Metallic Contaminant detection Systems for Foods and Beverages
by High Tc SQUID Magnetic Sensor
Saburo Tanaka*, H. Fujita*, Y. Hatsukade*, T. Nagaishi**, K. Nishi**, H. Ota**, T. Otani***and S. Suzuki***
Abstract – A computer controlled contaminant detection systems based on high-Tc Superconducting Quantum Interference Device(SQUID) for food and drink have been designed and constructed. The systems we have developed are the High-Tc SQUID based
systems, which are covered with waterproof stainless steel plates and acceptable to HACCP (Hazard Analysis and Critical Control
Point) program. One system is for foods and the other is for beverages. The systems employed double-layered permeable metallic
shield plates as a magnetically shielded box. The distribution of the magnetic field in the box was simulated by FEM; the gap
between each shield layer was optimized before fabrication. Then the shielding factor of more than 700 in Z-component wasachieved. This value is good enough to operate the system in a factory. As a result, we successfully detected a steel ball as small
as 0.3 mm in diameter by ether of the system. We believe that these systems are the fist practical SQUID based metallic
contaminant detectors for food and beverages.

Keywords: SQUID, contaminant, detection, food safety, NDE, shield.

1. Introduction
The consumption of processed foods such as ahamburger is a common feature of daily life. There isa possibility that unfavorable contaminants are
accidentally mixed with foods although great efforts are
made to exclude such a chance in the factory.
Examples of these contaminants are small chips like finewires of a strainer element of processing machines andbroken syringe needles used for immunization or
hormone injections to food animals. Because of theincrease in international concern regarding food safety,
we should develop a highly sensitive detector to ensure
food and drug safety. Several detection methods
currently exist, such as eddy current detector and X-ray
imaging. The eddy current method is widely used in
the world; however, the sensitivity is much affected by
the conductivity of the contaminants and the food itself.
Since wires of strainer elements or syringe needles are
made of stainless steel, their conductivities are lower
than carbon steel. Therefore it is hard to detect
stainless steel contaminants by the eddy current method.
X-ray imaging is strong candidate and is getting popularin food factories. It can detect not only metallic but*

Toyohashi University of Technology, 1-1 Hibarigaoka Tempaku-cho
Toyohashi Aichi 441-8580, Japan.(tanakas@eco.tut.ac.jp)** Sumitomo Electric Hightechs Co., Ltd., Itami, Hyogo 6640016, Japan.( nagaishi@shs.co.jpT)** * Advance Food Technology Co., Ltd., Toyohashi, Aichi 4418113, Japan.(suzuki@aftweb.co.jp)

ceramic contaminants. However, maintenance costs
for X-ray equipment are high, and the lower detection
limit for practical X-ray usage is on the order of 1 mm.
Along with others, the authors have proposed thedevelopment of a detection system using a SQUID
magnetic sensor to circumvent the difficulties outlined
above [1]-[5]. The detection technique is based onrecording the remanent magnetic field of contaminant
by SQUID sensors. Here we describe contaminantdetection systems for foods and beverage with a robust
magnetically shielded box.

2. Principle
A block diagram of the detection system is shown in Fig.
1. It consists of a permanent magnet, a conveyor, amagnetically shielded box and SQUIDs. All of the objects
move from left to right and pass through the magnet tunnel
before the detection. An austenitic stainless steel material
is originally nonmagnetic. However, it shows properties
similar to those of a ferromagnetic material after
martensitic transformation by history during its fabrication.
Therefore the magnetization prior to the detection is also
effective for austenitic stainless steel contaminants. The
remanent magnetic field from a metallic contaminant infood is detected by the SQUID magnetometers when it
passes below the sensor.

aluminum shield panels which prevent electromagnetic
disturbance outside the system. The appearance of thesystem is shown in Fig. 2.

