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Fruit ripeness checked via shockwave technique

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Researchers at Shibaura Institute of Technology in Tokyo have developed a convenient method for measuring the ripeness of fruit, using laser-induced plasma (LIP) shockwaves and the resulting vibrations on the surface of the fruit.

Picking fruit at the right stage in its ripening is essential for preventing consumers buying overripe or underripe fruit, resulting in unnecessary food waste. In agriculture, harvests are timed precisely so that the harvested fruit has reached optimal ripeness when it is sold.

There are several techniques used to assess the ripeness of fruit, which vary in effectiveness depending on the type of produce they are used on. Although biochemical and optical methods exist, mechanical techniques are most widely used. This involves applying a force to the fruit with a device such as a hammer, pendulum or speaker and observing the vibrations that occur on the fruit. This allows for a measurement of firmness, from which ripeness can be assumed.

Unfortunately, this technique is suboptimal for soft types of fruit which generally do not take so well to being knocked by a hammer or other device.

Now, a team of Shibaura Institute of Technology scientists have addressed this issue by presenting a new, contactless method for assessing fruit firmness. This method, which uses LIP, follows on from a previous study in which LIP was used to assess the firmness of hard fruits.

Plasma is a state of matter, a gas for which most atoms have been stripped of their electrons, leaving them with an electric charge. The state can be produced in ordinary air by focusing a high-energy laser beam onto a small volume of air. This “bubble” of plasma immediately expands, emitting shockwaves at ultrasonic speeds.

During their previous study, the researchers generated these shockwaves near to the surface of fruit to excite a type of vibration nicknamed “football mode vibration”, due to the deformation of spherical bodies resulting in a shape reminiscent of a football. They then verified that the frequency of these vibrations was correlated with the firmness of the fruit.

However, soft fruits do not exhibit this type of vibration, so the team instead turned to Rayleigh waves - waves which occur on the surface of a body. Using Kent mangoes to demonstrate the technique, they verified that the speed of the Rayleigh waves is directly correlated to the firmness of the fruit. This provides a reliable way to indirectly assess the ripeness of soft fruit, without any touching.

They also found that the large seeds inside mangoes – as well as some other soft fruits – can alter the propagation of surface waves, ruining their measurements. Measuring Rayleigh waves along the 'equator' of the mango produces more consistent results than those along the 'prime meridian' of the fruit. They also found that cavities within the flesh, or decay, can also affect measurements. Over time, they intend to identify the best practices for measuring soft fruit ripeness.

“Our system is suitable for non-contact and non-destructive firmness assessment in mangoes and potentially other soft fruits that do not exhibit the usual [football mode] vibrations,” explained Professor Naoki Hosoya.

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