Judging how ripe a piece of fruit is can be a frustrating process for everyone from farmers to shoppers. Now, researchers at Shibaura Institute of Technology (SIT) in Japan have developed a new high-tech way to check, without touching or damaging soft fruit like mangoes. All it takes is a laser and some plasma shockwaves.

Firmness is one of the most reliable indicators of ripeness, but it’s time-consuming (not to mention unhygienic) for everyone along the chain to give it a squeeze. Plus, soft fruits can be damaged by repeated handling and pressure.

So the team on the new study set out to come up with a contactless method to measure ripeness. The researchers used a high-intensity laser to create a bubble of plasma near the surface of mangoes. As the plasma bubble expands it produces shockwaves, which in turn create vibrations across the fruit. An instrument called a laser Doppler vibrometer (LDV) can then measure those vibrations and infer the firmness, and by extension the ripeness, of the fruit.

Similar methods have been used before to, for example, check the ripeness of avocados, but that involved mechanically striking the fruit, which doesn’t work as well on softer fruit like mangoes.

A diagram of how the new fruit ripeness measuring device works Shibaura Institute of Technology

The SIT researchers previously used their plasma method on apples, but again, it didn’t work on soft fruit because they don’t produce the right type of vibrations. For this new study, the team found that plasma shockwaves can create a type of vibration called Rayleigh waves, which only propagate across the surface.

Tests revealed that these Rayleigh waves could still indicate firmness and ripeness with the help of the LDV. The results were clearer when the waves were sent around the “equator” of the mango, rather than along the vertical axis. This seems to be because of the seeds in the center.

The team did point out that the measurements can be thrown off by cavities inside the flesh or by decay. In future work the researchers plan to continue investigating how to get around these problems by targeting different parts of the fruit, and hopefully extending the method to other soft fruits.

Other contactless methods to measure ripeness include sensors and handheld "electronic noses" that can detect specific compounds that fruit gives off as it ripens.

The new research was published in the journal Food.