Quality-checking packaged dairy products

Metal detectors and X-ray machines both have a place on the production line. When used together, they’re a formidable force.

With a metal detector, metals that have one or both conductive magnetic characteristics — for example, brass, bronze and ferrous metals — will create a detectable signal. Nonmagnetic 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.

This type of product effect has for years posed a challenge for North American dairy producers, notably in areas such as cheese, butter, yogurt and ice cream, where moisture content can be high. With cheeses, variations in salt content and product density can contribute to the product effect.

Technology, however, 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.

Room for both

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. Typically, gravity systems are used for dry ingredients. For dairy, this tends to be limited to milk powder

X-ray machines are getting better, but they’re typically limited to 250 feet, around 75 meters, per minute. A gravity application has products passing through at some 800-1,500 feet per minute, over 450 meters, so at that point the metal detector is the best performer.

Products and their 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. This tended to trigger excessive numbers of false positive rejects. Simultaneous multi-frequency scanning now makes it much easier for dairy processors to identify hard-to-find metals in products with high conductivity, as it’s able to “tune out” the product effect.

These new levels of performance and reliability have been a game-changer for dairy producers, and one that can be applied to metal detection not just of finished packs, but also of free-flowing pumped liquid or powdered product. Other valuable benefits include the ability to more accurately inspect product packaged in metalized film or foils of the type commonly used to wrap butter and some soft cheeses.

Products that differ in density can impact an X-ray machine’s performance. Although the product’s total density can be calibrated for, cereals or dried fruits in a yogurt might not be. As the X-ray is measuring the total density, if there’s a contaminant in the yogurt container and the machine lands on a low-density cereal 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 nonferrous contaminates, particularly aluminum foil — a big contaminant that 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 quite often is 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 sliced cheeses products while detecting metal contaminants.

An X-ray has even more potential. It can look for missing components — for example, a preserve missing in a flavored yogurt, chocolates missing from a presentation box or identify a broken or mispackaged product.

In conclusion, 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.