With a metal detector, metals that have one or both conductive magnetic characteristics — such as brass, bronze, ferrous metals, etc. — will create a detectable signal. Non-magnetic stainless steel is harder to pick up — it’s a bad conductor — and certain food products with added iron, moisture, salt and acids tend to mask metal detection. Known as the “product effect,” this can impact inspection performance.
However, technology has moved forward and detectors are now suited to wet and dry applications. Even conductive products consistently presented in small packaging will be picked up.
With an X-ray machine, light waves pass through the product to be inspected. Anything very dense will block the waves. Yet, low-density materials, such as wood, feathers, bugs, some stones/glass, magnesium and aluminum, allow the light waves to pass straight through.
When working together, the X-ray machine will favor an in-line inspection position while the metal detector is better at the end of the line, typically where products have been packaged. Gravity systems are the one exception for in-line processing, as the products move through the machine at such a fast speed, this type of application doesn’t lend itself well to X-ray detection.
X-ray machines are getting better, but they’re typically limited to 250 ft, around 75 m per min. A gravity application has products passing through at some 800-1500 ft per min, over 450 m, so at that point the metal detector is the best performer.
Products and its packaging clearly affect detectability. A metal detector was, until quite recently, limited in how it performs on products with added iron and any salt/moisture. Manufacturers of “wet” products often had little choice but to calibrate their metal detection in a way that reduced margins of error. The consequence of this was it tended to trigger excessive numbers of false positive rejects. Simultaneous multi-frequency scanning now makes it much easier for food manufacturers to identify hard-to-find metals in food products with high conductivity, as it’s able to “tune out” the product effect.
Products that differ in density can impact an X-ray machine’s performance. Although it can be calibrated for the product’s total density, cereal ingredients, raisins and bran for example, have variable densities. Speckles of raisin will be quite high, the bran low and the box it’s in will have the same density. As the X-ray is measuring the total density, if there’s a contaminant in the box and the machine lands on a low density bran flake, it may not detect the sample in the same way it would when landing on a higher density raisin.
When it comes to packaging, recycled cardboard can cause issues for both machines. This cardboard tends to contain ferrous and non-ferrous contaminates, particularly aluminum foil — a big contaminant which an X-ray will overlook — and staples.
Both machines have potential bonus features for specific products and packaging. A metal detector offers "reverse detection." A cereal box for example, may have a toy that is quite often identified by a metalized filmstrip. The detector will ensure the toy is there, acknowledging the strip while inspecting for other metals. Similarly, it can ensure oxygen absorbers are packed into meat products while detecting metal contaminants.
An X-ray has even more potential: it can look for missing components, for example missing ingredients in ready meals or identify a broken or mispackaged product. Both machines work well together, however the metal detector should be the critical control point on the line and the X-ray machine a bonus.
For more details, listen to the latest Fortress Technology food safety podcast featuring Steve Gidman.