04-10 rotten apples

[Image above] Food waste is a major problem in the United States—a new sensor that monitors the ripeness of fruits and veggies could help. Credit: Cristie Guevara, PublicDomainPictures.net (CC0 1.0)

Have you ever opened the fridge and been greeted with the smell of a very, very ripe vegetable that lay forgotten on the back shelf?

Whenever I discover spoiled produce in my fridge, I always feel a twinge of guilt. Food waste is a major problem in the United States—it accounts for 30%–40% of the food supply, based on estimates from the U.S. Department of Agriculture.

Consumers play a small role in this problem by buying or cooking more food than they can consume. But food loss is endemic to the entire food production and supply chain, from farm (e.g., problems drying, milling, transporting, or processing food) to retail (e.g., equipment malfunction, over-ordering, and culling of blemished produce).

One cause of food waste is premature ripening or wilting, which occurs when plant stress triggers overproduction of ethylene, a hormone that stimulates growth, ripening, and other key stages of a plant’s life cycle.

“To manage any kind of produce that’s stored long-term, like apples or potatoes, people would like to be able to measure its ethylene to determine if it’s in a stasis mode or if it’s ripening,” Timothy Swager, John D. MacArthur Professor of Chemistry at Massachusetts Institute of Technology, says in an MIT press release. Unfortunately, “There still is not a good commercial sensor for ethylene.”

In 2012, Swager’s lab developed an ethylene sensor using an array of carbon nanotubes doped with carbon atoms.

“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,” an MIT press release explains.

However, this sensor only detects ethylene concentrations to 500 parts per billion (ppb)—the concentration required for fruit ripening can start as low as 100 ppb. Additionally, because the sensors contain copper, they likely would corrode and stop working.

In a new paper published last month in ACS Central Science, Swager and his team created a new ethylene sensor. This one is also based on carbon nanotubes, but it works by an entirely different mechanism: Wacker oxidation.

“Instead of incorporating a metal such as copper that binds directly to ethylene, they used a metal catalyst called palladium that adds oxygen to ethylene during a process called oxidation,” an MIT press release explains. “As the palladium catalyst performs this oxidation, the catalyst temporarily gains electrons. Palladium then passes these extra electrons to carbon nanotubes, making them more conductive. By measuring the resulting change in current flow, the researchers can detect the presence of ethylene.”

In initial experiments, the researchers made an exciting discovery—their new sensor could potentially monitor ethylene concentrations as low as 15 ppb!

“A clear signal is attained after a 1 min exposure to 500 ppb ethylene in air, and the responses are linear from 500 ppb to 50 ppm [parts per million] with a calculated limit of detection (LOD) of 15 ppb,” they write in the paper.

To demonstrate the sensor’s capabilities, they monitored ethylene production in plant senescence, “a process that is mediated by low ppb concentrations of ethylene.” They monitored two types of flowers—red carnations and purple lisianthus—over five days and found a rapid spike in ethylene concentration on the first day, after which the flowers quickly bloomed.

In the press release, the researchers explain their findings are not only useful for preventing food waste.

“In addition to its natural role as a plant hormone, ethylene is also the world’s most widely manufactured organic compound and is used to manufacture products such as plastics and clothing,” they explain. “A detector for ethylene could also be useful for monitoring this kind of industrial ethylene manufacturing.”

The 2012 paper, published in Angewandte Chemie, is “Selective detection of ethylene gas using carbon nanotube-based devices: Utility in determination of fruit ripeness” (DOI: 10.1002/anie.201201042).

The 2020 open-access paper, published in ACS Central Science, is “Trace ethylene sensing via Wacker oxidation” (DOI: 10.1021/acscentsci.0c00022).