Within the total spectrum of electromagnetic waves X-rays comprise the area of wave lengths between about 10 and 0.001 nanometres, i.e. shorter than ultra violet light. As in the case of light, effects such as absorption, dispersion, diffraction, polarisation and interference can be observed with x-rays. What distinguishes x-rays from visible light, however, is their ability to penetrate animate and inanimate media. During this process the shortwave rays are differently absorbed and weakened by substances of different density, thereby creating an image of the inner structure of the x-rayed medium. This principle has been used in medicine for over 100 years to show bone systems, the heart, lungs and other organs. The invisible rays are also often used during fish research to show the fine bone structure of individual species. With the help of a contrast medium it is even possible to examine functional correlations within digestive processes, for example, or details of blood circulation in live fishes.
At the Skretting Aquaculture Research Centre (ARC) x-rays are used to enable more exact analysis of the structure of feed pellets, particularly the size and distribution of the inner pores that develop during extrusion. The production of feed pellets for fishes is a very complex process and demands more discerning expertise and attention to detail than the production of a lot of feeds for terrestrial farm animals. The structure of the fish pellets and their inner pore pattern is in a sense the matrix upon which the feed’s nutritional value and its floating/ sinking behaviour in the water will depend. The pores absorb a large part of the fat with which the pellets are enriched after extrusion. The desired density and porosity of the pellets is difficult to adjust, however, because they shrink and change their structure during cooling after extrusion. With the help of x-ray technology feed researchers want to find out which influencing factors and components in the feed play a significant role during these processes. Special software today enables them to translate the x-ray data into three-dimensional pellet models and so identify fat distribution via different colour codes in the pellet matrix. Such tests are becoming increasingly important because traditional feed components such as fishmeal and fish oil sometimes have to be replaced by alternative raw materials from one batch to another. Apart from that, today’s fish feed has to be more robust and more resilient to mechanical strain than in the past because it is stored in increasingly large silos and transported to the net cages through pipes before being distributed using compressed air which can easily damage less resistant pellets and cause losses through friction.
Trimming, cutting, removing pinbones
During machine trimming of beef and pork x-ray technology is used to enable accurate adjustment of the share of fat that has to be cut away to achieve the exact target settings. The machine’s x-ray eye recognises how the fat is distributed and cuts away exactly that amount which will meet the targets. Similar systems are already in use for fish processing but in this case they do not offer support for trimming the fatty edges but for removing the bones. Marel’s automatic SensorX Fish Bone Detection System scans the fish fillets using low-energy x-rays as they pass through the machine and spots any bones and other foreign bodies that they may contain. The captured data are then transmitted from the sensor to the computer. Trimming is still carried out by hand but with the support of monitors which are installed at all work stations. The high-resolution display terminals show the appropriate x-ray image for every fillet that arrives for trimming and the area that requires attention or removal is clearly marked. Marel’s Bone Detection System shortens the time-consuming search for leftover bones, reduces the error rate and improves product quality.
The brand-new x-ray guided cutting machine that was developed by the Icelandic manufacturer Valka together with HB Grandi goes one step further. The automatic robotic system combines x-ray technology with an image processing system. X-rays transilluminate the fillet and identify the position of the pinbones and the image processing system gives a 3D picture of the outer shape of the fillet. Analysis of the X-ray and 3D images of the fish allows data on the precise position of the pinbones to be fed to the robotic system’s fine high-pressure water jets. These trim away the areas containing pinbones and cut the fillets into predetermined portion sizes with exceptionally high accuracy and without any major yield losses, or divide the fillet into portions. Trials carried out so far on a prototype of the system under careful observation by Syni Laboratory Service had gone well, announced Valka. The trials had focused on cutting accuracy, i.e. minimum fillet losses and, in particular, on precision during removal of the pinbones. Tests carried out on 500 redfish fillets resulted in 94% of the fillets being absolutely boneless, equal to about 0.6 bones per kilogram of fillet. On top of that, yield losses were only half as high as what is to be expected during manual trimming.
