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Determination of optimum ripeness for edibility of postharvest melons using nondestructive vibration
Food Research International 42 (2009) 137–141
Contents lists available at ScienceDirect
Food Research International journal homepage: www.elsevier.com/locate/foodres
Determination of optimum ripeness for edibility of postharvest melons using nondestructive vibration Mitsuru Taniwaki a,*, Masahiro Takahashi b, Naoki Sakurai b a b
Collaborative Research Center, Hiroshima University, VBL Office, 2-313, Kagamiyama, Higashi-Hiroshima 739-8527, Japan Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan
a r t i c l e
i n f o
Article history: Received 3 June 2008 Accepted 17 September 2008
Keywords: Melon Fruit ripening Postharvest quality Vibration technique Nondestructive measurement Laser Doppler vibrometer
a b s t r a c t We investigated time-course changes in the elasticity index (EI) of two melon (Cucumis melo L.) cultivars (‘‘Andes” and ‘‘Quincy”) during their postharvest period. The EI was determined using the formula EI ¼ f22 m2=3 , where f2 and m were the second resonance frequency and the mass of the sample, respectively. A nondestructive vibrational method with laser Doppler vibrometer (LDV) was used for measuring the second resonance frequency (f2) of the melon samples. The changes in the EI of both cultivars showed quasi-exponential and biphasic decays. Along with sensory tests, we determined the optimum ripeness for edibility of the melons in terms of their EI to be 4.2–6.3 104 kg2/3 Hz2 (‘‘Andes”) and 4.5– 5.6 104 kg2/3 Hz2 (‘‘Quincy”). Therefore, predetermined EI of two melon cultivars enables consumers to predict the time range of optimum ripeness. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Consumers are interested in the period of optimum ripeness for edibility of melons as they continue to ripen even after harvest. However, melons exhibit various degrees of ripeness in a market; thus, making it difficult for consumers to assess the optimum ripeness for eating. To help solve this problem, we investigated the period of optimum edibility of melons using a nondestructive method. This method is based on the fact that melons loose firmness in postharvest ripening. The predetermined period of optimum ripeness helps consumers to choose the timing for optimum edibility. Moreover, the data provides distributors with a base for monitoring the ripening process and helps determine when to ship commodities. Many researchers have developed various methods of measuring the firmness of a fruit. Mechanical methods include the measurement of: elastic deformation, velocity of sound transmission in fruit, and acoustic resonance. Takao and Ohmori (1994) developed a firmness tester that was based on measuring elastic deformation. They confirmed the use of the device for measuring the firmness of kiwifruits and melons during postharvest ripening. A similar method was used by Davie, Banks, Jeffery, and Studman (1996) for measuring deformation under constant load, which caused negligible damage to fruit. A more sophisticated method is a laser air-puff detector developed by Hung, Prussia, and Ezeike * Corresponding author. Tel./fax: +81 82 424 7889. E-mail address: [email protected] (M. Taniwaki). 0963-9969/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2008.09.007
(1999) for nondestructive measurement of fruit firmness. In this method, the firmness of peaches was determined by the amount of surface deformation made by an air-puff. The method was used to investigate the firmness of kiwifruits by McGlone, Ko, and Jordan (1999). Sugiyama, Otobe, Hayashi, and Usui (1994) studied the velocity of sound transmission in fruit and found that the velocity slowed down as the fruit ripened. Later, Sugiyama, Katsurai, Hong, Koyama, and Mikuriya (1998) developed a more practical device for evaluating the firmness of fruit based on the same principle. Muramatsu, Sakurai, Yamamoto, et al. (1997) also used the velocity of sound transmission to measure the firmness in kiwifruit. Yamamoto and Haginuma (1982), and Yamamoto, Iwamoto, and Haginuma (1981) used an acoustic impulse response resonance method to evaluate the firmness of fruit. Muramatsu, Sakurai, Wada, et al. (1997) showed that a method involving the use of laser Doppler vibrometer (LDV) was advantageous for determining the firmness of fruit. This method has been applied to monitoring the ripeness of kiwifruits (Terasaki, Sakurai, Yamamoto, Wada, & Nevins, 2001; Terasaki, Wada, et al., 2001) and pears (Terasaki et al., 2006). More practical devices have been developed for evaluating the ripeness of melons. For instance, the elasticity index of melon fruit being grown in a greenhouse was measured using a portable device based on an acoustical vibration technique (Kuroki, Tohro, & Sakurai, 2006). Therefore, in the present study, we measured time-course changes in the elasticity index (EI) and sensory test index of melons at the postharvest stage. Using the correlation between the EI and the sensory test index, we determined the period of
138
M. Taniwaki et al. / Food Research International 42 (2009) 137–141
optimum ripeness for edibility of melons, which serves as an excellent indicator for optimum quality for eating.
