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Nondestructive dynamic testing of apples for firmness evaluation
Postharvest Biology and Technology 29 (2003) 287 /299 www.elsevier.com/locate/postharvbio
Nondestructive dynamic testing of apples for firmness evaluation I. Shmulevich a,*, N. Galili a, M.S. Howarth b a
Faculty of Agricultural Engineering, Technion / Israel Institute of Technology, Haifa, Israel b Sinclair International Ltd, Jarrold Way, Bowthorpe, Norwich, Norfolk NR5 9JD, UK Received 14 May 2002; accepted 24 February 2003
Abstract Two nondestructive dynamic test methods, low-mass impact and acoustic response, were tested and compared with destructive compression and penetration tests to evaluate apple firmness. The purpose of the study was to analyze the performance of the impact test methods for nondestructive firmness evaluation, and to assess whether the acoustic tests could add sorting capacity to low-mass impact testing in apples. A laboratory-instrumented impact hammer and a lowmass impact firmness sensor produced by Sinclair International (SIQ-FT) were used to perform the impact tests, and a piezoelectric-film transducer was applied for the acoustic tests. A new Sinclair internal quality index IQ and two conventional impact parameters C1 and C2 were extracted from the impact signals, and a firmness index FI was calculated from the acoustic signal of the samples. The nondestructive tests were followed by parallel-plate compression tests and Magness /Taylor penetration tests. Three apple varieties, ‘Golden Delicious’, ‘Starking Delicious’ and ‘Granny Smith’, were tested by the impact and acoustic methods. The performance of the experimental systems and the test methods were evaluated first by rubber ball calibration tests. The elastic modulus of all apples was well predicted by the acoustic firmness parameter FI (R /0.91). The impact parameter IQ was equivalent to FI in predicting the elastic modulus of ‘Golden Delicious’, and less accurate in ‘Starking Delicious’ and ‘Granny Smith’. The conventional indices C1 and C2 performed similarly to the FI and IQ in the rubber balls and the ‘Golden Delicious’ tests, but failed to predict the elastic modulus of ‘Starking Delicious’ or ‘Granny Smith’ apples, probably because of their less uniform shape, and the sensitivity of these empirical parameters to test conditions. The changes of the penetration force during the test period were very low, and their correlation with the elastic modulus and the firmness parameters of all apples was poor. These previously reported findings might indicate that the Magness /Taylor penetration test is not an appropriate measure of apple firmness during shelf life. The test results indicated that the new SIQ-FT device and the IQ parameter offer a new technology that may overcome some basic difficulties associated with the low-mass impact test methods, which have been reported in the literature. The results also indicated that the acoustic method might add sorting capacity to the impact method in some apple varieties. Further R&D efforts are needed, however, to verify these results and adapt the acoustic method for on-line operation. # 2003 Elsevier B.V. All rights reserved. Keywords: Nondestructive; Impact; Acoustic; Firmness; Apple; Quality
* Corresponding author. Tel.: /972-4-8292626; fax: /972-4-8221529. E-mail address: [email protected] (I. Shmulevich). 0925-5214/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-5214(03)00039-5
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1. Introduction Fruits and vegetables in today’s modern market must be of high quality standards, especially when proposed for export. Fruit texture and firmness are among the several qualitative terms to describe internal fruit quality. Hence, fast sorting of each individual fruit and vegetable according to firmness is very important. Low-mass impact analysis in the time domain is a simple and quick method for determining local fruit properties. The method has been successfully applied for on-line sorting operation of various fruits, but not yet in apples. Analysis of the acoustic fruit response to mechanical impulse in the frequency domain detects internal properties of the whole fruit, including firmness. The acoustic test method has been applied successfully in laboratory conditions, but still faces difficulties in implementation due to technical limitations. The motivation of the present work is to investigate the performance of the two dynamic test methods for firmness evaluation in apples. The two techniques and their basic principles are explained below. 1.1. Quality detection by impact force Numerous researchers have evaluated fruit firmness using impact techniques. Various shape characteristics of impact force and acceleration signals were developed as firmness predictors in the time and frequency domains. The most frequently used empirical impact parameters for firmness evaluation are C1 and C2 (Rohrbach et al., 1982; Delwiche et al., 1987): P C1 max (1) T and C2
Pmax T2
(2)
where Pmax is the peak amplitude of the impact pulse and T, an impact characteristic time, such as Tp, time to peak amplitude, Tc, pulse duration, or Tm, width of the impact at half of the peak amplitude. These impact parameters and addi-
tional characteristic values of the impact signal in time or frequency domain were used to evaluate the firmness of various fruits, as described below. 1.1.1. Dropping the fruit on a flat force transducer One technique for firmness evaluation of fruits and vegetables has been to drop the product on a force transducer. De Baerdemaeker et al. (1982) measured the impact force of apples and evaluated several characteristics of the force signal in the frequency domain as firmness indicators. Rohrbach et al. (1982) studied the firmness of berries by measuring their response when dropped on a rigid surface. Various shape characteristics were developed as firmness predictors and compared to parameters extracted from the compression test by a parallel-plate. The correlations between berry firmness and its ‘peak impact force parameter’ C2 was low (R /0.52). Yet, it was concluded that C2 could be a rapid means for determining fruit firmness. Delwiche (1987) studied a theoretical model of homogeneous elastic spheres to analyze the effect of the model parameters and impact velocity on its impact characteristics. The investigation showed that both the time /domain parameters (C1 and C2) and frequency /domain characteristics (F250 / F340) of the simulated impact force are strongly dependent on the elastic modulus E of the sphere, and also on the impact contact velocity that should remain constant in any practical implementation of this method. Variation in fruit size (mass or dimensions) had a moderate effect on the frequency indices and very little on C2. These findings were applied by Delwiche et al. (1987) to measure the impact response of peaches striking a rigid surface. The highest correlation with elastic modulus and penetration readings of flesh firmness was that of C2 and F295. Delwiche et al. (1989) reported implementation of the impact technique in a sorting line of peaches and pears, at a rate of 5 fruits s 1. In their report, both indices C1 and C2 correlated well (0.86 and 0.84, respectively) with the destructive indices. The results were slightly better with elastic modulus as the texture index rather than penetration. Molto et al. (1998) tested apples, peaches and pears, looking for a reference for firmness evaluation. Fruit were
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dropped from a given height on a load cell. Pmax, Tp and Tc, C1 and C2, and some additional impact characteristics were compared with compression and penetration firmness tests, sugar content, and expert classification. Low correlations were found between the impact parameters and the destructive tests. The distractive MT measurements did not offer good correlation either with sugar or with expert classification of apples. 1.1.2. Tapping the fruit with a firmness sensing element Another impact technique was to tap the fruit with a medium or small impact device. Delwiche and Sarig (1991) developed a firmness sensor of 63 g to impact the fruit. The acceleration of the mass gave a measure of the impact force that was analyzed to obtain the impact parameters C1 and C2. These parameters were normalized by the dropping height h . The correlation obtained between C2, C2/h and a standard compression test was higher for peaches (R /0.80), lower for pears (R /0.68) and very low for ‘Red Delicious’ apples (R /0.53). Similar results were obtained by Ruiz-Altisent et al. (1993) for pears, avocados and apples. Chen and Tjan (1996, 1998) and Chen et al. (1996) introduced a new mechanical system for low-mass impact based on a swing-arm sensor. They reported good performance of the system when testing rubber balls, kiwifruit and peaches. Preliminary tests showed that the sensor could sense fruit firmness at a speed of 5 fruit s 1. OrtizCanavate et al. (2001) adapted a modified version of the low-mass impact method for an experimental fruit packing line with an operation speed of 5/ 7 fruit s 1. ‘Golden Delicious’ apples and several peach varieties were tested dynamically by the system and by destructive compression and MT penetration tests. Several impact characteristics were compared with the destructive test results. The correlation coefficients between the impact parameter C1, when compared with force/deformation slope and MT penetration force, was quite high in peaches (0.93 and 0.87, respectively) but much lower in ‘Golden Delicious’ apples (0.74 and 0.43, respectively). The authors reported that values obtained by the impact tests were very
