Try Laser-Induced Plasma Shockwaves

In order to save you from this pain, farmers are working hard to find the most suitable harvest time, so that the fruit will reach the best maturity when it is sold.

Not only will this reduce waste, but it will also allow you to buy the best quality fruit. Various methods have been invented to achieve this goal, each with pros and cons.

While both biochemical and visual recognition methods can be used to judge fruit ripeness, measurement using mechanical techniques is the most commonly used method.

The latter mainly indirectly determines the ripeness of the fruit by measuring the firmness of the fruit, which usually requires measuring the vibration of the fruit when it is hit by an implement such as a hammer pendulum.

However, such a direct contact method usually causes damage to the softer fruit and is therefore limited in scope. For example, you can tell if a watermelon is raw or ripe by feeling the vibrations when you slap it, but this method obviously won't work when picking strawberries.

Recently, a team from Shibaura Institute of Technology in Japan has invented a method for measuring the firmness of softer fruits based on previous research using laser-induced plasma to measure hard fruits. The related research was published in MDPI's open access journal "Food" .

Wait, what the heck is a laser-induced plasma? How can it still be used to measure whether the fruit is ripe?

A plasma state is a state of matter similar to a gaseous state. Particles of matter in the plasma state are usually charged.

This energetic state of matter can be created by irradiating a small volume of air with a high-intensity laser beam. Because the "plasma bubbles" created by this method are very unstable, these "bubbles" quickly "burst" into supersonic shock waves.

Professor Naoki Hosoya of Shibaura Institute of Technology and colleagues have successfully generated a laser-induced plasma shock wave very close to the surface of an apple. When the shock wave touched the apple, it triggered a vibration known as the 0S2 mode.

Because the deformation caused by the vibration of this mode is the same as when a football is kicked, it is also called "football mode vibration". Subsequently, they confirmed that the vibration pattern of 0S2 was related to the firmness of the fruit.

Softer fruits, however, do not exhibit 0S2 vibrations, so the team could only analyze them using another type of vibration known as Rayleigh waves.

Such traveling waves usually travel only on the surface of an object and do not penetrate deep into the object.

With the help of a laser-induced plasma launcher, a commercially available laser seismometer, and a few Kent Mounts, the scientists determined that the propagation speed of Rayleigh waves is directly related to the firmness of mangoes .

As the mangoes were stored for longer, the wave propagation speed decreased significantly. This shows that they have found a reliable way to indirectly measure the ripeness of mangoes.

The team further tested where on the mango's surface it would be most appropriate to measure the Rayleigh waves.

Because mangoes have a large core, this could have implications for the propagation and measurement of Rayleigh waves.

"We found through experiments that measurements along the 'equator' part of the mango would be better than measurements along the 'prime meridian'," Hosoya said. They also found that cavities in the fruit or spoilage of the fruit can significantly affect the measurements. result. So they continue to look for the best areas on the mango to measure.

▲The laser pulse is focused through the lens at a place close to the surface of the mango. Shock waves from laser-induced plasma excite Rayleigh waves on the surface of mangoes. Laser Doppler seismometers will measure this fluctuation. Photo credit: ?SHIBAURA INSTITUTE OF TECHNOLOGY

In short, the Shibaura Institute of Technology team built an innovative solution for measuring the ripeness of soft-textured fruits from the outside.

Hosoya emphasized: "Our detection system can measure mangoes and other fruits that do not cause vibrations in the OS2 mode by non-contact, non-destructive means."

For practical application in agricultural production, the feasibility and reliability of this measurement method need to be improved. If you're lucky, the widespread availability of this technology will allow you to never have to worry about buying raw fruit again.

Editor: Li Chenyan

Responsible editor: Tang Wenjia

Source: Research Circle

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