Introduction
Since the international market has become more demanding in terms of quality, safety and delivery, more sensitive and sophisticated techniques of inspection are introduced. Metal detection technique is one of them. Metal fragments often get into the product through moving machines and other sources including raw material. Therefore it is necessary to segregate any metal fragments at all stages of food chain.
The role of metal detection in the food industries has changed dramatically over recent years. The Food Safety Act and the increasing demands imposed by supermarket retailers has led to food manufacturers installing factory-widesystems that comply with the Hazard Analysis and Critical Control Points (HACCP). These regulations introduced a systematic and preventative approach to food production that would identify physical, chemical and biological hazards during the production process rather than in the finished product itself. The aim of the new standards was to identify potential food safety hazards and eliminate them before any risk could be realised.
To comply with these new standards, metal detection has changed and new machinery has been developed. Food products are now not simply checked for metal contamination at the end of the production process, but monitored throughout. There are three main groups of metallic contaminants: Ferrous; Non-Ferrous; and Stainless Steel.
Detection depends on the magnetic or conductive properties of the contaminant.
To ensure that such products comply with the HACCP standards, and reach the consumer free from metal and other contaminants, the products need to be checked at all stages of the production process where metal contamination could conceivably occur.
Installation and Positioning of Metal Detector
The metal detector should therefore form a seamless part of the free-flowing production process. Most of the products used in these instances are gravity fed or ‘free-fall’: this allows the product to be piped directly through the aperture of the metal detector search head.
It is very important to select a suitable metal detector for a particular type of food industry. It depends on size of packing. The size of packing also determines the type of metal detector to be purchased. The metal detection system should be placed in line with the main production flow, after or at the end of the finished packing point. Conveyor-based detectors include the following, for the most efficient performance:
An efficient automatic rejection system
A lockable box to receive the rejected product
A full enclosure between the search head and the rejection bin
A device to confirm that the contaminated products have been successfully rejected into the bin
An automatic belt stop failsafe system
For optimum sensitivity, the search head must be of the size appropriate for the specified food product. It's important that the best attainable sensitivities are established and set for each product, relating to product size, type and packaging. This process should only be carried out in consultation with the manufacturer of metal detector. Sensitivity adjustment controls must not be accessible to untrained employees. Access should only be given to nominated, fully trained staff and for additional security, the controls should be password-protected or kept locked.
Operation and Maintenance
There are many types and makes are available in the market. One of the basmatirice exporters installed a metal detector for final packing. Every packet of 25 kg will have to pass through the metal detector. After some time they received a complaint that there were some metal fragment were found. The company was puzzled why it could happen when there a metal detector. It was discovered that the machine was not properly maintained with the frequency specified by the manufacturer and employee operating it were not well trained. Therefore it is not only selection of machine is important but also it proper upkeep by the trained personnel. For maximum efficiency and safety, all relevant staff should be properly trained in the principles and use of metal detection equipment and the use of testing routines.
As with any piece of vital precision machinery, high performance can only be assured if metal detector is regularly and properly maintained. So, it's worth instigating a planned programme of preventative maintenance to take place at regular intervals, in accordance with the manufacturer's recommendations.
Maintenance should be carried out by the equipment's manufacturer; it can also be done by company engineers, provided that they have been trained by the manufacturers. After any repairs, maintenance or adjustments, it is necessary to ensure that a full metal detector test is carried out before using the system again.
Limitation in Metal Detection
The most widely used modern metal detectors in the food industry are still unable to detect every particle of metal passing through them. The physical laws applied in the technology limit the absolute capability of the instrument. Consequently, as with any measuring instrument, metal detectors have restrictions on accuracy. These restrictions vary depending on the application, but the main criterion is the size of the detectable metal particle. Despite this, though, metal detectors perform a valuable and essential role in process quality control.
Equipment Testing Procedures
Metal detection testing procedures must be clearly documented, and communicated to all relevant staff. Testing should take place at the start of each shift, between each change of product, and in any case, at least hourly. Intervals between tests need to be short enough that, if a fault is found, products potentially affected have not left the premises and can be identified, recalled and retested.
