Scientists develop convenient technique to measure ripeness using laser-induced plasma shockwaves and the ensuing vibrations on the fruit’s surface

Most people are probably familiar with the unpleasant feeling of eating overripe or underripe fruit. Those who work in agriculture are tasked with ensuring a timely harvest so that ripeness is at an optimal point when the fruit is sold, both to minimize the amount of fruit that goes to waste and maximize the quality of the final product. To this end, a number of techniques to assess fruit ripeness have been developed, each with their respective advantages and disadvantages depending on the type of produce.

In a recent study published in Foods, a team of scientists from Shibaura Institute of Technology (SIT), Japan, addressed this issue through an innovative method for measuring the firmness of soft fruits using laser-induced plasma (LIP). This work is a sort of follow-up of a previous study in which LIP was used to quantify the firmness of harder fruits.

But what is LIP and how is it used? Plasma is a state of matter similar to the gaseous state but in which most particles have an electric charge. This energetic state can be produced in normal air by focusing a high-intensity laser beam onto a small volume. Because the generated plasma “bubble” is unstable, it immediately expands, sending out shockwaves at ultrasonic speeds. Professor Naoki Hosoya and colleagues at SIT had successfully used LIP shockwaves generated close to the surface of apples to excite a type of vibration called ₀S₂ mode, colloquially referred to as “football mode vibration” because of how the resulting deformation looks on spherical bodies. They then verified that the frequency of the ₀S₂ mode vibrations was correlated with the firmness of the fruit.

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