Food Engineering

Tech Update: Metal Detection and X-ray Equipment

November 1, 2009
Contaminated products can severely damage any manufacturer’s reputation. Adding metal detection and X-ray equipment to your lines helps increase product value.

MDP metal detector performs final inspection at a food plant in Germany. Source: Sartorius Mechatronics.


Vision systems guide robots. Vision systems also ensure that tomatoes are ripe, lean beef has little fat, and hamburger rolls are properly baked. But vision systems can’t tell if there’s a splinter in a tomato, a broken hypodermic needle in prime beef or a stone in a hamburger bun destined for a major chain restaurant.

“Seeing” embedded, invisible objects in food is the work of metal detection systems and X-ray equipment. And seeing these "hidden" objects is something that needs to happen in your plant-not at your customer’s location. After all, you don’t want consumers ingesting an unseen foreign object. Consumers have long memories when they bite into a contaminated product, and hungry lawyers are waiting to take the case.

While regulations such as BRC, IFS, SQF and ISO 22000 encourage processors to explicitly define HACCP plans for foreign object contaminants, there’s nothing like customer pressure to encourage the use of metal detection and X-ray systems to put HACCP into practice. “What drives processors toward X-ray or metal detection?” asks Doug Pedersen, S+S Inspection inspection systems sales manager. His client’s customers are beginning to demand that all products leaving the plant be X-rayed for foreign object contaminants. In fact, Costco has been moving suppliers toward X-ray systems for some time.

When asked whether paying the price for an X-ray system is like buying risk insurance, Mettler-Toledo Safeline Head of Sales and Marketing Mike Bradley responds, “Yes, bigger [retail] organizations consider brand protection of primary importance. In fact, processors won’t be able to sell to the Wal-Marts without this equipment.” He notes that large retailers look at it from a compliance perspective. There is always the threat of litigation if someone comes across a stray piece of metal in a product. He points out that it’s not just large processors who will have to comply to keep the Costco and Wal-Mart business; small- and medium-sized processors will also be expected to comply.

Purchasing metal detection and X-ray equipment to please a customer like Costco has differing ramifications, however, for small and large processors, says Robert Scott, product manager, X-ray Inspection, Mettler-Toledo Safeline. To gain advantage over its competitors, a small processor will buy an X-ray system to supply product to Costco. The X-ray system provides two solutions. First, it allows the processor to inspect product, which, for example, might be in an aluminum tray where a metal detector would be unable to detect a metal fragment. Second, by purchasing this equipment, the processor keeps the account.

The ramifications for the billion-dollar processor, however, are more encompassing. If the processor decides to put an X-ray system on a particular production line in a single plant, then it’s obligated to put the system in all the same or similar processes within the enterprise.

Any foreign object close to a specific gravity of 1.0 (water) will not show up clearly on X-ray systems. Source: Smiths Detection

Teaming two technologies

US retailers are not the only ones demanding knowledge of the unseen. While the European Community has always maintained high standards, “… in Japan, food safety is the world’s most stringent, with a zero tolerance policy,” says Erik Brainard, Anritsu Industrial Solutions USA general manager. He explains that Japanese retailers demand metal detectors and X-ray systems be used in tandem. Japanese processors typically use a metal detector to find metal dust and low-density aluminum. Then they use X-ray systems afterward to find wire, stainless, ferrous and non-ferrous contaminants at levels the metal detectors typically miss.

For Accu-Ray Inspection Services, a third-party foreign-object inspection service company, using both systems in tandem makes good sense. “When someone sends product to us,” says Bob Loerop, vice president and technical director, “we’ll always try to incorporate both technologies.” The two technologies compliment each other’s capabilities in many ways. For example, aluminum is generally not detectable with X-rays; however, it is easy for a metal detector. Stainless steel is easy to detect with an X-ray system and more difficult for a metal detector-especially for conductive products. If metal isn’t associated with the product, a metal detector will often outperform an X-ray system for all types of metals, especially for high-density products (like a 50 lb. bag of ingredients).

