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Detection Technologies: What Works, What Doesn’t
by Bob Ries
Metal detection and X-ray inspection traditionally have been the first line of defense to identify the presence of physical contaminants in food products before they leave the processing plant.
For food safety and quality professionals, process engineers, and others who decide which technology will best protect against contaminants, choosing a detection system is typically based on three things: The optimum detection point, overall application capability, and total cost/benefit.
However, even though detection systems have been used by food processors for decades, engineering and software improvements continue to set new standards. This has led to confusion regarding which technology to employ and why.
In security applications, such as airport screening, metal detectors use radio frequency signals to react to moving metal (i.e. coins in your pocket). X-ray systems produce density images for specific elements that are analyzed for irregularities.
Deploying these technologies for food applications is more complex. The size and type of anomaly being detected is more challenging and the rapid speed in which the detection needs to take place makes the process more difficult. In fact, in many cases, the real challenge isn’t finding the contaminant; it’s ignoring the product, packaging, or environment. False detections add up to big costs and high frustrations.
Metal detectors and X-ray systems for food applications must be very sensitive, easy-to-use, fully automatic, fast, robust, reliable, and cost effective. This is a tall order for any automated system that must run for many years in a harsh factory environment, and make reliable pass/fail decisions on literally millions of products.
Foreign object detection performance is determined in three ways: Detectable contaminant types, minimum contaminant size, and probability of detection.
The best practice prior to deployment is always to test many samples with different contaminants. This helps you understand how the product and contaminant react when in the detection system. Minimum contaminant size depends on the system design/technology and the product effect (how much the food itself “looks like” a contaminant to the system). Probability of detection means the chance of missing a contaminant in real production with real products running at real speeds. Typically, the larger the contaminant the higher the probability of detection.
This fundamental trade-off is addressed by building in margin for error, setting periodic mandatory audits, and performing preventative maintenance.
Selecting the Detection Point
Companies typically use Hazard Analysis and Critical Control Point (HACCP) methodology to manage food safety. The first part of the process (HA) identifies which contaminants are most likely to occur as part of the process or ingredients used. Next is the determination of the CCP, or in the case of contaminants, the best detection point. CCPs can occur in multiple places—at beginning of the process; after cutting, sifting or mixing; immediately after a bag/box is filled; or at end of the line.
Ideally, the goal is to find problems early in the process to reduce the cost of rework or scrap while still ensuring the final product is safe. Inspecting large cases immediately prior to shipment is not always the right decision.
The optimum detection point can influence which technology should be employed. Metal detectors can be installed almost anywhere, but their performance depends on the size of the aperture (hole) the product passes through. In general, they work best for bulk conveyed or piped product or products in small packages.
X-ray systems are also dependent on product size but have greater sensitivity with large products than metal detectors. Due to the basic detector sensor scanning rate, X-ray systems are limited by speed. They are typically found closer to the end of the line. Because X-ray systems need a constant known speed to construct images, they cannot be used in gravity flow applications. Metal detectors are ideal for these types of products.
Decision-Making Check List
Determine what contaminants you want to find and where do they come from.
Given all the factors that affect application performance, the best way to select a technology and specific system is to run a test. Try everything to make the system fail. Strive for near 100% probability of detection with no false readings. Make sure you have enough detection margin so the system can run trouble free for hours without false rejects or the need for calibration.
Guidelines for X-Rays
X-ray systems create grayscale images corresponding to density. To detect a contaminant in those images, the contaminant must have significant contrast compared to the product the contaminant is inside.
Table 2 shows some typical contaminant material densities compared to water (i.e water density = 1.0).
The only way to definitively determine what can and cannot be detected (material and contaminant size) is have an application specialist run a test.
Metal Detection Capacity
Sensitivity decreases for wet/variable products due to their product effect. Table 3 is for dry products that aren’t conductive. Note X-ray systems also can detect metals—typically in 0.5 mm to 2 mm range. Capability is dependent on density and texture of the product, not aperture size.
Package Material Trends
The need to market products in packaging materials which cost-effectively enhance shelf life has led many brand owners to convert to metalized film or foil-based structures. These materials not only provide better oxygen, moisture, and UV-light barriers, but also improve shelf presence. However, metal-based packages are not compatible with metal detectors. On the other hand, X-ray systems have no problem seeing through these packages and can detect very small contaminants inside.
Packaging material trends will continue to be a critical factor in contamination detection choices.
Last but not least, fully educate staff on the use and operation of whichever technology you employ. Audit the system regularly to assure policies and procedures are being followed. A thorough and thoughtful analysis prior to selecting a detection system will ensure many years of trouble-free operation and fundamentally safer products.
Reis is the lead product manager, metal detection and X-ray inspection, at Thermo Fisher Scientific. Reach him at 763-783-2500.