Every
year, U.S.
supermarkets lose roughly 10% of their fruits and vegetables to spoilage,
according to the Department of Agriculture. To help combat those losses, Massachusetts
Institute of Technology (MIT) chemistry professor Timothy Swager and his
students have built a new sensor that could help grocers and food distributors better
monitor their produce.
The
new sensors, described in Angewandte Chemie, can detect tiny amounts of
ethylene, a gas that promotes ripening in plants. Swager envisions the
inexpensive sensors attached to cardboard boxes of produce and scanned with a
handheld device that would reveal the contents’ ripeness. That way, grocers
would know when to put certain items on sale to move them before they get too
ripe.
“If
we can create equipment that will help grocery stores manage things more
precisely, and maybe lower their losses by 30%, that would be huge,” says
Swager, the John D. MacArthur Professor of Chemistry.
Detecting
gases to monitor the food supply is a new area of interest for Swager, whose
previous research has focused on sensors to detect explosives or chemical and
biological warfare agents.
“Food
is something that is really important to create sensors around, and we’re going
after food in a broad sense,” Swager says. He is also pursuing monitors that
could detect when food becomes moldy or develops bacterial growth, but as his
first target, he chose ethylene, a plant hormone that controls ripening.
Plants
secrete varying amounts of ethylene throughout their maturation process. For
example, bananas will stay green until they release enough ethylene to start
the ripening process. Once ripening begins, more ethylene is produced, and the
ripening accelerates. If that perfect yellow banana is not eaten at peak
ripeness, ethylene will turn it brown and mushy.
Fruit
distributors try to slow this process by keeping ethylene levels very low in
their warehouses. Such warehouses employ monitors that use gas chromatography
or mass spectroscopy, which can separate gases and analyze their composition.
Those systems cost around $1,200 each.
“Right
now, the only time people monitor ethylene is in these huge facilities, because
the equipment’s very expensive,” Swager says.
Detecting ripeness
Funded by the U.S. Army Office of Research through MIT’s Institute for Soldier
Nanotechnologies, the MIT team built a sensor consisting of an array of tens of
thousands of carbon nanotubes: Sheets of carbon atoms rolled into cylinders
that act as “superhighways” for electron flow.
To
modify the tubes to detect ethylene gas, the researchers added copper atoms,
which serve as “speed bumps” to slow the flowing electrons. “Anytime you put
something on these nanotubes, you’re making speed bumps, because you’re taking
this perfect, pristine system and you’re putting something on it,” Swager says.
Copper
atoms slow the electrons a little bit, but when ethylene is present, it binds
to the copper atoms and slows the electrons even more. By measuring how much
the electrons slow down—a property also known as resistance—the researchers can
determine how much ethylene is present.
To
make the device even more sensitive, the researchers added tiny beads of
polystyrene, which absorbs ethylene and concentrates it near the carbon
nanotubes. With their latest version, the researchers can detect concentrations
of ethylene as low as 0.5 ppm. The concentration required for fruit ripening is
usually between 0.1 and 1 ppm.
The
researchers tested their sensors on several types of fruit—banana, avocado,
apple, pear, and orange—and were able to accurately measure their ripeness by
detecting how much ethylene the fruits secreted.
Lead
author of the paper describing the sensors is Birgit Esser, a postdoc in
Swager’s laboratory. Grad student Jan Schnorr is also an author of the paper.
John
Saffell, the technical director at Alphasense, a company that develops sensors,
describes the MIT team’s approach as rigorous and focused. “This sensor, if
designed and implemented correctly, could significantly reduce the level of
fruit spoilage during shipping,” he says.
“At
any given time, there are thousands of cargo containers on the seas,
transporting fruit and hoping that they arrive at their destination with the
correct degree of ripeness,” adds Saffell, who was not involved in this
research. “Expensive analytical systems can monitor ethylene generation, but in
the cost-sensitive shipping business, they are not economically viable for most
of shipped fruit.”
Swager
has filed for a patent on the technology and hopes to start a company to
commercialize the sensors. In future work, he plans to add a radio-frequency
identification (RFID) chip to the sensor so it can communicate wirelessly with
a handheld device that would display ethylene levels. The system would be
extremely cheap—about 25 cents for the carbon nanotube sensor plus another 75
cents for the RFID chip, Swager estimates.
“This
could be done with absolutely dirt-cheap electronics, with almost no power,” he
says.