According to Steve Gidman, Fortress Technology president, ten to fifteen years ago most metal detectors were end-of-line. Now, he says, processors are placing about 30% upstream to catch contaminants before they wind up in smaller pieces of product. An upstream machine allows a processor to track its suppliers to make sure they’re in compliance.

KD 7405 series X-ray inspection systems from Anritsu can reveal misshapen or lacked products. Missing detection is achieved through mass conversion. Source: Anritsu Industrial Solutions.

Multitasking metal detection

With metal detectors becoming more sensitive and because they are also sensitive to salt brines, conductive liquids and changes in pH (especially acidic) or conductivity, they have been known to trip when there is no metal to be found. Kevin Jesch, Heat and Control inspection systems product manager, says this issue is known industry wide as the “product effect,” and can also be attributed to different temperature levels of frozen food passing through the detector. Jesch notes that today’s metal detectors can “learn” about products with changing water content, salt levels and conductivity or pH so they don’t trigger falsely.

To Bradley, a falsely-tripping metal detector is an unacceptable situation. But there is no reason, however, not to make use of these wandering signals that really don’t show the presence of a metal contaminant. The signals and the data they create can be recorded to generate a pictorial representation of the whole hour’s or day’s worth of production-whatever the time base is. This trend information can be linked with products that pass through the detector and used as a powerful diagnostic tool that allows users to look into their process and ascertain what is happening in the system. “The detector is going beyond its number one mission, which is to detect metal,” says Bradley. “It’s now able to be used as a process control device to give greater control over the manufacturing process.”

A metal detector checks for the presence of fragments in fresh fruit packages. Source: Heat and Control.

X-ray: Vision on steroids

In some cases, X-ray systems can do some of the jobs of a vision system. X-ray systems generally detect stone, bone, metal, agglomerates, voids and cracks in products and in some cases can count the number of items in a pack, says Steve Dowd, sales manager, meat and processing, for Smiths Detection. But X-rays would be hard pressed to find wood, hair, gloves, non-metallic earplugs, cloth, insects and other materials with a specific gravity less than 1.0.

According to Ray Spurgeon Jr., product manager with Eriez Magnetics, “X-ray systems will most often detect any foreign object in food that has a differentiating grey scale and/or atomic number from what has been established as a norm. For example, at the atomic level, food consists of hydrogen, carbon and oxygen elements, which have an atomic number of one, six and eight respectively.” With glass fragments, silicon-the principle element of glass-has an atomic number of 14, whereas food adds up to 15, providing enough difference for glass to be detected in food.

X-ray systems are good in applications with metalized film and closed packaging, and in situations where a processor wants to verify length, width, count, size, voids, weight and contaminant detection, according to Dowd. In other applications, like hamburger patties, X-rays can identify meat flakes on top or below a patty, which can be a health hazard if they affect cooking time in an automated process. X-ray systems can also perform measurements like fat analysis, an important feature since fat content is key to recipe controls for the meat industry.

X-ray systems can be used to verify the existence of proper packaging elements, food items, fill levels and weights, according to Martin Lymn, general manager, sales and service, Loma Systems. For example, a pastry’s packaging can be checked for the existence of just one metal tie; a loaf of garlic bread can be checked for the correct number of butter portions; and a tray of yogurt can be checked for less-than-filled containers.

While no one would suggest that an X-ray system is as accurate as a checkweigher for verifying weight of products, an X-ray system can verify, for example, that the correct number of sticks of gum are in a properly check-weighed tray pack of gum, says Brainard. Because the density of gum can vary, a pack of gum may come up short on the number of sticks yet weigh the proper amount. The X-ray machine has the capability to break the package down into 25 or 30 zones-however many may be required-and analyze each zone independently, to locate a missing stick of gum.

