Foreign body detection and identification

Consumer complaints regarding foreign bodies are a constant problem for virtually all food companies, and so their prevention and control should be high on the list for all Quality and Technical Managers. The effective management of prevention and control of foreign bodies requires attention to many parts of the business, including recruitment and training, sourcing of raw materials and packaging, factory design and the selection and installation of manufacturing equipment among others – and all this is before we have even considered items like metal detectors to detect and remove foreign bodies.

The law states that “…food fails to comply with food safety requirements if…it is unfit for human consumption; or it is so contaminated (whether by extraneous matter or otherwise) that it would not be reasonable to expect it to be used for human consumption in that state.”
(Section 8(2)(b and c) of the Food Safety Act 1990)

and

“Any person who sells to the purchaser’s prejudice any food which is not of the nature or substance or quality demanded by the purchaser shall be guilty of an offence.”
(Section 14(1) of the Food Safety Act 1990)

The law also allows defences of ‘all reasonable precautions’ and ‘all due diligence’ when a company is faced with prosecution over a foreign body complaint. In order to demonstrate that you have taken all reasonable precautions against a foreign body getting into your product and hence have shown all due diligence, a six-point plan may be used:

1. Consider the hazards – what foreign body hazards may your product be subjected to?
2. Judge the concerns – of these, what are the real concerns, in terms of both safety and likelihood?
3. Select and install suitable controls – metal detectors, magnets, sieves, etc.
4. Integrate the controls into a whole plan
5. Set up a review system and continuous improvement plan
6. Maintain full records

In order to identify the hazards, it is necessary to identify the foreign bodies being reported from your product, to find out when and where they are getting in and therefore whether or not it is something you could control. For example, glass fragments form a significant proportion of foreign body complaints, and an expert laboratory examination can use factors like shape, thickness, curvature and the nature of the various surfaces present to reconstruct the type of article the glass is likely to have come from. Surface scratching on a glass surface can often indicate how the original article was used, and probably indicate a domestic item which has been used repeatedly, whereas the absence of scratching on a surface such as a rim likely to be exposed to wear can suggest an item designed for single use, such as a jar or bottle. An elemental analysis using an X-ray analyser can tell you the composition of the glass – whether it is ‘ordinary’ soda lime glass such as that used to make domestic items such as drinking glasses or containers such as jars and bottles, or heat-resistant glass such as Pyrex, used to make domestic items such as casseroles, or a lead crystal glass used for high quality domestic glassware. Moreover, examination of surface deposits on the glass can often show whether the fragment has been in contact with the food product from which it was reported, or has come from somewhere else. Whilst some fragments of glass reported from food do come from glass breakages on filling lines or from raw material, experience shows that the vast majority come from accidental breakages in the consumer’s own home. These can range from chips from the rims of glass casseroles, to a piece from the bottom corner of a glass jar, forced out when the last remaining contents of the jar are being scraped out with a spoon or knife. Hence, whilst it may be possible to use on-line technology to detect and remove glass fragments coming in with raw materials or produced by on-line breakages, no such equipment can help to remove fragments which enter the food later in the chain.

The techniques outlined above can be used for many types of foreign body other than glass. Metal fragments can be approached in the same way, and in the case of metals such as stainless steels likely to be used in food factories, their elemental composition can often be checked against metal equipment to determine whether such equipment might be a likely source. Occasionally, recognisable pieces of metal such as nuts and bolts are reported as foreign bodies, and here, obviously, their size and shape are important identifying features. More frequently, however, the analyst is presented with a twisted shard of metal, where shape is of less use in its identification. However, even here, there is useful information. The metal may have had a surface coating of lacquer, tin or chrome, which may still be present on one surface of the shard. Whilst food debris does not adhere well to stainless steels, rusty metal surfaces will often carry clear evidence of materials with which they have previously been in contact.

Plastics are an increasing foreign body problem for a number of reasons. They are being increasingly used in food packaging and in many other spheres of everyday life, but they are generally very difficult to detect on-line. There are many different types of plastic, and the difficulty is exacerbated by the use of co-polymers (mixtures of different plastics) for some applications. Moreover, many of these plastics have a wide range of different uses. However, many food packaging materials are made of multilaminates – several layers of different plastics, each performing a particular function in the package. The different layers can be identified by Fourier Transform Infra-Red (FT-IR) Spectroscopy, and the thickness of each can be measured under the microscope. The data from such an examination can be compared with the package used for the food product itself or possibly one of the ingredients to check for a match. FT-IR Spectroscopy is a technique that can be applied to a wide range of organic materials besides plastics, including food materials and tablets and capsules of medical drugs. This is a particularly important technique for samples like this, where a single test is required for use on a very limited amount of sample to identify one amongst thousands of possible candidate sources.

The range of materials being reported as foreign bodies in food products is virtually limitless, and the analyst often has to draw upon specific techniques developed in a wide range of scientific disciplines such as geology, entomology or botany to make an identification.

