The team from the Department of Environmental and Life Sciences at the university developed the contaminant detector using three high-Tc RF (radio frequency) superconducting quantum interference devices (SQUIDs) for food inspection.
The detection technique is based on recording the remnant magnetic field of a contaminant using SQUID sensors.
SQUIDs is a high-sensitivity magnetic sensor based on superconductivity.
A strong magnetic field is applied to food to magnetize metal fragments inside, and if there are any, they can be detected by sensing magnetic fields using SQUID sensors.
The SQUIDs are cooled by liquid nitrogen, which can be supplied from a reservoir by a pump.
Contaminant detection
Professor Tanaka, whose team developed the method, said the inspection system permits contaminant detection in a food package with a height of 100mm with three high-Tc RF SQUIDs.
“To accurately detect even smaller metallic fragments, digital filters have also been used to improve the signal-to-noise ratio. The target size of the metallic contaminant in food with a stand-off distance of 100mm is 0.5mm."
Existing inspection methods such as the X-ray radiation method have limitations as they cannot detect contaminants with sizes smaller than 1mm with current practical X-ray levels.
It also cannot be applied to items that have lactic acid bacteria because X-ray radiation causes ionization of such foods.
A SQUID-based metal detector has been looked at before, one study from Bick et al was from 2005.
Sensitivity parameters
The SQUID system's sensitivity to stainless steel fragments was found to be significantly higher than coil systems if the samples are magnetized in a 100 mT magnetic field prior to detection.
They also found it has a higher sensitivity than X-ray systems, depending on the density distribution of the product under inspection.
Tanaka said to reduce impact of noise (magnetic fields that come from other sources) as much as possible the sensor is placed inside a square metallic box designed so food can be tested as it passes through.
“The box is made of 2-mm iron-nickel alloy plates. Magnetic fields have strong affinities to this iron-nickel alloy. Thus, magnetic fields around the sensor are concentrated in the walls of this box."
The researchers are working to improve the performance so even small metal fragments can be detected.
It has been able to detect a steel ball with a diameter as small as 0.3mm.
The technology, which can identify small metal fragments of about 300 microns in cheese passing through at a speed of 20m/min, involves a magnetic sensor, a triple-layered box, a strong magnet, and a digital filter.
Source: IEEE Transactions on Applied Superconductivity (Volume: 25, Issue: 3)
Online, DOI: 10.1109/TASC.2014.2361893
“Development of Metallic Contaminant Detection System Using RF High-Tc SQUIDs for Food Inspection”
Authors: Tanaka, S., Ohtani, T., Narita, Y., Hatsukade, Y. Suzuki, S.