Food Engineering

Tech Update: Metal detection & X-ray inspection

System enhancements keep a protective eye on quality

January 10, 2012
Try as hard as you can, but there will always be a foreign object or contaminant that slips past your human vision or the “eyes” of metal detection equipment and X-ray systems. With some food products, even vision systems might not see a potential interloper. The following three incidents are reminders that food processors must always look for an extra edge to help them spot and keep foreign bodies out of food—even when they have been diligent in trying to prevent a highly unusual occurrence.

• A Somerset, UK man was preparing a baby leaf and rocket (arugula) salad when he found the carcass of a small, dead bird among the salad materials. (BBC News, November 15, 2011).

• Kraft Foods Global, Inc. voluntarily recalled three varieties of Velveeta Shells & Cheese Single Serve Microwaveable Cups due to the possible presence of small, thin wire bristles (Kraft, September 30, 2011; FDA, October 3, 2011).

• A UK food production company was fined nearly £17,000 when a man found a dead mouse in the loaf of bread he was using to prepare sandwiches for his children (BBC News, September 27, 2010).

While the second incident above is an extremely difficult detection challenge, additional technology can solve the problem, and that may be how Kraft found the wire fragment and was able to alert the supply chain, keeping affected product to a relatively small number of UPCs (five) and 137,000 cases.

Metal detectors with improved sensitivity can help spot hard-to-find targets, but a single unit may not be enough. “When talking about wire contamination, it is important to understand ‘orientation effect,’” says Robert Rogers, Mettler-Toledo senior advisor for food safety and regulations. “A wire traveling through the detector in one orientation is easy to detect, yet in the opposite orientation it is difficult to detect. To improve the chance of detecting the material, one could place multiple detectors at different angles on a single production line. However this does not eliminate the need for a robust foreign body prevention program.”

One way to improve metal detector sensitivity with applications like sliced cheese, warm bread or filled bottles is to use multiple coil receivers, says Bob Ries, Thermo-Fisher lead product manager, metal detection and X-ray/product inspection business unit. “In the case of metal strands or wires, the only solution known today is to use two metal detectors adjacent to each other at 45° angles. This, in effect, covers all the possibilities [or orientation].”

X-ray systems are becoming sensitive enough to detect wires down to 0.3mm in diameter if they are long enough to create a visible, linear contaminant image the system can analyze and detect, says Ries.

However, X-ray systems are challenged by cases 1 and 3 above. Since X-ray detection is density based, objects like animal carcasses/bones—because they have little density—are difficult for X-ray systems to find. “Only the most dense bones and plastics are currently detectable,” adds Ries.

Retailers drive inspection

While government agencies may not be able to mandate the use of metal detection or X-ray systems, retailers have taken the lead in mandating this equipment because they know how important safe food is to their customers. Craig Wilson, Costco assistant vice president, food safety and quality assurance, circulated a letter more than six years ago to producers asking them to “raise the bar in the area of product inspection.” The letter states: “Our audit companies have been instructed to inform our vendors that dated metal detection devices should be replaced with X-ray units in all food processing facilities where applicable.”

“For customers like Costco, Sysco, airline companies and others, maintaining the highest product quality and food safety is critical,” says Adalberto Salce, plant engineer at Orval Kent’s Linares, Mexico facility. A large producer of prepared salads for foodservice customers in North America, Orval Kent installed a Mettler-Toledo Safeline X-ray system 18 months ago to monitor product quality.

Orval Kent produces more than 80 different types of fruit salads, which are packaged into 4- and 8-lb. PET jars and 1-, 3- and 5-lb. flexible pouches on five filling lines. “The Safeline X-ray system helps us achieve the highest product quality. It detects metal, glass, plastic and other types of foreign material and removes contaminated product from the production line,” adds Salce. 