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sensitive to variations in fruit form, location and impact angle. This work was repeated and extended by Homer et al. (2002), with similar results. Medium to high correlation coefficients were obtained between the impact parameters Pmax, C1 etc. and the destructive tests of nectarines and peaches; much lower correlation coefficients of 0.55 /0.64 were obtained for ‘Starking Delicious’ apples. It was observed that better performance was achieved for softer fruits. The research reported a series of difficulties associated with the swing-arm sensor and its high sensitivity to variations in fruit position and orientation in the conveying system. Recently, Sinclair International Ltd has developed the Sinclair IQTM-firmness tester (SIQ-FT) that is based on a low-mass impact sensor (Howarth, 2002). This on-line system measures firmness using a sensing element on the tip of a bellow. SIQ-FT takes advantage of Sinclair’s patented bellows delivery system, which is also used in fruit labeling and can be simply adapted to existing sorting lines. Medium to high correlation coefficients were obtained with penetration tests for nectarines (0.85/0.95), plums (0.80), avocados (0.81 /0.84) and kiwifruit (0.83 /0.92). The SIQ-FT on-line system currently operates at speeds up to 10 fruit s 1 and makes four independent measurements that are combined to estimate the fruit firmness. These measurements are made on 4 different quadrants around the fruit surface and can be combined as an average, median, etc. as defined by the user. According to the cited literature the impact techniques have reached good results in the firmness evaluation of peaches, pears and some tropical fruits, but not in apples. Hence, it is important to evaluate the performance of the new SIQ-FT device and the Sinclair IQ parameter for the firmness evaluation of apples. 1.2. Quality detection by acoustic techniques Acoustic techniques are an alternative method for evaluating fruit firmness (Shmulevich, 1998). The acoustic response, sensed by a microphone, an accelerometer or a flexible piezoelectric sensor, was analyzed using a FFT algorithm to extract the
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fruit’s resonant frequencies. Many researchers have investigated acoustic techniques using this approach (Abbott et al., 1968; Chen et al., 1992; Galili and De Baerdemaeker, 1996; Shmulevich et al., 1996; Galili et al., 1998; Shmulevich, 1998). This research reported good correlation between the acoustic fruit response and the firmness or softening of several fruits such as mango, melon, avocado, apple and tomato. Spherical fruits produced better results due to the fact that spherical modal shapes correlate to firmness. The limitations of this method for non-spherical fruit, such as avocado or mango, could be minimized by proper fruit orientation. Although good laboratory results were reported, there are still technical difficulties in the implementation of the method to on-line sorting. The general objective of the present research was to evaluate the performances of the low-mass impact and acoustic test methods for nondestructive firmness assessment of apples. The specific objectives were to: . evaluate the performance of the experimental systems and the various firmness algorithms by well defined rubber-ball calibration tests, . evaluate the performance of the nondestructive dynamic methods and the destructive tests in apples, . compare the new low-mass impact parameter IQ to the conventional C1 and C2 impact parameters, and . assess whether the acoustic firmness test, if successfully adapted to an on-line operation, can improve the firmness sorting capacity in dynamic test methods.
2. Method and materials The experimental study was performed in two stages. Four calibration rubber balls were tested in the first stage of the study, in order to examine the experimental set-ups and analysis methods. The calibration balls were 74.7 /75.1 mm in diameter and weighted of 262.0 /265.6 g, with a stiffness range of 35/70 Shore-A hardness. The tests were repeated 10 times for the nondestructive test
method and 5 times for the parallel-plate compression tests. The second stage included full-scale tests of three apple varieties: ‘Golden Delicious’, ‘Starking Delicious’, and ‘Granny Smith’. Two hundred and seventy apples from each variety were obtained from commercial controlled atmosphere storage facilities and were kept in laboratory conditions of about 20 8C and 50% RH during the test period. The apple mass, height (H) and two equator diameters (D1 and D2) are given in Table 1. Each fruit variety was divided into nine groups of 30 apples each, and was tested by a total of four nondestructive and destructive tests every 3 /7 days, during a period of up to 7 weeks. Four tests per fruit, equally spaced around its perimeter, were performed; the results were averaged and compared with the other tests. The experimental set-up included three nondestructive dynamic measurement systems that were designed to detect firmness and two standard destructive tests to validate fruit properties, as described below. 2.1. The Sinclair impact device A bench-top version of SIQ-FT was used for nondestructive impact firmness measurements. The SIQ-FT, based on a low-mass sensing element placed inside the Sinclair patented air bellow, hits the fruit by air pressure and captures the impact signal (Fig. 1). A special data acquisition and signal analysis program was employed to determine the internal quality index IQ of the tested sample. The IQ value is calculated according to the impact signal as a dynamic measure of fruit tissue spring constant (/N mm1), and can be expressed by the following equation: IQ C
Pmax
g
2
(3)
p(t)dt
where C is a system constant, Pmax the peak amplitude of the impact response and p(t) the impact response as a function of time. Typical impact responses are shown in Fig. 2, which illustrates the difference between a soft and
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Table 1 Mass and dimensions of the experimental apples Weight (g)
H (mm)
D1 (mm)
D2 (mm)
Golden delicious Max Min
163.5 75.0
60.8 46.1
71.5 52.4
72.2 54.0
Average S.D.
118.3 23.6
56.8 6.8
62.5 4.8
62.0 5.0
Starking delicious Max Min
266.0 119.2
79.5 58.4
69.6 63.9
72.7 61.8
Average S.D.
167.5 25.8
63.4 4.8
71.3 4.0
71.2 4.1
Granny Smith Max Min
250.4 95.7
75.4 50.3
80.3 57.4
83.3 59.8
Average S.D.
150.3 29.7
61.0 5.1
68.7 4.7
69.0 4.8
firm fruit sample. Both peak value and pulse duration depend on the firmness of the sample.
As the firmness of a fruit sample increases, the peak-value increases and the duration decreases. The air pressure setup of the bellow system was 15 cm H2O and the sensor height above the fruit was 15/35 mm. These operational conditions gave good reproducibility of the measurements and did not cause damage to the tested fruit. 2.2. The instrumented hammer impact device A low-mass impact apparatus, composed of an instrumented hammer with a miniature excitation force transducer (PCB-086C80), an electromechanical actuator and a 4-mm diameter hemispherical aluminum tip, was used to produce the impact signal (Fig. 3). The data acquisition and signal analysis system of the SIQ-FT device were used to extract the impact firmness parameters of the force signal (Eqs. (1) /(3)). 2.3. The acoustic device
Fig. 1. The Sinclair IQTM-Firmness Tester (SIQ-FT).
An upgraded FirmalonTM firmness tester (Eshet Eilon Ltd; Fig. 4) measured the acoustic response. This tester included a force transducer to measure the fruit mass, an instrumented fruit-bed equipped
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Fig. 2. Typical impact response for two fruit samples by the SIQ-FT. The pulse with the longer duration and smaller peak is less firm than the pulse with the shorter duration and larger peak.
Fig. 4. The acoustic test apparatus */‘FirmalonTM’.
2.4. Destructive test devices with three soft piezoelectric sensors which enabled free vibrations of the fruit, and three electromechanical low-mass strikers to excite fruit vibrations. Typical frequency domain signals of the three sensors are shown in Fig. 5. Only one sensor was active in the present study, as in the other test methods. A data acquisition program was used to select the lowest resonant frequency of the tested sample and to calculate the acoustic firmness index FI (104 kg2/3 s 2):
where f1 is the first resonant frequency and m is the fruit mass. Four measurements where performed per fruit and averaged to give the acoustic firmness index of each fruit.
An InstronTM 4204-Universal Testing Machine with a top load cell of 1000 N was used to perform parallel-plate compression tests of the fruit. The maximum load and displacement were limited to 150 N and 5 mm, respectively, at a head velocity of 50 mm min 1. Measurements of the radii of curvature were taken at the loading points. The force /deformation curves were analyzed after zero displacement adjustment. The confined elastic modulus E ? (MPa) was calculated by Eq. (5) to prevent error in estimating the Poisson ratio: E 0:338F 1 1 1=3 E? Ku 1 m2 D3=2 Ru R?u 1 1 1=3 3=2 KL (5) RL R?L
Fig. 3. The instrumented hammer.
where E is the apparent modulus of elasticity, D , deformation, m , Poisson’s ratio, F , force, Ru and R ? u radii of curvature of the convex surface of the fruit at the point of contact with the upper plate, and RL, R ? L radii of curvature of the convex surface of the fruit at the point of contact with the lower plate (Ru and RL are the minimum radii of curvature of the fruit at the point of contact; R ? u and R ? L are the maximum radii of curvature). The constants Ku and KL are determined from ASAE Standards (2001). Two compression tests were performed on the equator of each fruit in perpendicular directions. The two measurements were averaged and compared with the other tests.
