Food Metal Detectors

Metal detectors operate on the principles of electromagnetic induction. Each system features one or more inductor coils that interact with metallic elements within a product. As shown, a single-coil metal detector uses the same principles as a standard metal detector, just simplified. A pulsing current is applied to the coil, which then induces the magnetic field shown in blue. This magnetic field envelops the area surrounding the metal detector opening, and products pass through this field during inspection.

In addition to the transmitter coil, metal detectors have receiving coils that continuously analyze the magnetic field of the inspection zone.

When metal moves through the magnetic field of the metal detector, such as the coin in this illustration, the field induces electric currents (called eddy currents) in the coin. The eddy currents induce their own smaller magnetic field (shown in red), generating an opposite current that can be picked up by the receiving coil. The metal detector is then able to detect this signal from the eddy current, and will alert to the presence of metal in the inspection area.

Metal detectors for the food industry operate with this same principle and take advantage of the conductivity of the most common metal contaminants to provide a reliable inspection.

A metal detector for food can detect a wide range of metal contaminants and conductive materials, including both ferrous and nonferrous metals, and some other conductive compounds. Ferrous metals are conductive and contain iron, though many are composed of carbon or other elements combined with iron. The various types of steel are all categorized as ferrous metals. Ferrous metals are usually magnetic, and more prone to rust than nonferrous metals. Nonferrous metals are composed of elements other than iron, and include aluminum, copper, nickel, tin, brass and zinc. Stainless steel can be detected as well, but with varying levels of sensitivity depending on the stainless steel’s specific composition. 

All of these types of metal can be detected by inline metal detectors. The exact amount of each metal needed to trigger detection depends on the conductivity of the metal, as well as the conductivity and size of the product being inspected. Some metal particles that are highly conductive can be detected by an inline metal detector. 

Almost all types of food and beverage products can be inspected with an inline metal detector, as long as they are not packaged with any metallic material. Harvested goods, snack foods, bakery items, proteins, and frozen foods are all viable to inspect with metal detection. This means that a majority of packaged food products can be inspected with a standard food metal detector. 

However, products that are packaged with metallized foil, in metal containers, or with metallic inserts are not recommended to be inspected with food grade metal detectors. These products will conduct the magnetic field of a metal detector in the same way that an actual contaminant would, causing either high false-reject rates (of product without contaminants), or significantly reduced sensitivities of true foreign bodies. X-ray inspection systems are the preferred method for inspecting products packaged with metallic elements, as x-ray does not rely on conductivity during inspection.

The term “product effect” describes the natural conductivity of a product, and the impact it has on a metal detector system in food production. Product effect is observed even on products that are known to have no metal bodies in them.

Because metal detectors for food use conductivity to inspect for contaminants, products that are naturally conductive can trigger the metal detector as they pass through. Red meats, products with high salt or fat contents, and products with a lot of moisture all have above average product effects. These products can still be inspected using a metal detector system. However, it is important to select the right system when considering high product effects.

Certain metal detectors use multi-frequency inspection. This allows them to optimize sensitivity even on those more conductive products. RMD Series High-Configuration Metal Detectors from TDI Packsys allow the operator to modify the frequency of inspection, ensuring high-performance inspection with minimal product effect.

In some applications, product effects are simply too high, and will negatively impact inspection sensitivities with even the most advanced metal detectors. For these products, x-ray inspection systems are a perfect solution, as they do not experience product effect, and will not be affected by changes in the product.

Both metal detectors and x-rays can be used to inspect for foreign material in food processing. The main difference between metal detectors and x-ray inspection systems is the methodology of how each machine inspects for foreign materials. Metal detection uses conductivity to locate stainless steel, ferrous, and nonferrous metals in product. X-ray inspection uses density differences between the product and foreign bodies, allowing for a far greater range of foreign material detection – they can be used to find metal, glass, stone, bone, rubber, gasket (both metal detectable and non-metal detectable), plastic, and various other mid- and high-density items.

Some products are limited to certain inspection systems due to their composition and packaging. Products with metallic packaging, such as those with metallized foil, cannot be accurately inspected with a metal detector, and require the use of an x-ray inspection system.

The best way to determine which option is best for your application is to receive a consultation or free, no obligation validation testing. We can test your exact product on a variety of different systems using certified test cards, or foreign material samples you provide, to offer you a formal report on the capabilities and expected performance of each type of technology.

At TDI Packsys, we believe validation testing on your specific products is an integral step in choosing the right system for your needs.

Metal detectors for the food industry can be used in a variety of points in the production process. Metal Detection is often used as a critical control point in food production. 

Where and how a food grade metal detector can be used depends on a few factors:

1. Will the food be packaged or unpackaged during time of inspection?

For food that is packaged, conveyorized metal detector systems often provide the easiest installation with an existing line. For unpackaged food, running the product through a pipeline metal detector (for liquids) or a gravity fed/flow-through metal detector (for powders) might be a better option. TDI Packsys also offers throat-style food grade metal detectors that can be integrated with a packaging system for free falling products.

2. Where is the main concern for contamination?

It is always important to consider the point in the process that has the highest risk for contamination. Metal detecting product before it reaches this point might not ensure that your products are free from metal contaminants and might lead to issues with food safety and quality. It is always best to try and inspect product once it has been completely sealed, so that you have the confidence that the product remains metal free throughout the process. 

Metal Detectors in the food industry are designed to keep up with the normal flow of the production process.

Conveyorized metal detectors can run at speeds of up to 120 meters a minute, which is just over 393 feet! This allows food metal detectors to inspect hundreds of products a minute without skipping a beat. Pipeline and flow-through metal detectors are limited only by the speed of product moving through them,

For the fastest lines, TDI Packsys has optional high-speed reject mechanisms that can remove any contaminated production from the line without stopping or slowing down the system.

The standard types of metal seen in food processing are ferrous, non-ferrous, and stainless steel. However, there are some instances when aluminum can make its way into the process, and it is essential to ensure that whatever critical control points being used to inspect for foreign materials are able to successfully detect it.

Aluminum is a low density, highly conductive non-ferrous metal that tends to be found as small shavings or foil pieces when it contaminates food products. Although it is non-ferrous, aluminum is much less dense and significantly more conductive than the brass test standards often used to qualify metal detectors.

Because of its conductivity, food metal detectors are actually able to detect aluminum at much smaller sizes than other metals. In fact, 1 pound of aluminum can actually carry the same electrical current as 2 pounds of brass.

Because of its low density, x-ray inspection typically will not be able to detect aluminum down to the same size as metal detectors in the food industry, therefor making food grade metal detectors the best option for detecting aluminum in most food products.