“We considered metal detectors, but the X-ray system was the better choice for us for several reasons,” explains Salce. “The X-ray system enables us to inspect for non-metal contaminants such as glass, which allows us to achieve the highest specifications. It offers consistent inspection even with the natural variations in the conductivity of our products. Since the X-ray system can be used after we’ve sealed the PET jars with foil, we can inspect at the end of the line where there is no risk of subsequent contamination.”

Compared to the old X-ray system, the Safeline PowerChek is more sensitive and reliable, says Salce. He tests it at the beginning of every production day and after every changeover, which is typically four or five times a day. “In the 18 months that we’ve had it, it’s never failed a test, and we’ve had no unplanned downtime. This system helps us guarantee our customers receive the highest product quality and food safety.”

Planning and using metal detection systems

Preventing metal-contaminated product from reaching the consumer should be a systematic approach instead of just installing a piece of equipment into the production line, suggests Rogers. Proper installation and system design are critical, as outside influences within the production environment could cause issues. The goal is to have a system capable of running at maximum sensitivity, absent of product, with no false reject activity. Thus, if any product is rejected, it should be further tested to determine the source of the contamination. Then corrective action can be taken to prevent further occurrences of contamination.

If a high level of false reject activity occurs, it must be addressed since it is an indication the program may be faulty and not necessarily the equipment, offers Rogers. Metal detectors may be reaching a plateau in terms of sensitivity or the ability to find smaller targets, but other improvements are extending the usefulness of these detection systems. “Detectors offer significantly higher performance today than even two years ago,” says Steve Gidman, Fortress Technology president. “Detection levels in the 0.5 to 0.8mm range are easily achievable with modern metal detectors.” And modern metal detectors are much better at living with real-world interference sources. “Proper shielding and signal processing techniques allow excellent performance in electrically active [noisy] environments,” he adds. 

Last year, Lock Inspection introduced its Insight metal detector, which according to Lock President Mark D’Onofrio, uses advanced digital signal processing (DSP) to reduce unwanted product and environmental signals. This allows for more precise product setups and better overall performance. “The trick with metal detection is to run the electromagnetic field at super-high frequencies, but also effectively manage product signal and outside interference,” says D’Onofrio. Advancements in technology have contributed to improved detection for most metals.

In the recent past, several metal detectors used a couple of frequencies—usually one at a time—to obtain a signal from a target that was greater than the noise produced from the “product effect.” An example of product effect could be a moist bakery or salty product.  A benefit of using two frequencies simultaneously is that one can be used to detect ferrous metals and the other, stainless steel (SS), according to Christopher Young, Anritsu product manager. While certain SS alloys can be a challenge for metal detectors, aluminum, which is not “seen” by X-ray systems, can be detected by a metal detector.

Another approach to effective design is to pick a single frequency from a range of frequencies (e.g., 31-800 kHz) that’s right for each product, says Steven Bunnell, Loma Systems sales and marketing manager. In Loma’s IQ3+ system, users can select a different frequency for a given product, or they can let the metal detector pick the best frequency for each product during the product “learn” or calibration step, which reduces the possibility of operator error.

A metal detection system that simultaneously uses multiple inspection frequencies can improve detection sensitivity and make it easier to reduce strong product effects such as salinity and water concentration, according to Todd Grube, Heat and Control manager of inspection systems. “With single and three-frequency metal detectors, processors typically reduce sensitivity to achieve a compromise between safety and excessive false positives.” Multi-spectrum technology (multiple frequencies), according to Grube, significantly reduces product effect and tracks it during production, automatically adjusting for changes in the product and inspection environment.

Five Star Gourmet Foods (Ontario, CA), a provider of high-quality frozen products including entrées, custom sauces and marinades, snacks and sandwiches, decided to try the multiple frequency technology as implemented by Heat and Control’s CEIA THS121 detector. “Our line operators decided to give it the torture test,” says QA Manager Marissa Myat. “We mixed different types of fresh and frozen product in the same run to expose any shortcomings. To our surprise, the CEIA detector passed every test. Its performance really exceeded our expectations.” Five Star runs from 150 to 450 items per minute per production line, and each detector runs a minimum of 14 hours per day, five days a week.