3. System for Foods
3.1 General specification
The size of the whole system is 1500L mm x 477W mm
x 1445H mm, which is covered with stainless steel, andapprovable under the HACCP (Hazard Analysis andCritical Control Point). The magnet is made of Nd basealloy and its magnetic field is 0.1 T. The LN2 cryostat
used for maintaining the temperature of the SQUIDs at 77
K consists of three separated glass dewars. The size of theeach dewar is O.D. 70 mm x I.D. 50 mm x 300 mm. The
total volume of the cryostat is 1.5 liters and the liquid
nitrogen can be maintained for 12 hours without filling.
An automatic LN2 supply system is attached. Three highTc SQUIDs are installed in this system. The SQUID and
its driving electronics employed here were manufactured by
Sumitomo Electric Hightechs. The size of the pickup loop
is 10 mm x 10 mm square and of high-Tc directly coupled
type. The sensitivity of the SQUID is nominally 300
fT/Hz1/2 at 10 Hz. The SQUID driving eletronics is ofmodulation type with a bandwidth of 10 kHz. The signal
is passed through a low-pass filter (LPF) at a cutoff
frequency of 5 Hz.
The system was totally controlled bya PC and can be operated by touching the display panel infront of the system. All the electronics except for PC andthe glass dewars with SQUIDs were surrounded by
CryostatController

3.2 Magnetic shield design
Since the magnetic shield covering the sensors is the
crucial part of the system for practical use, special attention
was given to its design. Prior to system design, 3D
analy s e s of the magnetic field distribution inside themagnetic shield were carried out.
Firstly, a Finite Element Model (FEM) simulation was
performed on a PC for the simple tri-layered cylindricalmagnetic shield model shown in Fig. 3.
The model
dimensions were φ850 mm x φ750 mm x φ650 mm x 1600L
mm. The thickness of the material was 2 mm. The
simulation software (Ansoft Corporation, Japan) was used.
A dc magnetic field of 5.3 μT was given along Z-direction
and the field distribution inside was calculated as a functionof the relative permeability, μs, of the material. The
shielding factor (SF) was calculated as a ratio of theimposed external magnetic induction to the value of themagnetic induction at the center of the internal region of theshield. Fig. 4 shows the dependence of the SF on thePC

Magnetic shield
Magnet
Food

SQUID

N
S

Belt conveyer

N
S

Fig. 3. Simple tri-layered cylindrical magnetic shieldmodel for Finite Element Method (FEM) simulation.
ContaminantFig. 1. Block diagram of a contaminant detection
system for food.
SQUIDs (inside
the cover)Touch Panel

Magnet (0.1T)

SF for Z component

Magnetic Shield(inside the cover)

6000
5000
4000
3000
2000
1000
0
0

Belt
conveyer
PC controller(inside the cover)Fig. 2. Appearance of the whole system. The sizeof the system is 1500L mm x 477W mm x 1445H mm
and is covered with stainless steel.

1 2 3 4 5 6Relative permeability

7 8
4[x10 ]

9

Fig. 4. Dependence of the SF on the relative
permeability. It indicates that the SF increases linearly
along the relative permeability μs.

relative permeability. It indicates that the SF increases
linearly along the relative permeability μs. An actual trilayered cylindrical magnetic shield, having the same
dimensions as the model described above, was then built
using permalloy. The SF of this shield was obtained byapplying a dc field of 5.3μT; the SF was found to be 1250
at the center of the cylinder. We thus estimate the relative
permeability of the material to be about 20000 bycomparing the value shown in Fig. 4.
Secondly, a rectangular shape model as shown in Fig. 5,
which is more realistic and suitable for our system was
employed for the simulation. Two shield layers were used.
The same software was used for the FEM simulation. A
magnetic field of 50 μT was given along Y-direction with
angle of 10 degrees. The thickness of the permalloy layer
and the relative permeability were supposed as 1 mm and20000, respectively. The size of the inner shield box was
fixed as 1000L mm x 252W mm x 482H mm; the spacing
between inner box and outer box was changed from 10 mm
to 400 mm. The shielding factor SF was calculated as thesame manner as the cylindrical case. Figure 6 shows theshielding factor calculated as the ratio between the Zcomponent of magnetic field outside and at the center of theshield (z=140 mm from the bottom of the inner shield box).
The shielding factor presents a non-linear behavior andmaxima at around the layer separation of 100 to 200 mm.
Therefore the separation of 100 mm was used for the designof the shield box by the consideration of its cost of thematerial. In this model the shielding factor SF of 634 was
obtained. Then the dependence of the SF on the length of
the inner shield box along the Y direction was investigated.
The results are shown in Fig. 7. The Shielding factor SF
shows the plateau region around Y length of 700 to 1000
mm. Thus the Y length of 700 mm was employed for real
design because of the cost reduction. We also calculated
the SF when the height of the inner shield box was
extended for 50 mm, which gives more space forelectronics in the shield. The shielding factor SF of 523
was obtained at the Y length of 700 mm and the height of
532 mm, which is high enough for practical use. The final
dimensions of the inner shield (L x W x H) were set at 700
mm x 252 mm x 532H mm, while the outer shield was 902
mm x 454 mm x 734 mm..

