 |
|
|
 |
Cheesy grins all round |
|
 |
 |
|
 |
|
Who moved my cheese? Physicist Marcel Bick with his prototype stainless steel detector.
|
|
 |
|
|
Nobody wants to find metal filings in their food, but for food manufacturers to get to them before you do isn't easy. Fortunately, physicists at CSIRO reckon their superconducting SQUIDs could be the answer.
Most food handled by a machine - be it mixed, sliced, moulded or packed - is screened by a metal detector before leaving the factory. Metal detectors find metals because they conduct electricity. They also pick up magnetic metals, such as the iron in steel. The problem for the food processing industry is that a lot of their machinery is made of a special grade of stainless steel, which is an alloy with a lot of non-metals in it and it's only very weakly magnetic.
"It generates a magnetic field more than a million times weaker than the Earth's magnetic field," said Dr Marcel Bick from CSIRO.
Marcel's group has more than a decade's experience making the world's most sensitive magnetic sensors. Called Superconducting QUantum Interference Devices (SQUIDs) they harness the power of a superconducting ceramic, yttrium barium copper oxide, to detect very weak magnetic fields.
Marcel's prototype detector can pick up a fragment of stainless steel weighing less than one milligram.
"We can even detect stainless steel when it's wrapped in aluminium foil, such as you might find in the packaging around a block of chocolate. This is something a standard metal detector has lots of difficulty with," he said.
The project started when the team was approached by the Japanese beef industry. "Apparently they were sued by a consumer who found a piece of a needle in their steak. It turns out that it's quite common for syringes to break off in cows when they're being vaccinated," Marcel said.
The team are now focusing on the dairy industry because, if it weren't already hard enough finding tiny bits of a weakly magnetic substance in large volumes of food, dairy products, particularly cheeses, are the trickiest of the lot.
"Cheese is wet and therefore conductive," Marcel said. "This means eddy currents get set up in the cheese which triggers a false positive response in conventional metal detectors."
Known in the trade as the product effect, it means the cheese gives off a signal so it looks as though there's metal there when there's not. "They can turn down the sensitivity of the system so the cheese doesn't set it off, but then they miss the little bits," Marcel said.
Here's another of those complications: forming cheese is hot work so metal shavings and scrapings often come off the machines.
And another: not only is cheese conductive, but hot cheese generates a magnetic signature that's very similar to stainless steel.
Traditional metal detectors work well with dry products such as powdered milk, but Marcel says they can't even find a wire in the industry standard, twenty kilogram blocks of cheese. X-ray systems will find stainless steel objects as small as 1.5 millimetres across, but they cost hundreds of thousands of dollars. "And we've been asked to search bulk cheese for particles 0.5 mm in diameter," Marcel said.
Marcel and his team are busy refining the prototype conveyer belt system they made when they were looking at meat. They're setting up a magnet to magnetise the sample so the tiny fragments become easier to find. SQUIDs may be sensitive, but even they need help when it comes to cheese.
Who'd have thought there was so much science behind getting a cheese sandwich on the table? |
|
 |
|
|
 |
|
|
 |
 |
 |
Superconductors have no electrical resistance (their electrons travel freely through them) |
 |
 |
|
Superconductivity occurs at very low temperatures (although scientists are busy trying to get it to happen at room temperature). If you cool lead or aluminium to around 1 Kelvin (almost absolute zero) they turn into superconductors |
|
|
|
Ceramics such as yttrium barium copper oxide are known as high temperature superconductors because they only have to be cooled to 77 Kelvin (the temperature of liquid nitrogen) |
|
|
|
Not just in the oceans...
CSIRO's SQUIDs have two thin loops of yttrium barium copper oxide either side of an insulating layer. The insulator is so thin electrons can tunnel through it. Slight changes in the magnetic field affect the superconductors, which in turn affects the electron tunnelling. This is measured as a change in resistance and you've got what's known in the trade as a SQUID magnetometer. |
|
|
|
 |
|
Get it delivered
Learn how Australia's science in motion makes a difference to daily lives. Sign up today. Why sign up?
|
|
|
 |
|
