Smartphone app uses science to tell you whether your apple is ripe for eating

Scientists have developed a hand-held device that uses a tiny sensor to instantly determine the ripeness of a piece of fruit — no squeeze test necessary.

It could be a game-changer for grocery shoppers who feel the need to fondle several pieces of fruit in the store before deciding what ultimately makes it into the shopping cart.

The new instrument, described this week in Scientific Reports, is a small and inexpensive spectrometer, which can be constructed for just under $250 (Dh918) and interfaces wirelessly with a smartphone.

A spectrometer is a scientific tool that splits light signals into many components. Scientists use it to measure light that is emitted, absorbed or scattered by materials, which in turn helps them identify and study those materials.

To test its effectiveness, the science team decided to see how well it was able to detect the ripeness of a piece of fruit.

It turns out there is a relationship between the ultraviolet fluorescence of chlorophyll in the skins of fruits like apples, oranges and bananas and the softness of the fruit inside. Using a spectrometer to measure the fluorescence of chlorophyll allows scientists to tell whether the fruit is ready to eat, or if it could use a few more days to reach prime sweetness.

Although spectroscopy has not yet been used to determine fruit ripeness in the field, it has frequently been used in the lab.

“Ripeness testing using spectrometers is non-destructive and very fast,” said Anshuman Das, a postdoctoral researcher at the MIT Media Lab and coauthor of the new study. “It does not involve much sample preparation, so it’s an attractive approach.”

For their study, Das and his colleagues used the device on about 100 apples.

For each test, they held the spectrometer against the skin of the fruit and then activated an LED light built into the device. The chlorophyll in the apples’ skin re-emitted the light in an ultraviolet wavelength that was measured by the instrument. Those measurements were sent to a smartphone, which was able to analyse the information and determine whether the fruit was ripe.

To see if the device was effective on all types of apples, the researchers ran experiments on three varieties with different colouration patterns.

The spectrometer-cum-ripeness-detector worked best on Golden Delicious apples because their green and yellow skin has the most chlorophyll and gives off the strongest fluorescence signal.

Spectrometers have traditionally been quite expensive, costing several thousands of dollars. They have also generally been too large to use outside of a laboratory setting and require a laptop to operate.