In February 2026, Rosina Food Products recalled about 9,462 pounds of ready-to-eat frozen meatballs sold at Aldi stores across the US, after a consumer found metal fragments in the product, according to USDA-FSIS reporting via Food Safety News. The meatballs had a 15-month shelf life, so the problem surfaced months after production.
Stories like this are why every QA team asks the same question: how does a metal detector work in food industry lines, and what can it really catch? This guide answers that in plain language, from the coil system to the reject bin.
The balanced-coil principle
Almost every industrial metal detector for food and pharma uses a balanced-coil design. Inside the detector head are three coils wound around the aperture that the product passes through.
Stories like this are why every QA team asks the same question: how does a metal detector work in food industry lines, and what can it really catch? This guide answers that in plain language, from the coil system to the reject bin.
The balanced-coil principle
Almost every industrial metal detector for food and pharma uses a balanced-coil design. Inside the detector head are three coils wound around the aperture that the product passes through.
- Transmitter coil (centre): carries a high-frequency alternating current and creates an electromagnetic field across the aperture.
- Two receiver coils (either side): pick up that field. They are wound in opposite directions and set up so their signals cancel out exactly. With nothing in the aperture, the output is zero. The system is "balanced".
When a metal particle enters the aperture, it disturbs the field near the first receiver coil before the second. For a moment the coils are out of balance. The control electronics amplify that tiny difference, process it and, if it passes the set threshold, trigger the reject.
[Insert animated diagram of the coil system: transmitter in the middle, two receivers, product moving through.]
Ferrous, non-ferrous and stainless steel
Metal contaminants behave differently in the field, so detectors are always rated separately for each group.
Stainless steel is the hardest to find. Grades like 304 and 316 are weakly magnetic and poor conductors, so they produce only a small signal. This matters because most food and pharma equipment is made of stainless steel. When a screen tears or a blade chips, the fragment is usually stainless.
That is why sensitivity specs always list stainless steel as the largest size. TCT's Pharma Metal Detector, for example, is specified at 0.25 mm ferrous, 0.35 mm non-ferrous and 0.45 mm stainless steel. The Conveyor Metal Detector is specified at 0.5 mm, 0.7 mm and 1.0 mm respectively.
Frequency selection and multi-frequency
The frequency of the transmitter field changes what the detector sees best.
[Insert animated diagram of the coil system: transmitter in the middle, two receivers, product moving through.]
Ferrous, non-ferrous and stainless steel
Metal contaminants behave differently in the field, so detectors are always rated separately for each group.
Stainless steel is the hardest to find. Grades like 304 and 316 are weakly magnetic and poor conductors, so they produce only a small signal. This matters because most food and pharma equipment is made of stainless steel. When a screen tears or a blade chips, the fragment is usually stainless.
That is why sensitivity specs always list stainless steel as the largest size. TCT's Pharma Metal Detector, for example, is specified at 0.25 mm ferrous, 0.35 mm non-ferrous and 0.45 mm stainless steel. The Conveyor Metal Detector is specified at 0.5 mm, 0.7 mm and 1.0 mm respectively.
Frequency selection and multi-frequency
The frequency of the transmitter field changes what the detector sees best.
- Higher frequencies are more sensitive to small non-ferrous and stainless particles in dry products.
- Lower frequencies cope better with wet, salty or conductive products, which create their own signal at high frequency.
No single frequency is perfect for every product. That is why modern detectors either let you choose the frequency per product or run several frequencies at once (multi-frequency or simultaneous frequency). The goal is always the same: the highest sensitivity to metal with the lowest response to the product itself.
Product effect
The product itself can disturb the field. This is called product effect, and it is the main reason real-world sensitivity is lower than the test-piece figure on a spec sheet.
Products with strong product effect usually share one or more of these traits:
Product effect
The product itself can disturb the field. This is called product effect, and it is the main reason real-world sensitivity is lower than the test-piece figure on a spec sheet.
Products with strong product effect usually share one or more of these traits:
- Wet – fresh meat, cheese, sauces
- Salty – cured meats, brined vegetables, ready meals
- Warm – freshly baked or cooked items that are still cooling
- Conductive packaging – metallised film or foil
Modern detectors use digital signal processing to learn the signal of a good product and filter it out. TCT detectors use an ARM controller with digital signal processing and are engineered to suppress vibration, electrical noise and product effect. Even so, a wet, salty product will never reach the same sensitivity as a dry tablet. Always validate with your real product.
Aperture size and position
Sensitivity is best in the centre of the aperture and falls as the aperture gets bigger. A simple rule: the smaller the aperture relative to the product, the better the detector can find small metal.
Good practice is to:
Aperture size and position
Sensitivity is best in the centre of the aperture and falls as the aperture gets bigger. A simple rule: the smaller the aperture relative to the product, the better the detector can find small metal.
Good practice is to:
- Choose the smallest aperture your largest pack can pass through with a little clearance.
- Centre the product in the aperture height where possible.
- Test with test pieces placed at the least sensitive point, usually the geometric centre of the aperture.
This is why pharma detectors that sit after a tablet press have very small apertures.
Tablets pass through a narrow channel, which is how such small spheres can be detected.
TCT's Conveyor Metal Detector offers pusher, flipper, flap-down and drop-down belt rejects, so the reject can match the pack. The Pharma Metal Detector uses an electrically operated fast-acting flap to divert contaminated tablets immediately.
In the meatball case above, the contamination was found by a consumer, not a detector. A well-set, fail-safe detector with a validated reject is designed to make sure that kind of discovery never happens outside the factory.
Frequently asked questions
How does a metal detector work in the food industry? A transmitter coil creates a field, two balanced receiver coils cancel each other out, and any metal passing through unbalances them. The detector then rejects that pack.
Why is stainless steel hard to detect? Common grades are weakly magnetic and poor conductors, so they produce a much smaller signal than iron of the same size.
What is product effect? It is the signal created by the product itself, typically from moisture, salt, temperature or metallised packaging. It limits real-world sensitivity.
Does a smaller aperture improve sensitivity? Yes. Sensitivity is best when the aperture is as small as the product allows.
Can a metal detector find glass or plastic? No. Non-metallic contaminants need X-ray inspection.
Find the right detector for your product
Talk to a TCT application engineer about which technology fits your product.
Tablets pass through a narrow channel, which is how such small spheres can be detected.
TCT's Conveyor Metal Detector offers pusher, flipper, flap-down and drop-down belt rejects, so the reject can match the pack. The Pharma Metal Detector uses an electrically operated fast-acting flap to divert contaminated tablets immediately.
In the meatball case above, the contamination was found by a consumer, not a detector. A well-set, fail-safe detector with a validated reject is designed to make sure that kind of discovery never happens outside the factory.
Frequently asked questions
How does a metal detector work in the food industry? A transmitter coil creates a field, two balanced receiver coils cancel each other out, and any metal passing through unbalances them. The detector then rejects that pack.
Why is stainless steel hard to detect? Common grades are weakly magnetic and poor conductors, so they produce a much smaller signal than iron of the same size.
What is product effect? It is the signal created by the product itself, typically from moisture, salt, temperature or metallised packaging. It limits real-world sensitivity.
Does a smaller aperture improve sensitivity? Yes. Sensitivity is best when the aperture is as small as the product allows.
Can a metal detector find glass or plastic? No. Non-metallic contaminants need X-ray inspection.
Find the right detector for your product
Talk to a TCT application engineer about which technology fits your product.