Little improvements in metal detectors can mean a lot. With the pressure that processors have been feeling to include both metal and X-ray systems in HACCP plans, suppliers are listening. “To that end, what was a detection level just five years ago is no longer accepted, and if the detection level can be improved by just 0.1mm, most food processors will force metal detector manufacturers toward that mark,” says Wayne Eide, Advanced Detection Systems marketing manager.

With incremental improvements in head construction symmetry, product screening and software, an improvement in target size, for example, from 2.0mm to 1.5mm is about 40 percent, according to Doug Pedersen, S+S Inspection Systems sales manager. “Some companies will change out a metal detector for this level of improvement, and look at it as only 0.5mm, which is small. But let me ask this question: Would you change any other piece of production equipment for a 40 percent improvement in performance? Very likely.”

Extending metal detectors’ performance

While some would say that the basic physics of the metal detector is mature and as good as it gets, the emphasis, therefore, has been on improving the peripherals of metal detection systems. “Enhancements in metal detector software and coil configurations have resulted in modest improvements in sensitivities, and the best sensitivities are realized in the pharmaceutical industry as very small apertures are used to scan tablets,” says Ray Spurgeon Jr., Eriez X-ray Inspection Division product manager. “Typically, sensitivities in this industry range from 0.3 to 0.5mm diameter. In my opinion, metal detection technology is mature and will offer little or no future improvement to sensitivity.”

“There is always a point that the physics of the metal detector has reached its limits, and that is why you see the improvements in compensating for external factors such as vibration and product-effect variability,” counters Eide. “For example, our Acculearn technology is so smart that the product learn cycle that once took many passes is now completed in just two passes.” Older metal detectors took many passes, and some never learned the product as well as the machines today, claims Eide.

With the auto-learning capabilities of metal detectors, the ease of use is improving to a level where operators can easily set up a new product run without making mistakes in configuration, according to Young. A bigger issue, says Young, is the product itself changing in consistency. For example, a fresh-baked loaf of bread might be “learned” at one temperature, but over time, the product temperature or moisture content changes, potentially affecting the performance of a metal detector.

Most metal detection systems (and X-ray as well) are also network friendly and provide useful tools to track inspection results. For example, Lock’s Insight series delivers the streamlining of data management reports and product profiles through its Optix software and offers wireless Ethernet and SCADA packages as well, according to D’Onofrio. 

State-of-the-art X-ray system

Ries, who is responsible for both metal detection and X-ray systems, sees X-ray as a step beyond metal detection, but not a panacea for all inspection applications. “X-ray systems can typically find 10-30 percent smaller metal contaminants and many non-metal contaminants. The price is higher, usually two to three times, and system lifetime is shorter (five to seven years vs. 10 to 15 years for metal detectors).” Current state-of-the-art with X-ray is 0.5 to 0.7mm detection of metal or thin products. Because X-ray detection is density based, many other contaminants more dense than water can also be found. Glass detection is usually in the 1.5 to 3mm range, and stones with density similar to that of glass have comparable results under X-rays. However, only the most dense bones and plastics are currently detectable, as stated above.

X-ray technology is best when SS foreign objects need to be detected in products that are conductive, according to Spurgeon. He attributes this to the physics of the application. For example, SS is masked when scanned by a metal detector because conductive products and SS “look” the same to a metal detector. An X-ray system, however, uses contrast (which is not affected by the conductivity of products) to detect SS targets. But, if a product is dry, non-homogenous (e.g., a box of rigatoni pasta) and has no foil packaging or safety seals, a metal detector will consistently outperform an X-ray system, says Spurgeon.