1300
2. Materials and methods
1200
1100
1000
900
Mass (g)
We used two cultivars of melon (Cucumis melo L.), ‘‘Andes” and ‘‘Quincy”, for our investigations. The samples were grown and harvested in Kumamoto Prefecture, Japan. Twenty samples of each cultivar were used and stored at room temperature (ca. 20 °C and ca. 50% RH) throughout the measurements. The sensory test was performed by two experts (both male). Each panelist graded the samples for hardness, sweetness, fibrousness, thickness, fragrance, appearance, and overall acceptability. The samples were rated on a scale of 1–5 (1: overripe, 3: ripe, and 5: immature) every day or every other day for a total period of 10 days. The EI of each sample was determined nondestructively every day or every other day immediately before the sensory test, using a previously reported vibrational method ( Muramatsu, Sakurai, Wada, et al., 1997). The experimental setup is shown in Fig. 1a. A sample with a reflective film was set on an electrodynamic shaker
a
800
b 1200
1100
1000
900
800
2
0
4
6
8
Days Fig. 2. Time-course changes in the mass of the melon samples: (a) ‘‘Andes” and (b) ‘‘Quincy”.
Elasticity index (kg2/3·Hz2 ×10 4)
8.0
7.0 20 17 14
5.0
11
8
4.0
5
Quincy
3.0
2.0 Fig. 1. (a) Experimental setup for the nondestructive measurement of the EI of the melon samples. The sample was mechanically excited by a shaker that was driven by swept sine wave signals. The response at the opposite side of excitation was sensed by a laser Doppler vibrometer (LDV). (b) A typical response spectrum of an ‘‘Andes” melon sample; f2: the second resonance peak that was used for determining the EI.
Andes
6.0
0
2
2
4
6
8
10
Days Fig. 3. The time-course changes in the averaged EI of ‘‘Andes” and ‘‘Quincy” melons determined by the method described in Fig. 1. The bars represent the SE. The numbers represent the number of samples used for each measurement.
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M. Taniwaki et al. / Food Research International 42 (2009) 137–141
(513-B, EMIC Co., Tokyo, Japan), and the sample was excited using swept sine wave signals (frequency, 0–1 kHz) for 10 s generated by a PC. The vibrational response of the sample was sensed by a laser Doppler vibrometer (LDV; LV-1720, Ono Sokki Co. Ltd., Yokohama, Japan). The vibration of the shaker was simultaneously monitored using an acceleration pickup (NP-3211, Ono Sokki Co. Ltd., Yokohama, Japan). Both signals from the LDV and the accelerometer were transmitted to the PC through a signal separator (D2VOX, IO DATA Device Inc., Kanazawa, Japan). A fast Fourier
a
transform (FFT) algorithm (Spectra Pro, Sound Technology, Campbell, USA) was applied to the ratio of the response signals (Xsample) to the excitation signals (Xinput) in order to obtain a vibrational spectrum of the sample. A typical vibrational spectrum of an ‘‘Andes” sample is presented in Fig. 1b. EI was determined according to the formula EI ¼ f22 m2=3 (Cooke, 1972) using the second resonance of the vibrational spectrum, i.e., f2 (indicated by an arrow in Fig. 1b), and the mass of the sample m. Using the second resonance f2 was validated by the work of Yamamoto and
5 4.5
Hardness
Sweetness
r = 0.855
r = 0.813
Thickness
Fragrance
r = 0.897
r = 0.858
Fibrousness r = 0.886
4 3.5 3
Sensory test index [1-5]
2.5 2 1.5 1 4.5
Appearance r = 0.935
4 3.5 3 2.5 2 1.5 1
b
2
4
6
8
10
2
4
6
8
10 2
4
6
8
10
5 4.5
Hardness
Sweetness
r = 0.833
r = 0.846
Thickness
Fragrance
r = 0.819
r = 0.886
Fibrousness r = 0.810
4 3.5 3
Sensory test index [1-5]
2.5 2 1.5 1 4.5
Appearance r = 0.888
4 3.5 3 2.5 2 1.5 1
2
3
4
5
6
7
2
3
4
5
6
7
2
3
4
5
6
7
8
Elasticity index (×10 4 kg2/3·Hz2) Fig. 4. The correlations between the sensory test index of various attributes and the EI: (a) ‘‘Andes” and (b) ‘‘Quincy” melons (n = 20, P < 0.01).