FI f12 m2=3
(4)
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Fig. 5. A typical acoustic signal of apples in frequency domains.
The ultimate strength of the apple tissue was determined by the MT penetration test using the InstronTM 4204-Universal Testing Machine and a 11-mm diameter probe with a 19-mm radius of curvature tip. The head velocity and penetration depth were 50 mm min 1 and 8 mm, respectively. The maximum penetration force was recorded at the same locations as the low-mass impact firmness measure, after the fruit peel had been removed. Four penetration tests per fruit were averaged and compared with the other tests.
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mented hammer (Fig. 6b) were not the same as those from the SIQ-FT sensor (Fig. 6a). The SIQFT amplified signals showed a negative force at the end of the impact, most probably caused by the signal conditioning and amplification system. Since the algorithms ignore the negative portion of these signals, firmness calculated from the instrumented hammer and the SIQ-FT amplified signal was very similar (R /0.993). The confined elastic modulus E ? was calculated from the compression tests according to the ASAE 368.3 standard. After zero correction, the nonlinear force /deformation curve fitting of the balls represented very well the Hertz theory for the contact force of elastic bodies. The repeatability of 5 compression tests per ball was very high (S.D. of /2%), while the calculated E ? ranged between 15.0 MPa of the hard ball #1/2.4 MPa of the soft ball #4. Test results comparing firmness parameters FI, IQ* (notation for SIQ-FT measurement) and IQ** (notation for the instrumented hammer measurement) versus the elastic modulus E ? are presented
3. Results and discussion 3.1. Rubber ball calibration tests A preliminary examination of the experimental systems and firmness parameters was made by testing four calibration rubber balls. The balls have a stiffness range similar to that of the apples. However, the internal damping of the balls was much higher than that of typical fruit. As a result, the decay rate of the acoustic signal was very strong and the frequency analysis of the hard balls was difficult to achieve. Nevertheless, the repeatability of the acoustic firmness tests of the rubber balls was very high (S.D. of /2%). The impact parameters for the calibration balls were calculated according to the SIQ-FT and the instrumented hammer signals (10 tests per ball, S.D. of /2%). Fig. 6a and b illustrates results from the two low-mass impact systems. It should be noted that the impact signals from the instru-
Fig. 6. Impact signals of the four calibration rubber balls (a) from the SIQ-FT device and (b) from the instrumented hammer.
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only low-mass impact device for the second stage of testing. 3.2. Golden delicious tests
Fig. 7. Firmness parameters FI, IQ* (notation for SIQ-FT measurement), and IQ** (notation for the instrumented hammer measurement), versus the confined elastic modulus E ? for the calibrated rubber balls.
in Fig. 7. The linear relationship between E ? and the firmness parameters is obvious (R /0.997). Hence, the quasi-static compression test E ? was selected as the reference for the dynamic firmness measurements. A comparison of the calibration tests, including the calculated indices C1 and C2 (see Eqs. (1) and (2)) by the relevant impact signals, is presented in Table 2. The linear relationship between E ? and the dynamic firmness parameters in the calibration balls is demonstrated again here (R /0.996), except for C2 that correlated better to E ? by using a non-linear regression. Since the SIQ-FT performed nearly identical to the instrumented hammer, it was chosen as the
The ‘Golden Delicious’ apples were divided into nine groups of 30 apples each and evaluated by the nondestructive and destructive tests devices during a period of 3 weeks. Pearson correlation between the firmness parameters is presented in Table 3 (correlation is significant at the 0.01 level, 2tailed). The best fit of the data was found between E ? and the nondestructive acoustic parameter FI (R /0.913); very similar results were obtained by the IQ impact parameter (R /0.911). The performances of the impact parameters C1 and C2 calculated from the SIQ-FT signal were slightly lower (R /0.890 and 0.901, respectively). The changes in the MT penetration force during the test period were very low, and its correlation with E ? and all the other firmness parameters was quite poor (R B/0.55). Significant changes in the firmness parameters according to fruit softening are important for efficient sorting. These changes along the test period are presented in Fig. 8. The largest firmness change was that of the acoustic firmness index FI, with an average decrease of 3.43 during 21 days. The confined elastic modulus E ? decreased during the testing period by a factor of 2.75, the IQ impact parameter decreased by a factor of 1.78, and the MT by a factor of 1.41.
Table 2 Pearson linear correlation between indices measured on four rubber balls by: parallel-plate (E ?); acoustic device Firmalon (FI); lowmass impact SIQ-FT (IQ*); instrumented hammer (IQ**) and impact parameters for each impact method C1 and C2, respectively
E? FI IQ* C1 C2 IQ** C1 * C2 *
E?
FI
IQ*
C1
1
0.998 1
0.999 0.999 1
1.000 0.997 0.999 1
*SIQ-FT impact test. **Instrumented hammer impact test.
C2
0.993 0.987 0.987 0.991 1
IQ**
C1 *
C2 *
0.997 0.992 0.993 0.996 0.999 1
0.996 0.989 0.992 0.996 0.997 0.999 1
0.982 0.971 0.973 0.980 0.997 0.993 0.993 1
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Fig. 8. Acoustic firmness index FI (104 kg2/3 s 2), low-mass impact response IQ (N mm1), confined elastic modulus E ? (MPa) and MT penetration firmness (10* N) versus time at 20 8C and 50% RH. Each value is the mean of 30 ‘Golden Delicious’ apples. Error bars are 9/S.D.
3.3. Starking delicious tests The nine groups of ‘Starking Delicious’ apples, 30 fruit each, underwent nondestructive and destructive testing devices during a period of 6 weeks. The correlation between the firmness parameters is presented in Table 4. The best fit was found between E ? and FI (R /0.947). The data spread of IQ was wider, as reflected in its correlation coefficient with E ? (R /0.837). However, the IQ correlation with E ? was much higher than that of C1 and C2 (R /0.323 and 0.539, respectively).
As with ‘Golden Delicious’ apples, the changes in the MT penetration force during the tests were low. Its correlation coefficient with the elastic modulus and all the nondestructive firmness parameters was also low (R B/0.60). The changes in ‘Starking Delicious’ firmness parameters during the test period are presented in Fig. 9. The largest firmness change during the testing period was that of the acoustic firmness FI, with an average decrease factor of 4.93. The confined elastic modulus E ? decreased during that period by a factor of 3.21, the IQ firmness
Table 3 Pearson linear correlation between the destructive and nondestructive tested indices for 270 ‘Golden Delicious’ apples, each apple represented by the average value measured
Table 4 Pearson linear correlation between the destructive and nondestructive tested indices for 270 ‘Starking Delicious’ apples, each apple represented by the average value measured
E? MT FI IQ C1 C2
E?
MT
FI
IQ
C1
C2
1
0.542 1
0.913 0.520 1
0.911 0.517 0.923 1
0.890 0.478 0.906 0.985 1
0.901 0.506 0.921 0.988 0.995 1
E? MT FI IQ C1 C2
E?
MT
FI
IQ
C1
C2
1
0.597 1
0.947 0.598 1
0.837 0.532 0.829 1
0.323 0.121 0.203 0.358 1
0.539 0.270 0.444 0.619 0.951 1
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Fig. 9. Acoustic firmness index FI (104 kg2/3 s 2), low-mass impact response IQ (N mm1), confined elastic modulus E ? (MPa) and MT penetration firmness (10* N) versus time at 20 8C and 50% RH. Each value is the mean of 30 ‘Starking Delicious’ apples. Error bars are 9/S.D.
parameter decreased by a factor of 1.90, and MT by a factor of 1.47. 3.4. Granny Smith tests The nine groups of ‘Granny Smith’ apples, 30 fruit each, underwent the nondestructive and destructive testing devices during a period of 7 weeks. The correlation between the firmness parameters is presented in Table 5. As reported for ‘Starking Delicious’ apples, the best fit was found between E ? and FI (R /0.915). The data spread of IQ was higher and its correlation coefficient with E ? was lower (R /0.820). The firmness indices C1 Table 5 Pearson linear correlation between the destructive and nondestructive tested indices for 270 ‘Granny Smith’ apples, each apple represented by the average value measured
E? MT FI IQ* C1 C2
E?