1000 mm
800

P

Z
Belt conveyer

Y

Fig. 5. Rectangular shape model for FEM
simulation. This model is more realistic and suitable
for the system. The number of the shield layer is
two.

700
600
500
400
300

400

600

800

1000

Y length of inner layer [mm]Fig. 7. Dependence of the SF on the length of theinner shield box along the Y direction.
The
Shielding factor SF shows the plateau region aroundY length of 700 to 1000 mm.

800
700
SF for Z component

SF for Z component

Spacing

600
500

3.3 Characterization of the system

400
300
200
100
0

0

100

200

300

400

500

Separation between layers [mm]Fig. 6. Shielding factor as a function of layer
separation. Shielding factor presents a non-linear
behavior and maxima at around the layer separationof 100 to 200 mm.

The double-layered shield box described above was
manufactured. Then the actual shielding factor of theshield box was evaluated by applying dc magnetic field; SFof 732 was obtained and it was 40 % larger than thecalculated value.
This suggests that the relative
permeability of the shield materials is better than the value
we expected because it is dependent on the annealing
conditions. The performances of the total system were
tested. Steel balls and stainless steel balls with different
diameters were prepared as test samples. The samples

of the detection is the same as that of the detection system
for food described above with exception of no belt conveyer.
Figure 10 shows the appearance of the system for beverages.
A water circulation system with a reservoir tank was
prepared for the demonstration. A sample contaminant in
the water comes out from the reservoir tank and ismagnetized by the magnet. Then it passes below the
SQUIDs and finally it is retrieved at the strainer in thereservoir tank.

38
Tube SQUID

42

SQUID(ch1)4066were put on the conveyer and magnetized. Then they werepassed below the SQUIDs with speed of 14 m/min. The
signal was not affected by either electromagnetic radiationof a nearby mobile phone or the motion of a bulky steel cart
near the system.
Fig. 8 shows the dependence of the
SQUID output peak signal on the sample diameter.
Conversion factor of Volt to Tesla is 1.2 nT/V. For steel
balls (solid circle), signals scaled well with the cubic of thediameter. This suggests that the signal is proportional to thevolume of each sample. On the contrary, stainless steel
balls (open circles), which are nominally austenitic stainless,produced signals which deviated from expected values
because their magnetism depends on the martensitic phase
content which is affected by the mechanical history of thematerial [6].

Flow

SQUID(ch2)(a) Top View

100

Output signal [V]SteelStainless Steel(b) Side View

Fig. 9. Configuration of the SQUIDs.
Two
identical SQUIDs were employed so that they can keep
the sensitivity over the full width of the tube.

10

1

0.1

Touch Panel

0.01
0.1

1

SQUIDs (inside )10

Diameter [mm]Fig. 8. Dependence of the SQUID output peaksignal on the sample diameter. Conversion factor ofVolt to Tesla is 1.2 nT/V.