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M. Taniwaki et al. / Food Research International 42 (2009) 137–141
Haginuma (1984) proposing that the same formula, EI ¼ f22 m2=3 , was obtained assuming that the second resonance was the 0S2 mode (spherical type vibration; oblate–prolate mixed mode). The second resonance was later confirmed experimentally to be the 0S2 mode by Terasaki, Sakurai, Wada, et al. (2001). 3. Results Fig. 2 shows the time-course changes in the mass of ‘‘Andes” (a) and ‘‘Quincy” (b) samples. Each sample showed a linear decline in mass. Termination of the data in the period of measurement meant that the samples were being used for sensory test at the time. Fig. 3 shows the time-course changes in the averaged EI of the melon samples. The pattern of decline consists of two stages: the first two days and thereafter (‘‘Andes”), the first five days and thereafter (‘‘Quincy”). However, the overall decline pattern was quasi-exponential (r = 0.998) for both cultivars. Fig. 4 shows the correlations between the sensory indices with respect to six attributes (hardness, sweetness, fibrousness, thickness, fragrance, and appearance) and the EI. High correlations (significant at 1% level) were observed between the sensory indices and the respective EIs of both ‘‘Andes” and ‘‘Quincy” melons.
4. Discussion We investigated the time-course changes in the EI of two melon cultivars. The investigation employed a previously developed nondestructive method with an LDV and a shaker ( Muramatsu, Sakurai, Wada, et al. (1997)). The method used swept sine wave signals to excite a melon sample. The swept sine waves method is advantageous for accurate determination of the resonance of a sample as compared to the impulse method (Yamamoto et al., 1981) because the former enables the excitation energy to be concentrated within a small frequency band at a particular time. In contrast, in the latter method, the excitation energy spreads over a wide range of frequency in a limited time period. The pattern of decline of the EI appeared to consist of two stages (Fig. 3). Similar patterns have been observed for pears (Murayama, Konno, Terasaki, Yamamoto, & Sakurai, 2006; Terasaki et al., 2006) and kiwifruits ( Terasaki, Sakurai, Yamamoto, et al., 2001). Taniwaki, Hanada, and Sakurai (in press) measured the timecourse changes of EI of persimmons. There are some differences among various types of fruit such as skin thickness: skins of kiwifruits and pears were thinner than those of melons. Despite of these differences, above-mentioned studies showed that the
12.0
a
10.0
8.0
Elasticity index (×104 kg2/3·Hz2)
6.0
4.0
2.0
b 7.0
6.0
5.0
4.0
3.0
2.0 Fig. 5. The correlations between the sensory test index of overall acceptability and the EI: (a) ‘‘Andes” and (b) ‘‘Quincy” melons. The bars represent the SE. (n = 20, P < 0.01). Dotted lines are for determining the EI which corresponds to the period of optimum ripeness.
0
1
2
3
4
5
6
7
8
9
Days Fig. 6. The changes in EI of individual melon samples: (a) ‘‘Andes” and (b) ‘‘Quincy”. The shaded areas are the determined periods of optimum ripeness.