MT
FI
IQ
C1
C2
1
0.500 1
0.915 0.461 1
0.820 0.426 0.883 1
0.609 0.261 0.657 0.856 1
0.693 0.321 0.752 0.926 0.982 1
and C2, calculated from the same impact signal, could not adequately predict the elastic modulus of ‘Granny Smith’ apples (R /0.609 and 0.693, respectively). As illustrated in the other apple varieties, changes of MT during the tests were very low, and its correlation coefficients with E ? and all the other firmness parameters were very poor (R B/0.51). The fruit firmness of ‘Granny Smith’ apples decreased linearly during the test period (Fig. 10). The largest firmness change was that of FI, with an average decrease factor of 3.13. The confined elastic modulus E ? decreased during the same period by a factor of 3.00. The IQ firmness parameter decreased by a factor of 1.54, and MT by a factor of 1.25. 3.5. Results and analysis The purpose of the study was to analyze the performance of the low-mass impact and acoustic test methods to evaluate the firmness of apples. The experimental systems and the dynamic firmness parameters were first evaluated by calibration tests of the rubber balls. All the dynamic firmness parameters correlated well with the elastic modulus E ?. This justifies the selection of the compres-
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Fig. 10. Acoustic firmness index FI (104 kg2/3 s 2), low-mass impact response IQ (N mm 1), confined elastic modulus E ? (MPa) and MT penetration firmness (10* N) versus time at 20 8C and 50% RH. Each value is the mean of 30 ‘Granny Smith’ apples. Error bars are 9/S.D.
sion test to serve as a reference for the dynamic firmness tests. The apple tests indicated that the new impact parameter IQ was equivalent to the acoustic parameter FI in predicting the elastic modulus and firmness of the ‘Golden Delicious’ apples, but was less effective in the firmness prediction of the ‘Starking Delicious’ and ‘Granny Smith’ varieties, most probably due to less uniform fruit shape and test conditions in these varieties. The empirical impact parameters C1 and C2, which performed similarly to IQ in the ‘Golden Delicious’ apples, could not adequately predict the elastic modulus of the ‘Starking Delicious’ and ‘Granny Smith’ fruit. Similar low results have been reported by Delwiche and Sarig (1991) for ‘Red Delicious’ apples, Ortiz-Canavate et al. (2001) for ‘Golden Delicious’ apples, and Homer et al. (2002) for ‘Starking Delicious’ apples. The IQ impact parameter improved firmness prediction of these apple varieties, as mentioned above. Examination of the experimental data showed that all the dynamic firmness parameters, including C1 and C2, were in sensitive to changes in fruit
mass or size. Yet, it was found that C1 and C2 were very sensitive to variations in the impact impulse amplitude. These amplitude variations were more pronounced in the ‘Starking Delicious’ and ‘Granny Smith’ fruit, and could be explained by the less uniform fruit shape, resulted in less uniform test conditions in these varieties. Unlike IQ measures, the impact firmness parameters C1 and C2 are strongly dependent on the peak impact amplitude (see Eqs. (1) and (2)). This relates to a strong dependence on the velocity of the impacting mass (Delwiche, 1987). Under well-defined test conditions of the calibration balls (same shape, size and location of the balls, same sensor height, bellow air pressure, etc.) both C1 and C2 predicted adequately the elasticity or firmness of the balls. However, this was not the case with the ‘Starking Delicious’ and ‘Granny Smith’ apples. Hence, under on-line conditions the IQ firmness parameter may perform better than the conventional C1 and C2 parameters. The changes in the penetration force MT during the test period were very low, and their correlation with the elastic modulus and the firmness para-
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meters of all of the apple varieties was poor. This finding agrees with previous observations by Shmulevich (1998) and Molto et al. (1998) and others. This also suggests that the MT test, which is a measure of the fruit yield strength, does not adequately predict the firmness or the elastic properties of the apples.
4. Conclusions Three apple varieties, ‘Golden Delicious’, ‘Starking Delicious’ and ‘Granny Smith’, were tested in laboratory conditions by low-mass impact and by acoustic response methods, to evaluate nondestructively the firmness of apple tissue. The performances of the experimental systems and test methods were evaluated first by rubber ball calibration tests. The dynamic impact and acoustic tests were followed by quasi-static compression and penetration tests. The confined elastic modulus E ? of fruit tissue, measured by the compression test, was selected as the preferred physical measure of fruit firmness. The IQ impact parameter and the acoustic firmness index FI predicted very well the elastic modulus of the calibration balls (R /0.99). Good correlation was found between E ? and the acoustic firmness FI of the three apple varieties (R /0.91). The correlation of the new IQ firmness parameter with E ? was similar to that of FI in ‘Golden Delicious’ apples, and slightly lower in the ‘Starking Delicious’ and ‘Granny Smith’ apples (R / 0.82). This was most probably because of the less uniform fruit shape and test conditions of these apple varieties. The conventional impact parameters C1 and C2 calculated from the impact signals produced by the instrumented hammer or by the SIQ-FT system were equivalent to the FI and IQ firmness parameters when compared to the elastic modulus E ? of the rubber balls (R /0.98). These parameters could predict the E ? of the ‘Golden Delicious’ apples (R /0.89), but failed to predict that of ‘Starking Delicious’ (R B/0.54) or ‘Granny Smith’ apples (R B/0.70). This was most probably due to their sensitivity to the impact conditions of the less uniform fruits, as also reported in the cited
literature. The changes in the penetration force MT during the test period were very low, and their correlation with the elastic modulus and the firmness parameters of all apple varieties was poor (R B/0.60). This finding, which has been reported in the past, may indicate that the MT test is not an appropriate measure of apple firmness during shelf life. The test results indicated that the new Sinclair IQ firmness tester offers a new technology that may overcome some technical difficulties indicated by Ortiz-Canavate et al. (2001) and Homer et al. (2002) and others, when studying the performances of the low-mass impact test methods. The SIQ-FT bellows and impact sensor design offer hardware which is easy to apply and less sensitive to variation in fruit size, shape and test location. These design features combined with the IQ firmness parameter performed better in firmness evaluation than the conventional impact parameters C1 and C2. The results also indicated that the acoustic method might improve firmness evaluation in ‘Starking Delicious’ and ‘Granny Smith’ apples. Further effort is needed to verify these results and to adapt the acoustic method for on-line operation.
Acknowledgements The research was supported by the Fund for Promotion of Research of the Technion R&D Foundation Ltd. and Sinclair International Ltd.
References Abbott, J.A., Bachman, G.S., Childers, N.F., Fitzgerald, J.V., Matusik, F.J., 1968. Sonic techniques for measuring texture of fruits and vegetables. Food Technol. 22, 635 /646. ASAE Standards, 2001. Compression test of food materials of convex shape. ASAE S368.4 DEC00, American Society of Agricultural Engineering, pp. 580 /587. Chen, P., Tjan, Y., 1996. A low-mass impact sensor for highspead firmness sensing of fruits. Paper 96F-003. AgEng96, Madrid, Spain. Chen, P., Tjan, Y., 1998. A real-time impact sensing system for on-line firmness sensing of fruits. AgEng’98 Paper No. 98F-006. AgEng Oslo 98, Conference on Agricultural Engineering, Oslo, Norway.
