Foreign Body Detection: Risk Assessment, Validation and Testing

InsightsForeign Body Detection2026

How to choose, position, validate and test metal detectors, X-ray systems, magnets and sieves, and what to do when they fail

Key takeaways

• Foreign body detection starts with a risk assessment: which materials could enter the product, where, and which technology can find them.
• Metal detectors and X-ray systems have different strengths; neither replaces good prevention.
• Sensitivity must be validated in the actual product, at line speed, with test pieces at the least sensitive position.
• Every test needs a defined failure procedure: stop, segregate product since the last good test, investigate, re-inspect.
• FSSC 22000 Version 7 (2.5.11) and BRCGS Food Safety both expect a documented risk assessment, testing procedure and handling of failures and rejects.

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1. Prevention first, detection second

Detection equipment catches what prevention missed. The foreign body programme should therefore include:

• Supplier controls and specifications for raw materials.
• Glass, brittle plastic, ceramics and wood registers with breakage procedures.
• Knife, blade and small-parts control in maintenance and production.
• Hygienic equipment design and preventive maintenance to avoid wear parts entering product.
• Personal effects rules (jewellery, pens with metal clips).

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2. Choosing the right technology

Technology	Detects	Strengths	Limitations
Metal detector	Ferrous, non-ferrous and stainless steel metals	Low cost, proven, fast	Product effect in wet, salty or warm products; foil packaging limits use; stainless steel harder to detect
X-ray	Dense materials: metal, glass, stone, calcified bone, dense plastics	Works through foil; can check fill level and missing items	Higher cost; low-density materials (wood, light plastic) often not detected
Magnets	Ferrous particles in free-flowing products	Simple, continuous	Only magnetic metals; need regular cleaning and pull testing
Sieves and filters	Oversized particles in powders and liquids	Simple, removes many types of contaminants	Mesh integrity must be checked
Optical sorters	Colour and shape defects, some foreign materials	Good for raw agricultural products	Limited for hidden contaminants

The choice depends on the hazards identified, the product characteristics and the packaging.

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3. The risk assessment

A documented risk assessment should cover:

1. Hazard sources: raw materials, equipment, packaging, environment, people.
2. Process steps: where contamination could enter and where detection is possible.
3. Detection point: usually as late as possible, ideally after final packing, so that later contamination is also covered.
4. Product effect: moisture, salt, temperature and density affect performance.
5. Expected sensitivity: what size of each contaminant the system can reliably detect in that product.
6. Reject system: automatic rejection into a locked bin, with confirmation sensors where possible.

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4. Setting and validating sensitivity

Validation demonstrates that the system detects the target contaminants in the real product. Good practice includes:

• Using certified test pieces for ferrous, non-ferrous and stainless steel (metal detectors) or appropriate test pieces for X-ray.
• Placing test pieces in the product, at the position of lowest sensitivity (often the centre of the aperture for metal detectors).
• Testing at normal line speed and with the heaviest and largest pack sizes.
• Recording the achievable sensitivity per product and fixing it in the product settings.

Many retailers publish their own codes of practice with target sensitivities; where customers specify them, they become requirements.

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5. Routine testing

Element	Typical practice
Frequency	At start-up, at product change, at regular intervals during production (e.g. hourly) and at the end of production
Method	Test pieces passed in or on a pack, in the least sensitive position, through the full reject cycle
Checks	Detection, rejection into the bin, bin-full and fail-safe alarms, belt stop where fitted
Records	Time, test pieces, result, operator, corrective action

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6. When a test fails

1. Stop the line and prevent further product from passing.
2. Hold all product produced since the last successful test.
3. Re-inspect held product through a correctly working detector.
4. Investigate the cause: settings, product effect, reject mechanism, test procedure.
5. Record the incident, the quantity re-inspected and the outcome.
6. Review the failure in trend analysis and, where needed, the risk assessment.

Rejected product must also be investigated: the contaminant found tells you where prevention failed.

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7. Common audit findings

• Tests done on the belt edge or outside the product instead of in-product.
• No defined action when a test fails, or product since the last good test not held.
• Reject bins unlocked, or rejects returned to the line without investigation.
• Sensitivity settings changed without authorisation.
• X-ray assumed to detect all plastics.

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Conclusion

Foreign body detection is a system: prevention, the right technology, validated sensitivity, disciplined testing and clear failure procedures. When any part is weak, the detector gives false confidence. When all parts work together, it is one of the most reliable controls a food plant has.

Walk to your detector now and ask the operator to show you what happens if the hourly test fails.

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