Reservoir Tank
for Circulation

Magnet (0.1T)
Outlet

Inlet
Plastic Tube
(inside)Pump

4. System for Beverages
4.1 General Specification
Since the system with a conveyer can not be applied to aliquid material in process, there is a strong demand for
detection of metallic contaminants in a beverage such as
juice with pulp because a strainer cannot be applied to sucha pulpy liquid. Therefore we developed the detection
system based on high-Tc SQUID for a beverage. The
outer dimensions of the system are 800 mm length x 530
mm width x 1560 mm height. A straight plastic tube witha dimension of 26 x 66 mm, in which a beverage flows ispenetrating the system. The two identical SQUIDs were
employed so that they can keep the sensitivity over the full
width of the tube. The configuration of the SQUIDs isshown in Fig. 9. The separations of the SQUIDs are 40
mm in the direction across the tube and 42 mm in the
direction parallel to the flow, respectively. The principle

Fig. 10. The appearance of the system for beverages.
A water circulation system with a reservoir tank was
prepared for the demonstration.
4.2 Characterization of the system
The performances of the system were examined.
Stainless steel balls with diameter of 0.3mm were prepared
as test samples. The sample was put in a small capsulewith some copper weight. Before the measurement, it was
confirmed that the copper weight does not affect the output
signals. Water with a capsule run in the tube with speed of
0.88 m/s. The signals from the SQUID output weredirected to the A/D converter of PC through a band passfilter with frequency of 0. 2 Hz and 100 Hz to eliminate adc offset and noise. Fig. 11 shows the time trace of
signals from the SQUID ch1 and ch2, respectively.

1V

Conversion factor of Volt to Tesla is 30 pT/V. Clear peaks
can be seen in the figure. The difference of the peak values
between ch1 and ch2 is due to the position of the capsule.
In this case, the distance between the capsule and the ch1
must be closer than the ch2. The signal is large enoughand S/N ratio is more than 5. This detection level was
hard to be achieved by a conventional eddy current or Xray detection method.

100ms

ch2
ch1

Fig. 11. Time trace of the signals from SQUIDs.
Conversion factor of Volt to Tesla is 30 pT/V. Clear
peaks can be seen.

5. Conclusion
We have designed and constructed contaminant detection
systems for foods and beverages using high Tc SQUID
magnetometers. The detection technique is based onrecording the remanent magnetic field of contaminant by
SQUID sensors. The magnetic shield which covers the
SQUID sensors was designed in cautious manner because it
is one of the crucial parts of the system for practical use.
After the FEM simulation, the double-layered rectangularmagnetic shield using permalloy metal was made; itsshielding factor SF of more than 700 was obtained. Test
spherical stainless steel ball as small as 0.3 mm in diameter
was successfully detected with more than S/N of 5. The
systems were robust and not affected by the disturbance ofelectromagnetic wave from cell phone or motion of an ironmade bulky carrier. This detection level was hard to beachieved by a conventional X-ray detection method. We
believe that this system is the fist SQUID based metallic
contaminant detector for a beverage.

Acknowledgements
This work was supported in part by Ministry of
Education, Culture, Sports, Science and Technology

References[1] S. Tanaka, S. Kudo, Y. Hatsukade, T. Nagaishi, K.
Nishi, H. Ota and S. Suzuki: IEICE Trans. on
Electronics Vol. E88-C, pp. 175-179, 2005.
[2] S. Tanaka, M. Natsume, T. Matsuda, Z. Aspanut and M.
Uchida: Superconductivity Science and Technology
Vol. 17, pp. 620-623, 2004.
[3] M. Bick, P. Sullivan, D.L. Tilbrook, J. Du, B. Thorn, R.
Binks, C. Sharman, K.E. Leslie, A. Hinsch, K. Macrae
and C.P. Foley: Extended abstracts of 9th International
Superconductive Electronics Conf. (ISEC03) PTh06,
2003.
[4] G.B. Donaldson, A. Cochran and D. McKirdy,“Fundamentals and Applications” ed. Weinstock H,
Kluwer Academic Publishers 1996, pp. 599.
[5] H.J. Krause, G.I. Panaitov, N. Wolter, D. Lomparski,
W. Zander, Y. Zhang, E. Oberdoerffer, D. Wollersheim
and W. Wilke: IEEE Trans. on Applied
Superconductivity Vol. 15, pp. 745-748, 2005.
[6] S. Tanaka, H. Fujita, Y. Hatsukade, T. Nagaishi, K.
Nishi, H. Ota, T. Otani and S. Suzuki, “A foodcontaminant detection system based on high-Tc
SQUIDs”, Supercond. Sci. Technol., Vol. 19, pp.
S280-S283, 2006.