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Naoki Sakurai and Donald J. Nevins

During fruit softening, tissue texture characteristics may be documented by stress-relaxation analysis. Data were collected when a conical probe was inserted into tomato tissue to a prescribed depth. Five parameters were resolved during the load decay. By this method the elastic (firmness or softness) and viscous characteristics of the fruit tissue were identified. The initial load required to insert the probe into the fruit reflects tissue elasticity, while the two time-dependent parameters, one reflecting the initial response time and the other corresponding to the point when the tissue ceases to accommodate the imposed load, reflect tissue viscosity. The technique is also applicable for the measurement of processed materials, viz. canned tomatoes and paste, when modified probes are used.

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Noboru Muramatsu, Naoki Sakurai, Ryoichi Yamamoto and Donald J. Nevins

A nondestructive, acoustic method was applied to evaluate firmness of nectarines (Prunus persica Batch.), apricots (Prunus mume Sieb. et Succ.), plums (Prunus salicina Lindl.), and tomatoes (Lycopersicon esculentum Mill. `Beiju'). Sound with frequencies from 200 to 2000 Hz, generated by a miniature speaker attached to the fruit surface, was received by a small microphone attached to the opposite side. The signal was monitored by an oscilloscope. Sound frequency did not change during propagation in the fruit. However, as the microphone was moved along the circumference of the fruit, a phase shift in the received signal was observed. When the distance the microphone was displaced along the surface of the fruit corresponded to a shift of exactly one wavelength, the sound wavelength propagated within the fruit could be determined. The number of sound waves within the fruit over half its circumference was calculated as a function of this distance. Mature fruit propagated shorter wavelengths and consequently more sound waves than immature fruit, indicating that the sound velocity in the mature fruit was lower than in immature fruit. This relatively simple method for measuring lower frequency suggests that the sound velocity propagated through fruit can be determined without measuring the absolute velocity.

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Noboru Muramatsu, Naoki Sakurai, Naoki Wada, Ryoichi Yamamoto, Keiichi Tanaka, Toshikazu Asakura, Yuko Ishikawa-Takano and Donald J. Nevins

Developmental changes in fruit texture during ripening were determined based on remote sensing of surface vibrations. The technique was evaluated with fruit having a range of firmness and textural characteristics including kiwifruit [Actinidia deliciosa (A. Chev.) Liang et Ferguson, `Hayward'] treated with ethylene, apple (Malus ×domestica Borkh. `Ourei') stored at 10 or 20 °C and persimmon (Diospyros kaki L. `Fuyu') stored at 10 °C. In each case fruit were placed on a stage capable of imparting sine wave vibrations with frequencies ranging from 5 to 2,000 Hz. The vibration transmitted through the fruit to the top surface was precisely measured without any direct contact with the Doppler laser vibrometer. The perceived fruit surface signal was corrected by subtraction of the stage vibration based on an accelerometer signal, hence the true vibrational signal of the fruit mass was determined. The phase shift at selected frequencies was based on the difference between the input and output vibration. The phase shift significantly increased in the range of 1,200 to 1,600 Hz in all three kinds of fruit analyzed as a function of maturation. The resonance frequency, peak height, and peak width of second resonance peak were also determined. The resonance frequency decreased in all fruit as a function of maturation. In apple, the peak height decreased as a function of storage duration, but in kiwifruit and persimmon the peak height fluctuated and a consistent pattern in this particular parameter was not observed. The amplitude of vibration decreased as a function of maturation when the imposed vibration exceeded 1,200 Hz. Data clearly showed that the Doppler laser vibrometer is capable of detecting the phase shift and vibration amplitude of fruit, and can be used as a versatile remote sensory tool for determining fruit firmness and for evaluations of maturity.

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Noboru Muramatsu, Keiichi Tanaka, Toshikazu Asakura, Yuko Ishikawa-Takano, Naoki Sakurai, Naoki Wada, Ryoichi Yamamoto and Donald J. Nevins

To examine the feasibility of using a laser Doppler vibrometer (LDV) for fruit quality evaluation, measurements of firmness derived by this method were compared with those acquired using a contact accelerometer. Apples (Malus pumila Miller var. Domestica Schneider `Fuji'), kiwifruit [Actinidia deliciosa (A. Chev.) Liang et Ferguson, `Hayward'], Japanese pear [Pyrus pyrifolia (Burm. f.) Nakai var. Rehd. `Nijusseiki'], and Hassaku (Citrus hassaku Hort. ex Tanaka) were used. Fruit were subjected to sine waves at frequencies from 5 to 2000 Hz at the basal surface, and the vibrations resulting from these transmissions were precisely montitored at the upper surface with a LDV monitor. Measurements on all of the tested single fruit exhibited a distinct phase shift in the applied sine wave and in the responance frequency, dependent on frequency used. These shifts were also detected by an accelerometer, but in this case the range of frequency was restricted to an upper limit of 400 Hz for kiwifruit and 800 Hz for Japanese pear and Hassaku. Efforts to extend the range using a greater vibrational mass with the accelerometer resulted in anomalous tissue behavior, most likely due to excesive compression when the weight exceeded 1 g. Hence firmness measurements of fruit depended on the phase shift and resonance frequency, which were achieved with more precision by LDV than accelerometer. Since LDV measurements of fruit firmness were made without directly contacting the fruit surface, it could be potentially used for on-line quality evaluation and fruit sorting.