M. Taniwaki et al. / Food Research International 42 (2009) 137–141
1250
showed overall pattern of EI change, the optimum edibility should be determined using a set of individual data (Fig. 5). Determined optimum edibility in terms of EI enables prediction of the time required for a melon to reach optimum edibility. Such information regarding the predicted time or date of optimum ripeness may gain consumer acceptance. The EI of melon samples on day 0 was independent of their mass (Fig. 7), suggesting that fruit size of the samples was independent of maturity and larger melons did not always show higher degree of maturity than smaller ones. We employed a previously developed nondestructive method using a vibrational technique together with the sensory test to investigate the changes in the degree of ripeness of melons after harvest. This method enabled the determination of the period of optimum ripeness of melons in terms of EI. Thus, consumers and distributors will find our nondestructive method useful for predicting the optimum ripeness for edibility of melons.
a
1200
1150
1100
1050
1000
950
Mass (g)
r = 0.15 900 1250
141
References
5
6
7
8
9
10
11
b
1200
1150
1100
1050
1000 r = 0.31 950 4.5
5
5.5
6
6 .5
7
7.5
Elasticity index Fig. 7. The correlation between the EI of melon samples at day 0 and their mass: (a) ‘‘Andes” and (b) ‘‘Quincy”.
nondestructive method of measuring the firmness of fruit is applicable to various types of fruit. Fig. 4 shows that the EI highly correlated with the sensory test indices for both cultivars. Therefore, the results can be used to determine the period of optimum ripeness for edibility, which is an excellent indicator of optimum quality for consumers. If this period is defined in terms of the sensory test index of overall acceptability, which lies between 2.5–3.5 the corresponding EI can be derived as shown in Fig. 5. The EI for optimum ripeness was calculated as 4.2 – 6.3 104 kg2/3 Hz2 for ‘‘Andes” and 4.5 – 5.6 104 kg2/3 Hz2 for ‘‘Quincy”. These values overlapped well but ‘‘Andes” showed a slightly wider range of EI for the period of optimum ripeness for eating quality. Fig. 6 shows the variation of the samples’ EI. The EI of both ‘‘Andes” and ‘‘Quincy” melons was significantly scattered on day 0. Some samples were already in the period of optimum ripeness, while others were immature. These data imply that melons of different degrees of ripeness occur simultaneously in the market. Although the time-course changes in the averaged EI (Fig. 3)
Cooke, J. R. (1972). An interpretation of the resonant behavior of intact fruits and vegetables. Transactions of the ASAE, 15, 1075–1080. Davie, I. J., Banks, N. H., Jeffery, P. B., & Studman, C. J. (1996). Non-destructive measurement of kiwifruit firmness. New Zealand Journal of Crop and Horticultural Science, 24, 151–157. Hung, Y. C., Prussia, S. E., & Ezeike, G. O. I. (1999). Nondestructive firmness sensing using a laser air-puff detector. Postharvest Biology and Technology, 16, 15–25. Kuroki, S., Tohro, M., & Sakurai, N. (2006). Monitoring of the elasticity index of melon fruit in a greenhouse. Journal of the Japanese Society for Horticultural Science, 75, 415–420. McGlone, V. A., Ko, S. M. W., & Jordan, R. B. (1999). Non-contact fruit firmness measurement by the laser air-puff method. Transactions of the ASAE, 42, 1391–1397. Muramatsu, N., Sakurai, N., Wada, N., Yamamoto, R., Tanaka, K., Asakura, T., et al. (1997). Critical comparison of an accelerometer and a laser Doppler vibrometer for measuring fruit firmness. HortTechnology, 7, 434–438. Muramatsu, N., Sakurai, N., Yamamoto, R., Nevins, D. J., Takahara, T., & Ogata, T. (1997). Comparison of a non-destructive acoustic method with an intrusive method for firmness measurement of kiwifruit. Postharvest Biology and Technology, 12, 221–228. Murayama, H., Konno, I., Terasaki, S., Yamamoto, R., & Sakurai, N. (2006). Nondestructive method for measuring fruit ripening of ‘La France’ pears using a laser Doppler vibrometer. Journal of the Japanese Society for Horticultural Science, 75, 79–84. Sugiyama, J., Katsurai, T., Hong, J., Koyama, H., & Mikuriya, K. (1998). Melon ripeness monitoring by a portable firmness tester. Transactions