Top Editors ... Foreign body, detection & renoval

Title: Foreign body, detection & renoval
Date: 05/07/2006
Autor: By Dr. Mike Edwards

Aforeign body can be defined as anything that a consumer perceives as being alien to his or her food, and it is the perception of the consumer that is all important since not all foreign bodies are in fact alien, yet all have the potential to give rise to a consumer complaint. Hence, foreign bodies can range from items that are demonstrably alien to the
food, such as pieces of glass, metal or plastic; through items related to the food, such as fragments of bone in meat products; to part of the food itself, such as crystals of sugar or salt that are mistaken for glass.

Sources of foreign bodies

Foreign bodies can get into food at any stage from initial harvesting to final processing or even preparation by the consumer. Harvesting of field crops may result in pieces of soil or grit being inadvertently collected with the crop, for example, and in the factory itself, many foreign bodies can be traced back to pieces of food processing machinery. Incorrect storage of raw materials and final products can result in infestation by storage pests, resulting in the presence of insects or rodent droppings in the food. Food packaging is a further potential source of foreign bodies, as
is treatment by the consumer, for example, metal slivers getting into a product from a can opener or glass shards from cooking dishes. Fragments of tooth or dental filling from consumers themselves may be reported as foreign bodies, and unfortunately the possibility of malicious contamination also cannot be discounted.

Detection and removal systems for foreign bodies

Approaches to the technical methods of combating foreign bodies on the food production line fall into two main categories: Separation Systems and Detection and Removal Systems. 

 
Separation systems such as sieving and flotation aim to separate foreign bodies from the food as a result of basic physical differences. In many cases these methods are integral to the production system itself.
Detection and removal systems, in contrast, are systems designed specifically to detect the presence of a foreign body in the food and remove it.

Metal detection systems

There are two main types of metal detectors. The most common is the Balanced Coil type, in which the food product passes through an aperture surrounded by two receiver coils and a transmitter coil. The transmitter coil generates a field similar to that generated by a radio transmitter. The field is disturbed by a metal particle travelling though it, and this change is detected by the two receiver coils.
The second type is the Magnetic Field type, where the food product passes through a tunnel subjected to a strong magnetic field, which magnetises any magnetic metal particle passing through it. When the magnetised particle passes under coils incorporated in the tunnel, a current is generated in the coils and detected. This system will not detect non-ferrous metals or most stainless steels. Consequently, the major application of this type of detector is in food products packed in aluminium foil. 

Metal detectors have a number of limitations. They cannot detect metal particles below a certain size. Many food products are electrically conductive or have magnetic properties, reducing the sensitivity of the detector to metal fragments. Some metals are more readily detected than others, and the ease of detection of a metal fragment other
than a perfect sphere will depend upon its orientation as it passes through the detector.

X-ray detection systems

Initially very costly, over recent years the price of X-ray systems has fallen as a result of advances in technology
and a highly competitive environment.
All X-ray detection systems have the same basic components. They rely on the X-ray beam being either partially absorbed by the material through which it travels, in proportion to the density of the material, or on inducing fluorescence in the material through which it passes. A linear sensor is located under the X-ray beam, restricted to a
narrow fan shape perpendicular to the direction of flow of the product. Each of the discrete units of the sensor array converts the energy of the X-ray beam falling on it into an electrical signal. The processor examines each scan line and outputs a signal when an abnormal value is detected. 


X-ray systems can be tailored to detect a range of foreign body types that cannot be readily detected by any other method. Thus ferrous and non-ferrous metals, stone, glass, and bone are usually readily detectable, but dust, debris, insects, low-density rubber and plastic, plastic films, wood, paper, oil, fabrics and extraneous vegetable matter are not usually detectable.

Optical systems

Optical inspection systems range from simple colour sorters to complex image recognition systems able to sort by shape. The basic principle is to illuminate the item and examine the reflected light in relation to a pre-set standard.
The product is fed towards the illuminator by a feed system in an appropriate fashion, for example arranging peas in a single line. The illuminator may include one, two or three different coloured light sources. Monochromatic units are only capable of sorting dark and light coloured objects, whilst the introduction of two or more colours allows more sophisticated functions.
The detection system may vary from a simple diode able to detect light or dark to an array of charge-coupled devices capable of creating an image. Although no optical system is perfect, they can offer efficiencies from 85 to over 99% in some cases, and have a consistent level of effectiveness over a long period.

Rejection systems

All of the automatic systems outlined above require a rejection system to remove the product containing the contaminant from the production line. A range of methods is available, each having different applications and
limitations. Having removed the package from the line, it is essential that it is held in a reject bin until it can
be examined, investigated and disposed of by an authorised person.

Manual systems

Humans are far more versatile and adaptable than machines, using up to five different senses and having the dexterity to manipulate the sample, coupled with the ability to judge. For short spells, therefore, humans can carry out inspection work to detect foreign bodies with great skill, care and sensitivity, readily adapting to new or previously
unthought-of foreign body types. However, human inspectors cannot continue to work uninterrupted for long periods of time without a decrease in their efficiency.

Equipment for separation of foreign bodies

Air separation systems
Air separation systems are dry cleaning methods that are rarely used in isolation, normally being combined with other removal systems. There are two main types: Aspirators usually have specific applications for the separation of materials of different weights or densities such as the separation of chaff from wheat or shell fragments from nuts. A strong current of air passing through the product carries lighter material off, separating it from the heavier
material.
Abraders and graders are useful for removing surface contaminants of food material such as soil or husk.

Liquid separation systems
Liquid separation systems involve a wide range of wet cleaning methods, which are usually used in conjunction with other separation techniques. The washing of food is frequently one of the first stages of processing, particularly for agricultural crops. Common methods include: Washers and sprayers, in which the product is carried in or through clean water to remove light or surface contamination.
Settlers and flumes are used to remove either light or heavy contamination. The density difference between food and water allows food to float off, whilst heavier contaminants sink to the bottom.

 

Centrifuges work by separating food material with different phases or densities by centrifugal force. The operating conditions must be chosen very carefully to optimise separation of the phases from each other.
All liquid separation systems are limited by the damage that immersion in water can do to many food products.

Sieves and filters
Sieves and filters remove foreign matter on the basis of size, and are equally applicable to both wet and dry systems. They range from simple mobile hand-operated systems to integrated in-line installations.

Magnetic grids and permanentmagnets
Magnetic separators are normally powered by permanent magnets. Low intensity magnetic fields from ceramic magnets are used to attract larger ferrous particles such as nuts and bolts, whilst higher intensity magnetic fields from rare earth magnets are required for attracting small or weakly magnetic particles, including rust scale and some stainless steels. There are four basic types of magnetic separator:
Plate magnets are usually enclosed in a box, with one surface acting as a magnetised plate.

Rod magnets are cylindrical permanent magnets placed in the product flow, either singly or in multiples, or built into grids through which the food material flows.
Pipeline traps are used for liquids transported in pipelines, and are arranged as groups of magnets through which the food flows.
Magnetic Drum Separators consist of a non-magnetic rotating drum containing a magnetic unit extending approximately 180o around the periphery. Typically this may form the end of a conveyor belt.
Some foreign bodies remain particularly difficult to remove, especially insects and plastics. While the available technology may not eliminate all foreign bodies from food, the correct application of technology will assist in removing many of them. A vital part of this is incident investigation involving the accurate identification of the foreign body.
Once the source of the foreign body is known, control measures can be implemented which will assist in preventing a
recurrence of the incident.