The time is ripe! An innovative contactless method for the timely harvest of soft fruits

Although biochemical and optical methods exist, mechanical techniques are the most widely used. They indirectly assess ripeness based on the fruit's firmness. In turn, firmness is quantified by observing the vibrations that occur on the fruit when mechanical energy is accurately delivered through devices such as hammers, pendulums, or speakers. Unfortunately, these approaches fall short for softer fruits, which are more readily damaged by the contact devices used.

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.

However, soft fruits do not exhibit ₀S₂ mode vibrations, so the team had to analyze an alternative type of oscillation: Rayleigh waves. These are waves that occur exclusively on the surface of bodies without penetrating far into the interior. Using Kent mangoes, a setup for generating LIP, and commercially available laser-based vibrometers, the scientists verified that the velocity at which Rayleigh waves propagate is directly related to the firmness of the mangoes. Because the propagation velocity markedly decreases with storage time, it provides a reliable way to indirectly assess ripeness.

The team went further and looked for the best position on the mangoes' surface to determine the velocity of Rayleigh waves. Mangoes, as well as other soft fruits, have large seeds inside, which can alter the propagation of surface waves in ways that are detrimental to measurements. "The results of our experiments indicate that Rayleigh waves along the 'equator' of the mango are better for firmness assessment compared to those along the 'prime meridian'," explains Hosoya. The experiments also revealed that cavities within the fruit's flesh or decay can greatly affect the results of the measurements. Thus, as Hosoya adds, they will keep investigating which is the best area to measure firmness in mangoes using their novel approach.

In short, the team at SIT has engineered an innovative strategy to assess the ripeness of soft fruits from outside. "Our system," remarks Hosoya, "is suitable for non-contact and non-destructive firmness assessment in mangoes and potentially other soft fruits that do not exhibit the usual ₀S₂ mode vibrations." Further refinement of such firmness assessment methods will hopefully make them more reliable and accessible for the agricultural industry. With any luck, their widespread adoption will ensure that fruits reach your plate only when the time is ripe!