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Supermarket recalls: Why detectors sometimes fail

15 February 2016

Last month, Sainsbury’s made national headlines when it issued a recall for bread that may have been contaminated with metal fragments. Sarah Ketchin, Managing Director for Fortress Technology, discusses the impact of metal contamination on brand reputation and how to mitigate the risks. 

Any contamination is a food safety issue that can have an incendiary effect on a food business’s reputation. Of all the potential contaminants, metal is still the most likely contaminant risk in a food processing and packing plant and the preparation of bread means that it can be exposed to a variety of mixing, sieving and baking processes where metal fragments could potentially enter the food chain.

On 18 January 2016, the Food Standards Agency announced that Sainsbury’s, as a precautionary measure, was recalling nationwide it’s own-branded wholemeal bread medium sliced and thick sliced with a best before date of 19 January 2016. This was due to a possible presence of tiny pieces of metal in the bread. No other products or date codes were affected by this recall.

The supplier will certainly have had metal detection equipment in place, so how could this happen? The answer is that no system can entirely eliminate the risk of metal contamination. However, optimising metal detection systems can manage that risk more effectively and reduce it as far as is practical. Food operators should understand the factors that can leave them vulnerable, even those that already have metal detectors in place.  

Material difference

First, there’s the widespread use of stainless steels in the food industry. These are more difficult to detect than ferrous metals such as iron and steel or non-ferrous metals such as copper or zinc. That’s because metal detectors work by spotting materials that create a magnetic or electrical disturbance as they pass through an electromagnetic field. 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. In practice this means that in a sphere of stainless steel hidden in a dry product typically needs to be 50 per cent larger than a ferrous sphere to generate a similar signal size. That disparity can rise up to 200 to 300 per cent in wet products, and bread falls into this category. 

Understanding product effect

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 electromagnetic field generated by the metal detector. In the bakery category, bread, muffins, cakes, pastries, raw dough, chilled pastry and fortified cereal bars have this conductive property.

The scientific reason behind this ‘product effect’ occurrence is largely due to physics. Water and mineral content can create a large conductive signal that the detector must overcome in order to detect small pieces of metal.

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. Plus, a lot of freshly baked goods, cakes included, are baked in tins, so there’s another potential source of metal contamination.

Orientation effect

The signal produced from a wire shape will vary greatly depending on the type of metal it is and on its angle when it passes through the detector. This is known as orientation effect. For example, a stainless steel wire that passes through the aperture upright or sideways generates a higher signal than a straight.  In the worst case a wire may produce a signal no bigger than a sphere of the same size as the diameter of the wire. It’s therefore important to optimise the performance of the detector to cope with the worst-case scenario. 

An improvement in sphere size from 3mm to 2.5mm may not sound like much but it can be the difference between success and failure when trying to spot an irregular fragment. 

Test and record

It’s also vital to check that any metal detection system is fail-safe. So, for example, if a fault with the reject system means that a contaminant is detected but not rejected, the line should stop automatically until the situation is resolved. 

Both the detector performance and fail-safe capability should be tested regularly and full records kept to support traceability. 

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