When testing conventional metal detection systems, it is necessary to use both ferrous and nonferrous test packs. These should be made up from packs that are proven to be free from metal, and be clearly marked and labelled so they cannot be packed inadvertently for despatch. It is also necessary to make up fresh test packs at a frequency that reflects the nature, durability and shelf life of the product concerned.
Dealing with Rejected Products
Needless to say, no rejected product must ever be returned to the production line. However, this does not include products rejected during normal test procedures. If these are in a sound condition, you should replace them in the product flow for them to be redetected.
Rejected packs must be investigated by a suitable, trained person, within one hour of rejection.
Frozen products must still be frozen, or refrozen. The investigation should be carried out using the metal detector system, which initially rejected the products, but not while it is being used in production. If you cannot stop the production line, use an off-line detector with at lease the same, if not higher sensitivity.
Pass the rejected products through the detector positioned in the same way as they were when they originally went through the search head. Then pass the same products through the search head twice more, each time positioned in different ways.
If at any stage the products are rejected again, it's essential to find the contaminant and identify it. Then, take any necessary action to ensure similar contamination does not recur.Having more than one metal-contaminated product rejected on a single production line within a shift, is a matter of great concern. Every effort must be made to ensure the identification and elimination of the cause. And if you are manufacturing private label goods, you should inform your retailer customers, in writing, about the incident.
Documents and Record-Keeping
It's important to keep all relevant documentation and records covering a number of areas. These include commissioning and sensitivity tests and records for new equipment, and also those following the movement/relocation of equipment:
Results of routine tests showing time, result, sensitivity, product, and any action taken
Number of rejected packs each shift
Number and details of detected contaminants
Action taken to trace source of contaminants
Planned preventative maintenance programme and service work
Personnel training
Operating Procedure
All magnets are cleaned at the start of work every day. This includes: Cleaning of surface; Magnet setup; and Soundness of the setup.
Once in a month the setup is checked with iron specimen to see effectiveness of operation of magnetic separation. In accordance with the HACCP Manual (HACCP-01), all CCPs are validated by the Validation Team. This takes into account ongoing verification and maintenance records.
Metal Separation Standard
Sensitivity - Ferrous: 1.5 mm diameter and greater
Non-ferrous: 2.5 mm diameter and greater
Non-magnetic Stainless Steel: 2.5 mm diameter and greater
The system must also be challenged with non-spherical samples or sample shapes historically found in a particular product type. The wider and higher the aperture, the lower the sensitivity of the device, therefore to achieve the greatest sensitivity the aperture should be as narrow as the nature of the product will allow.
Placement - Consider whether
Product packaging contains metal. Where metal is included in product packaging, the metal separation system shall be placed directly prior to product packaging.
For all other packaging, the metal separation system may be positioned as desired by engineering. Ideally, equipment should be positioned as close to the end of the process as possible, or even once product is packed/wrapped and no more risk is present.
The metal separation system shall be integrated into the process line control system, so that the line stops/alarms when the metal separation system fails or is subject to intermittent problems.
Mode of Operation
Because foods may contain varying amounts of water, a metal separation system must be purchased with the correct sensitivity and capable of being calibrated to suit each specific product. Separation of suspect product from the process, once the metal detector has identified a packet/pouch containing metal, should occur automatically, and not on operator removal, since this could lead to human error.
In addition, product passing through a metal detector may create small changes in field strength that are measurable. The metal detector must be sensitive enough to detect these changes in field strength. The metal separation system must have the correct sensitivity and be capable of being calibrated to suit each specific product run through the system. The metal separation system should be easy to clean, so as to minimise the potential for product contamination.
Conclusion
Metal detection technique is one of the important aspect of modern day food inspection system. It must be integrated with overall testing and inspection in the food chain.
References
The Food Safety and Standards Act 2006, Food Authority of India
(The writer is MD, Quality Care Services Pvt. Ltd, New Delhi, and food safety expert of UNIDO, Vienna. He can be contacted at qualitycare1995@gmail.com)
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