The Anristu X-ray system, according to Young, can typically find 0.8mm SS in most food products (both reactive and non-reactive). On thinner products, detection down to 0.4mm SS is possible, and thin wire can be detected when it’s long enough and covering some area of the detector. Young says how well an X-ray system performs depends upon how the image is generated and the image processing software behind it. Several software tools are available that make it easy to adjust one control without compromising other tools, and this can be done automatically during the auto-learn process.However, thin wire strands are problematic for metal detectors because of orientation.

The same phenomenon is also prevalent for X-ray systems, says Spurgeon. With an X-ray machine, when the wire is perpendicular to the beam, the end of the wire will be represented by a single pixel, or none at all if the wire falls in between sensors in the matrix. In a metal detector, the problem occurs when the sliver is on plane with (vs. perpendicular to) the metal detector coils. Consequently, neither technology may detect a wire strand the first time it passes through a system.

Many products that are packaged in metalized films, foils or aluminum trays cannot be inspected with a traditional metal detector. Similarly, SS is commonly used in food equipment and can end up being a contaminant in the food product. Unfortunately, it is the hardest form of metal for a traditional metal detector to find. But an X-ray system can see though all types of product packaging and also find very small SS contaminants, according to Young.

Cooked bone is very difficult to detect with X-rays because the calcium is cooked out, reducing bone density. On the other hand, while bone is not expected in a chicken breast fillet, bone content is in sausage due to the nature of the ground meat/bone that goes into the product.

Where it gets tricky for X-ray detection is in bagged applications where the product is not flat, says Grube. With a Hershey bar, it’s easy to set up the X-ray system and get consistent results. But imagine a bag of walnuts. While the X-ray system can penetrate the bag, the software will trigger on edges of the nuts if the sensitivity is turned too high. With a lower sensitivity, the system may miss a contaminant, especially if it’s nearly the same density as the nuts.<br><br>
At times, a bag of nuts or salad materials can fool the X-ray system into thinking a contaminant exists when it doesn’t. Typically, this occurs when an X-ray system algorithm detects a darker gray value (something with more density or contrast) than what is deemed as normal. To avoid such an occurrence, the proper inspection parameters must be set with a margin of error to accommodate product variations, according to Spurgeon.

 Non-detection applications

“Metal detectors can be used to ensure package contents, often referred to as ‘reverse detection.’ This could be ensuring the presence of a toy in a cereal carton or counting foil pouches within a larger package,” says Gidman. “Similarly, the inclusion of an oxygen scavenger in packaged meat can be confirmed while simultaneously inspecting for contaminants.”

X-ray systems, too, can do more than just inspect for contaminants, and often these additional features can help pay for the cost of the system. Because X-ray systems analyze product images, they do many other things—like finding missing pieces, measuring fill levels, estimating product mass and finding damaged or incorrectly sealed packaging, explains Ries. “These applications will evolve and increase in the future as sensors, images and software improve.” In many cases, adding these inspection capabilities to an X-ray system already finding contaminants is very low cost or even free, he suggests.

For meat processors and packagers, X-ray technology can be used to measure fat content in meats, in addition to the more common uses—finding metal contaminants, stone and calcified bone, says Terry Woolford, general manager, Eagle Product Inspection (formerly Smiths Detection PID). The technology, called dual energy X-ray absorptiometry (DEXA), can measure fat content online in real time and can be used in place of Anyl-Ray and NIR/NIT (near infrared reflectance/transmission) techniques. Basically, DEXA machines measure the amount of X-rays that are absorbed by fat in the meat as it passes through the system. 

“DEXA machines are not only accurate and repeatable for measuring fat content in meats, they function dually as management tools and process control tools, providing management the information it needs to make informed decisions and guarantee compliance,” says Woolford. “Intuitive management software, bar code readers and trace servers, which allow quality personnel to remotely access real-time data and reports for production, rejection, weight and trending, can greatly improve manufacturing processes, cutting costs and meeting regulatory standards.”