For these applications, the X-ray machine is much easier to understand and explain to processors because they can see an image, says Brainard. X-ray machines can be up and running in five minutes once they’re plugged in, yet with all the sophisticated applications they provide, food processors are often not using them to their advantage. The primary application for many food processors is simply to identify metal fragments or other foreign bodies, but there are additional features available, such as missing item analysis, broken product detection, multi-lane inspection, product in the seal detection, virtual weighing and other features.

Brainard suggests saving the reject images for traceability purposes and to confirm that a contaminated product was actually rejected. When all the contaminated products for a day are compared to what’s in the bin, the numbers should match. Or, if a product is not rejected, the reject bin is full or the programmed reject limit is exceeded due to a major contamination problem, the system can be programmed to shut down the line until quality control has an opportunity to investigate the situation.

While metal detectors and X-ray systems can find numerous kinds of contaminants, they also are suitable for detecting the size of foreign objects. The important thing to remember is that this specification is very application dependent. According to Lymn, perhaps “best case” today in metal detection systems may be 0.8 mm in some applications, while “worst case” may be 10 mm stainless steel in some very large aperture applications. A common requirement in a retail packaged goods application might be 1.5 mm ferrous metals, 2.0 mm most non-ferrous metals, and 2.5 mm non-magnetic stainless steel. Metal sensitivity with X-ray inspection (other than aluminum) is more consistent across metal types-1.0 – 1.2 mm is not uncommon, while other common contaminants may be detected in 2x or 3x multiples of the metal size. According to Brainard, some X-ray systems with 0.4 mm detector arrays can see contaminants down to 0.4 mm, prompting processors and vendors to raise their expectations and improve their contaminant detection goals.

Operating frequencies are another source of considerable discussion about metal detection systems. Some units use a single operating frequency at a time; others typically use a frequency selected from either a set of multiple, fixed frequencies or from a variable frequency range. Anritsu’s detector is capable of running two frequencies simultaneously, says Brainard. While the machine picks the frequencies for the application, it runs two to maximize the detection of different metals.

According to Bradley, operating at higher frequencies improves sensitivity for hard-to-detect metals such as stainless steel. Providing machines with multiple, available frequencies lets processors tune the detector to their specific application. But the machine still needs to be adjusted to provide the maximum sensitivity at any chosen frequency.

Through several years of experience, Loerop has found that with conductive products such as cheese, operating at a frequency of 300 kHz will give good results. Increase the weight and size of the cheese and you’ll have to reduce the frequency to get the best performance. The frequency used ultimately depends on the product. There is no “one size fits all.” And a single frequency metal detector tuned to the specific application is the best way to ensure optimum performance for a particular product, adds Loerop.

There is no reason to use a standalone vision inspection system, metal detector or an X-ray machine when they’re all capable of being tied into a plant network. When combined with intelligent rejection systems and data collection equipment, the information gained from these inspection systems can be saved and applied to record-keeping systems to enable track-and trace if the need should arise. Don’t depend just on supplier’s specifications. Test the equipment yourself in the real world with your application.

For more information:
Doug Pedersen, S+S Inspection, 716-297-1922, doug.pedersen@sesotec.com
Mike Bradley, Mettler-Toledo Safeline, 44 (0) 161 8488636, ext. 1301, mike.bradley@mt.com
Robert Scott, Mettler-Toledo Safeline, 813-889-9500, robert.scott@mt.com
Erik Brainard, Anritsu, 847-419-9729, ext. 301, ebrainard@us.anritsu-industry.com
Bob Loerop, Accu-ray, 800-378-1226, gtobob@aol.com
Kevin Jesch, Heat and Control, 800-227-5980, kevinj@heatandcontrol.com
Steve Dowd, Smiths Detection, 865-934-2275, steve.dowd@smithsdetection.com
Martin Lymn, Loma Systems, 763-656-7808, martin.lymn@loma.com
Steve Gidman, Fortress Technology, 416-754-2898, steve@fortresstechnology.com
Ray Spurgeon Jr., Eriez Magnetics, 814-835-6298, rspurgeon@eriez.com