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Sorting the wheat from the chaff

06 February 2015

As a company with more than 10 years experience in the development of tailor-made sorting solutions for the food industry, Nicholas Stein, Director of Stein Solutions explains why sorting is a vital part of the food processing line.

Since most produce is harvested by machines, the likelihood of having defects and foreign matter in the product stream is high. Optical sorting will remove these products of foreign bodies before they can reach the consumer. It is vital to protect the consumer against harm, which could occur by biting on or swallowing a foreign object such as a stone or a piece of glass and it is equally important to protect the customers’ brand reputation by ensuring that blemished, discoloured or misshapen product is removed or reduced to an absolute minimum.

Where in the food chain is sorting used?
Optical sorting can be implemented at various stages in the food chain and different areas in a production process. Growers or farmers, food processors, re-packers, food manufacturers, etc. will all have benefits from using optical sorting to provide their customers with the highest product quality. The requirements differ depending on the customers’ activity or sector; hence dedicated machines have been developed for a variety of applications.

In a production line, the same type of sorter can be used at different stages of the process, for example on raw, cooked or frozen product. Here, the decision is made depending on the value that is being added to the product at various stages and ensuring that the final inspection is carried out as close as possible to the packing process. For example, it is better to remove blemished product before adding the energy and cost of blanching or freezing, but at the same time it is good practice to do a final FM inspection just before the product is packaged to ensure that any potential impurities, which may even have been introduced after the first sorter, are detected and rejected reliably. 

How can efficiency be improved by sorting?
Installing an optical sorter will certainly result in an increase of the overall line efficiency and product output quality. On higher volume lines - anything more than a few 100kg/hr - the manual inspection stage can become a bottleneck for the process. The number of people hand sorting, along with the space which needs to be made available for them to perform their job, can become an issue. In addition, manual sorting will always allow some level of subjective interpretation of the requirements, whereas an optical sorter will always provide a reliable and constant sorting result, independent from the time or day of the week and person on the line.

What technologies are involved in sorting?
As with hand pickers on a line, an electronic sorter needs ‘eyes’ to look at the product, sufficient light to see what needs to be inspected and ‘hands’ to remove the impurities or defects. Basic sorters rely on traditional sensors or cameras to inspect the product and will only detect colour variations. They look at the colour of an object, for i.e. they will detect a brown blemish on a pea, but would struggle to correctly identify a pea pod. More advanced cameras are able to see outside of the visible light spectrum and will detect differences under infrared light. Lasers can also be used as light sources and to see differences in the surface structure of an object, making it an ideal foreign material detection technology. Finally, X-ray inspection can be used to find dense foreign bodies such as metals, stones or glass.

When a defect is correctly identified, it will trigger a signal to the ejection system, which will then move the object out of the product stream into a reject conveyor or bin. This is done using a mechanical ‘finger’ or ‘paddle’ rejector for heavier objects or a more precise, perfectly timed burst of air from a pneumatic valve. The reject mechanisms used are very fast and accurate, which is important to avoid good product ending up in the waste stream.

By what criteria can foods be sorted? 
Defects and impurities can be detected through a variety of criteria. Discolorations are currently still one of the most important product related defects as they indicate blemish, rot, immaturity or other. This is why colour sorting is widely used, however nowadays more in combination with other more advanced detection technologies. Laser sorters for example can find impurities by assessing the surface structure of an object, independent of its colour. This makes it possible to find transparent foreign bodies, such as glass and plastic foils, or foreign material with a very similar colour to the good product, such as shells in hazelnuts or white stones in navy beans. In some cases, naturally occurring phenomena such as fluorescence or bioluminescence can unveil differences which are not visible to the naked eye. Chlorophyll in green produce for example will fluoresce when illuminated with the correct type of light. Density variations can also be detected by X-ray or infrared transmission and the product can also be sorted by shape or form.

More and more, optical sorters can also be used to collect data on product quality such as length, product defects and foreign bodies. This data can then be used by the customers to assess raw material or finished product quality and will enable them to make quick changes to the production process, if required.

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