Food producers use various techniques to sort out physical contaminants from bulk agricultural products to help ensure that a consumer does not ingest stones, metals, or shells while eating a handful of nuts, seeds, or dried fruits. A screen, for example, allows bigger items to stay on one side while smaller ones fall to the other side. Separating magnets and gravity have been exploited as well to remove ferrous metals and dense materials, respectively.
There are advanced technologies but they also may be inadequate for the job. Optical sorters can only detect and reject objects that look different than the product being inspected. High-sensitivity food-grade metal detectors only find metal. Trained workers can visually inspect for just about anything, but can be costly and less accurate than machines as people can tire. This is why advanced bulk flow X-ray inspection systems are being used to augment other inspection techniques.
A special-use X-ray system designed to inspect bulk ingredients can find a range of objects potentially missed by other technologies. For bulk food inspection, X-ray machines are particularly useful because they can:
- detect many of the contaminants likely to be found in the production line, nonmetallic objects such as stones and glass in particular.
- uncover contaminants that are visually similar to the food being inspected unlike optical detectors.
- find metals that contain non-ferrous material better than metal detectors.
X-ray systems work by comparing densities of materials. The denser the material, the easier it is for the machine to detect. We measure density as grams per cubic centimeter (gm/cm3) and materials such as metals are quite dense – weighing well over 7 gm/cm3. Even if the metal is lacking ferrous content, an X-ray machine will likely catch it based on its density.
But many of the materials likely to be in the process line are not metal. The density of items such as bone, shells, Teflon® and glass is such that an X-ray system has a good chance of detecting it. The key is the size of the foreign object relative to the amount of product being inspected by the X-ray beam.
Here’s something surprising about this application: Rocks and stones vary so much in density that typically a company installing an X-ray machine at a production facility will test the density of rocks and stones in the field where the harvesting takes place. A field that lies near granite deposits will have far heavier rocks than one lying near a sandstone formation, for example.
Of course, there are limitations. It’s all but impossible for an X-ray inspection system to flag such items as hair, wood, insects and small plastic bits. But X-ray systems can detect metal items as small as 1-2 millimeters and glass and rock as small as 3-6 mm.
Detection works best when it is designed into a plant’s overall production system and the machine is configured to efficiently track and remove unwanted items. There are five elements that constitute a typical bulk flow X-ray system:
- The infeed – to place the product on the belt continuously and efficiently.
- A metering system – to ensure that product height is consistent prior to the X-ray beam.
- Belt guides – to keep the product flowing smoothly, free of jams and trapped food items.
- Inspection software/hardware – to detect unwanted material.
- The rejection system – to reject the contaminant and minimize good product waste.
You can read more about bulk flow x-ray inspection systems in this white paper.