The food we eat must be safe, free from hazardous foreign bodies, and pose no risks. These basic requirements, which in most countries are required by law, must be met by all companies involved in the food industry. Anyone wanting to offer their fish products on the market thus has to comply with requirements such as those defined in Directive 21CFR Part 11 of the Food and Drug Administration of the USA (FDA). The number of inspections, tests, laboratory analyses and certificates required of food manufacturers to market their products has grown greater and greater over time. Some of the necessary controls are concerned only with the finished products, others have to be carried out continuously during the ongoing production process. Testing for metallic impurities is a particularly frequent necessity. Food producers employ highly sensitive metal detectors to detect such impurities so that they can remove the products that contain them from the production chain.
In doing so, they meet statutory requirements and prevent potential risks to consumers. At the same time, however, they are also protecting their processing machines from possible damage which could result from undetected metallic foreign bodies. Metal detectors are an integral part of companies’ HACCP concepts and are often regarded as an essential prerequisite for successful certification, e.g. according to IFS, ISO or BRC standards. European Directive 93/43 / EC, which obliges European food producers to establish effective control systems for food hygiene based on HACCP systems, does not explicitly stipulate the use of metal detectors. However, this is not necessary since the HACCP principles themselves already specify effective control of the possible physical risks. According to HACCP hazard analysis, all sensitive areas within the production chain that could present physical, chemical or biological hazards are considered "critical control points" (CCPs) that must be monitored on an ongoing basis. Possible physical hazards include not only bits of broken glass, tiny stones or splinters of bone, but above all product impurities caused by metallic particles.
Metallic impurities can enter foods in various ways, for example due to abrasion of moving parts or broken machine parts, especially of the knives and cutting tools used. Metal fragments can enter the process chain during maintenance work or as a result of carelessness of employees or targeted sabotage. In the fish processing sector hooks that have been left in the fish are often to be found. Metal particles can thus already be contained in the raw material deliveries or they can enter the product later on during the production process. Even if they cause no harm to consumers the consequences are often serious and costly because recall actions damage a brand’s image and reduce trust in the products concerned. If metal fragments damage the processing machines in any way repairs can be very time-consuming, and the search for the source of contamination often results in production losses. A lot of fish processors do not only use a metal detector at the end of the production chain but install several such devices at various points along the processing line right from the start when the raw materials arrive so as to be able to detect and effectively remove any defective material wherever it occurs.
Growing spectrum of metal detectors
Modern metal detectors are able to detect a variety of different metallic contaminants, both ferrous and non-ferrous metals, aluminum, and even stainless steel. The detection method uses the contaminants’ magnetic properties or their electrical conductivity. Ferrous contaminants are both magnetic and conductive, non-ferrous contaminants are not magnetic but they are good conductors, so both groups are relatively easy to detect. Stainless steel, however, is available in various qualities, many of which are non-magnetic or are poor electrical conductors and so difficult to detect. The ability to detect stainless steel is also hindered if the products are wet or have a high salt content, something which is often the case during fish processing. In addition to salt and water, normal detectors also respond to an increased sugar or mineral content in a product which in this context is then sometimes termed “product effects”. In order to eliminate such effects without sacrificing the detector’s sensitivity special metal detectors are required which are equipped with multicoil systems and evaluation electronics. Electric smog or strong vibrations at the installation site can also reduce a detector’s sensitivity.
Not every metal detector is therefore suitable for every application. A number of features and characteristics must be taken into account when selecting them. For one thing it is necessary that the sensors are adapted to the size of the products and their expected metallic contaminants. Large detectors are usually less sensitive when used with relatively small products or cost considerably more when fitted with accordingly higher sensitivity. But not only the size of the detector is important: the temperature at the installation site, the degree of protection of the sensor and the material it is made of also play an important role. Hygiene standards are high in the food sector and this means that appliances have to be made of stainless steel or food-grade plastics. The range of available accessories and optional equipment should also be considered, for example, if the detectors are to be integrated into a processing line or a company’s data networks. Irrespective of the material throughput, the size and condition of individual products, it must be guaranteed that all metal particles are reliably detected and removed from the processing chain. The detection limits are based on the latest state of the art and are defined, for example, by GMP standards (Good Manufacturing Practice). Frequent checks at different points of the production process are important in order to prevent a metallic impurity from being increasingly fragmented during processing so that it is no longer reliably detected during final inspection.
Metal detectors are used particularly frequently on conveyor belts. They usually surround the conveyor belt in the form of a frame through whose opening the products for inspection have to pass. These units always include a mechanism that selectively removes metal-contaminated parts from the belt. There are various options for achieving this, for example using pushers or air blast rejection which directs an air jet precisely at the undesired product. Metal detectors are also used in downpipes, through which loose or powdery materials pass. To detect metal particles in liquid or paste-like products such as sauces or dips so-called inline systems are used. Here the impurities are removed by means of separating filters or special 3-way valves.