Sorting, checking fill level, identifying foreign bodies
Perhaps with the help of x-rays it will soon be possible to carry out routine tasks more quickly, more exactly and more gently than previous technology has allowed. Sorting sensitive fish species like herring with mechanical devices often causes damages and quality losses in the fishes, for example. The Norwegian research institute SINTEF has investigated whether normal x-ray images might be suitable for estimating the weight of round herring. To achieve this herring were first x-rayed and then the areas of lighter and darker colouring in the images of the fishes measured with an image recognition program. The results correlated extremely well with the actual weight of the fishes which suggests that x-rays would be very useful for estimating fish weight and thus for fish sorting processes.
X-ray images thus offer a great number of possibilities. They can be used for quality inspection and mass assessment, or for controlling fill levels in canned products. Or they can count the number of components in non-transparent bags or cans as well as identify products that are missing or damaged. X-ray inspection systems offer food producers greater certainty during production and make a decisive contribution towards preventing complaints and product recalls.
Just how efficient x-ray technology is can be seen for example during recognition of non-metallic foreign bodies such as glass, stone or plastic in various packagings. Because modern x-ray systems do not measure conductivity but absorption differences they can also recognise iron and non-iron metals in foil packagings even if the packaging material – as is often the case with Tetra Paks, for example – is metal laminated. X-ray inspection systems are reliable and very precise, also because recognition of foreign bodies is hardly influenced by external conditions such as moisture or product temperature.
Mettler Toledo, a well-known manufacturer of precision instruments for research purposes, industry and trade, has some x-ray inspection systems in its range that fulfil all food industry requirements. These include models with horizontal and combined x-rays which can be adapted to numerous applications and achieve high recognition accuracy even in high-speed operation with maximum product throughput. X-ray inspection systems with fixed, invariable x-ray beam geometry can control practically any packaging and any container that is covered by the radiation angle. There are also models such as the Inspire X R50S in which the focal length is adjustable. This makes it possible to inspect containers of different sizes on the same production line.
In the meantime there are hardly any upper limits to the size of x-ray inspection systems. Mobile x-ray systems can even be used to scan trucks in a car park. In the south of Germany the customs authorities just a few months ago invested in a mobile x-ray system which makes time-consuming unloading superfluous when searching for smuggled goods. The system which cost 1.5 m EUR is ready for operation in 20 minutes and will mainly be used in motorway car parks. The x-ray truck shows an x-ray image of the vehicle that is being checked as it drives slowly past, similar to luggage controls at airports.
Sterilisation and preservation with ionising radiation
One potential application field for x-rays is in the irradiation of foods. In theory this could be of great significance but up to now it has only played a subordinate role. It is estimated that every year about 500,000 t of irradiated products are placed onto the market; about half of these are spices. Not only x-rays are used for food irradiation but also electron and gamma irradiation that is usually obtained by breaking down the isotope cobalt 60 or caesium 137. It is possible to treat whole pallets using x-ray and gamma irradiation because their penetration is deeper. The penetration depth of electron irradiation is lower and its use thus limited to smaller single packs.
The aim of ionising radiation is to kill microorganisms such as bacteria and fungi, but also insects and other undesired organisms, so that in a broader sense it constitutes a method of preservation. To achieve the desired sterilisation effect the radiation dose has to be set at a level above the organisms’ resistance level. 0.1 to 1.0 kGy is needed to combat insects and parasites effectively. The dose of radiation is measured in the SI unit known as the gray (Gy), named after the British radiologist Gray. One Gray of radiation is equal to 1 joule of energy absorbed per kilogram of food material. During x-ray radiation one Gray is equal to the more widely known Sievert, both being exactly equal to 1 Joule per kg. In order to kill pathogenic microorganisms 0.5 to 10 kGy, i.e. slightly higher energy doses, are necessary.