Once the most important types of foreign body to be controlled have been identified, the appropriate controls can be selected and installed. One of the most common controls applied is a metal detector, and it is not always appreciated that these are of two quite distinct main types, each with their advantages and limitations. The most common type is the Balanced Three Coil detector, in which an electric current is passed through one wire coil (the transmitter coil) surrounding an aperture through which the food product passes. The magnetic field so produced induces a current in wire coils (the receiver coils) placed either side of the transmitter coil. The presence of a piece of metal produces changes in the current induced in the receiver coils to enable detection of the metal. This type will detect a range of different metal types, but must be ‘tuned’ to the par­ticular metal type regarded as the major risk – hence the need to identify the most important risks before deciding on the necessary controls. The sensitivity of this type is affected by the type of product, the metal being detected, the size, shape and orientation of the metal fragment, and the position of the metal fragment within the product. A recent advance in this type of detector is the development of variable frequency instruments, selecting the optimum detection frequency for the metal to be detected. The second type is the Magnetic Field System or ‘Ferrous in Foil’ Detector, which is simple to set up, having a single control to adjust the sensitivity. In this case, the magnetic field is provided by permanent magnets, and a piece of metal passing through the field induces a current in wire coils surrounding the food package. However, its major limitation is that it does exactly what its name suggests – it is incapable of detecting even large pieces of non-ferrous or stainless steel metals, and is unsuitable for other types of container.

X-ray machines have the advantage over metal detectors of being able to detect a much wider range of foreign bodies than metal detectors, the important factor being that the foreign body must be significantly different from the food material in X-ray density in order to be detected. In practice, this means that an X-ray machine will be capable of detecting items such as metal, glass, stones, bone and some denser plastics such as PVC, but not plastic films, paper, cardboard, wood, insects, string or fruit stones. There are two basic types of X-ray machine: the Image-Intensifier or ‘Airport-style’ system, and the Line Scan System, which is much more appropriate for food production lines. Instead of producing an image, these latter systems have an array of individual sensor elements which pick up a difference in X-ray density. These systems are capable of working at much higher lines speeds. Limitations of X-ray machines include product packs such as glass or metal which are themselves X-ray dense, and the size, shape and orientation of the foreign body. Particular advantages include the ability to detect non-metallic contaminants, high sensitivity when used in conjunction with foil containers, and the fact that they are unaffected by moisture content. They also have the ability to detect other quality problems such as underfill and missing product. X-ray machines have come down in cost over the years, although they are still relatively expensive, so are used by some companies on a consultancy basis to check batches of product when contamination is suspected.

Optical Inspection systems are useful for checking particulate raw materials such as rice or peas for the presence of foreign bodies where there is a difference in external appearance between the product and the foreign body. They offer a high rate of throughput and accuracy, but require a very clean operating system, so are limited to specific applications.

A new system developed in Sweden uses microwave absorption to detect a wide range of foreign bodies from glass through plastics to insects. A limitation of this system is that it is only applicable to liquid foodstuffs in a pipe, but the ability to detect foreign matter such as thin plastic and insects is an enormous advantage over other systems.

There are, of course, many ‘low-tech’ means of removing foreign bodies from food pro­duction lines, including liquid separation systems such as washers, sprayers, settlers and flumes. These are often one of the first stages in cleaning agricultural crops, but of course can only be used with materials which will not be damaged by water. They are typically used to remove light contaminants, but bring with them the problems of handling and disposing of large volumes of water. Dry separation systems such as aspirators are often used in conjunction with abraders or grinders to remove surface contaminants and are relatively cheap and convenient. Systems such as sieves and screens may be used either wet or dry, depending on the particular application, and are often used in conjunction with other methods. Magnets are found on many production lines for the removal of ferrous metal contaminants. Conventional ceramic permanent magnets are used to remove relatively large contaminants such as nuts and bolts, whilst the more powerful rare earth magnets are much more effective at removing small items such as rust particles which may be only weakly attracted to a magnet. Magnets can be arranged in a wide variety of configurations depending on whether the food material is wet or dry, in a pie or loose on a conveyor belt.

Manual inspection of the production line should not be forgotten. It is important to provide good conditions for the inspectors, so that they are not distracted by noise, poor lighting or discomfort, and to recognise that human inspection is only effective for short periods at a time. Advantages include the ability to detect the unexpected, including the detection of a range of other quality problems as well as foreign bodies.

The range of potential foreign bodies is almost limitless, and it has only been possible here to consider a few of the most common types. However, the key to controlling foreign body contamination remains the same: accurate identification of the foreign body problem and its source, leading to the application of suitable controls. This approach will control the foreign body risk in the vast majority of cases, the main exception being those which are difficult to detect such as most plastics and insects, and those introduced accidentally (or sometimes deliberately) by the consumer.