Fish forms an important part of diets around the world — with global fish consumption last year around 46 pounds per head. In North America, the figure is estimated at 52 pounds per head.
Major improvements in processing — as well as in refrigeration, ice making and transportation — have allowed increasing commercialization and distribution of fish in a greater variety of product forms in the past few decades. But for many people there remains one big drawback — the presence of fish bones. Thin but long in nature, these sharp bones are at best a frustration and at worse can cause serious health problems if they are swallowed and get stuck in the throat.
As the production of fish has become increasingly mechanized, manufacturers of deboning machines are striving to increase the performance and throughput of their systems — but inevitably some bones occasionally make it through. Often the deboner has lifted the bone and removed a section of it, leaving a smaller section behind — making it even more challenging to detect.
For high-value fish such as salmon, or for high-value products such as sushi, increasing the customer's confidence level of receiving a bone-free piece of fish is crucial. Supermarkets are very aware of the aversion many of their customers have to fish bones — and, as a result, are pushing processors to make a product that can be "guaranteed bone free". And that's when X-ray inspection often offers a helping hand.
X-ray inspection works by passing low-energy X-rays through a product. Different elements absorb different amounts of X-ray, so in the resulting images metals, glass and bone appear darker than the surrounding fish. But although X-ray inspection is widespread in the food industry more generally, the challenge of detecting fish bones remains. This is primarily because fish bones are low density, with low mineral contents, and are very thin. This all makes them difficult to detect.
Standard end-of-line X-ray inspection equipment uses a 0.4 mm or 0.8 mm resolution sensor in order to detect contaminants. However, this is insufficient to detect pin bones and other sub-milllimeter (mm) diameter bones. To detect such tiny bones requires a 0.05 mm (50 micron) resolution area sensor more typically seen in the medical or electronics industries. This unparalleled level of resolution, combined with low-energy X-ray generators, gives rise to images which can finally reliably show even the smallest of bones in fish.
Having created a high-quality image of the fish — and potentially the bone — it must now be processed via automatic inspection software requiring no human interaction. At its most simple level, this involves looking for features which follow a straight line.
However, the separation of muscle fibers in fish means that there are long straight features present naturally. The software must therefore be able to differentiate between the dark features which are caused by bone and the lighter features caused by the natural texture. This process is made more difficult if the fish has been frozen and then defrosted — or has been excessively processed — as this can cause separation of the muscle, giving rise to false rejects.
Once any bones have been identified, manufacturers have a choice — the production line can be programmed to reject any products that are shown to contain bones or additional control procedures can be put in place. This may simply be to inform the operator that the deboning machine has fallen out of range and needs attention. Alternatively, the technology can identify which region of the fish still has bones in — and determine whether this is a high-risk area where no bones can be tolerated or a lower-risk region where the bones are likely to disappear during cooking. In addition to being able to select a maximum bone size — most food standard agencies and retailers allow very small bones which pose little threat to health — it's also possible to count the number of bones, thereby allowing through a certain number of bones. Additionally, this information can be used to grade fish — sending truly bone-free fish along lane one, while products containing a low number of bones go to lane two, leaving fish with a large number of bones to go to lane three and be reworked.
If fish is reworked, the X-ray image can be provided to a rework station — providing operators with an image showing the location of the bone, and so speeding up the process. A return conveyor can then be used to reinspect the fish, ensuring all the bones have been removed correctly.
Metal contaminant detection has long been the primary reason for installing X-ray equipment. But the ability to detect ever-smaller bones is now playing a key role in the constant battle to improve product quality. X-ray inspection is giving suppliers a new perspective when it comes to satisfying public demand for bone-free fish.
David Bosworth is a senior program manager at Cambridgeshire X-ray equipment specialist Cheyney Design. As well as working with customers to determine how best to solve their needs, he heads up the company's North American operations. David has a PhD in material science from the University of Cambridge.
