Metal Detection: Multiscanning Turns Up Frequency for Food Industry

Imagine watching the news and a story entitled “Check your fridge and pantry for food products recalled this week” brings up a picture of your brand’s packaging. It’s not that uncommon. For example, in late 2018 one manufacturer recalled 29,028 pounds of frozen, ready-to-eat poultry and pork sausage links after five people called USDA’s Food Safety and Inspection Service to let the agency know they found metal pieces in their sausage.

While regulatory pressure and the risk of financial loss have pushed many manufacturers to invest in detection technologies, mitigating risk entirely remains a challenge. Thankfully, today’s metal detectors and X-ray detection systems offer higher levels of sensitivity, but that wasn’t always the case, and not every manufacturing facility has the latest technology.

This article will explore the evolution of metal detection technology for food safety. Metal detection systems provide reliable, cost-effective protection from even the smallest metal contaminants found anywhere in a food production process. They can also help improve operational efficiency and eliminate expensive downtime, service costs, and repair bills. And metal detectors are suited for a wide range of demanding food processing applications and packing environments.

Metal Detector Technology

Metal detectors are common across food processing facilities to meet HACCP (Hazard Analysis and Critical Control Point) requirements. Most often they are placed at the end of the line as the last defense against escape before a packaged product is sent on its way to the consumer. The core technology, though, has always had limitations, such as the so-called “product effect,” where a detector cannot differentiate between a conductive product or one with high mineral content and the metal contaminant and susceptibility to “noise” coming from many possible sources in the typical harsh, industrial food production environment.

Basic metal detector technology relies on coils that are wound on a non-metallic frame and connected to a radio frequency transmitter and receiver. The transmitter “excites” any unexpected metal objects and generates very small changes in return signals to detect foreign contaminants. Digital signal processing algorithms are used to differentiate between the expected product signal and that of an unexpected foreign object. The technology works, but historically performance can be inconsistent and sometimes even unpredictable. Recently, with the introduction of multiscan metal detection technology, this is starting to change.

The Evolution of Frequencies

Early metal detection technologies for the food industry were limited to single, fixed frequencies. A manufacturer could best detect a piece of stainless steel using a high frequency, but when a wet, warm, or salty product was introduced it would be forced to reduce the frequency and thus the sensitivity due to the product effect. This simple frequency change required setup by skilled technicians who might spend hours selecting the “best” frequency for detection of all metal types. A user could not make this change themselves.

Single, fixed-frequency metal detectors had limitations for the typical food manufacturing environment given the range of products to be tested and the variability of metal contaminants that could enter the process. That’s why manufacturers started adding second and third frequency choices (always running just one frequency at a time), giving users more flexibility. Manual frequency switching became more common but was only marginally less onerous: Expertise was still needed to optimize detection. Nonetheless, this was an advancement since it introduced more frequency flexibility to metal detection.

The next advancement in metal detection was the development of frequency selection via software. The “best” single frequency for a given application could then be selected prior to production by scanning a product many times and testing detection. This was known as variable frequency metal detection, and it enabled setup without the need for a specialist. Manufacturers still were forced to live with the “best” single frequency compromise, however, and accept its lower overall performance.

A recent advancement in metal detection enabled detection at two frequencies simultaneously, essentially performing like a low and high frequency detector in one. Although the dual-frequency metal detection approach improved overall sensitivity, the combination of frequencies that could run simultaneously was still limited. The opportunity to miss metals with frequencies between or on either side of the dual setting still led to compromise that left quality managers wanting more.

The Advent of Multiscanning

Multiscan technology is said to be the long-awaited innovation in metal detection. Metal detectors with this capability can identify contaminants that are up to 50 percent smaller in volume than previous technologies, including food items with high product effect. With multiscan technology, the CCP can scan up to five adjustable frequencies, raising the probability of detection exponentially. Essentially, it’s the equivalent of having up to five completely adjustable metal detectors back to back in a production line.

