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From: Food Quality & Safety magazine, October/November 2007

Sort out the Best Methods for Foreign Object Detection

Today, food manufacturers have a wide range of technologies available for foreign matter detection, but the bottom line is the type of matter expected

by Giri Veeramuthu, PhD

Foreign matter contamination is the main source of recalls and rejections and may lead to injury to customers, loss of brand loyalty, and large recall expenses. These undesirable additions differ from food groups and, depending on the type of food product, can be anything from stem stalks to bone fragments.

Numerous emerging technologies and identification methods, ranging from physical examination to ultrasound to sophisticated nuclear magnetic resonance techniques, can detect foreign matter. PacMoore Products (Hammond, Ind.), a dry repack and blending contract manufacturer, meets various client needs that involve repackaging or blending products that include dry ingredients for baking, dairy, meat, or ready-to-eat, micro-sensitive applications. PacMoore utilizes various detection methods to control and detect foreign matter, including visual examinations, screens, magnets, and metal detectors.

A Combination of Characteristics

Product type, flow characteristics, particle size, density, blended components, metal detector capabilities, screen size, and magnet strength all play roles in foreign matter detection, says Scott Reid, PacMoore’s general manager.

The food industry chooses the mesh sizes of its screens based on the product. Figure 1 (p. 51) compares the approximate size of particles that can be seen with the naked eye with different types of detection tools used in the food industry. Mesh sizes denote the number of small squares in a one-square-inch area. The larger the mesh sizes, the finer the screen and detection capability. There are no standards designating which screens are to be used for different food applications; this is dictated instead by the flow characteristics of the product. For instance, it is common to use a 20-mesh screen (840 µm in opening) for dry food applications.

Particles of foreign matter in processing utilities—water; gas; heating, ventilating and air conditioning (HVAC) systems; manufacturing equipment; container closures; and packaging—are intrinsic. And in a dry ingredient process, a food manufacturer can expect some metal-to-metal contact, which may produce a fine dust.

Is Zero Defect a Myth?

Preventive measures like filtration, in-line filtration, at-the-point use filters, screens, and magnets can prevent these issues, but it may still be difficult for a food manufacturer to nullify or control all intrinsic foreign matter. Practical limits based on risk assessment and process capabilities need to be set specific to a food process, and the risk assessment should consider the type of process being used (e.g., grinding versus milling versus just repacking).

“In a company where there are few metal-to-metal moving parts, where equipment handling is minimal, and where gravity and pneumatics are used for material transport, the metal contamination is very low,” says Bill Moore, PacMoore’s president.

But as a proactive approach, any amount of metal dust collected is measured, and product history is developed. “This helps us identify client issues when we handle their products,” says Bob Lyman, vice president for sales at PacMoore. Knowing this type of history helps clients tweak their processes. Magnets are placed prior to introducing the product into repacking/blending operations, something that helps identify the source of metal contamination.

It is common to find metal dust when there is new machinery or when a new supplier is qualified. “We ask if preventive maintenance was done on the equipment recently or if any repair was conducted. If the answer is no, most often the contamination is external to our systems,” Lyman says.

Establishing a Baseline

Inherent process assessments need to be different for different food manufacturers. Most hazard analysis and critical control point (HACCP) plants and clients require metal detection as a critical control point or set criteria for it without establishing a baseline. Even though the Food and Drug Administration (FDA) has established defect action levels, these have mostly been for extrinsic foreign matter such as insect parts and fragments, and the limits are vague.

Various mitigation tools are used to control foreign matter. Magnets are a simple, effective, and inexpensive way of identifying metal in any industry. The performance of magnets—whether ceramic, plate, or rare earth—depends on their strength. Strength is measured by a magnetic pull test that measures the relative magnetic attraction of a separator on a ferrous test piece. The test is based on the idea that the magnets should be strong enough to remove the ferrous matter from a product stream, something that depends on reach-out distance and the collection area of the magnetic face.

Damaging magnets can reduce their life expectancy; magnets can also lose potency if the process they are used in involves excessive heating and/or cooling. A good HACCP plan should include the magnet pull strength to validate such findings on the magnets used. Once again, industry standards vary based on the product category. Some industries replace magnets when pull strength is lowered by 10%, while others admire the fact that such tests are being conducted.

