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Addressing foreign body contamination in dairy products

01 July 2019

Dairy products are at high risk of foreign body contamination so producers should have appropriate in-line insurance against a wide variety of foreign bodies. A recent whitepaper from Ishida looked at this subject in detail. 

Product contamination is a constant threat for the dairy sector. It can occur at many points – via processing machinery, factories, transportation, or through the addition of non-dairy ingredients. 

Retailers ‘Codes of Practice’ have always been very prescriptive in specifying how the desired foreign body detection should be achieved. However, this is now changing, with retailers increasingly indicating an acceptable quality level and leaving it to producers to implement red processes and equipment to meet these standards. There are two main technologies to consider – Metal detection and X-ray technology.

Metal detection
Metal detecting machines offer an established and reliable solution. Systems are easy to maintain, require a low initial investment. However, if the dairy product is contaminated with stones, glass, plastics, or rubber, the metal detector will let it through.

Even if the foreign body is metallic, the metal detector may still miss it. If, for example, the contaminating item is smaller than the sensitivity set for, or achievable by, the detector – which can be a particular issue for dairy products with high moisture and salt content.

Detectors may also have problems finding metals according to their ferrous or non-ferrous nature: it is easier for them to pick up a ferrous metal than a non-magnetic metal such as stainless steel. So, while a detector may be able to find a ferrous metal particle of 1.5mm, a foreign body made of stainless steel may have to be 3mm or greater to be detected by the same machine. Metal detectors may also have problems with items packaged in metallised packs, such as foil for yogurt pots or packages insulated with metallic materials.

X-ray technology
X-ray systems measure the density of a product as it crosses the inspection area, creating an image that is then analysed to identify anything that might be a foreign object. Like metal detection, X-ray technology can be applied during production, to bulk flow materials prior to packaging, and to packaged retail goods before shipping.

The technology provides a wide detection spectrum and can pick up contamination by foreign objects made of stone, glass, dense plastic/rubber and much else — including metal. In general, X-ray technology can detect foreign body contaminants that have a density greater than water.

X-ray technology is also versatile and can be used on the production line for counting, weight estimation, detecting fill levels, identifying product flaws and ensuring the integrity of packaging as well as for detecting foreign bodies.

So, while the capital investment cost of an X-ray machine may be greater than a metal detector, the added capability can often offset that extra expense.

False positives
X-ray systems can detect extremely small particles lies in tolerance levels both now and in the future. For example, if a machine can detect items of 1.2mm and the retailer specifies detection of 1.5mm, there is little difference between the sizes and there will be a substantial number of ‘false positive’ detections. As a result, perhaps one in 5,000 packs could be incorrectly rejected, which, over a production run or shift will add up to considerable cost.

However, using a system that detects particle sizes as small as 0.6mm if the retailer still requests 1.5mm, then there is a much bigger difference and the producer could reduce that number to one in every 20,000. The producer would still be confident of hitting the retailer’s targets but would reduce the volume of waste and thus financial loss.

Furthermore, if the retailer decides to reduce the tolerance levels in the future or targets smaller particles, the dairy product producer with the better X-ray system will already have the correct technology in place.

Primary processing 
At a primary processing site cheese will be produced and aged in large blocks. Many processes are required to produce cheese and each of these has the risk of adding foreign bodies. At the primary phase, before the cheese is cut for retail, blocks of cheese must be checked for any metal contamination, to protect the integrity of the cutting blades. If there is metal in the block then damage to the blades in the cutting operation may occur and this, in turn, can cause a wider contamination issue and potential recall. There will of course also be losses in terms of machine downtime, lost production and maintenance costs.
Metal detectors are commonly used for this task. However, because of the large block size a larger aperture metal detector is required, which reduces potential sensitivity.
This, coupled with the product effect caused by the salt and moisture levels in cheese, means detection limits are high – typically around 5.0mm or greater for stainless steel. Even a 3.0mm piece of metal can do significant damage if contacted by the cutting blades. Here, X-ray systems can provide detection down to 1.5 - 2.0mm on the largest blocks and down to below 1.0mm in most cases, providing a high level of protection.

Cutting the blocks may take the form of slicing for single serve portions, cutting into typical retail block sizes or grating. For sliced operations, inspection of the final package can provide benefits. From a foreign body perspective the sliced product gives a uniform density and the relatively thin nature of sliced cheese packaging means that a reasonable contrast between the product and potential contaminants is available.

Any non-metal contaminants that may have been present in the block can be detected as well as very small metals with stainless steel detection levels readily to 0.6mm – metal detection will be typically around 1.5mm. Additional benefits that can also be employed at this stage include product in seal detection, should a small piece encroach upon the sealing area of the tray this can be easily detected to prevent possible product spoilage on the shelf.

Other additional inspections – such as weight by mass – can also be used to ensure the correct number of slices are in each pack. These additional inspections can occur simultaneously with the foreign body inspection and rejects can be separated between quality defects and high-risk contamination.

In the block cheese operation inspection is often limited to foreign body control. However, X-ray inspection can also be utilised to confirm the block size and check that the block is whole and is not split or broken.

The greatest inspection challenges comes when cheese is grated as it has a highly variable density with strands overlapping and combining with each other. Here the differential with metals is still large and because of the lower height and overall density the metal, detection levels achievable with X-ray inspection will still be consistently below 1.0mm and still below the 0.6mm level in most cases. However, lower density contaminants will be much more difficult to detect.

Yoghurt production
Yoghurt production takes many forms, from single pot filling to large integrated machines filling and sealing many rows of pots at a time and automatically packing these in to the final case. For this reason inspection methods are equally varied. Many yoghurt products are foil sealed to ensure the integrity of the product is maintained throughout its shelf life so metal detection of the final sealed product is not always an option. In such applications X-ray inspection can reduce need for converging solutions, maximising throughput and saving production line space.

A fully automated line where pots are filled, sealed and packaged within the same machine can use X-ray systems to detect foreign and to confirm the presence and fill level of the pots within the tray as a whole.

A copy of the original whitepaper entitled ‘ Dairy production; Eliminating foreign body contamination and ensuring brand integrity, can be downloaded from

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