X-ray detection can be a very effective method for identifying polymer contaminants within food products and preventing them from reaching the consumer chain. However, in order to ensure reliable and repeatable detection, the polymer itself must be correctly designed and enhanced with specific detectable additives. Even then there are many other factors which need to be considered when designing for effective foreign body detection, including but not limited to, food product itself, containment material and the limitations of the detection system.
All work was carried out on a Minebea Intec Dylight top-down system and findings are specific to the test set-up and food products utilised.
Foreign bodies entering the food processing chain and passing undetected through to consumers is clearly a major concern for the food industry. Examples of foreign bodies include metal, wood, ceramic, glass, rubbers and plastics and, due to the varying nature of these, often different detection techniques are necessary to target each. One of the most difficult categories to detect is rubbers and plastics (grouped together as polymers) due mainly to their comparative low density (similar to many foods) and non-conductive/non-magnetic nature. Polymers are used extensively throughout food production lines and hence being able to detect foreign body fragments of them is an important consideration.
Early revisions of ‘detectable’ polymers relied purely on visual detection to prevent foreign body contamination. Polymer products were (and commonly still are) coloured blue to render them easier to detect on a food line due to the lack of naturally occurring blue in food products. Further enhancements lead to the introduction of modified polymers, detectable by conventional metal detection systems. This remains probably the most common format of detectable polymer.
Moving on from metal detection systems, X-ray detectors are becoming increasingly popular in the food industry. Compared to metal detection systems, they have the advantage that they primarily rely upon density differences to food products and hence are able to detect a range of additional foreign bodies such as glass, ceramic, bone etc. With the growth in popularity of X-ray, it is important to consider the suitability of this technique for detection of polymer foreign bodies and the important points to take into account when doing so.
X-RAY DETECTION OVERVIEW
In basic terms, the X-ray beam passes through the food product and is absorbed at differing rates, determined principally by product/foreign body thickness and density. The transmitted beam is detected at the opposite side by an array of sensors, whereby the signal intensity is converted to a grey value. The more X-rays that are absorbed by a particular material, the darker shade of grey it will appear on the final image.
DETECTION OF POLYMER CONTAMINANTS
X-ray detection principally relies upon a differential in density (and atomic number) between food product and foreign body generating a detectable difference in absorption. A ‘typical’ value for density of food products would be around 1.00g/cm3. Due to the make-up of most polymers and the fact that many utilised on a food production line have a density below 1.30g/cm3, this differential is typically not sufficient to allow reliable and repeatable detection of standard polymers.
The greater the difference in density between foreign body material and food product, the greater the detection sensitivity. This is highlighted in the images below, where test cards containing standard sized balls (ferrous and nylon) were run through the machine on a box of mixed nut muesli.
The left hand image shows ferrous balls being detected by the machine at 2.0mm, 1.5mm, 1.2mm and 1.0mm diameter. The right hand image is the same product with a test card of nylon balls. In this instance, not even the maximum size of 6.0mm diameter was detected and such a foreign body would have passed by undetected on an actual food line.
Clearly the reliable detection of polymer products by the X-ray method requires suitable material modification, just as is the case in adding metal powders to allow detection by metal detectors. In optimising the polymers, there are several points for consideration:
1. Design Of Polymer
The actual formulation of any polymer used to manufacture a component on a food production line is a key factor in its ability to be detected. Many products sold as ‘X-ray visible’ rely solely upon traditional metal detectable content to add density to the polymer and hence enhance X-ray detectability. To an extent this will work, however, in order to optimise X-ray detectability fully, specific high density non-metallic additives are required. A true dual detectable solution would require a combination of these additives.
The example below shows a range of polymer ‘fragments’ (10mm diameter x 2mm thick) within a 50mm thick layer of sugar. The first image is the actual greyscale output created by the X-ray machine:
The second image is a screenshot showing which fragments were actually detected and would, on a line, have caused a rejection.
This example clearly shows that, under these conditions, only the polymers containing X-ray additives were detected and by increasing the focus on X-ray the level of detectability is increased. For optimum performance, it is key to correctly design the polymer system to optimise detectability by X-ray systems and not simply rely upon traditional metal detectable additives.
2. Base Polymer
The density of an optimised detectable polymer is principally influenced by the combination and addition rate of the specific additives. However, there is also an effect of the density of the base polymer utilised. Polypropylene, for example, has a particularly low base density of around 0.93g/cm3, whereas a polymer such as polyamide (nylon) has a higher base density of around 1.14g/cm3. Such a difference will still be evident in the final compound once detectable additives have been incorporated and will result in a higher level of detectability for the polyamide compound.
The example below shows this effect with 2mm thick fragments of polyamide and polypropylene detectable compounds (comparative levels of detectable content) imaged against an egg and potato salad.
The polyamide fragment was detected, whereas the polypropylene fragment remained undetected.
3. Material thickness
The thicker the fragment of polymer, the more X-rays it will absorb and hence the greater the detection sensitivity against the food product. The image below shows 2mm and 1mm thick fragments of the same detectable polymer against a 50mm thick block of cheese. The 2mm thick was detected (uppermost), but the 1mm was not.
4. Food product
As the X-ray machine is looking for a difference in density/absorption between food product and foreign body, the food product itself has a large effect on detectability levels. It is typically easier to detect foreign bodies against consistent product than against product made up of multiple ingredients with variations in density and dispersion. The examples below show results of a 2mm thick detectable polypropylene fragment being passed through the machine with a 50mm thick block of cheese and a mixed nut muesli. Detection was only possible against the consistent cheese product.
5. Location within product
The vertical position of the contaminant in the product has little or no effect on levels of detectability as the X-ray must travel through the contaminant and food product to reach the detectors regardless of where it is located.
Orientation of contaminant can though have a large effect on detectability. Imagine a fragment which is 10mm long, 3mm wide and 2mm thick. Were the fragment laying flat to the plane of the X-rays then they would only be absorbed by the 2mm thickness. Were the fragment laying vertical then the X-rays would be absorbed by the 10mm dimension, generating much more absorption and a much higher level of detectability.
6. Food product containment
X-ray absorption by thin plastic packaging/films is extremely negligible and has no real effect on contaminant detection levels. Containments such as glass jars and metal cans show high density and absorption levels and will dramatically effect sensitivity levels for contaminant detection.
There is clearly a growth in reliance upon X-ray technology in the food industry due, in part, to the flexibility and user friendliness of these systems and also to equipment advances in recent years making them more openly accessible for mass usage. X-ray systems can be used to successfully detect a wide range of foreign body materials including metals, ceramics, glass, bone, stone etc. When it comes to standard polymers, these are often very difficult to detect or even totally undetectable in many food products by conventional X-ray systems. The utilisation of metal detectable polymers will improve this sensitivity, but optimised foreign body detection of polymeric materials can only be achieved by the correct design and use of specific additives which focus on X-ray absorption.
Even with the optimisation of detectable polymers utilised on a production line, it is important to consider the other factors contained in this report and to fully understand the X-ray inspection system of choice and its limitations via strong links with the manufacturer. Detectable materials should always be verified on the specific line of interest, utilising the actual food products and detection system(s).