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Article derived from Food Quality & Safety.

With over 23 years’ experience in food manufacturing, including holding various roles in quality assurance for several Fortune 500 companies, the one thing that always surprises me is that many manufacturing facilities still get customer complaints for metal in their products even though the companies have systems in place to prevent foreign material contamination. It begs the question, how can metal get through their process with metal detection equipment in place?


There are many ways that a piece of metal can make it through a manufacturing system if the proper systems are not put in place. Below are just a few examples of the most common system failures that may have caused metal to make it to a customer.


One of the most preventable failures tends to be the timing being off on the rejection system, which would allow product to pass or not get rejected. This can happen due to many factors. For instance, in air reject systems, the air pressure could be too low or the air blast not effective for the product to fully reject. Also, the timing for the rejecting system could be set up incorrectly. Be sure to set up a testing protocol to have one of your test spheres placed at the leading edge of the product and one product with the test sphere placed at the trailing edge of the product. This will assure your timing covers the whole range of the product length depending on where the metal is located in the product. Many manufacturers, such as Anritsu Infivis, have systems in place to assure contaminated products do not pass. For example, photo eyes assure that product is actually rejected in the rejection bin if the metal detector triggers a contaminated product. Or an alarm sounds if the air pressure gets low in the reject system. These features are ideal but there are many systems and options available to reject product from the metal detector so examine your own setup to determine the best method and ask your metal detector suppliers what they would recommend for your specific application.


Another failure could occur from a large piece of metal, such as a large bolt, making it past the reject mechanism. This occurs because a large piece of metal may be sensed by the machine at a different time than a small piece of metal and this would affect the timing of the machine for rejection. When a large piece of metal approaches the aperture, it could be detected earlier than your standard metal test sphere and if the timing delay is not long enough for the reject mechanism, it could reject other product before the actual metal piece gets there. This is how it could be missed and make it to the customer. The only way to assure this does not happen is to test a large piece of metal as part of your protocol. Many companies use a larger bolt or metal washer as part of their test procedure to replicate potential risk factors. Walking through a manufacturing line, you may find a spot where a nut or bolt is missing. It may have been removed intentionally but if not, how would you know where it will be found? Hopefully, not in transit to a customer. This is why it is important to review your lines daily for missing parts before product leaves your facility.


Double the Rejection Detection

The rejection of one piece of metal is fairly easy to assure with a good standard operating procedure. However, what happens when there are two rejections simultaneously on the line? If you have the proper type of reject mechanism and the timing is set up correctly, you should be good. However, picture a plunger-type reject mechanism, which is pneumatic. If the plunger pushes to remove one product by the time it retracts and pushes again, it may not be able to reject the next product depending on the type of product and set up of the machine. Many companies are now running test spheres at the leading and trailing ends of the product and they are running multiple samples in a row with their test spheres to assure that multiple contaminates will reject.


Train People on Handling Rejected Product

When product is rejected, make sure only trained people can access the rejected product. Employees with good intentions may put product back on the line if they see it sitting in an open rejection bin. That is why products should be rejected into a locked reject bin so only qualified employees with a key have access. The employees with access to the rejected product should be trained on how to evaluate the rejected product for foreign material and how to properly dispose of it. Many progressive companies are now taking the stance that any product that rejects from the metal detector is to be disposed of even if it is run back through the metal detector five additional times with no reject. They still evaluate the product to determine the source of contamination when possible, but no rejected product goes back on the line. All the proper controls must be in place to prevent false-rejects or you will spend your days evaluating and throwing out rejected products.


Placement of Metal Detector

The placement of the metal detector is just as essential as it functioning properly when it comes to metal detection. The best practice from a HACCP standpoint is to place the metal detector in the last position in the process to prevent a possible hazard. For instance, if you package bulk product into bags or bulk cases, it is best to have the product metal detected in the finished package if at all possible. If the product is metal detected before it is packaged then you take a risk that there could be an occurrence of metal happening after the metal detector and before it is packaged. Contamination of finished product can occur after the metal detection if “metal detectable” belting breaks and ends up into a bulk case or tote with what was considered good product. This can be prevented by implementing metal detection after the product is packaged. Some companies have two metal detectors in line at different locations. One metal detector is located up front in the process where it is able to detect to a more sensitive level before it is packaged and then one metal detector placed after it is packaged where it may not be able to be as sensitive.


Don’t forget, a metal detector’s sensitivity and capability can be affected by the size of the product being detected. For instance, if running a 4-ounce package of product through a 4-inch metal detector aperture, you are generally going to pick up much smaller pieces of metal contamination than if you run a 30-pound case with multiple packages through a larger aperture metal detector. Therefore, you have to decide the best place in the line to place a metal detector or detectors to eliminate the hazard. If you have any questions on the best placement, speak to others in the industry or reach out to a metal detector supplier for advice.


Chris Hetherman, an operating partner in X-ray inspection for food at Service Cold Storage, has 23 years of experience in the food manufacturing industry. Reach him at chetherman@servicecold.biz or 715-600-4657.


What is Phase Change?

In terms of metal detection, phase change is simply the change from a solid to liquid (frozen to unfrozen or in metal detector terms, wet to dry). Think of frozen and dry ingredients as one in the same as far as the settings on the machine and refrigerated and wet ingredients in the same way. Once a machine is set up for a frozen product and the product gets warmer, it will change to a more liquid state. The machine’s sensitivity will change compared to when the product was initially set up. Once this happens, it is very likely for good product to be rejected by the metal detector as if it had a contaminant in it. This event tends to have so many rejections that it will be hard to keep the process in control because product will be overwhelming the rejection systems and the line will most likely need to be shut down to get the process back in control.


As a solid frozen product begins to change to a thawed product, the water molecules become free-flowing and change the conductivity of the field by the metal detector. Also, different types of metals with different density and conductivity properties will detect differently in metal detection because of how they are sensed by the magnetic field. This is similar to why certain metals such as copper are used for electrical wiring because of their special properties for conducting electricity. Based on the phase change (frozen to liquid), the conductivity will change and affect the magnetic field. Most manufacturers who produce frozen products have experienced this event when products on the edge of being frozen have been falsely rejected from the metal detector.


When these events occur, it is important to remember the product could actually have metal in it. Don’t just assume the issue is being caused by the temperature or phase change. The facility may be running more product than the capacity of their freezer system, or the line may break down and the product may start to defrost before the line is up and running. If these situations happen frequently, the employees will become desensitized to having the metal detector reject a lot of good product and then when they have a real metal issue, they won’t react as quickly or appropriately as they should to control the issue. Product still needs to be checked to assure there is not metal present.—C.H.