of the ASAE, 41, 121–127. Sugiyama, J., Otobe, K., Hayashi, S., & Usui, S. (1994). Firmness measurement of muskmelons by acoustic impulse transmission. Transactions of the ASAE, 37, 1235–1241. Takao, H., & Ohmori, S. (1994). Development of device for nondestructive evaluation of fruit firmness. Japan Agricultural Research Quarterly, 28, 36–43. Taniwaki, M., Hanada, T., & Sakurai, N. (in press). Postharvest quality evaluation of ‘‘Fuyu” and ‘‘Taishuu” persimmons using a nondestructive vibrational method and an acoustic vibration technique. Postharvest Biology and Technology. Terasaki, S., Sakurai, N., Wada, N., Yamanishi, T., Yamamoto, R., & Nevins, D. J. (2001). Analysis of the vibration mode of apple tissue using electronic speckle pattern interferometry. Transactions of the ASAE, 44, 1697–1705. Terasaki, S., Sakurai, N., Yamamoto, R., Wada, N., & Nevins, D. J. (2001). Changes in cell wall polysaccharides of kiwifruit and the visco-elastic properties detected by a laser Doppler method. Journal of the Japanese Society for Horticultural Science, 70, 572–580. Terasaki, S., Sakurai, N., Zebrowski, J., Murayama, H., Yamamoto, R., & Nevins, D. J. (2006). Laser Doppler vibrometer analysis of changes in elastic properties of ripening ‘La France’ pears after postharvest storage. Postharvest Biology and Technology, 42, 198–207. Terasaki, S., Wada, N., Sakurai, N., Muramatsu, N., Yamamoto, R., & Nevins, D. J. (2001). Nondestructive measurement of kiwifruit ripeness using a laser Doppler vibrometer. Transactions of the ASAE, 44, 81–87. Yamamoto, H., & Haginuma, S. (1982). Vibrating reed method and non-destructive acoustic impulse response method for measuring textural quality of apple flesh. Journal of the Japanese Society for Horticultural Science, 51, 210–218. Yamamoto, H., & Haginuma, S. (1984). Estimation of the dynamic Young’s modulus of apple flesh from the natural frequency of an intact apple. Report of the Food Research Institute, 44, 20–25. Yamamoto, H., Iwamoto, M., & Haginuma, S. (1981). Nondestructive acoustic impulse response method for measuring internal quality of apples and watermelons. Journal of the Japanese Society for Horticultural Science, 50, 247–261.
Contents lists available at ScienceDirect
Food Research International journal homepage: www.elsevier.com/locate/foodres
Determination of optimum ripeness for edibility of postharvest melons using nondestructive vibration Mitsuru Taniwaki a,*, Masahiro Takahashi b, Naoki Sakurai b a b
Collaborative Research Center, Hiroshima University, VBL Office, 2-313, Kagamiyama, Higashi-Hiroshima 739-8527, Japan Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan
a r t i c l e
i n f o
Article history: Received 3 June 2008 Accepted 17 September 2008
Keywords: Melon Fruit ripening Postharvest quality Vibration technique Nondestructive measurement Laser Doppler vibrometer
a b s t r a c t We investigated time-course changes in the elasticity index (EI) of two melon (Cucumis melo L.) cultivars (‘‘Andes” and ‘‘Quincy”) during their postharvest period. The EI was determined using the formula EI ¼ f22 m2=3 , where f2 and m were the second resonance frequency and the mass of the sample, respectively. A nondestructive vibrational method with laser Doppler vibrometer (LDV) was used for measuring the second resonance frequency (f2) of the melon samples. The changes in the EI of both cultivars showed quasi-exponential and biphasic decays. Along with sensory tests, we determined the optimum ripeness for edibility of the melons in terms of their EI to be 4.2–6.3 104 kg2/3 Hz2 (‘‘Andes”) and 4.5– 5.6 104 kg2/3 Hz2 (‘‘Quincy”). Therefore, predetermined EI of two melon cultivars enables consumers to predict the time range of optimum ripeness. