Metal Detectors for Food Processing

Introduction

Why should a food processor consider a metal detector?  Some of the reasons are:

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• Profuct safety
• Equipment protection
• Contract (customer) requirement
• Regulatory compliance

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.

1. How a metal detector works
2. What can be detected
3. Product conditions
4. Rejection mechanisms
5. Purchase and installation
6. Metal detection program

1. How a metal detector works

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.

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.

2. What can be detected

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

3. Food product conditions

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.

4. Purchase and installation

Some of the information needed to specify a metal detector includes:

• Thorough definition of product (or ingredient) to be inspected, including an ingredient list, physical properties and a description of the package if the product is packaged. One of the most important physical properties is moisture content.
• List of available utilities at installation site (e.g. compressed air and electric).
• Description of the operating environment (temperature, humidity, corrosive materials, etc.).
• Description of the process (how product is being made and handled in the area where the metal detector is to be installed).
• Description of controls desired (manual, automatic, semiautomatic).
• Description of the desired reject mechanism (if purchasing from the same source as the metal detector).

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.

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5. Metal detection program

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.

Conclusion

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.

Reference

Lock, Andrew. . The Guide to Reducing Metal Contamination in the Food Processing Industry. Safeline Inc., Tampa, Florida.

The United Kingdom's highly mechanised commercial food processing industry is fast and efficient. It delivers a wide range of wholesome foods, including dried, canned and bottled ingredients, to retail outlets throughout Britain and abroad.

Industrial food processing involves multiple techniques performed by many different types of machinery. Ingredients are constantly in contact with metal implements such as blades, conveyor belts and sealing equipment.

Although the machinery is usually dependable and robust, the potential for metal contamination remains. Therefore, reliable metal detectors are a vital part of commercial food processing. They alert production lines to metal contamination before the products are distributed. Metal detection machinery enables food processing factories and retailers to present the public with foods that are safe and trustworthy.

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How Widespread is Food Contamination?

As of , there were around 1,685 manufacturing plants in the UK. The food and drinks manufacturing industry contributed approximately £30bn to the UK economy in . According to the Government's statistics, 93% of consumers trust food that's processed in the UK is perfectly safe to eat. However, contamination from various sources can occur at any stage. Hygiene regimes or bacterial contamination may pose a problem if routines are not strictly adhered to.

In , 106 incidents were reported concerning foreign bodies being found in food. Some of these may have included plastic, aluminium foil or glass. However, metal contamination is a prime concern due to the high use of metals in the complex production lines and in the canning process.

In , one major incident involving fragments of metal occurred in a whey protein product sold by Tesco, one of the UK's leading supermarkets. However, the relatively low number of cases involving food contaminated by metal is due to the efficiency of sophisticated metal detecting machines.

Are Metal Detectors Used at any Stage of Food Production?

Metal detecting equipment is frequently stationed at Critical Control Points (CCP). These are usually breaks between the different procedures that occur throughout food processing routines.

For instance, baked beans undergo several different procedures, beginning with blanching the dried haricot beans with steam. They are then soaked at least once to rehydrate them before they are added to a rich, lightly spiced tomato sauce. The mixture is poured into metal tins, which are then sealed and cooked under high pressure.

Baked beans are in high demand as they are the most popular processed food in the United Kingdom. In fact, more baked beans are consumed in Britain than in any other country. According to the Guinness Book of Records, Britons enjoy 11 pounds 10 ounces or 5.3 kilograms of baked beans per capita every year.

To satisfy the demand for producing more than 1.5 million tins per day, the processing of baked beans has been refined to a two-hour procedure. Conveyor belts transport the beans at up to five miles per hour. Metal detecting machines are highly proficient at dealing with the intensity of such speeds at any stage of the procedure.

Metal Detecting Machinery Used in Food Processing

Food metal detecting equipment manufactured is specifically designed for the food industry. The machines are precision engineered to the highest specifications, making them highly reliable and efficient. They are also compliant with current legislation regarding food safety. In addition, machines can be custom-built to suit individual food processing requirements.

Independent, stand alone metal detectors offer versatility as they can be placed at any CCP throughout the food processing procedure. They can also be used as a final end-of-procedure safeguard, screening finished containers for pieces of metal.