Multiscan detectors don’t continuously broadcast the five frequencies simultaneously. If they did, the power requirement would be too high and expensive. Instead, the frequencies are scanned thousands of times per second, equivalent to broadcasting simultaneously without requiring as much energy.

Another benefit of multiscan technology is complete flexibility to set frequencies and the associated detection parameters. This is important given that the interaction of the product and metal in all applications is different, depending on factors such as the ingredients in the product, the type of packaging, the product temperature, and variation in all of the above. Most times these interactions are impossible to predict too. With multiscan technology users can make changes in software, selecting the appropriate five frequencies in the 50 to 1,000 kHz range. If a quick test shows detection for an application is best in the 400 to 600 kHz range, the user can easily select five frequencies in that range to maximize performance. To counteract product effect, the user can simply select a lower frequency range, such as 100 to 250 kHz. Different combinations can be selected for different products and they can be changed at will at any time. Multiscan detectors are based on the idea that there is no perfect frequency, and that the best range of frequencies changes depending on the application.

A not-so-obvious benefit of multiscan technology is that it can be used to address an all-too-common metal detection problem–electromagnetic interference (EMI), which can happen in almost any factory at any time. EMI is an invisible field typically generated by a motor or variable frequency drive that moves through the air into the metal detector aperture, causing interference with the detection signals. EMI can come from a variety of other sources in a harsh industrial setting and the aperture can’t be shielded because it’s where the products pass through. Users can simply look at the screen on an advanced multiscan detector to see which frequency or frequencies are affected by EMI and adjust accordingly. This can be done in a matter of minutes and doesn’t require a specialized skill set.

Finding the Best Metal Detection Solution

There is no “one size fits all” approach to metal detection. The best protection against metal escapes is ensuring that the solution you implement is the right one for your products. Even with advanced multiscan technology, it’s critical that manufacturers consider their unique systems, processes, equipment, and product types before making a final decision about which technology to deploy and how.

To ensure future detection performance, a best practice is to have the metal detector manufacturer conduct controlled tests on the detection equipment of interest. The test must simulate, as closely as possible, how product will ultimately be inspected on an actual processing line. Product-specific factors such as temperature and package configuration must be replicated.

While no product test can replicate actual conditions, the more rigorous the test, the better. A testing process should specify performance requirements to provide confidence that the inspection solution will be suited to a specific application. Even for an advanced metal detector, such as one with multiscan capability, it’s important to follow a strict process to ensure each requirement is addressed. At a minimum, the testing should consider the following.

Product presentation and orientation. Results could be invalid if the product passes through the metal detector in the wrong way.

Production conditions. Temperature, pitch, and speed should match the actual production environment. Because temperature affects the electromagnetic signal given off by products, failing to factor in the unique signal of a hot versus cold product on a production line would lead to false rejects. Pitch should also be tested to understand the total amount of signal in the detector at any time and how many products might be detected at a time.

Placement of metal. Testing should be performed by placing metal in multiple locations on a package, including the center of the aperture, the weakest detection point because it is the farthest away from the metal detector coils. A thorough assessment should include tests on leading, trailing, absolute center, and sides to ensure metal is detected anywhere in the package.

Analysis of results. After testing is complete, a formal report should provide recommendations for each tested product, including recommended conveyor speed, frequencies, and setup parameters.

Finding the best metal detection solution is certainly easier than it once was. The most advanced instruments are now more reliable and versatile, bringing greater efficiency to manufacturers while requiring fewer trade-offs. It’s possible to have confidence, high throughput, and flexibility at the same time. Today, the high bar is multiscan technology, yet future advancements are inevitable. It may never be possible to make escapes 100 percent preventable, but today’s technology—supported by best practices—is already saving millions for manufacturers by avoiding costly recalls and, most importantly, ensuring food is safer for consumers.

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Ries, the lead product manager for metal detection and X-ray inspection at Thermo Fisher Scientific, advises customers on specifying, installing, and using metal detection and X-ray systems to improve food safety and quality. Reach him at [email protected].