Device can tell if an apple’s ripe from its ‘glow’

A hand-held device can test whether or not an apple is ripe just by looking at it – no peeling required – and while it’s still on the tree.

Developed by a team from the US and India, the unit exploits how the apple’s skin interacts with light – which indicated if the fruit if ready to be picked or should be left on the tree – and wirelessly send the results to a smartphone app. 

The prototype, by Anshuman Das from Massachusetts Institute of Technology in Cambridge, US and Akshat Wahi from WeSchool in Mumbai, India and colleagues, was unveiled in Scientific Reports.

At its core, the device is a spectrometer – an apparatus that measures how light waves bend and scatter. It takes advantage of the fact that, like green leaves, fruit contains chlorophyll – the sunlight-absorbing pigment that gives plants their verdant hue. 

When chlorophyll molecules absorb sunlight, they become energised. As they return to a lower energy state, they release a halo of fluorescent light.

While this glow is far too weak to see with the human eye, it can be detected by a spectrometer, which evaluates those chlorophyll light levels and links it to ripening stages. Less chlorophyll fluorescence, for instance, means riper fruit. 

And in apples, the bulk of chlorophyll is found in the skin. Simply holding the nozzle of the device against an apple is enough to get a ripeness reading sent to a smartphone via Bluetooth.

There are already devices already use chlorophyll fluorescence to measure if fruit is ready to eat. But they’re typically clunky and often tethered to a computer.

Das and Wahi’s unit can be popped in a pocket and weighs just 48 grams.

The smaller size doesn’t detract from its light-analysis power. Its nozzle shields instruments inside from stray light, regardless of how sunny the weather.

And when they used the device over 11 days for three types of apple – Golden Delicious, Empire and McIntosh – and compared it to standard ripeness tests, its churned out consistent results each time.

Das says by allowing farmers and distributors to keep track of the state of their fruit’s ripeness, they can better manage and store their produce.

Growers can test the same apple many times without destroying it, and by monitoring the ripening process, they can predict the best harvest date and improve their fruit’s quality when it hits the market. 

Agriculture Victoria’s Dario Stefanelli, who uses and promotes a similar device, believes such light-measuring units will be the dominant method of measuring fruit ripeness. 

“Moving to non-destructive technologies that will make understanding fruit ripening easier along the entire supply chain, consumer included,” he says.

But for Das, the connection to smartphones is the most exciting aspect to the study.

“We live in an exciting time where hardware is becoming so accessible and the combination of software on mobile platforms can lead to powerful products that can improve our quality of life,” he says.