Metal detection in packaged food: how to avoid false rejects

metal detection in packaged food

The challenge of metal contamination in packaged food is a big one. With metal and non-magnetic stainless steel among the biggest culprits for food contamination today, many food processors choose a metal detection system to seek out even the smallest metal contaminants in their product.

While metal detection is incredibly beneficial to food processing and packaging lines, it must be the right system, with the proper installation and ongoing corrective maintenance.

If you fail to invest this time up-front, you risk a high number of false detections, which can slow down the line, waste product, and ultimately impact on profits and productivity.

Some manufacturers who want to improve detection of contaminants may increase metal detector sensitivity, resulting in increased false rejects of good product. At the same time, there are those who attempt to reduce product loss by reducing detector sensitivity, therefore passing more products but potentially risking that some metal contaminants are released onto the shelves.

So how do you find the right balance and avoid false rejects? In this article, we’ll explain how metal detectors work and five ways to ensure maximum success.

How metal detectors work

Metal detectors can find small particles of ferrous, non-ferrous and stainless steel in products. They use coils wound on a non-metallic frame connected to a high-frequency radio transmitter. When a particle of metal passes through the coils, it disturbs the high frequency field under one coil, which then changes the voltage by just a few microvolts – enough to signify the presence of foreign material.

Metal detectors in the packaged food industry need to be highly sensitive to pick up on contaminants – especially when particles may be just 1mm in diameter. However, there are lots of factors that can influence their sensitivity. For example, ferrous metal is more easily detected than non-ferrous metals, such as brass, copper and aluminium, because it is magnetic and a good electrical conductor. Non-ferrous metals on the other hand are relatively easily detected in dry applications but less so in wet application, as they are non-magnetic. Then there’s the shape and orientation of metal: a non-spherical particle or wire tends to be easier to detect at a certain angle compared with another, something known as the “orientation effect”.

In fact, the real challenge isn’t finding the contaminant, but ignoring the packaging, product and environment.

But here’s where you don’t want to ignore your packaging, product or its environment.This free whitepaper takes you through how packaging improvements help to optimise your supply chain. It describes the 3 types of packaging, the relationship between packaging & logistics efficiencies, the consequences of ineffective packaging, how to be innovative with packaging & more! Download Now


Want to reduce false detections? Consider these 5 factors:

1. Environmental conditions

Line vibrations, temperature fluctuations and air-borne electrical interference can all make a difference to the performance of the metal detection system. For example, ovens, freezing tunnels and hot water wash-down will create thermal shock that can result in false reject signals. Therefore, selecting a metal detector that suits your operating environment is critical to its performance.

Evaluate your inspection area:

  • Is it wet or dry?
  • Is wash-down high or low pressure?
  • What are the temperature variations?
  • What equipment is nearby? As far as practically possible, a metal detection system shouldn’t be installed close to sources of vibration and mechanical shock.
  • Are inverters, cables and variable-speed drives nearby? Other equipment using similar technology nearby may cause electromagnetic interference (EMI), which will cause erratic operation and false rejections.

2. Product characteristics

The “product effect” is a big factor causing false rejections. This is caused by certain characteristics of the product being inspected (such as high salt or moisture), which are able to disrupt the voltage balance between the receiver coils. As a result, the metal detector reacts as it would to a metal contaminant.

Ask these questions:

  • Does product have a high salt and/or iron content? Many snacks, such as potato chips and nuts, contain high doses of sodium and are therefore conductive. When they pass through a metal detector, they can create a disturbance of the detection field, which triggers false rejects.
  • Does the product have high moisture content? Fresh meat, poultry, cheese and fish tend to have higher moisture content, which creates a “product effect” signal in the detector; this needs to be cancelled out before inspection can begin.
  • Are there variations in the products temperature? As warm products cool and frozen products begin to thaw along the line, these changes can affect the product’s signal.
  • Is the product packaged in metal film? This is an obvious one; if so, it will be extremely hard for a metal detector to inspect the product.