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Consumers are interested in the period of optimum ripeness for edibility of melons as they continue to ripen even after harvest. However, melons exhibit various degrees of ripeness in a market; thus, making it difficult for consumers to assess the optimum ripeness for eating. To help solve this problem, we investigated the period of optimum edibility of melons using a nondestructive method. This method is based on the fact that melons loose firmness in postharvest ripening. The predetermined period of optimum ripeness helps consumers to choose the timing for optimum edibility. Moreover, the data provides distributors with a base for monitoring the ripening process and helps determine when to ship commodities. Many researchers have developed various methods of measuring the firmness of a fruit. Mechanical methods include the measurement of: elastic deformation, velocity of sound transmission in fruit, and acoustic resonance. Takao and Ohmori (1994) developed a firmness tester that was based on measuring elastic deformation. They confirmed the use of the device for measuring the firmness of kiwifruits and melons during postharvest ripening. A similar method was used by Davie, Banks, Jeffery, and Studman (1996) for measuring deformation under constant load, which caused negligible damage to fruit. A more sophisticated method is a laser air-puff detector developed by Hung, Prussia, and Ezeike * Corresponding author. Tel./fax: +81 82 424 7889. E-mail address: [email protected] (M. Taniwaki). 0963-9969/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2008.09.007
(1999) for nondestructive measurement of fruit firmness. In this method, the firmness of peaches was determined by the amount of surface deformation made by an air-puff. The method was used to investigate the firmness of kiwifruits by McGlone, Ko, and Jordan (1999). Sugiyama, Otobe, Hayashi, and Usui (1994) studied the velocity of sound transmission in fruit and found that the velocity slowed down as the fruit ripened. Later, Sugiyama, Katsurai, Hong, Koyama, and Mikuriya (1998) developed a more practical device for evaluating the firmness of fruit based on the same principle. Muramatsu, Sakurai, Yamamoto, et al. (1997) also used the velocity of sound transmission to measure the firmness in kiwifruit. Yamamoto and Haginuma (1982), and Yamamoto, Iwamoto, and Haginuma (1981) used an acoustic impulse response resonance method to evaluate the firmness of fruit. Muramatsu, Sakurai, Wada, et al. (1997) showed that a method involving the use of laser Doppler vibrometer (LDV) was advantageous for determining the firmness of fruit. This method has been applied to monitoring the ripeness of kiwifruits (Terasaki, Sakurai, Yamamoto, Wada, & Nevins, 2001; Terasaki, Wada, et al., 2001) and pears (Terasaki et al., 2006). More practical devices have been developed for evaluating the ripeness of melons. For instance, the elasticity index of melon fruit being grown in a greenhouse was measured using a portable device based on an acoustical vibration technique (Kuroki, Tohro, & Sakurai, 2006). Therefore, in the present study, we measured time-course changes in the elasticity index (EI) and sensory test index of melons at the postharvest stage. Using the correlation between the EI and the sensory test index, we determined the period of
138
M. Taniwaki et al. / Food Research International 42 (2009) 137–141
optimum ripeness for edibility of melons, which serves as an excellent indicator for optimum quality for eating.
1300
2. Materials and methods
1200
1100
1000
900
Mass (g)
We used two cultivars of melon (Cucumis melo L.), ‘‘Andes” and ‘‘Quincy”, for our investigations. The samples were grown and harvested in Kumamoto Prefecture, Japan. Twenty samples of each cultivar were used and stored at room temperature (ca. 20 °C and ca. 50% RH) throughout the measurements. The sensory test was performed by two experts (both male). Each panelist graded the samples for hardness, sweetness, fibrousness, thickness, fragrance, appearance, and overall acceptability. The samples were rated on a scale of 1–5 (1: overripe, 3: ripe, and 5: immature) every day or every other day for a total period of 10 days. The EI of each sample was determined nondestructively every day or every other day immediately before the sensory test, using a previously reported vibrational method ( Muramatsu, Sakurai, Wada, et al., 1997). The experimental setup is shown in Fig. 1a. A sample with a reflective film was set on an electrodynamic shaker
a
800
b 1200
1100
1000
900
800
2
0
4
6
8
Days Fig. 2. Time-course changes in the mass of the melon samples: (a) ‘‘Andes” and (b) ‘‘Quincy”.
