As fruit matures, it releases a gas known as ethylene, that causes the ripening process to begin. Once that process is under way, more ethylene is released, kicking the ripening into high gear. Currently, produce warehouses use expensive technologies such as gas chromatography or mass spectroscopy to measure ethylene levels, in order to gauge the ripeness of fruits that are in storage. A scientist from MIT, however, is developing small, inexpensive ethylene sensors that could be used in places such as supermarkets. There, they could let shopkeepers know which batches of fruit need to sold the soonest, in order to minimize spoilage.
Developed by chemistry professor Timothy Swager, each sensor utilizes an array of tens of thousands of carbon nanotubes, which have had copper atoms attached to them. While electrons ordinarily flow freely through the nanotubes, any ethylene molecules present in the vicinity will bond with the copper atoms, obstructing the flow of those electrons. Tiny beads of polystyrene are also used, which absorb ethylene and concentrate it near the nanotubes.
By measuring how much the electron flow has been slowed, the sensors are able to determine ethylene levels. As a result, the sensors can reportedly measure concentrations as low as 0.5 parts per million – for context, a concentration of between 0.1 and one part per million is what is generally required for most types of fruit to ripen.
The sensors were tested on bananas, avocados, apples, pears and oranges, and were able to accurately gauge the ripeness of all of them. Swager now envisions the sensors being built into the cardboard boxes used to store fruit, and equipped with radio-frequency identification chips that would allow them to transmit ripeness data to handheld reading devices used by shopkeepers.
Each sensor and chip combined should cost about US$1, as opposed to the $1,200 or so that gas chromatography or mass spectroscopy systems currently run at.
Another system, developed at the UK's National Physical Laboratory, uses radio frequencies, microwaves, terahertz radiation and far-infrared light to determine the ripeness of strawberries – although it’s intended more for use in the field.