I. Shmulevich et al. / Postharvest Biology and Technology 29 (2003) 287 /299 Chen, P., Sun, Z., Huarng, L., 1992. Factors affecting acoustic response of apples. Trans. ASAE 35, 1915 /1920. Chen, P., Riuz Altiset, M., Barriero, P., 1996. Effect of impacting mass on firmness sensing of fruits. Trans. ASAE 39, 1019 /1023. De Baerdemaeker, J., Lemaitre, L., Meire, R., 1982. Quality detection by frequency spectrum analysis of the fruit impact force. Trans. ASAE 25, 175 /178. Delwiche, M.J., 1987. Theory of fruit firmness sorting by impact forces. Trans. ASAE 30, 1160 /1166, 1171. Delwiche, M.J., Sarig, Y., 1991. A probe impact sensor for fruit texture measurement. Trans. ASAE 34, 187 /192. Delwiche, M.J., MacDonald, T., Bowers, S.V., 1987. Determination of peach firmness by analysis of impact forces. Trans. ASAE 30, 249 /254. Delwiche, M.J., Tang, S., Mehlschau, J.J., 1989. An impact force response fruit impact sorter. Trans. ASAE 32, 321 / 326. Galili, N., De Baerdemaeker, J., 1996. Performance of acoustic test methods for quality evaluation of agricultural products. Proceedings of the ISMA21 */International Conference on Noise and Vibration Engineering, Leuven, Belgium. Vol. 3. Katholieke Universiteit Leuven / Dept. of Werktuigkunde, Heverlee, Belgium, pp. 1959 /1972. Galili, N., Shmulevich, I., Benichou, N., 1998. Acoustic testing of avocado for fruit ripeness evaluation. Trans. ASAE 41, 399 /407. Homer, I., Ortiz-Canavate, J., Moreda, G., Morabito, S., RuizAltisent, M., 2002. Evaluation of non-destructive impact
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sensor on-line for determination of firmness of peaches and apples. Paper Number 02-PH-026, AgEng */International Conference on Agricultural Engineering, Budapest, p. 8. Howarth, M.S., 2002. Sinclair IQ firmness tester. Paper Number 02-IE-006, AgEng */International Conference on Agricultural Engineering, Budapest, p. 8. Molto, E., Selfa, E., Pons, R., Fornes, I., Juste, F. 1998. A firmness sensor for quality estimation of individual fruits. Acta Hort. 421, 65 /72. Ortiz-Canavate, J., Homer, I., Garcia-Ramos, F.J., RuizAltisent, M., 2001. Determination of firmness in fruit packing line using a non-destructive impact sensor. In: Fruits, Nuts, and Vegetable Production Engineering, Sept. 11 /14, Potsdam, Germany, pp. 457 /462. Rohrbach, R.P., Franke, J.E., Willits, D.H., 1982. A firmness sorting criterion for blueberries. Trans. ASAE 25, 261 /265. Ruiz-Altisent, M., Jaren, C., Correa, P., 1993. Fruit quality sensing: post harvest ripeness. In: Proceedings of the 14th International Symposium on Fruits, Nuts, and Vegetable Production Engineering, vol. 2, Valencia /Zaragoza (Spain), 1993, pp. 211 /222. Shmulevich, I., 1998. A review: firmness quality measurements in fruits and vegetables. Sensoral 98: Proceedings of the International Workshop on Sensing Quality of Agricultural Products, Montpellier, France, pp. 1 /22 (on CD). Shmulevich, I., Galili, N., Rosenfeld, D., 1996. Detection of fruit firmness by frequency analysis. Trans. ASAE 39, 1047 /1055.
Nondestructive dynamic testing of apples for firmness evaluation I. Shmulevich a,*, N. Galili a, M.S. Howarth b a
Faculty of Agricultural Engineering, Technion / Israel Institute of Technology, Haifa, Israel b Sinclair International Ltd, Jarrold Way, Bowthorpe, Norwich, Norfolk NR5 9JD, UK Received 14 May 2002; accepted 24 February 2003
Abstract Two nondestructive dynamic test methods, low-mass impact and acoustic response, were tested and compared with destructive compression and penetration tests to evaluate apple firmness. The purpose of the study was to analyze the performance of the impact test methods for nondestructive firmness evaluation, and to assess whether the acoustic tests could add sorting capacity to low-mass impact testing in apples. A laboratory-instrumented impact hammer and a lowmass impact firmness sensor produced by Sinclair International (SIQ-FT) were used to perform the impact tests, and a piezoelectric-film transducer was applied for the acoustic tests. A new Sinclair internal quality index IQ and two conventional impact parameters C1 and C2 were extracted from the impact signals, and a firmness index FI was calculated from the acoustic signal of the samples. The nondestructive tests were followed by parallel-plate compression tests and Magness /Taylor penetration tests. Three apple varieties, ‘Golden Delicious’, ‘Starking Delicious’ and ‘Granny Smith’, were tested by the impact and acoustic methods. The performance of the experimental systems and the test methods were evaluated first by rubber ball calibration tests. The elastic modulus of all apples was well predicted by the acoustic firmness parameter FI (R /0.91). The impact parameter IQ was equivalent to FI in predicting the elastic modulus of ‘Golden Delicious’, and less accurate in ‘Starking Delicious’ and ‘Granny Smith’. The conventional indices C1 and C2 performed similarly to the FI and IQ in the rubber balls and the ‘Golden Delicious’ tests, but failed to predict the elastic modulus of ‘Starking Delicious’ or ‘Granny Smith’ apples, probably because of their less uniform shape, and the sensitivity of these empirical parameters to test conditions. The changes of the penetration force during the test period were very low, and their correlation with the elastic modulus and the firmness parameters of all apples was poor. These previously reported findings might indicate that the Magness /Taylor penetration test is not an appropriate measure of apple firmness during shelf life. The test results indicated that the new SIQ-FT device and the IQ parameter offer a new technology that may overcome some basic difficulties associated with the low-mass impact test methods, which have been reported in the literature. The results also indicated that the acoustic method might add sorting capacity to the impact method in some apple varieties. Further R&D efforts are needed, however, to verify these results and adapt the acoustic method for on-line operation. # 2003 Elsevier B.V. All rights reserved. Keywords: Nondestructive; Impact; Acoustic; Firmness; Apple; Quality
* Corresponding author. Tel.: /972-4-8292626; fax: /972-4-8221529. E-mail address: [email protected] (I. Shmulevich). 0925-5214/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-5214(03)00039-5
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1. Introduction Fruits and vegetables in today’s modern market must be of high quality standards, especially when proposed for export. Fruit texture and firmness are among the several qualitative terms to describe internal fruit quality. Hence, fast sorting of each individual fruit and vegetable according to firmness is very important. Low-mass impact analysis in the time domain is a simple and quick method for determining local fruit properties. The method has been successfully applied for on-line sorting operation of various fruits, but not yet in apples. Analysis of the acoustic fruit response to mechanical impulse in the frequency domain detects internal properties of the whole fruit, including firmness. The acoustic test method has been applied successfully in laboratory conditions, but still faces difficulties in implementation due to technical limitations. The motivation of the present work is to investigate the performance of the two dynamic test methods for firmness evaluation in apples. The two techniques and their basic principles are explained below. 1.1. Quality detection by impact force Numerous researchers have evaluated fruit firmness using impact techniques. Various shape characteristics of impact force and acceleration signals were developed as firmness predictors in the time and frequency domains. The most frequently used empirical impact parameters for firmness evaluation are C1 and C2 (Rohrbach et al., 1982; Delwiche et al., 1987): P C1 max (1) T and C2
Pmax T2
(2)
where Pmax is the peak amplitude of the impact pulse and T, an impact characteristic time, such as Tp, time to peak amplitude, Tc, pulse duration, or Tm, width of the impact at half of the peak amplitude. These impact parameters and addi-
tional characteristic values of the impact signal in time or frequency domain were used to evaluate the firmness of various fruits, as described below. 1.1.1. Dropping the fruit on a flat force transducer One technique for firmness evaluation of fruits and vegetables has been to drop the product on a force transducer. De Baerdemaeker et al. (1982) measured the impact force of apples and evaluated several characteristics of the force signal in the frequency domain as firmness indicators. Rohrbach et al. (1982) studied the firmness of berries by measuring their response when dropped on a rigid surface. Various shape characteristics were developed as firmness predictors and compared to parameters extracted from the compression test by a parallel-plate. The correlations between berry firmness and its ‘peak impact force parameter’ C2 was low (R /0.52). Yet, it was concluded that C2 could be a rapid means for determining fruit firmness. Delwiche (1987) studied a theoretical model of homogeneous elastic spheres to analyze the effect of the model parameters and impact velocity on its impact characteristics. The investigation showed that both the time /domain parameters (C1 and C2) and frequency /domain characteristics (F250 / F340) of the simulated impact force are strongly dependent on the elastic modulus E of the sphere, and also on the impact contact velocity that should remain constant in any practical implementation of this method. Variation in fruit size (mass or dimensions) had a moderate effect on the frequency indices and very little on C2. These findings were applied by Delwiche et al. (1987) to measure the impact response of peaches striking a rigid surface. The highest correlation with elastic modulus and penetration readings of flesh firmness was that of C2 and F295. Delwiche et al. (1989) reported implementation of the impact technique in a sorting line of peaches and pears, at a rate of 5 fruits s 1. In their report, both indices C1 and C2 correlated well (0.86 and 0.84, respectively) with the destructive indices. The results were slightly better with elastic modulus as the texture index rather than penetration. Molto et al. (1998) tested apples, peaches and pears, looking for a reference for firmness evaluation. Fruit were
