Japanese scientists have presented a new acoustic technique for the evaluation of the texture firmness of watermelon flesh. The technique was first developed by one of the authors for the estimation of elasticity in gelatin gels in 1999. The technique consist in surface elastic waves at fixed frequencies ranging from 800 to 2400 Hz propagate on watermelon flesh. Detecting the propagation times of the surface waves at different positions gives the velocity, which, along with knowledge of the Poisson ratio, yields the shear elasticity of the flesh. Although the shear elasticity does not directly correspond to the sensory firmness, it provides a good representation of the firmness. Previous acoustic and ultrasonic techniques have not yielded the shear elasticity of fruits.
For the study, scientists examined two kinds of Japanese watermelons, namely Matsuribayashi777 and Wasenissho varieties, which in sensory tests have relatively hard and soft flesh, respectively. The Matsuribayashi777 variety has a crisper texture compared with Wasenissho. Compression tests were also made to obtain Young's moduli and Poisson ratios for the two varieties, and these results were compared with those determined by surface wave measurements.
In the paper published on Journal of Food Engineering, the experimental system used for measuring is described. Scientists explain that the surface elastic waves were excited by an oscillator connected to a shaker. The edge of the oscillator gently touched the cut surface of the watermelon flesh. A pulsed-wave signal with a duration of 20 cycles at 800–2400 Hz was applied to the oscillator by using a function generator and a power amplifier. The pulsed waves were detected by using a piezo bimorph sensor. The measurements were made near the center of a half-cut flesh where seeds were scarce.
Scientists determined the shear elasticity from the surface-wave velocity for Matsuribayashi777 and Wasenissho, to be 1.18 and 0.74 MPa, respectively. These results correlated well with the sensory firmness evaluation. The values of Young modulus obtained by the surface-wave measurements were nine times larger than those by the compression tests performed, which can be explained by the differences in measurement displacement and frequency.
This technique based on the surface elastic waves was applied for the evaluation of fruit firmness for the first time. Scientists found that the velocity for Matsuribayashi777 was larger by 25% than that for the Wasenissho variety, indicating that Matsuribayashi777 has a harder texture. The results of compression tests and sensory evaluation of the firmness supported this conclusion. The effect of storage on the surface-wave velocity for Matsuribayashi777 was also investigated and it was found that the surface-wave velocity decreased by about 10% after ten days of storage.
Scientists conclude that this technique may provide a novel firmness-monitoring tool for watermelon and fruits with a thin peel such as tomato and apple.
Source: Takashi Ikeda, Pak-Kon Choi, Toshio Ishii, Ikko Arai, Masako Osawa, 'Firmness evaluation of watermelon flesh by using surface elastic waves', September 2015, Journal of Food Engineering, 160, pages 28–33. http://www.sciencedirect.com/science/article/pii/S0260877415001193
Contacts:
Takashi Ikeda
School of Agriculture, Meiji University
Kawasaki, Kanagawa 214-8571
Japan
Tel.: +81 044 934 7171
Fax: +81 044 934 7902
Email: agrisys@meiji.ac.jp