Metal detectors are installed on most of the world's food production lines and provide a highly reliable and relatively low-cost solution to ensure metal free product reaches the consumer and supports the user's desire for maximum brand protection. Metal detectors work by creating a magnetic field that the presence of metal contaminants disturbs, leading to their detection. However, in many situations the characteristics of the products themselves can also disturb the magnetic field.

Known as “Product Effect” it can impact the detection levels of metal contaminants within the product being inspected. Mettler-Toledo’s Mike Bradley, Metal Detection Systems Product Inspection Specialist explains that Product Effect is more complicated than many food manufacturers realise and explains what the suppliers of Metal Detections systems are doing to minimise its impact.

Product Effect is a not considered fact of life for many food manufacturers. If left unchecked it can lead, on the one hand, to good, contaminant free products being incorrectly rejected by metal detection systems, and, on the other hand, to actual contaminated products not being identified, and potentially reaching the consumer.

Many food manufacturers understand Product Effect in simple terms: there are “wet” products with high moisture or salt content that have a strong Product Effect; and there are “dry” products, such as bags of flour, that do not. It might be broadly true, but other factors – the type of packaging used, the orientation the product travels down the production line, the temperature of the product for instance – can also be contributing factors to the "Product Effect", and negatively affect the detection of metal contaminants. These factors are generally much less well understood in the market.

So, what is Product Effect, what causes it, and how can it be mitigated against? To begin with, we must quickly look at how a metal detector works.

Metal detectors typically have a coil arrangement with a transmitter coil loop running parallel with and between two receiver coil loops. The transmitter coil is energized with a high frequency current that generates a magnetic field, which in turn produces electrical signals in the receiver coils. These are connected in opposition to each other, so the resultant signal is close to zero. This state is referred to as balanced, and such metal detectors are therefore known as “balanced coil” metal detectors. When a metal particle or anything that is magnetic or electrically conductive passes through the coil arrangement it interferes with the magnetic field, causing a signal to be detected by the receiver coils, which is evaluated by the metal detector to confirm that metal is present.

It is this possibility of interference by substances that are magnetic or electrically conductive, but not necessarily metal, that gives rise to Product Effect, thereby complicating the detection of metal contaminants in product inspection.

The following factors can all cause Product Effect, and should be kept in mind when using metal detection in product inspection:

Moisture or salt content

Many food products are able to conduct electricity and disturb magnetic fields through the levels of moisture of salt present within the product. These effects might not be as strong as those generated by most metals. But while passing through highly sensitive metal detectors they are strong enough to make an interference in the magnetic field and therefore produce a Product Effect that disrupts contaminant detection.

Complicating the matter further is the amount of moisture or salt which can vary significantly from one pack to the next. Beef, for example, will have variations in moisture and salt content based on the cut of the beef and even the animal that the meat originated from. If a product has been marinated this also causes inconsistent levels of conductivity.

Temperature

The temperature of the product being inspected can have a major impact on its ability to affect the metal detector. When frozen, the conductivity of a wet product drops to almost zero, and generates a much smaller signal compared to a chilled product. However, as a frozen product begins to thaw, the situation changes. Condensation forms on the outside of the product, increasing its conductivity and product signal.

Size and shape of the product

Uniformity in the shape and size of a stream of products is another consideration to keep in mind. Generally, with packaged products, there is a good degree of uniformity, and the packs therefore give a consistent product signal. However, think of a conveyor belt of whole fresh chickens heading into the metal detection system: will they all be of the same size, shape and weight? Quite conceivably they will vary in some or all of these features, and a bigger chicken typically gives a stronger product signal than a smaller chicken, potentially confusing the metal detector as it looks for contaminants.

Position and orientation of the product through the detector

On the production line, it can be difficult to control the orientation of a product as it goes through the metal detector. It can also be difficult to ensure that products are consistently positioned on the same area of a conveyor – for example, centrally, or at the sides. Both of these factors can significantly impact the signal that a product gives. To return to the chicken example, a chicken that passes through a metal detection system with its short edge leading (i.e., head-first) will present a smaller signal to the detector than a chicken passing through with the long edge leading.

This complicates the detection of metal contaminants, as does the position of the chicken on the conveyor. The magnetic field in a metal detector aperture is weakest at its centre, and strongest close to the sides. A product passing through the centre of the aperture is therefore exposed to a weaker magnetic field than a product passing near the sides, and the effect of its own level of conductivity will be different on the effectiveness of metal contaminant detection.

Consistency and density of the product

This is a key consideration. The consistency and density of the product will affect the signal that the product delivers to the metal detector. With food products, variations in the ratio of certain ingredients – bone content in meats, for example – affect density and consistency, and therefore the Product Effect on the metal detector. This is one of the biggest challenges in inspecting products such as ready meals, where variations in the quantity and ratio of elements with different densities and consistencies (for example, mashed potatoes, sausages and gravy in the same tray, for example) could present very different product signals as they pass through the detector’s aperture.

Packaging material

This is an often-overlooked factor, possibly because many packaging materials have little or no effect on a metal detector’s sensitivity. However, if the packaging includes metallised film, which is typically a thin aluminium coating on material such as polypropylene or polyethylene terephthalate, this will have relatively good conductivity levels. It allows the metallised film to disturb a magnetic field, and that could have a significant impact on metal detection performance, making it more difficult to detect actual metal contaminants. One way around this potential problem is to inspect products before they are packaged into metallised film. Using a throat metal detection system can also help in effective contaminant detection of products packaged in metallised film, although these types of systems are not suitable for all applications.

How Technology helps to mitigate Product Effect

Using the right metal detection technology is essential to overcome Product Effect and getting this right can also have a significant positive impact on your operational efficiency. For example, the False Reject Rate in your factory, and the amount of waste you generate as a consequence of this, will both reduce substantially with the right system. You will also have greater confidence in both your product inspection regime, and your ability to comply with regulations.

One way to deal with Product Effect is through changing the frequency at which a metal detector operates. It is possible to set a system up to operate as low as 25 kHz, and as high as 1 MHz, but there is a trade-off involved in choosing either a high or a low frequency. High frequencies are more sensitive to small metal contaminants however they are also more sensitive to product effect, which can severely compromise detection capability if the effect is significant. Lower frequencies tend to be used as product effect increases with a smaller compromise in detection capability.

In recent times, the advent of MSF (Multi-Simultaneous-Frequency) metal detectors means that these systems can operate using combinations of two or more frequencies at the same time, reducing this trade-off between Product Effect, maximum operating frequency and metal detector sensitivity. Advanced discrimination techniques allow the use of multiple frequencies, which minimise the resultant product signal, and hence allows the user to realise the benefit of higher frequency detection capability when inspecting products with high or even gross Product Effects

The most sophisticated metal detectors available today have a dedicated Digital Signal Processor, that handles phase discrimination and other advanced signal processing techniques. Allied with MSF technology, these systems are able to effectively remove product signals, allowing for much smaller metal contaminants to be detected.

A common technique is called “phase discrimination”. It separates, or “discriminates”, between signals, allowing it to reduce or ignore a product signal once it has learned its characteristics, while amplifying a metal contaminant signal.

Such technological developments should give food manufacturers much greater confidence that they can overcome the challenges of Product Effect, however it is caused, and deploy metal detection systems as a key component in product inspection. With so many factors to consider, they should seek to discuss their needs with a good metal detection system supplier, helping to identify the right choice of technology, which leads to better contaminant detection, regardless of Product Effect, and the consequent upholding of high food safety standards.

To download a whitepaper on this subject click here.

Author - Mike Bradley, Metal Detection Systems Product Inspection Specialist, Mettler Toledo