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dropped from a given height on a load cell. Pmax, Tp and Tc, C1 and C2, and some additional impact characteristics were compared with compression and penetration firmness tests, sugar content, and expert classification. Low correlations were found between the impact parameters and the destructive tests. The distractive MT measurements did not offer good correlation either with sugar or with expert classification of apples. 1.1.2. Tapping the fruit with a firmness sensing element Another impact technique was to tap the fruit with a medium or small impact device. Delwiche and Sarig (1991) developed a firmness sensor of 63 g to impact the fruit. The acceleration of the mass gave a measure of the impact force that was analyzed to obtain the impact parameters C1 and C2. These parameters were normalized by the dropping height h . The correlation obtained between C2, C2/h and a standard compression test was higher for peaches (R /0.80), lower for pears (R /0.68) and very low for ‘Red Delicious’ apples (R /0.53). Similar results were obtained by Ruiz-Altisent et al. (1993) for pears, avocados and apples. Chen and Tjan (1996, 1998) and Chen et al. (1996) introduced a new mechanical system for low-mass impact based on a swing-arm sensor. They reported good performance of the system when testing rubber balls, kiwifruit and peaches. Preliminary tests showed that the sensor could sense fruit firmness at a speed of 5 fruit s 1. OrtizCanavate et al. (2001) adapted a modified version of the low-mass impact method for an experimental fruit packing line with an operation speed of 5/ 7 fruit s 1. ‘Golden Delicious’ apples and several peach varieties were tested dynamically by the system and by destructive compression and MT penetration tests. Several impact characteristics were compared with the destructive test results. The correlation coefficients between the impact parameter C1, when compared with force/deformation slope and MT penetration force, was quite high in peaches (0.93 and 0.87, respectively) but much lower in ‘Golden Delicious’ apples (0.74 and 0.43, respectively). The authors reported that values obtained by the impact tests were very
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sensitive to variations in fruit form, location and impact angle. This work was repeated and extended by Homer et al. (2002), with similar results. Medium to high correlation coefficients were obtained between the impact parameters Pmax, C1 etc. and the destructive tests of nectarines and peaches; much lower correlation coefficients of 0.55 /0.64 were obtained for ‘Starking Delicious’ apples. It was observed that better performance was achieved for softer fruits. The research reported a series of difficulties associated with the swing-arm sensor and its high sensitivity to variations in fruit position and orientation in the conveying system. Recently, Sinclair International Ltd has developed the Sinclair IQTM-firmness tester (SIQ-FT) that is based on a low-mass impact sensor (Howarth, 2002). This on-line system measures firmness using a sensing element on the tip of a bellow. SIQ-FT takes advantage of Sinclair’s patented bellows delivery system, which is also used in fruit labeling and can be simply adapted to existing sorting lines. Medium to high correlation coefficients were obtained with penetration tests for nectarines (0.85/0.95), plums (0.80), avocados (0.81 /0.84) and kiwifruit (0.83 /0.92). The SIQ-FT on-line system currently operates at speeds up to 10 fruit s 1 and makes four independent measurements that are combined to estimate the fruit firmness. These measurements are made on 4 different quadrants around the fruit surface and can be combined as an average, median, etc. as defined by the user. According to the cited literature the impact techniques have reached good results in the firmness evaluation of peaches, pears and some tropical fruits, but not in apples. Hence, it is important to evaluate the performance of the new SIQ-FT device and the Sinclair IQ parameter for the firmness evaluation of apples. 1.2. Quality detection by acoustic techniques Acoustic techniques are an alternative method for evaluating fruit firmness (Shmulevich, 1998). The acoustic response, sensed by a microphone, an accelerometer or a flexible piezoelectric sensor, was analyzed using a FFT algorithm to extract the
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fruit’s resonant frequencies. Many researchers have investigated acoustic techniques using this approach (Abbott et al., 1968; Chen et al., 1992; Galili and De Baerdemaeker, 1996; Shmulevich et al., 1996; Galili et al., 1998; Shmulevich, 1998). This research reported good correlation between the acoustic fruit response and the firmness or softening of several fruits such as mango, melon, avocado, apple and tomato. Spherical fruits produced better results due to the fact that spherical modal shapes correlate to firmness. The limitations of this method for non-spherical fruit, such as avocado or mango, could be minimized by proper fruit orientation. Although good laboratory results were reported, there are still technical difficulties in the implementation of the method to on-line sorting. The general objective of the present research was to evaluate the performances of the low-mass impact and acoustic test methods for nondestructive firmness assessment of apples. The specific objectives were to: . evaluate the performance of the experimental systems and the various firmness algorithms by well defined rubber-ball calibration tests, . evaluate the performance of the nondestructive dynamic methods and the destructive tests in apples, . compare the new low-mass impact parameter IQ to the conventional C1 and C2 impact parameters, and . assess whether the acoustic firmness test, if successfully adapted to an on-line operation, can improve the firmness sorting capacity in dynamic test methods.
2. Method and materials The experimental study was performed in two stages. Four calibration rubber balls were tested in the first stage of the study, in order to examine the experimental set-ups and analysis methods. The calibration balls were 74.7 /75.1 mm in diameter and weighted of 262.0 /265.6 g, with a stiffness range of 35/70 Shore-A hardness. The tests were repeated 10 times for the nondestructive test
method and 5 times for the parallel-plate compression tests. The second stage included full-scale tests of three apple varieties: ‘Golden Delicious’, ‘Starking Delicious’, and ‘Granny Smith’. Two hundred and seventy apples from each variety were obtained from commercial controlled atmosphere storage facilities and were kept in laboratory conditions of about 20 8C and 50% RH during the test period. The apple mass, height (H) and two equator diameters (D1 and D2) are given in Table 1. Each fruit variety was divided into nine groups of 30 apples each, and was tested by a total of four nondestructive and destructive tests every 3 /7 days, during a period of up to 7 weeks. Four tests per fruit, equally spaced around its perimeter, were performed; the results were averaged and compared with the other tests. The experimental set-up included three nondestructive dynamic measurement systems that were designed to detect firmness and two standard destructive tests to validate fruit properties, as described below. 2.1. The Sinclair impact device A bench-top version of SIQ-FT was used for nondestructive impact firmness measurements. The SIQ-FT, based on a low-mass sensing element placed inside the Sinclair patented air bellow, hits the fruit by air pressure and captures the impact signal (Fig. 1). A special data acquisition and signal analysis program was employed to determine the internal quality index IQ of the tested sample. The IQ value is calculated according to the impact signal as a dynamic measure of fruit tissue spring constant (/N mm1), and can be expressed by the following equation: IQ C
Pmax
g
2
(3)
p(t)dt
where C is a system constant, Pmax the peak amplitude of the impact response and p(t) the impact response as a function of time. Typical impact responses are shown in Fig. 2, which illustrates the difference between a soft and
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Table 1 Mass and dimensions of the experimental apples Weight (g)
H (mm)
D1 (mm)
D2 (mm)
Golden delicious Max Min
163.5 75.0
60.8 46.1
71.5 52.4
72.2 54.0
Average S.D.
118.3 23.6
56.8 6.8
62.5 4.8
62.0 5.0
Starking delicious Max Min
266.0 119.2
79.5 58.4
69.6 63.9
72.7 61.8
Average S.D.
167.5 25.8
63.4 4.8
71.3 4.0
71.2 4.1
Granny Smith Max Min
250.4 95.7
75.4 50.3
80.3 57.4
83.3 59.8
Average S.D.
150.3 29.7
61.0 5.1
68.7 4.7
69.0 4.8
firm fruit sample. Both peak value and pulse duration depend on the firmness of the sample.
As the firmness of a fruit sample increases, the peak-value increases and the duration decreases. The air pressure setup of the bellow system was 15 cm H2O and the sensor height above the fruit was 15/35 mm. These operational conditions gave good reproducibility of the measurements and did not cause damage to the tested fruit. 2.2. The instrumented hammer impact device A low-mass impact apparatus, composed of an instrumented hammer with a miniature excitation force transducer (PCB-086C80), an electromechanical actuator and a 4-mm diameter hemispherical aluminum tip, was used to produce the impact signal (Fig. 3). The data acquisition and signal analysis system of the SIQ-FT device were used to extract the impact firmness parameters of the force signal (Eqs. (1) /(3)). 2.3. The acoustic device
Fig. 1. The Sinclair IQTM-Firmness Tester (SIQ-FT).
