Metal Off The Menu

Stuart Pritchard March 2, 2016

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Fortress’s Sales Director, PHIL BROWN, examines the science behind screening ‘wet’ products using metal detectors and counts the costs of false rejects…

One of the biggest challenges when using metal detectors to inspect food for contaminants has long been ‘product effect’. It occurs when a product has a conductive property, such as high moisture or mineral content, which affects the magnetic field generated by the metal detector.

Make certain food is contaminant-free

Make certain food is contaminant-free

Until recently, a metal detector’s ability to discriminate between the signal generated by the product and the metal contaminant has been a problematic issue for pre-prepared produce, bread, dairy, meat, ready meals and even supplements and breakfast cereals. However, last year, Fortress Technology unveiled its global solution: the Interceptor range.

The reason why false rejects are more likely to occur in ‘wet’ products comes down to basic physics – water, like metal, is a conductive, and the industry term ‘wet’ extends to any higher-conductivity product, be it wet, moist, with a high salt content or containing other conductive compounds.

Taking the example of bread, a fresh batch is more vulnerable to product effect. As well as salt content, warm baked bread can impact the metal detector’s ability to distinguish between any actual stainless steel metal contaminants that may have been introduced during the mixing process and the false signal given by the combination of product attributes. What’s more, the air bubbles and density of a loaf of bread will vary, even in the same batch.


Naturally, brand-owners and retailers want to eliminate the cost and reputational damage of undetected foreign bodies, and the recalls that may result from them. Equally, food manufacturers are under pressure to reduce – or eliminate – false rejects. With margins on product tighter than ever, there is the cost of the rejected item itself. More importantly, where a fault recurs, stopping the line may incur further cost.

In fact, the real cost-burden extends beyond this most immediate level. False rejects do cost the customer money, which primarily occur when a metal detector cannot discriminate between ‘product effect’ and a metal contaminant. Equally, the man-hours spent checking the performance of equipment and investigating false rejects are bound to impact line productivity and OEE.

Precise figures for rejected products will depend on the product and, clearly, the volume of false positives. Industry estimates regularly put the potential costs at up to £14,000 per year per production line, which Phil believes is realistic.


No nasty surprises assured with the Interceptor

No nasty surprises assured with the Interceptor

Back to physics, there are two distinct components to metal detection: magnetic permeability and conductive effect. This is where an understanding of the material differences in metals is essential. Most detectors will exhibit a different level of sensitivity for the three main groups – ferrous (such as iron or steel), non-ferrous (including copper or brass) and stainless steel. Ferrous metals are both magnetic and good electrical conductors so they’re relatively easy to spot. Non-ferrous metals aren’t magnetic but they’re good conductors. Stainless steels are not magnetic and are also poor conductors, so they present an added challenge. Because metal detectors work by spotting materials that create a magnetic or electrical disturbance as they pass through an electromagnetic field, stainless steel is typically the most troublesome.

In practice this means that a sphere of stainless steel hidden in a dry product typically needs to be 50% larger than a ferrous sphere to generate a similar signal size. That disparity can rise up to 200 to 300% in wet produce.


Fortress is the force behind the rise of the machines

Fortress is the force behind the rise of the machines

In recent years most developments in metal detection have focused on the coils that transmit and receive multi-frequency signals. However, even these most advanced product compensation and phasing methods have limitations.

Fortress’s new Interceptor metal detector takes a new approach to Simultaneous Multi-Frequency, splitting the product and metal detection signals and then linking the readings back together. The low-frequency range can be used to eliminate the product effect, leaving any stainless steel signal in the higher-frequency range more readily identifiable. It takes into account the background noise from the product and looks for an additional ‘blip’ beyond that which it then eliminates.

With this latest way of applying metal detection technology, Fortress calculates that it can identify metal contaminants in wet products down to half the size of those detectable with the previous generation of equipment, with the same degree of reliability. Although a test sample sphere isn’t a real-world contaminant, halving it to 0.5mm equates to a wire length contaminant measuring around 2.5cm. From a risk assessment perspective this makes a huge difference and goes a long way to protecting brand reputation and keeping metal off the menu.