Detecting glass splinters in food

Product recalls of food items can not only damage companies financially, they also result in a loss of consumer confidence. Manufacturers therefore have a keen interest in inspecting their products for foreign objects.

While X-ray machines can easily identify metals, they can often have difficulty with plastics, wood and glass. This means that, despite inspections, there is still a certain residual risk for manufacturers.

SAMMI is a prototype radar technology developed at the Fraunhofer Institute for High Frequency Physics and Radar Techniques FHR. It can now be used to complement X-ray machines to provide greater security in food production.

“Our system is based on millimeter waves and can augment established X-ray techniques,” said Daniel Behrendt, business unit spokesperson at Fraunhofer FHR. “It detects the foreign matter that X-ray techniques can easily overlook — that is, glass splinters, plastics and wood. However, it is not able to penetrate metals, which in return is detected by X-ray techniques.”

Another advantage of the technology is that the millimeter waves used to inspect the foods pose no health risk.

How does it work?

The inspection works as follows: the food is placed on a conveyor belt and transported through the machine. Above the conveyor, the transmitting antenna rotates and transmits its waves through the product; below it, the receiving antenna receives these waves. The millimeter waves are uniquely attenuated by each of the different food materials and undergo a specific delay in their transit time. This makes it possible to identify not only the structure and composition of the food, but also the slightest deviations from those — such as are caused by foreign objects, for instance.

Assigning a pixel to each measurement point and encoding the changes with different colours produces an image of the investigated object on which the foreign matter is immediately evident. Even packaged goods can be inspected in this way, non-destructively and without physical contact.

The prototype the researchers have set up at Fraunhofer FHR measures 40 x 40 x 30 centimetres and can be used to analyse foods measuring up to 30 x 30 x 5 centimetres. “But from a purely technological standpoint, there is no limit here,” Behrendt said.

Feasibility study on sandwich cookies

Initial feasibility studies have already been completed. The researchers used SAMMI to inspect sandwich cookies in which they had previously placed a glass splinter in the chocolate filling.

Their efforts met with success: the prototype reliably detected the foreign objects.

This radar image shows a glass splinter that was previously placed in the chocolate filling.

The technology also performed well when inspecting an advent calendar: the radar image showed clearly that three pieces of chocolate were missing, but that all others were present and correctly positioned.

In an additional step, the research team now wants to further improve the inspection speed and the precision.

Other applications

But food inspection is just one application for which this technology is suitable. The system offers advantages, for instance, in non-destructive product testing: in an advent calendar inspection, it shows, for example, whether the adhesive dots were applied with sufficient thickness to hold the calendar together. Hübner Photonics is already marketing this system for the inspection of letters and small packages, calling it T-SENSE.

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