An upgraded FirmalonTM firmness tester (Eshet Eilon Ltd; Fig. 4) measured the acoustic response. This tester included a force transducer to measure the fruit mass, an instrumented fruit-bed equipped
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Fig. 2. Typical impact response for two fruit samples by the SIQ-FT. The pulse with the longer duration and smaller peak is less firm than the pulse with the shorter duration and larger peak.
Fig. 4. The acoustic test apparatus */‘FirmalonTM’.
2.4. Destructive test devices with three soft piezoelectric sensors which enabled free vibrations of the fruit, and three electromechanical low-mass strikers to excite fruit vibrations. Typical frequency domain signals of the three sensors are shown in Fig. 5. Only one sensor was active in the present study, as in the other test methods. A data acquisition program was used to select the lowest resonant frequency of the tested sample and to calculate the acoustic firmness index FI (104 kg2/3 s 2):
where f1 is the first resonant frequency and m is the fruit mass. Four measurements where performed per fruit and averaged to give the acoustic firmness index of each fruit.
An InstronTM 4204-Universal Testing Machine with a top load cell of 1000 N was used to perform parallel-plate compression tests of the fruit. The maximum load and displacement were limited to 150 N and 5 mm, respectively, at a head velocity of 50 mm min 1. Measurements of the radii of curvature were taken at the loading points. The force /deformation curves were analyzed after zero displacement adjustment. The confined elastic modulus E ? (MPa) was calculated by Eq. (5) to prevent error in estimating the Poisson ratio: E 0:338F 1 1 1=3 E? Ku 1 m2 D3=2 Ru R?u 1 1 1=3 3=2 KL (5) RL R?L
Fig. 3. The instrumented hammer.
where E is the apparent modulus of elasticity, D , deformation, m , Poisson’s ratio, F , force, Ru and R ? u radii of curvature of the convex surface of the fruit at the point of contact with the upper plate, and RL, R ? L radii of curvature of the convex surface of the fruit at the point of contact with the lower plate (Ru and RL are the minimum radii of curvature of the fruit at the point of contact; R ? u and R ? L are the maximum radii of curvature). The constants Ku and KL are determined from ASAE Standards (2001). Two compression tests were performed on the equator of each fruit in perpendicular directions. The two measurements were averaged and compared with the other tests.
FI f12 m2=3
(4)
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Fig. 5. A typical acoustic signal of apples in frequency domains.
The ultimate strength of the apple tissue was determined by the MT penetration test using the InstronTM 4204-Universal Testing Machine and a 11-mm diameter probe with a 19-mm radius of curvature tip. The head velocity and penetration depth were 50 mm min 1 and 8 mm, respectively. The maximum penetration force was recorded at the same locations as the low-mass impact firmness measure, after the fruit peel had been removed. Four penetration tests per fruit were averaged and compared with the other tests.
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mented hammer (Fig. 6b) were not the same as those from the SIQ-FT sensor (Fig. 6a). The SIQFT amplified signals showed a negative force at the end of the impact, most probably caused by the signal conditioning and amplification system. Since the algorithms ignore the negative portion of these signals, firmness calculated from the instrumented hammer and the SIQ-FT amplified signal was very similar (R /0.993). The confined elastic modulus E ? was calculated from the compression tests according to the ASAE 368.3 standard. After zero correction, the nonlinear force /deformation curve fitting of the balls represented very well the Hertz theory for the contact force of elastic bodies. The repeatability of 5 compression tests per ball was very high (S.D. of /2%), while the calculated E ? ranged between 15.0 MPa of the hard ball #1/2.4 MPa of the soft ball #4. Test results comparing firmness parameters FI, IQ* (notation for SIQ-FT measurement) and IQ** (notation for the instrumented hammer measurement) versus the elastic modulus E ? are presented
3. Results and discussion 3.1. Rubber ball calibration tests A preliminary examination of the experimental systems and firmness parameters was made by testing four calibration rubber balls. The balls have a stiffness range similar to that of the apples. However, the internal damping of the balls was much higher than that of typical fruit. As a result, the decay rate of the acoustic signal was very strong and the frequency analysis of the hard balls was difficult to achieve. Nevertheless, the repeatability of the acoustic firmness tests of the rubber balls was very high (S.D. of /2%). The impact parameters for the calibration balls were calculated according to the SIQ-FT and the instrumented hammer signals (10 tests per ball, S.D. of /2%). Fig. 6a and b illustrates results from the two low-mass impact systems. It should be noted that the impact signals from the instru-
Fig. 6. Impact signals of the four calibration rubber balls (a) from the SIQ-FT device and (b) from the instrumented hammer.
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only low-mass impact device for the second stage of testing. 3.2. Golden delicious tests
Fig. 7. Firmness parameters FI, IQ* (notation for SIQ-FT measurement), and IQ** (notation for the instrumented hammer measurement), versus the confined elastic modulus E ? for the calibrated rubber balls.
in Fig. 7. The linear relationship between E ? and the firmness parameters is obvious (R /0.997). Hence, the quasi-static compression test E ? was selected as the reference for the dynamic firmness measurements. A comparison of the calibration tests, including the calculated indices C1 and C2 (see Eqs. (1) and (2)) by the relevant impact signals, is presented in Table 2. The linear relationship between E ? and the dynamic firmness parameters in the calibration balls is demonstrated again here (R /0.996), except for C2 that correlated better to E ? by using a non-linear regression. Since the SIQ-FT performed nearly identical to the instrumented hammer, it was chosen as the
The ‘Golden Delicious’ apples were divided into nine groups of 30 apples each and evaluated by the nondestructive and destructive tests devices during a period of 3 weeks. Pearson correlation between the firmness parameters is presented in Table 3 (correlation is significant at the 0.01 level, 2tailed). The best fit of the data was found between E ? and the nondestructive acoustic parameter FI (R /0.913); very similar results were obtained by the IQ impact parameter (R /0.911). The performances of the impact parameters C1 and C2 calculated from the SIQ-FT signal were slightly lower (R /0.890 and 0.901, respectively). The changes in the MT penetration force during the test period were very low, and its correlation with E ? and all the other firmness parameters was quite poor (R B/0.55). Significant changes in the firmness parameters according to fruit softening are important for efficient sorting. These changes along the test period are presented in Fig. 8. The largest firmness change was that of the acoustic firmness index FI, with an average decrease of 3.43 during 21 days. The confined elastic modulus E ? decreased during the testing period by a factor of 2.75, the IQ impact parameter decreased by a factor of 1.78, and the MT by a factor of 1.41.
Table 2 Pearson linear correlation between indices measured on four rubber balls by: parallel-plate (E ?); acoustic device Firmalon (FI); lowmass impact SIQ-FT (IQ*); instrumented hammer (IQ**) and impact parameters for each impact method C1 and C2, respectively
E? FI IQ* C1 C2 IQ** C1 * C2 *
E?
FI
IQ*
C1
1
0.998 1
0.999 0.999 1
1.000 0.997 0.999 1
*SIQ-FT impact test. **Instrumented hammer impact test.
C2
0.993 0.987 0.987 0.991 1
IQ**
C1 *
C2 *
0.997 0.992 0.993 0.996 0.999 1
0.996 0.989 0.992 0.996 0.997 0.999 1
0.982 0.971 0.973 0.980 0.997 0.993 0.993 1
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Fig. 8. Acoustic firmness index FI (104 kg2/3 s 2), low-mass impact response IQ (N mm1), confined elastic modulus E ? (MPa) and MT penetration firmness (10* N) versus time at 20 8C and 50% RH. Each value is the mean of 30 ‘Golden Delicious’ apples. Error bars are 9/S.D.
3.3. Starking delicious tests The nine groups of ‘Starking Delicious’ apples, 30 fruit each, underwent nondestructive and destructive testing devices during a period of 6 weeks. The correlation between the firmness parameters is presented in Table 4. The best fit was found between E ? and FI (R /0.947). The data spread of IQ was wider, as reflected in its correlation coefficient with E ? (R /0.837). However, the IQ correlation with E ? was much higher than that of C1 and C2 (R /0.323 and 0.539, respectively).