Vague Guidelines for Metal Detection

Metal detection is another tool used to locate foreign material in food products. The FDA has set guidelines for metal detection, but these are vague for each food category, varying from 0.08” in ground meat operations to sharp objects 7 to 25 mm in size in ingredients that the FDA considers adulterated. Making the picture even murkier is the fact that the definition of adulteration depends on the industry group doing the defining.

Although the food industry is aware of metal-detection capabilities, it is important to understand the operating mechanics of metal detectors and signals on different product types. For example, does the HACCP plan encompass details such as adjustment of calibrations based on product type?

Metal detector calibration validations include signal strength, product prototype characters, and signaling pattern. Adjustments of metal detectors for a multi-product line, packaging material, and the size and shape of the package have to be considered. Are the belts used in the conveyors suitable? Antistatic or colored belts cannot be used because they may contain carbon or metallic pigments.

Are the rollers isolated on one end to avoid any looping—electrical disturbances that may affect metal detection capabilities? Even a closed loop system or a grounded system can cause electrical disturbances that may affect detection capabilities. A company that is knowledgeable about these minor details guarantees that its efforts will not only meet prescribed standards but will also be capable of accurate and precise detection every time.

Technologies Inherent to Process

Various technologies such as density separation, X-ray of product streams, automated color and shape recognition, microscopic examination for microphysical contaminants, and analytical test methods for determining the origin of macroscopic contaminants, are available to food professionals. But each processor needs to determine the extent to which a technology is inherent in a process before installation. A good HACCP plan will not only prevent any type of contaminant; it will also help all companies manage foreign matter from different sources and ensure its effective removal.

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are new technologies that can be used to identify foreign matter. These tools improve in-depth investigations and liability issues. For example, SEM and EDS can help determine if a glass-like foreign matter is glass or a fatty acid. This may lead the investigation to ingredients other than glass. These technologies can detect components of one type of glass and compare it to a different type of glass, helping to narrow the source of contamination to one of two different plants, both of which have glass in their production areas. In addition, metal from different sources can be easily identified using these technologies. For example, a tin-soldered metal container can be differentiated from a lead-soldered container.

Simple but Expensive

A simple, common tool that remains expensive and is mostly used by meat packers is X-ray technology. Every image that passes through the eye of the camera is converted into light intensity levels ranging in value from 0 (grey) to 255 (white); several readings of density levels are recorded, and the system’s computer monitors the data and determines if foreign matter is present. Not everyone can adopt such costly technology, but it is a good tool to have in grinding operations, where metal-on-metal contact raises liability issues.

Other technologies use optical methods to differentiate a good product from a bad one. This is more common in sorting raw agricultural commodities. The optical sensors and the image processing modules use a mono- or dichromatic system and high, low, or band bass filter systems.

Newer technologies such as microwave reflectance and electrical impedance, which are intermediate between conventional metal detection systems and microwave-based approaches, have yet to be commercialized. Microwave reflectance or impedance works on the principle that portions of food will impede microwaves in a different manner than foreign matter. This discrepancy helps to determine the spatial, three-dimensional location of the foreign matter.

Nuclear magnetic resonance techniques, like magnetic resonance imaging used for humans, can detect and identify foreign matter, but it has practical limitations. The spatial resolution and three-dimensional images need to be improved, so this technique is not ready for prime time in the food industry.

The Last Word: Visual Examination

When all is said and done and all the technologies are considered, nothing compares with simply looking at the foreign matter, a technique employed in all food companies. Observation plays a major role in identifying foreign matter, but while this time-tested method is effective, its drawbacks, including employee fatigue and excessive time required, outweigh its advantages.

Although food manufacturers have a wide variety of choices when it comes to foreign object detection, the bottom line is knowing the type of foreign matter expected and being prepared for an unexpected event. In the future, technological advances may make faster, more sensitive tools affordable for food manufacturers. But for now, as food companies continue to work on understanding liability and the associated costs in detecting foreign matter, one of their greatest remaining challenges continues to be the lack of proper regulatory guidelines.

Veeramuthu is vice president of quality at PacMoore Products in Hammond, Ind. Reach him at (219) 932-2666 or


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