Industrial metal detectors for the food industry are used at various stages of the production process to detect metal contamination in food.
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When determining where on the food production line the metal detection system will be used, the first step is often to carry out an HACCP analysis and identification of the Critical Control Points for detection of physical contaminants.
Many manufacturers use metal detection systems at the start of the line to monitor the quality of raw materials from suppliers, and to protect expensive equipment downstream. Others choose to use them in the production line prior to final packaging, reducing the expense of product waste and rework of contaminated product. Some choose to use metal detectors in food packaging and packaging lines, or after the final packaging or cartoning of the product for peace of mind in knowing that there is minimal risk of additional contamination.
For maximum protection to customer and brand, many manufacturers decide to maximize their use of industrial metal detection to prevent product recalls by using detectors in two or more areas on the production line to further reduce the risk of a disastrous food recall.
Meeting food industry safety and compliance requirements often starts with product inspection equipment including industrial metal detectors.
There are many types of metal detection used in food processing. The right metal detection equipment to use for product inspection in the food industry depends on the type of food you are inspecting, as well as the environment that the inspection will take place.
For packaged and unpackaged food that runs on a conveyor, the best option is a conveyorized tunnel metal detector designed for inline product inspection on the manufacturing line. Our fully automated line of industrial metal detector systems for conveyor belts will safely detect and reject product contaminated with ferrous and non-ferrous metals, as well as hard-to-detect grades of stainless steel. For conveyorized food products that are wet, hot, chilled, cooling, or wrapped in metal film, our Profile Advantage metal detector is designed to deliver up to 50 percent improved sensitivity in these challenging food applications, leading to fewer false rejects.
For pumped food applications such as sausage, raw chicken, juices, or sauces, your best bet is to go with a pipeline-based metal detection system. Our line of pipeline metal detectors are built with advanced detection technology to satisfy the most stringent detection requirements in liquid, paste, or slurry applications.
For inspection of bulk gravity-fed powders, grains or granules, you need a metal detection system that has the ability to inspect product in free-fall. Our high performance, gravity fall metal detection systems seamlessly detect and reject metal contaminants, delivering maximum sensitivity and performance. They also support ATEX, IECEx, and HazLoc compliance for explosive atmospheres.
Bulk free-falling food items such as nuts, snacks or confectioneries which require integration with Vertical Form, Fill & Seal (VFFS) packaging equipment are best suited to a vertical throat metal detector. These systems use high levels of sensitivity to detect unwanted metal in the product flow before it enters the bag, overcoming interference from metalized film packaging. Our throat metal detectors offer the sensitivity you need with Zero Metal Free Zone technology, enabling the space between the weigher and VFFS to be kept to a minimum.
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An in-line tunnel metal detector integrated with a material handling system is the best solution for inspecting packaged, raw, or processed foods products on a conveyor – at the front end, in-process, or end-of-line. METTLER TOLEDO Safeline offers a wide variety of industrial tunnel metal detectors for use in these applications to meet different performance and budget needs.
Selection of the metal detector head depends on the specific characteristics of the product being inspected. For challenging applications that are subject to product effect where food is hot, wet, chilled, frozen, or packed in metalized film, industrial metal detectors using multi-simultaneous frequency such as Profile Advantage can greatly reduce false rejection rate. If budget is the top concern, you can go for metal detectors with dual-simultaneous frequency such as the M34R GC. For dry product inspection, choose metal detectors with high tuned frequency such as the Profile or M33R GC.
We also offer a number of other tunnel metal detection solutions for packaged bulk as well as unpackaged bulk foods. We offer combination product inspection systems where a metal detector is integrated with a dynamic checkweigher to provide multiple product inspection capabilities in a single unit, with a compact footprint.
Why should a food processor consider a metal detector? Some of the reasons are:
Of the reasons listed, perhaps the most compelling is found in the Federal Register (Vol. 51, No. 118):
“Effective measures shall be taken to protect against the inclusion of metal or other extraneous material in food. Compliance with this requirement may be accomplished by using sieves, traps, electric metal detectors or suitable means.”
Metal detectors will not guarantee a metal-free food product, but a properly designed, installed and maintained unit, along with a metal contamination control program, can be extremely effective. This fact sheet discusses the following topics regarding metal detection and removal.
Most metal detectors use a balanced, three-coil, system to detect small particles of non-ferrous and stainless steel. The coils are wound on a non-metallic frame, each parallel with the other. The center coil is connected to a high frequency radio transmitter. Coils on either side of the transmitter coil are receivers. As these two coils are identical and placed the same distance from the transmitter, they receive the same signal and produce an identical output voltage. When the coils are connected in opposition, the output is cancelled, resulting in a zero value. A schematic of the coil configuration is shown in figure 1.
