Soft Mango Firmness Assessment Based on Rayleigh Waves Generated by a Laser-Induced Plasma Shock Wave Technique

Abstract

Many methods based on acoustic vibration characteristics have been studied to indirectly assess fruit ripeness via fruit firmness. Among these, the frequency of the 0S2 vibration mode measured on the equator has been examined, but soft-flesh fruit do not show the 0S2 vibration mode. In this study, a Rayleigh wave is generated on a soft mango fruit using the impulse excitation force generated by a laser-induced plasma shock wave technique. Then, the flesh firmness of mangoes is assessed in a non-contact and non-destructive manner by observing the Rayleigh wave propagation velocity because it is correlated with the firmness (shear elasticity), density, and Poisson’s ratio of an object. If the changes in the density and Poisson’s ratio are small enough to be ignored during storage, then the Rayleigh wave propagation velocity is strongly correlated to fruit firmness. Here, we measure the Rayleigh wave propagation velocity and investigate the effect of storage time. Specifically, we investigate the changes in firmness caused by ripening. The Rayleigh wave propagation velocity on the equator of Kent mangoes tended to decrease by over 4% in 96 h. The Rayleigh wave measured on two different lines propagated independent distance and showed a different change rate of propagation velocity during 96-h storage. Furthermore, we consider the reliability of our method by investigating the interaction of a mango seed on the Rayleigh wave propagation velocity.

Non-contact acoustic method to measure depth-dependent elastic properties of a kiwifruit

“…5A). This observation is similar to that seen in a study by Arai et al, in which two waves propagate in mango flesh at different speeds, both with a roughly linear relationship between angle and arrival time [16]. The interpretation given is that both are surface waves, where one is confined to the skin, the other to the outer flesh layer, and with v skin ∼ 10v flesh .…”

“…For other types of fruit, a very wide range of values of viscoelastic parameters have been reported. Shockwaveinduced Rayleigh waves have been used to measure v R ∼ 25 − 50 m/s in the outer flesh of mangos, finding that v R decreases as the mango ripens [16]. We also observe this trend for kiwifruit ageing, indicating that v R could be a useful obserable for predicting overall kiwifruit age/ripeness.…”

“…Over time, this could potentially result in some damage to the fruit at the source point. Future experiments will investigate the lower limits for source power, or employ alternate methods of non-contact excitation which remove the possibility of ablation [16,43].…”

“…It is now well-established that features of this spectrum correlate with the overall fruit firmness [4]; in addition, this type of measurement can be used to estimate a range of viscoelastic parameters [5][6][7][8][9], each giving slightly different information on fruit flesh properties. Acoustic velocity (measured in the temporal domain) has also been shown to correlate with fruit age and/or firmness [9][10][11][12][13][14][15][16]. It remains, however, an ongoing research question as to which parameter may be the most relevant in terms of fruit quality monitoring.…”

“…However, a few recent studies have exploited the sensitivity of acoustic waves to density variations in the propagation medium for non-destructive evaluation of different flesh layers. A few groups have used acoustic waves propagating on the fruit surface to estimate the properties of the outer flesh layer [14,16], and Yoshida et al have employed Doppler ultrasound imaging in fruit to reveal its inner structure [25]. These studies show that, while vibrational methods are effective for determining some properties of the fruit as a whole, and to signal the presence of defects [26][27][28], acoustic measurements in the temporal domain may have more potential to examine the inner structure of fruit in detail.…”