PRINCETON, N.J. – A research group at Princeton University has developed a sticker that can accurately detect fruit ripeness without cutting the fruit open.

“There’s no way right now to sense (fruit ripeness) other than manually touching the food and seeing if it’s ripe or not,” said Yasaman Ghasempour, assistant professor of electrical and computer engineering at Princeton.

However, touching produce isn’t always the best or easiest way to find out if the fruit is ready to eat. Anyone who’s bought avocadoes duped into thinking the produce is perfectly ripe, only to find that inside, it’s a brown, mushy mess. Or you’ve picked the brightest orange on display to find it severely lacking in flavor. Oftentimes, people aren’t sure what key traits they should be looking for when picking produce, and visual or tactile cues aren’t always accurate.

Enter the Meta-Sticker engineered by Ghasempour’s lab. Meta-Sticker is a sub-terahertz (sub-THz) metamaterial sticker made of simple materials like paper and aluminum that can be attached to fruit. By using a device that can send and detect sub-THz signals, it’s possible to determine how ripe a fruit is.

But what are sub-THz signals? Sub-THz signals are electromagnetic waves with frequencies that lie between microwave and infrared radiation on the electromagnetic spectrum. When sub-THz waves shine on an object, different frequency components interact with the object in different ways. Some waves get absorbed more and some get reflected, based on the material composition of the object.

Similarly, when sub-THz waves are passed through a piece of produce, the waves are absorbed differently. This is because sub-THz waves are sensitive to water and organic materials like sugar, which impact how they are absorbed and reflected.  Meta-Sticker highlights these frequency signatures that are ripeness dependent. Then, a sub-THz scanner can analyze how the waves are absorbed or reflected.

“Sub-terahertz waves can penetrate into the peel of the fruit and bring information back to us without needing to cut into the fruit,” Ghasempour said.

Let’s say researchers apply a Meta-Sticker to an unripe apple. Unripe fruits have higher levels of water and lower levels of sugar. As sub-THz waves pass through the fruit, they are highly absorbed by the water present. The sub-THz scanner would detect a high level of absorption, indicating a high concentration of water in the fruit. From this information, researchers could conclude that the fruit is unripe. As the apple ripens, it will contain less water and more sugar, which will influence how sub-THz waves are absorbed and reflected, enabling researchers to assess the fruit’s ripeness.

Ghasempour and her team studied the accuracy and efficiency of the Meta-Sticker in detecting fruit ripeness by testing the sticker on persimmons, pears and mangoes. The study results demonstrated that Meta-Sticker is 99.46% accurate in predicting fruit ripeness in the fruits studied.

Detecting fruit ripeness is one way to potentially combat food waste, Ghasempour noted. Globally, 30 percent of all food produced is wasted, and uncertainty in produce ripeness is a contributing factor. For example, grocery store managers and customers often discard bananas with bruises because they think they’re overripe. By scanning a Meta-Sticker, people could see that the fruit is still good to eat. Meta-Stickers could also help optimize supply chains by categorizing and shipping fruits according to their ripeness. Ripe fruit could be labeled ready-to-eat and sent to local stores immediately. Additionally, Meta-Stickers can provide consumers with information about the sugar content of fruits. Ghasempour said that this information can help people trying to control their sugar intake make informed decisions about the fruits they are consuming.

“Another interesting application we plan to look into is assessing the ripeness of crates or boxes of produce without placing stickers on each fruit,” Ghasempour added. “We are also interested in going beyond ripeness and trying to detect certain things about the health of their crop that farmers might be interested in tracking about their crops, trees and produce.”

Ideally, Ghasempour envisions a world in which people could use a smartphone app to scan Meta-Stickers and receive information about produce. Unfortunately, the chips within smartphones produced today are not advanced enough to send and detect sub-THz waves. For now, Meta-Stickers and other sub-THz technologies are limited to the lab, but this technology may become part of 6G soon, finding its way to our smartphones.