Figure 1.
When a particle of metal passes through the coils of a metal detector, the high frequency field is disturbed under one coil, changing the voltage by a few microvolts. The state of balance is lost and the output from the coils is no longer zero. It is this phenomenon that is used to detect metal.
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An important aspect of metal detector operation is the metal free zone, which is needed for proper operation. The zone includes a space on each side of the aperture that must be free from any metal structure such as rollers and supports. As a general rule, this needs to be approximately 1.5 times the aperture height for fixed structures and 2 times the aperture height for moving metal such as reject devices or rollers.
All metals are either ferrous, nonferrous or stainless steel. The ease of detection will depend on their magnetic permeability and electrical conductivity. Table 1 shows metal types and their ease of detection. The size, shape and orientation (with respect to the detector coils) of the metal particle also is important. Since size, shape and orientation of metal contaminants is not possible to control, it is best to operate a metal detector at the highest possible sensitivity setting.
Table 1. Metal types and their ease of detection
Food product conditions can have a major effect on metal detection. Electrical conductivity in foods such as cheese, fresh meat, warm bread, jam and pickles can generate a signal in a metal detector even though metal is not present. This phenomenon is known as the “product effect.” It is best to be aware of this phenomenon and to work with your metal detector supplier or manufacturer to determine the best means to compensate for product effect.
Figure 2.
Conveyor detectors are by far the most common; examples are given in figure 3. Many variations of manual, semi-automated and fully automated rejection mechanisms are possible, such as air blow (figure 3a), push arms, retractable conveyor bed (figure 3b), reversible conveyor (figure 3c), slider gate, ink marker or coder, diversion conveyors and robotic grippers.
Figure 3a.
Figure 3b.
Figure 3c.
Free-fall detectors are often found on dry product fillers and are useful for any material that can be dropped through an aperture. A fast-acting valve or deflector plate is used to divert detected material to a separate container.
Some of the information needed to specify a metal detector includes:
Table 2 lists some manufactures of metal detectors. For a more detailed list, visit the website maintained by BNP Media (Troy, Mich.) at www.foodmaster.com and search the food master equipment database for “metal detectors.” Installation of the metal detector will require a qualified electrician and a mechanical contractor familiar with metal detectors and the type of conveyors used in the operation. If there is a chance that the metal detector or the rejection mechanism could impede the normal flow of production, it may be desirable to install and test the unit during a scheduled down time to avoid production losses. An alternative is to install and test the unit on a mock production line.
Table 2.
*Oklahoma State University and FAPC do not endorse any specific equipment or manufacturer. The listing is given for information purposes only.
A formal metal detection program will help to insure product quality. Metal detectors may be used in various phases of production. A combination of finished and bulk ingredient and product inspection gives the best performance. Detectors may need to be placed after certain process equipment (such as size-reduction) that are prone to breaking or chipping metal materials.
A sensitivity standard (or standards) should be set for the entire facility. An important aspect of this is to identify an agreed upon minimum particle type and size. For example, a typical detection standard for finished product might be to remove all spherical, non-magnetic particles larger than 2.0-mm and all spherical, magnetic particles larger than 1.5-mm. Only detectors that meet these standards would be considered for purchase and installation. The conditions should be clearly marked on the side of any installed detector and samples of the correct diameters should be available for testing the unit.
Metal detectors should be operated at the maximum sensitivity setting for a given product. The maximum acceptable sensitivity setting will allow the detector to perform reliably for extended periods of time without excessive false rejects. Scheduled testing of the detector and reject device (with ferrous and nonferrous metal samples) will confirm proper operation. Intervals between tests can be determined by the consequence of a failed test. Testing every two to four hours is typical. A testing procedure should be established and followed. A plan of action must be specified for failed tests. Every effort should be made to identify, document and correct the source of detected metal.
Metal detectors are an important part of a comprehensive contamination control program. If they are specified, installed, operated and maintained correctly, they will contribute to improving product quality and reducing losses. A properly used metal detector should not be considered as insurance against metallic contamination. Rather, it is a diagnostic device that can help to discover accidental metallic contamination. Plans and procedures should be in place to prevent contamination. If contamination does occur, contaminants must be identified immediately, traced back to their source and eliminated.
This fact sheet has focused on metal detectors; however, other devices are available to remove metallic and other product contaminants. Line strainers are useful for removing foreign particles from liquid products and magnets have long been used to remove ferrous materials from particles and pumpable liquids.
Lock, Andrew. . The Guide to Reducing Metal Contamination in the Food Processing Industry. Safeline Inc., Tampa, Florida.
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