Metal Detection in the Food Industry.

March 25, 2020

Metal detectors as part of quality assurance

A metal detector detects metallic foreign objects in non-metallic products and is used in many industrial production processes. Especially food production places high demands on product quality and hygiene.

How does a metal detector enhance your quality assurance?

A metal detector ensures:

  • Purity of raw materials – incoming goods inspection
    Those who can precisely check their raw materials are able to reduce defect costs and evaluate suppliers through early detection.
  • Purity of the finished product – final goods inspection
    The customer may only receive pure, metal-free products. The degree of metal-free products must always be as high as technically feasible.

  • Safeguarding of the production facilities – Process control
    Production facilities can be sensitively disturbed, damaged or even destroyed by metal parts that have been introduced. Machine and plant downtimes have a negative impact on the consistency of product quality and production volume.

  • HACCP – Critical Control Point (metal foreign bodies)
    A critical control point is a point in the production chain at which a lack of monitoring represents an unacceptable risk to health.

How does a metal detector work?

Metal detectors work according to an inductive measuring principle. A high-frequency, electro-magnetic alternating field is generated by a transmitter coil. If a metal part moves through the metal detector, this field undergoes a change which corresponds to the magnetic and electrical properties of the metal part.

The change in field is registered by a matched pair of receiver coils and then processed and digitally evaluated in the electronics

This measuring principle reacts to electrical conductivity and magnetism and thus detects all types of metal, including non-ferrous metals (e.g. aluminum or brass) and stainless steel. However, magnetic metals can be detected slightly better than non-magnetic ones. The measuring field penetrates the tested food and detects metal particles even inside the product or in packaging.

The food itself is not affected or changed in any way by the measurement.

Technical limits

The maximum sensitivity is limited by the physical laws underlying the measuring principle. Food often has a more or less strong electrical conductivity due to salt, sugar, minerals and water. In practice, the attainable sensitivity mainly depends on how well the metal detector can compensate for these product effects.

Classical metal detection systems

  • Metal detection conveyors: A metal detector is integrated into a conveyor belt, which encloses the conveyor belt in a frame or is mounted as a surface underneath it. There are various methods available to separate detected metal particles (e.g. pusher).
  • Downpipe: Giant material falls in a pipeline through frame or ring-shaped metal detectors. Detected metal particles are separated by a separation flap.
  • Inline detector as a component of a pipe conveyor system: This inline system is used to detect metal particles in pasty or liquid products and is integrated into the conveyor line (e.g. with a milk pipe fitting). Product parts contaminated with metal are discharged via special valves.

Place of installation

Packaged as well as unpackaged products can be examined for metal. The inspection of already packed goods is usually preferable, because a new contamination with metal is almost impossible at this stage. Unpackaged goods, however, sometimes take up considerably less space – so a much smaller metal detector can be used. Under certain circumstances, this can also detect significantly smaller metal parts.

Unpackaged goods or packed goods that cannot be monitored sensibly (e.g. because of metal-containing packaging or too large packaging dimensions) should be checked additionally. If a metal detector is primarily used for machine protection, it should be installed as close as possible to the machine to be protected.

Sensitivity and reliability

The basis for optimum sensitivity is the right choice of search coil. The size of the search coil must be adapted to the product characteristics. A rule of thumb is valid for most applications: The smaller the metal detector, the higher the sensitivity.

Optimum sensitivity is achieved when the metal detector detects the smallest possible test ball without causing false alarms due to external influences or fluctuations in product properties.

The highest priority is that the metal detector works reliably and without false alarms.

Changes to the detector settings may only be carried out by trained personnel. Every change should be documented so that the current operating status is always clear.

Test procedures and test intervals

The function of the metal detector is tested during operation with the help of metal test balls in defined sizes. A test ball is added to the product and both are passed together through the search coil. If the ball is detected by the detector in the process, it is ready for use.

The test intervals must be defined so that if an irregularity is detected, all products that have passed through the metal detector since the last test can be checked again. In addition, a test should be carried out at the beginning of each shift and at each product change.


The metal monitoring must be documented in writing in a company operating according to HACCP or ISO 9000. This makes the metal monitoring traceable and verifiable for inspection bodies or in audits.

A complete automatic documentation of the metal detection, the tests as well as all set parameters is carried out via digital interfaces or, if necessary, an additional protocol printer.

Metal contamination? Find the source and mark contaminated products

If a metal particle is detected in the course of production, it is imperative that research be carried out into its origin. The cause must be consistently eliminated so that further contamination from the same source can be excluded.

It is imperative to ensure that products detected as containing metal are not inadvertently reused or delivered to customers. These products must therefore be clearly labelled accordingly, e.g. with a red sticker with the inscription “Locked – metal!”


A modern metal detector is equipped with an automatic self-control and is basically maintenance-free.

Preventive maintenance by the detector manufacturer means consulting and checking on site whether the device is still correctly installed after long operation. It is examined whether external disturbing influences could be present and whether the search sensitivity is still optimal for current products and the current location. This also includes the training and instruction of new employees as well as the integration of the metal detector into the company’s quality management.

Product liability law

Already since 01.01.1990, food producers have been liable, irrespective of fault, for damage caused by their defective product. This can lead to considerable costs for recall actions or even legal proceedings.


HACCP (Hazard Analysis and Critical Control Points) is a fault prevention concept for the food industry.

For metal detection, HACCP:

  • The metal detector is a critical “physical” control point against the risk of metal contamination, because it is likely that a health risk for the end customer will arise if control is lost.
  • The critical limit for calibration to the smallest metal test ball to be detected is detectability. A product must be as free of metal as “Good Manufacturing Practice” (GMP) and state of the art. In relation to the metal detector, this means: All metal particles should be removed from the product, which a modern metal detector can just about reliably detect with optimal installation.
  • In any case, the critical limit value must be defined in such a way that the metal detector operates stably, reliably and without false alarms.
  • It must be ensured that all products are checked by the metal detector.
    The operation and results of the metal detector must be fully documented.
  • Corrective action after metal detection: see Causal Analysis.
  • Preventive maintenance: see Maintenance.

DIN ISO 9000

It is advisable to draw up an operational working instruction for the metal detector which, for example, specifies the measures to be taken after metal detection and the regular testing procedure. A sample working instruction is available from CASSEL Inspection on request. All points mentioned under HACCP also apply to ISO 9000.

European Food Hygiene Regulation

The EU directive on food hygiene 93/43/EG obliges food producers to set up a self-control system based on the HACCP system. A metal detector is not explicitly required by the directive, but the HACCP principles include the control of physical hazards such as metal contaminants.

The above-mentioned guideline has been adopted into German law by the Lebensmittelhygiene-Verordnung (LMHV) of August 05, 1997. Food companies are obliged to carry out in-house control measures on the basis of the five principles of the HACCP concept listed in §4 Para.1 LMHV. It is strongly recommended – also from a product liability point of view – to document the performance of the internal control.