A place for magnets

While magnets do not technically constitute a detection or inspection system, they can be used early in the line to remove ferrous contaminants. “Magnets can be used on wet applications using traps, and on dry applications, [the use of] plates/grates is always a good idea,” says Spurgeon.

Magnets do not work well on many whole products like nuts, fruits, vegetables and meats, and they have problems with non-magnetic metals such as most of the 300 series of SS alloys, according to Young.

“What has to be brought to the forefront is that the strength of a given magnetic circuit is ‘as is,’” says Dennis O’Leary, Industrial Magnetics general manager. “A magnet doesn’t concern itself with the amount or grade of ferrous or 400 series work-hardened stainless that is present; it simply catches it.” Conversely, O’Leary explains that metal detection relies on operator input and an algorithm of potential “blueprints” or make-ups so the unit can be “dialed in.” “Dialed in very often means desensitized so as best to capture a broader array of contaminants with vastly differing metallurgical characteristics. The result is a metal detection unit that doesn’t function well and allows smaller materials to pass through,” adds O’Leary.

Magnets remove ferrous iron and work-hardened SS from processing lines, and they can remove particles much smaller (0.0001 in.) than a metal detector will find, according to Marc Trusdale, Cesco Magnetics operations manager. A magnet is not a replacement for a metal detector, but a complementary piece of equipment. Placed before the metal detector, magnets remove magnetic materials, decreasing the number of rejections by the metal detector. The metal detector removes contaminants that are not magnetic, e.g., aluminum and 300 series SS.

Combined systems and applications 

You can never be too safe. So for many applications, the use of magnets before a metal detector on a line can provide some extra assurance that you’re creating safe product. But is that enough, now that retailers want to see X-ray systems in place?
“Normally an X-ray system would replace a metal detector on a line as it is more flexible and can find smaller metals and other non-metallic contaminants that a metal detector can’t find,” says Young. However, a traditional metal detector still has value in the production line and can detect items that are quite thin but have a large surface area, for example, a metalized gum wrapper, suggests Young.

“A metal detector should always be used to cover the X-ray system’s weakness in detecting aluminum,” says Gidman. “They can be installed right next to each other without concern for interference. If the packaging is metallic, the metal detection system will have to be located upstream of, or within, the packaging machine.”

Pedersen points out a perfect application for the two systems in tandem. “In sausage production, I’d put a pipeline metal detector on the stuffer and X-ray after packaging. In areas where aluminum is a risk, a metal detector can do a better job than X-ray, so having them side-by-side on a packaging line is an alternative.” Pedersen suggests processors will have to decide which combinations—including magnets—make sense and are cost-effective.

Finding the widest range of contaminants possible would be the reason that Ries would recommend a combined system. “The metal detector can easily find aluminum while the X-ray cannot. The X-ray can easily find glass and stone while the metal detector cannot. The X-ray can easily (and usually at no cost) inspect the inside of a package to assure product integrity. This configuration is rare today but will become more popular as food safety standards increase, and the cost of ownership of X-ray decreases. Combination systems can also potentially be designed to share one user interface.” 

For more information:

Bob Ries, Thermo-Fisher, 763-783-2500,

Steve Gidman, Fortress Technology,416-754-2898, ext. 317,

Mark D’Onofrio, Lock Inspection USA, 978-829-0222,

Todd Grube, Heat and Control, 717-519-5240,

Wayne Eide, Advanced Detection Systems,414-672-0553,

Doug Pedersen, S+S Tech, 224-208-1900,

Ray Spurgeon, Jr., Eriez, 814-835-6298,

Christopher Young, Anritsu,847-419-9729,

Steve Bunnell, Loma Systems, 800-872-5662,

Robert Rogers, Mettler-Toledo, 813-342-9138,

Terry Woolford, Eagle Product Inspection, 877-379-1670,

Dennis O’Leary, Industrial Magnetics, 231-582-3100,

Marc Truslow, Cesco Magnetics, 877-624-8727,