With the increasing complexity of production and packaging processes in the manufacture of food, the risk of product contamination by foreign bodies has also increased. After microbiological pathogens, contamination with foreign bodies such as glass, metal or plastic particles is now the second most common reason for food alerts and recalls in Europe. The European Rapid Alert System for Food and Feed (RASFF) lists 856 entries of foreign body warnings for the first quarter of 2018 alone.

The number of contaminations caused by foreign bodies has increased twelvefold across the EU compared to 1970 to 1990 and 2011 to 2014. The latest RASFF annual report states that the three most common types of foreign bodies in food are metal, plastic and glass.

Risk analysis with HACCP or HARCP audit

Thanks to rapid advances in product inspection technology, many types of foreign bodies can now be reliably detected in food production. The main question for manufacturers is whether they should install a metal detector, an X-ray inspection system or both. The easiest way to decide between metal detection and x-ray inspection is to take a close look at the application. The first step should be a HACCP (Hazard Analysis and Critical Control Points) or HARCP (Hazard Analysis and Risk-based Preventive Controls) audit.

An HACCP audit identifies contamination risks in the manufacturing process and the types of foreign bodies that can occur. Appropriate product inspection technology must be installed at the critical control points (CCPs) identified in the audit to minimize risk. A HARPC audit also covers possible contamination hazards in the manufacturing process, but also takes into account other safety-relevant factors such as visitor access and control. In any case, it is advisable to also carry out product tests to determine the most suitable technology. The international trend is for large supermarket chains to increasingly require their suppliers to comply with strict codes of practice that exceed international food safety regulations.

Foreign body detection with metal detection technology

Metal detection systems are one option for product inspection: they identify all metallic foreign bodies, including ferrous (such as chrome or steel) and non-ferrous metals (such as brass or aluminum) as well as magnetic and non-magnetic stainless steels. The systems can be installed to inspect raw materials supplied for food before processing, at numerous points in the production process and at the end of the production or packaging line. The metal detection technology is suitable for inspecting a wide range of different product types, including loose, unpackaged or pasty products, pumped products such as liquids, bulk goods, loose powders and solids in free-fall systems, as well as packaged products. It can also be used to inspect large, robust containers such as bottles, jars and composite containers. However, care must be taken to ensure that the product is checked for metallic foreign objects before any metal closures are attached.

Influence of product properties

Products with a high moisture content generate similar signals to metallic foreign objects when they pass through a metal detection system. This so-called product effect can be eliminated using software algorithms.

© Mettler-Toledo

When deciding whether metal detection technology is the method of choice, the nature of the product - in particular its dryness or conductivity - must be taken into account in addition to the manufacturing and packaging process.

As a general rule, the detection of metallic foreign bodies is relatively easy with dry products. Due to the lack of moisture content, these products are generally non-conductive and the metal detection technology achieves a very high detection accuracy even for extremely small metallic foreign bodies. The situation is different for products with a high moisture content, such as chilled and salty products and products with a high water content: these are particularly challenging for foreign object detection, as they are conductive like metal itself. When passing through the metal detection system, these products cause a disturbance in the detection field and thus generate signals similar to metallic foreign objects. This so-called product effect can be largely eliminated by using software algorithms and setting the optimum operating frequency. A high-quality metal detector can therefore be operated at different frequencies and therefore offers a sufficiently high search sensitivity for the detection of small metal contaminants, even with moist and conductive products.

Problem case metallized foil

Products in metallized film packaging can usually be reliably inspected with metal detectors in low-frequency mode. However, depending on the thickness of the film, this can lead to a reduction in the achievable detection sensitivity. In some cases, it is therefore advisable to check very thick metallized film packaging for metallic foreign bodies before packaging.

A general advantage of metal detection technology is that the system can be designed for very different product sizes. The metal detector airlocks at airports clearly show which packaging and product sizes can be detected for foreign objects using metal detection technology if required. The rule of thumb here is: the higher the opening for the product to pass through, the lower the detection sensitivity of the metal detector. However, this reduction in detection sensitivity can be largely compensated for by adjusting the distance of the coil system in the metal detector. The use of variable frequency technology, in which the most suitable frequency is automatically set for each application and therefore a variety of products with varying moisture content can be inspected, also helps to solve any problems with detection sensitivity.