The germ-killing effect of ionising radiation is partly based on the destruction of the genetic makeup that is indispensable for the control of living processes and reproduction in organisms. DNA is usually the largest molecule in living cells and it reacts particularly sensitively to energy-rich radiation. Irradiation also splits water molecules in the cells, and this leads to the development of free radicals which attack enzymes, proteins and fats in the membranes and reduce their ability to function. All other properties of the food – its composition, its nutritional value and its identity (raw materials remain raw during irradiation and are not “cooked”) – remain unchanged. This is a considerable advantage that irradiation has over conventional preservation methods such as heating, drying or salting in which proteins, fats and carbohydrates are modified. High irradiation levels can destroy some of the sensitive vitamins, however, particularly A, C and E. Here, irradiation does not differ from thermal preservation.
When assessing the safety of irradiated foods it is important to note that up to now probably no other method of treating foods has been so thoroughly examined. In over 50 years of research which also included experiments on animals and voluntary persons, no negative health effects were recorded. Numerous internationally recognised institutions such as the World Health Organization (WHO), the US Food & Drug Administration (FDA), the Center for Disease Control (CDC), the International Atomic Energy Agency (IAEA), and the European Commission’s Scientific Committee on Food (SCF) have agreed that foods can be irradiated up to a total dose of 10 kGy without having to fear health risks to consumers. Sometimes irradiation can even constitute a health benefit, as scientific experiments have confirmed. For example, x-rays kill such dangerous bacteria as Listeria monocytogenes, as microbiologists at Mississippi State University were able to show in smoked mullets in vacuum packaging. An irradiation dose of 2.0 kGy inactivates all Listeria so that even days later no growth can be identified. And the flavour, consistency and appearance of the smoked products are not affected by irradiation, as sensory tests have shown.
Using oysters, the food technologist Barakat Mahmoud was able to show that x-rays can even destroy harmful bacteria in living organisms. Whilst the oysters themselves suffered no harm from irradiation and lost none of their freshness or quality (apart from a slight reduction in vitamin A and C content), pathogens such as Salmonella, E. coli, Vibrio, Shigella and Listeria were completely destroyed. Since oysters can constitute a relatively high risk of bacterial illness Mahmoud thus recommends irradiation as a good and effective method to make them safer for consumers and increase their product shelf-life. The irradiation method was simple, effective and could be carried out relatively inexpensively on an industrial scale.
Irradiation of foods seen critically in Europe
Whilst sterilisation via irradiation is relatively far advanced in technical and medical fields and can be seen as an established technique, irradiation of foods is still in its infancy in the EU. Although the Scientific Committee on Food (SCF) basically gave the go-ahead for irradiation of fruit, vegetables, cereals, poultry, fresh meat, fish and shellfish in 1986, 1992 and 1998 EU member states have only partially implemented the necessary national authorization. Whilst in France, for example, a relatively large amount of foods already undergo irradiation, other countries are a long way behind. The Codex Alimentarius recognises ionising irradiation as a method of treatment for foods and has defined standards and a Code of Practice. No WTO member state should refuse to import irradiated foods if they are properly declared (the labelling with the Redura symbol which indicates irradiation treatment is not sufficient since it is not authorized in the EU).
|Irridation of seafood products in EU member states (Food Irradiation Directive)|
|Product||Country and maximum dose (kGy)|
|Fish and shellfish (incl. crustaceans and molluscs)||3|
|Frozen peeled shrimps||5||5|
|Frozen frog legs||5||5||5|
The lack of conformity within the EU stems from problems arising during implementation of Directive 1999/2/EG, according to which the Commission was originally to present a common list naming all the foods that were authorized for irradiation (positive list) by the end of 2000. Up to this day, however, it has not been possible to achieve an agreement on this positive list. The Commission has suggested as a compromise that initially at least those foods in which there are known to be hygiene problems and that are irradiated in large quantities in at least one member state could be listed. This would include flog legs and shrimps. However, this proposal too met with criticism and has not been implemented. That is why the European positive list so far only contains “Dried aromatic herbs, spices and vegetable seasonings” that have been irradiated with a total dose of at most ten kGy. Only these products can enjoy unlimited marketing in Europe.