Elasticity index (kg2/3·Hz2 ×10 4)
8.0
7.0 20 17 14
5.0
11
8
4.0
5
Quincy
3.0
2.0 Fig. 1. (a) Experimental setup for the nondestructive measurement of the EI of the melon samples. The sample was mechanically excited by a shaker that was driven by swept sine wave signals. The response at the opposite side of excitation was sensed by a laser Doppler vibrometer (LDV). (b) A typical response spectrum of an ‘‘Andes” melon sample; f2: the second resonance peak that was used for determining the EI.
Andes
6.0
0
2
2
4
6
8
10
Days Fig. 3. The time-course changes in the averaged EI of ‘‘Andes” and ‘‘Quincy” melons determined by the method described in Fig. 1. The bars represent the SE. The numbers represent the number of samples used for each measurement.
139
M. Taniwaki et al. / Food Research International 42 (2009) 137–141
(513-B, EMIC Co., Tokyo, Japan), and the sample was excited using swept sine wave signals (frequency, 0–1 kHz) for 10 s generated by a PC. The vibrational response of the sample was sensed by a laser Doppler vibrometer (LDV; LV-1720, Ono Sokki Co. Ltd., Yokohama, Japan). The vibration of the shaker was simultaneously monitored using an acceleration pickup (NP-3211, Ono Sokki Co. Ltd., Yokohama, Japan). Both signals from the LDV and the accelerometer were transmitted to the PC through a signal separator (D2VOX, IO DATA Device Inc., Kanazawa, Japan). A fast Fourier
a
transform (FFT) algorithm (Spectra Pro, Sound Technology, Campbell, USA) was applied to the ratio of the response signals (Xsample) to the excitation signals (Xinput) in order to obtain a vibrational spectrum of the sample. A typical vibrational spectrum of an ‘‘Andes” sample is presented in Fig. 1b. EI was determined according to the formula EI ¼ f22 m2=3 (Cooke, 1972) using the second resonance of the vibrational spectrum, i.e., f2 (indicated by an arrow in Fig. 1b), and the mass of the sample m. Using the second resonance f2 was validated by the work of Yamamoto and
5 4.5
Hardness
Sweetness
r = 0.855
r = 0.813
Thickness
Fragrance
r = 0.897
r = 0.858
Fibrousness r = 0.886
4 3.5 3
Sensory test index [1-5]
2.5 2 1.5 1 4.5
Appearance r = 0.935
4 3.5 3 2.5 2 1.5 1
b
2
4
6
8
10
2
4
6
8
10 2
4
6
8
10
5 4.5
Hardness
Sweetness
r = 0.833
r = 0.846
Thickness
Fragrance
r = 0.819
r = 0.886
Fibrousness r = 0.810
4 3.5 3
Sensory test index [1-5]
2.5 2 1.5 1 4.5
Appearance r = 0.888
4 3.5 3 2.5 2 1.5 1
2
3
4
5
6
7
2
3
4
5
6
7
2
3
4
5
6
7
8
Elasticity index (×10 4 kg2/3·Hz2) Fig. 4. The correlations between the sensory test index of various attributes and the EI: (a) ‘‘Andes” and (b) ‘‘Quincy” melons (n = 20, P < 0.01).
