, including worst-case scenarios where contaminants pass through the least sensitive area of the detector.
Retailer Requirements and Additional Test Routines
Many large, high-street retailers have strict codes of practice that outline additional test routines beyond standard procedures. These are designed to strengthen quality control measures and improve detection reliability. Some common examples include:
- Standard Tests: Conducted regularly, these tests assess the detector’s ability to identify contaminants across different areas of the product, offering consistent performance during production.
- Consecutive Tests: These involve passing multiple contaminated and non-contaminated samples through the metal detector consecutively to verify consistent detection performance and rule out false rejects or missed contaminants.
- Memory Tests: These involve running an alternating sequence of three contaminated and two non-contaminated packs through the metal detector to assess whether it can accurately distinguish between them. This helps verify that detection settings are optimised and that the system does not produce false rejects, i.e. rejecting good product, or allow contaminated products through unchecked.
- Large Metal Tests: This checks that the photogating system is working correctly by introducing a significantly larger metal test piece, typically a 20mm ferrous contaminant. This means that the detection system does not overlook obvious contamination and that alarms and reject mechanisms trigger appropriately.
Frequency of Testing
Retailer requirements often specify the frequency of routine tests, which should align with production cycles. Testing could be conducted at the beginning and end of shifts, during product batch changes, after machine setting adjustments and following any maintenance downtime, depending on risk assessments and specific requirements.
Best practise is that the product should be quarantined between tests and only released after a test with a pass result is completed.