Detection sensitivity has increased significantly
Today there are numerous possible applications and installations available that make metal detection both fast and precise. Although there are different types of metal detectors most of those used in the fish industry are based on the transmitter-receiver principle ("balanced coil system") which has been known for a long time but has only been used for metal detection for just over 60 years. These detectors are based on sensors with one or more coils and operate according to an inductive measuring principle. They are usually equipped with one transmitter and two receiver coils. The transmitter coil generates a permanent, high-frequency electromagnetic alternating field, comparable to that of a radio transmitter. The two receiver coils are usually arranged in front of and behind the transmitter coil. When a metal part passes the detector the magnetic field is changed, and this is registered by the receiver coils. The reaction depends on the conductive and magnetic properties of the detected metal. The field lines can, for example, be deflected and altered in shape, or the field strength can be slightly attenuated. Sometimes the signal in the receiver coil is hardly more than a fraction of a volt. Nevertheless, these effects are registered by the sensitive electronics, electronically amplified and digitally processed in such a way that the intended all-or-nothing reaction takes place. In the end, this corresponds to the decision whether the product remains in the process or is eliminated from the belt.
Another detection technology, which is only suitable for detecting ferrous metals, however, is based on magnetic field systems. When a metal-contaminated product passes through the tunnel of the sensor, all the iron-containing particles are magnetized by the generated magnetic field. During their further passage they subsequently induce a weak current pulse in the subsequent coils of the detector, and this is amplified by the measuring electronics and serves as a trigger for the output signal of the sensor. Here again, the decision is always plus or minus: the product remains within the process or is removed.
As might be expected, there have been considerable technical advances, particularly in the field of electronics, which have significantly improved the performance, sensitivity and reliability of metal detectors in recent years. Nevertheless, the performance of these systems has certain limits which must be taken into account to ensure detection accuracy. These include, for example, product effects resulting from substances with dielectric or electrical conductivity properties which can produce effects similar to metal particles. Detection ability is also influenced by the size ratio between the opening width of the detector tunnel and the product dimensions. The smaller a product and the larger the tunnel, the less certain is the detection of metal particles. In general, the sensitivity of the detectors is inversely proportional to their size. In order to be sure, therefore, their accuracy must always be checked at the geometric centre of the tunnel opening, the least sensitive position.
Qualified staff needed for care and maintenance
To ensure the functioning of the sensitive systems, metal detectors must be serviced by professionally qualified staff – not only in the event of a disturbance in work routines, but also on a regular basis within the framework of a maintenance programme as a means of problem prevention. Drilling, welding and soldering work on the sensors must be avoided as well as provisional repairs, for example using wire, additional screw connections or adhesive strips. Nuts, screws, and washers that are required during repair work on the detector must be stored in labelled containers. Abrasive dust and other metallic residues should never be blown off with compressed air but must be removed immediately, completely and safely. Regular inspections which, depending on product throughput and risk of hazards, often have to be carried out and protocolled several times per day, also include checks on the cutting tools and fixtures, sieves and metal conveyor belts for breakage, excessive wear or visible splintering. The company’s cleaning and disinfecting staff should also be made aware of the peculiarities of metal detection and its susceptibilities.
Before production is resumed after repair, maintenance or cleaning work has been carried out it is necessary to check the correct functioning of the metal detector. However, such controls should also be carried out during the ongoing production process, at the beginning and the end of the day's production, at change of shift, or in the case of new products or product batches. The shorter the interval between controls is, the smaller are the product quantities which have to be rechecked or even recalled if the control system fails. Most manufacturers of metal detectors also offer test specimens made of different materials in various shapes and sizes which can be used to determine the correct functioning of the sensor or to identify any possible reduction in performance. Usually, these are spherical or strip-shaped structures which are passed through the detector in a product dummy. If the foreign body is detected, the test is considered passed.
In standard detection practice it is usually sufficient to carry out one test per test specimen and position. However, if the sensitivity of the system is to be tested, e.g. for a BRC certification ("verification tests"), several tests are necessary. This is usually done according to the "worst case scenario" and so the tests will be carried out under conditions that place the highest demands on the metal detector. In this case test specimens will be used that are particularly difficult to identify, or the products will be placed at locations where the detector is most insensitive, or several contaminated products will be sent through the sensor directly one after the other. Only if, in spite of these harsh conditions, all test objects are recognized and cleared from the belt or alarm signals are triggered is the detector suitable for the intended application in practice.
As in the case of foods contaminated with metal the test products containing the test specimens must not, of course, enter the supply chain or the market. This is why they are usually marked with distinct colours which will be noticed at the very latest when the products are packed. And it is no less important to ensure that the metal-contaminated products which are removed from the ongoing production process by the automatic sorting mechanism are correctly dealt with. In order to prevent an employee from accidentally putting them back on the belt they, too, must be clearly marked (e.g. with a warning label) and stored separately.
All companies – not only those that work according to HACCP, ISO 9000 or BRC standards – should document their package of metal monitoring measures completely and in writing. Above all they should keep records of the detector’s standard function, the performed tests as well as the set test parameters. This documentation is useful not only in the context of product liability but also gives the producers the desired security and simplifies procedures during possible recall actions. Many manufacturers of metal detectors now equip their devices with data interfaces which enable integration into company management systems or connection to protocol printers.