As with ‘Golden Delicious’ apples, the changes in the MT penetration force during the tests were low. Its correlation coefficient with the elastic modulus and all the nondestructive firmness parameters was also low (R B/0.60). The changes in ‘Starking Delicious’ firmness parameters during the test period are presented in Fig. 9. The largest firmness change during the testing period was that of the acoustic firmness FI, with an average decrease factor of 4.93. The confined elastic modulus E ? decreased during that period by a factor of 3.21, the IQ firmness
Table 3 Pearson linear correlation between the destructive and nondestructive tested indices for 270 ‘Golden Delicious’ apples, each apple represented by the average value measured
Table 4 Pearson linear correlation between the destructive and nondestructive tested indices for 270 ‘Starking Delicious’ apples, each apple represented by the average value measured
E? MT FI IQ C1 C2
E?
MT
FI
IQ
C1
C2
1
0.542 1
0.913 0.520 1
0.911 0.517 0.923 1
0.890 0.478 0.906 0.985 1
0.901 0.506 0.921 0.988 0.995 1
E? MT FI IQ C1 C2
E?
MT
FI
IQ
C1
C2
1
0.597 1
0.947 0.598 1
0.837 0.532 0.829 1
0.323 0.121 0.203 0.358 1
0.539 0.270 0.444 0.619 0.951 1
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Fig. 9. Acoustic firmness index FI (104 kg2/3 s 2), low-mass impact response IQ (N mm1), confined elastic modulus E ? (MPa) and MT penetration firmness (10* N) versus time at 20 8C and 50% RH. Each value is the mean of 30 ‘Starking Delicious’ apples. Error bars are 9/S.D.
parameter decreased by a factor of 1.90, and MT by a factor of 1.47. 3.4. Granny Smith tests The nine groups of ‘Granny Smith’ apples, 30 fruit each, underwent the nondestructive and destructive testing devices during a period of 7 weeks. The correlation between the firmness parameters is presented in Table 5. As reported for ‘Starking Delicious’ apples, the best fit was found between E ? and FI (R /0.915). The data spread of IQ was higher and its correlation coefficient with E ? was lower (R /0.820). The firmness indices C1 Table 5 Pearson linear correlation between the destructive and nondestructive tested indices for 270 ‘Granny Smith’ apples, each apple represented by the average value measured
E? MT FI IQ* C1 C2
E?
MT
FI
IQ
C1
C2
1
0.500 1
0.915 0.461 1
0.820 0.426 0.883 1
0.609 0.261 0.657 0.856 1
0.693 0.321 0.752 0.926 0.982 1
and C2, calculated from the same impact signal, could not adequately predict the elastic modulus of ‘Granny Smith’ apples (R /0.609 and 0.693, respectively). As illustrated in the other apple varieties, changes of MT during the tests were very low, and its correlation coefficients with E ? and all the other firmness parameters were very poor (R B/0.51). The fruit firmness of ‘Granny Smith’ apples decreased linearly during the test period (Fig. 10). The largest firmness change was that of FI, with an average decrease factor of 3.13. The confined elastic modulus E ? decreased during the same period by a factor of 3.00. The IQ firmness parameter decreased by a factor of 1.54, and MT by a factor of 1.25. 3.5. Results and analysis The purpose of the study was to analyze the performance of the low-mass impact and acoustic test methods to evaluate the firmness of apples. The experimental systems and the dynamic firmness parameters were first evaluated by calibration tests of the rubber balls. All the dynamic firmness parameters correlated well with the elastic modulus E ?. This justifies the selection of the compres-
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297
Fig. 10. Acoustic firmness index FI (104 kg2/3 s 2), low-mass impact response IQ (N mm 1), confined elastic modulus E ? (MPa) and MT penetration firmness (10* N) versus time at 20 8C and 50% RH. Each value is the mean of 30 ‘Granny Smith’ apples. Error bars are 9/S.D.
sion test to serve as a reference for the dynamic firmness tests. The apple tests indicated that the new impact parameter IQ was equivalent to the acoustic parameter FI in predicting the elastic modulus and firmness of the ‘Golden Delicious’ apples, but was less effective in the firmness prediction of the ‘Starking Delicious’ and ‘Granny Smith’ varieties, most probably due to less uniform fruit shape and test conditions in these varieties. The empirical impact parameters C1 and C2, which performed similarly to IQ in the ‘Golden Delicious’ apples, could not adequately predict the elastic modulus of the ‘Starking Delicious’ and ‘Granny Smith’ fruit. Similar low results have been reported by Delwiche and Sarig (1991) for ‘Red Delicious’ apples, Ortiz-Canavate et al. (2001) for ‘Golden Delicious’ apples, and Homer et al. (2002) for ‘Starking Delicious’ apples. The IQ impact parameter improved firmness prediction of these apple varieties, as mentioned above. Examination of the experimental data showed that all the dynamic firmness parameters, including C1 and C2, were in sensitive to changes in fruit
mass or size. Yet, it was found that C1 and C2 were very sensitive to variations in the impact impulse amplitude. These amplitude variations were more pronounced in the ‘Starking Delicious’ and ‘Granny Smith’ fruit, and could be explained by the less uniform fruit shape, resulted in less uniform test conditions in these varieties. Unlike IQ measures, the impact firmness parameters C1 and C2 are strongly dependent on the peak impact amplitude (see Eqs. (1) and (2)). This relates to a strong dependence on the velocity of the impacting mass (Delwiche, 1987). Under well-defined test conditions of the calibration balls (same shape, size and location of the balls, same sensor height, bellow air pressure, etc.) both C1 and C2 predicted adequately the elasticity or firmness of the balls. However, this was not the case with the ‘Starking Delicious’ and ‘Granny Smith’ apples. Hence, under on-line conditions the IQ firmness parameter may perform better than the conventional C1 and C2 parameters. The changes in the penetration force MT during the test period were very low, and their correlation with the elastic modulus and the firmness para-
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meters of all of the apple varieties was poor. This finding agrees with previous observations by Shmulevich (1998) and Molto et al. (1998) and others. This also suggests that the MT test, which is a measure of the fruit yield strength, does not adequately predict the firmness or the elastic properties of the apples.
4. Conclusions Three apple varieties, ‘Golden Delicious’, ‘Starking Delicious’ and ‘Granny Smith’, were tested in laboratory conditions by low-mass impact and by acoustic response methods, to evaluate nondestructively the firmness of apple tissue. The performances of the experimental systems and test methods were evaluated first by rubber ball calibration tests. The dynamic impact and acoustic tests were followed by quasi-static compression and penetration tests. The confined elastic modulus E ? of fruit tissue, measured by the compression test, was selected as the preferred physical measure of fruit firmness. The IQ impact parameter and the acoustic firmness index FI predicted very well the elastic modulus of the calibration balls (R /0.99). Good correlation was found between E ? and the acoustic firmness FI of the three apple varieties (R /0.91). The correlation of the new IQ firmness parameter with E ? was similar to that of FI in ‘Golden Delicious’ apples, and slightly lower in the ‘Starking Delicious’ and ‘Granny Smith’ apples (R / 0.82). This was most probably because of the less uniform fruit shape and test conditions of these apple varieties. The conventional impact parameters C1 and C2 calculated from the impact signals produced by the instrumented hammer or by the SIQ-FT system were equivalent to the FI and IQ firmness parameters when compared to the elastic modulus E ? of the rubber balls (R /0.98). These parameters could predict the E ? of the ‘Golden Delicious’ apples (R /0.89), but failed to predict that of ‘Starking Delicious’ (R B/0.54) or ‘Granny Smith’ apples (R B/0.70). This was most probably due to their sensitivity to the impact conditions of the less uniform fruits, as also reported in the cited
literature. The changes in the penetration force MT during the test period were very low, and their correlation with the elastic modulus and the firmness parameters of all apple varieties was poor (R B/0.60). This finding, which has been reported in the past, may indicate that the MT test is not an appropriate measure of apple firmness during shelf life. The test results indicated that the new Sinclair IQ firmness tester offers a new technology that may overcome some technical difficulties indicated by Ortiz-Canavate et al. (2001) and Homer et al. (2002) and others, when studying the performances of the low-mass impact test methods. The SIQ-FT bellows and impact sensor design offer hardware which is easy to apply and less sensitive to variation in fruit size, shape and test location. These design features combined with the IQ firmness parameter performed better in firmness evaluation than the conventional impact parameters C1 and C2. The results also indicated that the acoustic method might improve firmness evaluation in ‘Starking Delicious’ and ‘Granny Smith’ apples. Further effort is needed to verify these results and to adapt the acoustic method for on-line operation.
Acknowledgements The research was supported by the Fund for Promotion of Research of the Technion R&D Foundation Ltd. and Sinclair International Ltd.
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