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Haginuma (1984) proposing that the same formula, EI ¼ f22 m2=3 , was obtained assuming that the second resonance was the 0S2 mode (spherical type vibration; oblate–prolate mixed mode). The second resonance was later confirmed experimentally to be the 0S2 mode by Terasaki, Sakurai, Wada, et al. (2001). 3. Results Fig. 2 shows the time-course changes in the mass of ‘‘Andes” (a) and ‘‘Quincy” (b) samples. Each sample showed a linear decline in mass. Termination of the data in the period of measurement meant that the samples were being used for sensory test at the time. Fig. 3 shows the time-course changes in the averaged EI of the melon samples. The pattern of decline consists of two stages: the first two days and thereafter (‘‘Andes”), the first five days and thereafter (‘‘Quincy”). However, the overall decline pattern was quasi-exponential (r = 0.998) for both cultivars. Fig. 4 shows the correlations between the sensory indices with respect to six attributes (hardness, sweetness, fibrousness, thickness, fragrance, and appearance) and the EI. High correlations (significant at 1% level) were observed between the sensory indices and the respective EIs of both ‘‘Andes” and ‘‘Quincy” melons.
4. Discussion We investigated the time-course changes in the EI of two melon cultivars. The investigation employed a previously developed nondestructive method with an LDV and a shaker ( Muramatsu, Sakurai, Wada, et al. (1997)). The method used swept sine wave signals to excite a melon sample. The swept sine waves method is advantageous for accurate determination of the resonance of a sample as compared to the impulse method (Yamamoto et al., 1981) because the former enables the excitation energy to be concentrated within a small frequency band at a particular time. In contrast, in the latter method, the excitation energy spreads over a wide range of frequency in a limited time period. The pattern of decline of the EI appeared to consist of two stages (Fig. 3). Similar patterns have been observed for pears (Murayama, Konno, Terasaki, Yamamoto, & Sakurai, 2006; Terasaki et al., 2006) and kiwifruits ( Terasaki, Sakurai, Yamamoto, et al., 2001). Taniwaki, Hanada, and Sakurai (in press) measured the timecourse changes of EI of persimmons. There are some differences among various types of fruit such as skin thickness: skins of kiwifruits and pears were thinner than those of melons. Despite of these differences, above-mentioned studies showed that the
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2.0 Fig. 5. The correlations between the sensory test index of overall acceptability and the EI: (a) ‘‘Andes” and (b) ‘‘Quincy” melons. The bars represent the SE. (n = 20, P < 0.01). Dotted lines are for determining the EI which corresponds to the period of optimum ripeness.
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showed overall pattern of EI change, the optimum edibility should be determined using a set of individual data (Fig. 5). Determined optimum edibility in terms of EI enables prediction of the time required for a melon to reach optimum edibility. Such information regarding the predicted time or date of optimum ripeness may gain consumer acceptance. The EI of melon samples on day 0 was independent of their mass (Fig. 7), suggesting that fruit size of the samples was independent of maturity and larger melons did not always show higher degree of maturity than smaller ones. We employed a previously developed nondestructive method using a vibrational technique together with the sensory test to investigate the changes in the degree of ripeness of melons after harvest. This method enabled the determination of the period of optimum ripeness of melons in terms of EI. Thus, consumers and distributors will find our nondestructive method useful for predicting the optimum ripeness for edibility of melons.
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References
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nondestructive method of measuring the firmness of fruit is applicable to various types of fruit. Fig. 4 shows that the EI highly correlated with the sensory test indices for both cultivars. Therefore, the results can be used to determine the period of optimum ripeness for edibility, which is an excellent indicator of optimum quality for consumers. If this period is defined in terms of the sensory test index of overall acceptability, which lies between 2.5–3.5 the corresponding EI can be derived as shown in Fig. 5. The EI for optimum ripeness was calculated as 4.2 – 6.3 104 kg2/3 Hz2 for ‘‘Andes” and 4.5 – 5.6 104 kg2/3 Hz2 for ‘‘Quincy”. These values overlapped well but ‘‘Andes” showed a slightly wider range of EI for the period of optimum ripeness for eating quality. Fig. 6 shows the variation of the samples’ EI. The EI of both ‘‘Andes” and ‘‘Quincy” melons was significantly scattered on day 0. Some samples were already in the period of optimum ripeness, while others were immature. These data imply that melons of different degrees of ripeness occur simultaneously in the market. Although the time-course changes in the averaged EI (Fig. 3)
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