If the answer to one of more of the above questions is “yes”, you need to select the correct frequency in the equipment to move the product effect signal away from the signal of any contaminants. This is where a multi-spectrum metal detector is a smart choice. Those using a single frequency cannot accommodate much signal variation, which makes it ineffective at inspecting a variety of product types or products that vary in temperature. A multi-spectrum metal detector uses a spectrum of multiple frequencies at the same time to filter out product effect signals, which greatly reduces false alarms without reducing detection sensitivity.   

Also consider if you will change your product range in the near future. How will this impact both the processing environment and product characteristics? For example, a new product may require new packaging equipment, which may in turn impact the environment conditions.

3. Product uniformity

How many different products do you inspect? Do they change consistency through the day? For example, as the temperature and humidity in the plant changes, products with a high salt or moisture content may also change in conductivity or solidity. This affects the metal detector, as radio frequencies pass through product in a different way depending on the product make-up. This could result in false rejects.

4. Advanced technology

Technology has moved on in the past few years and the advanced metal detection units, such as the Bizerba VARICON+, have been designed to offer maximum sensitivity for contaminants while maintaining high resistance to interference. It is also easy to train the device for specific products, which means minimum manual adjustment is needed and product changeovers are quick and easy. For example, the Bizerba VARICON+ features a multi-product memory for up to 240 products.

5. Work with the experts

Working with the experts will ensure you have the correct inspection equipment and installation for your line, helping you get the best possible performance from the system while avoiding costly mistakes. With years of experience working with food processors of all types and sizes, Matthews Australasia can help you choose and install the right metal detection system for your line. Your equipment provider can also help train staff in operating the system and its nuances, and deliver a preventative maintenance programme to ensure the equipment is continuously working at its best.


Metal detectors can form an integral part of the inspection process for today’s packaged food processors. However, to avoid costly false rejections, it’s important to take some time up-front to make sure you have the right system and it is installed correctly.

Remember these top tips:

  • Work with the experts to choose the most appropriate technology for your line and products.
  • Ensure staff are properly trained in operating and maintaining the equipment.
  • Consider your product characteristics, both now and into the future.
  • Consider the environmental factors and how these might change in the near future.

Ready to talk about metal detection or other inspection systems for your packaging line? Speak to Matthews Australasia. Our team has nearly 40 years’ experience in the business, so we know how to help drive efficiency, accuracy and quality in your line.

And here’s how to drive efficiency, accuracy & quality in beverage manufacturing. This free whitepaper looks at the opportunities in Australia’s highly competitive market, through creating operational excellence, a cost-effective supply chain, recording & having access to quality data in real time, reducing waste & constantly innovating with products, including carving a niche. A must-read for beverage processors. Download Now


Here’s a good example of how and where metal detection can be used: an NFP packing organisation chose metal detection after gaining a new contract for packing baby food. This looks at what technology and solution they went with and why.

You may find this article comparing metal detection and X-ray inspection systems interesting, while this blog looking at which inspection technologies can be used for ready meals (no longer the old “TV dinner”) has a section on metal detection.

You will also find lots of valuable information in Matthewsexpansive resource library. And its all free to download! There are case studies, whitepapers, presentations we’ve done to industry bodies, infographics for manufacturing, articles from our thought leaders, vids showing solutions in action, lots of detailed of brochures and more!

Image credit: iStock / Freezingtime


Andrew Ball

Andrew Ball

Commercial Products & Marketing Manager at Matthews Australasia
Andrew Ball is an engineer and business professional with 25 years of broad industry experience. His career includes experience gained at Honeywell, Ford and Siemens, with over half of it invested in the coding and marking industry. At Matthews Australasia, Andrew and his team create and market innovative solutions, services and insights for Matthews’ customers and the broader industry alike. Aside from career and family, some of Andrew’s other passions include music, sport, fitness and reading.
Andrew Ball

by Andrew Ball

Andrew Ball is an engineer and business professional with 25 years of broad industry experience. His career includes experience gained at Honeywell, Ford and Siemens, with over half of it invested in the coding and marking industry. At Matthews Australasia, Andrew and his team create and market innovative solutions, services and insights for Matthews’ customers and the broader industry alike. Aside from career and family, some of Andrew’s other passions include music, sport, fitness and reading.

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