Typical package sizes when detecting fragments of metal often range from 120 millimetres to 400 millimetres. Sensitivity refers to the type of metal that can be detected. FoodMC.co.uk has machines capable of detecting ferrous metals (FE) from around 0.8 to 1 millimetre. Stainless steel, the most difficult metal to detect, can be located from sizes ranging between 1 millimetre and 3 millimetres.

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How Food Metal Detectors Work

Metal detectors for food are usually placed at vital CCPs where contamination can be readily located. Food processing systems usually include extensive roller conveyors with a variety of mechanisms such as gravity feed or vertical fall.

Liquids are usually pumped through a pipeline. Modern metal detectors for food have evolved a great deal since their development in . They are driven by sophisticated microprocessors that provide the highest level of accuracy.

Generally, metal detectors in the food industry operate through a balanced coil system that is shielded by an outer casing. The aperture through which food or packages pass contains two enclosed coils; a receiver coil and a transmitter coil.

They create a magnetic field that identifies metal particles in a process known as illuminating. The reject mechanism then propels the suspected package into a lockable container to prevent it being accidentally returned to the production line.

How Metal is Detected in Food

The search heads of commercial food metal detectors usually contain multi-scanning technology. They operate with at least twice the accuracy of models manufactured in the pre-computerised era. Systems that use multi-spectrum X-ray scanners are regarded as perfectly safe for the working environment as they don't use live radiation. This type of metal detector passes a computerised beam through the food and its container to create an on-screen image.

If a metal particle is detected, the system automatically rejects the container and food. Metal detectors carrying liquids or foods through pipelines usually include an audio alert. The system is ideal for checking foods contained in aluminium or tin cans. It can locate non-ferrous metals, and other contaminants such as rubber and plastic. An alternative solution for products destined to be wrapped in foil is to check the food before it is packaged.

The sensitivity of the search head should be a suitable match for the food it is checking. Some search heads detect large pieces of metal and are usually placed at a strategic CCP early on in the mechanised conveyor system. This prevents bulky contaminants infiltrating further down the system where the machinery may be more susceptible to damage. Metal detectors placed at later intervals may have increased sensitivity and be suitable for locating minute metallic particles.

Safety Guidelines

Commercial food processing carries enormous responsibility in maintaining safety standards in order to protect the consumer.

Food Machinery Company metal detectors for food processing comply with current government health and safety standards such as BS/EC 852/ and BS/EC 853/. However, it is important to adhere to a rigorous code of practice when operating metal detectors for food. Detailed records must be kept regarding the source of all the ingredients included in food processing. Computerised systems should be locked and protected by strong passwords. They should only be accessible to authorised personnel.

Avoiding False Readings

Metal detection machines contain highly sensitive components that can be affected by a number of conditions. To prevent false readings, the CCPs where metal detectors are positioned should be chosen with care. Try to avoid points along the production line where the machines could be affected by heavy vibrations. They also need to be placed out of reach of any electromagnetic interference from cables and electrical equipment.

The delicate mechanism of the metal detector's coils can be affected if foods are constantly changing temperature. This could be through cooling down from being heated, or if frozen products begin to thaw. High levels of salt can also cause fluctuations in the readings. Sensitivity adjustments can compensate for the discrepancies. Alternatively, a multiple search head can be programmed to compensate for minor variations.

Safety Tests

Metal detecting machines require a scrupulous maintenance regime to ensure safety guidelines are met at all times. Food metal detectors must be tested for accuracy at the start and end of each shift. They should also be tested every hour and whenever a different product is introduced to the production line. The metal detectors should be tested first with a safe, metal-free package, followed by one that includes appropriate metal pieces.

Some machines are supplied with specialised plastic strips fitted with metal. Conducting frequent tests ensures that if metal is discovered, a minimal amount of food is affected. It should be easily traced, preferably before it has left the factory.

Routine Maintenance

The machinery used in commercial food processing is heavy duty and designed for the stresses of a demanding regime. However, it is still constantly suffering wear and tear, particularly from the effects of relentless vibration. Routine maintenance should be carried out at planned, regular intervals. This prevents expensive downtime occurring through unexpected machine failure.

Replacement screws and rivets should be containerised and accounted for during repairs. Any work involving soldering, drilling or welding must be carried out off site.

What to Do if Your Product is Contaminated

When metal contamination of food occurs, swift action must be taken to minimise the impact on public health and safety. Hourly testing of metal detectors helps reduce the size of a batch, making the contamination easier to locate.