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 [email protected] 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.
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