Bananas are obvious. A nice, firm yellow peel equals ripe; brown spots equals overripe. But what of other fruits and vegetables? An inexpensive sensor developed by researchers at MIT could let you (and your grocer) know when produce is ready to eat. And it could help supermarket managers push fresh fruit before they spoil, avoiding waste.
The sensor, developed by MIT chemistry professor Timothy Swager and his students, picks up ethylene, a chemical that fruits give off as they mature. The riper the item, the more ethylene produced.
Ethylene is already monitored in the food industry, but the equipment used is expensive, on the order of $1,200 for a single unit. The detectors use gas chromatography or mass spectroscopy, which separate gases to analyze their composition.
Swager came up with a different idea: carbon nanotubes. The nanotubes allow electron flow, as they are conductive. Adding copper atoms slows the electrons down. When ethylene is present, it binds to the copper atoms and slows the electrons further, increasing the nanotubes' resistance. The amount of resistance reveals how much ethylene there is.
To boost the sensitivity of the device, the researchers added tiny beads of polystyrene, which absorbs the ethylene and concentrates it. The system can pick up ethylene concentrations down to 0.5 parts per million. The concentration required for fruit to ripen is usually 0.1 to 1 part per million. That means a gram of ethylene for the amount of air that would fill a room about 30 feet on a side. Swager told Disovery News that he can make the device small — about the size of a small computer chip. The only limit, he said, is that it has to be easy to handle for the user.
Swager and his students tested the device on bananas, avocados, apples, pears and oranges. In all five cases the amount of ethylene accurately measured ripeness.
Swager's previous work in the field was building sensors to detect chemical and biological warfare agents. In that sense this research is an extension of that.
A big advantage for this technology, Swager said, is cost. The carbon nanotube sensor would be about 25 cents. He plans to add an radio-frequency identification chip so the sensor can communicate wirelessly with a handheld device. That would add another 75 cents. (And yes, he has filed for a patent and hopes to commercialize the sensors).