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One of the most dynamic sectors in agriculture is the fresh fruit and vegetable postharvest, owing mostly to increasing consumer demand for quality produce Aleixos et al., 2002. Consideration of this topic and attention to the fact that machines are more reliable than humans (Njoroge et al., 2002; Aleixos et al., 2002), the shortage of worker in developed countries (Walsh, 2005), and the opportunity to reduce labor costs (Bato et al., 2000), has led to remarkable mechanization and automation in packinghouses during the past few decades. In the past few years, considerable effort has been made to develop indices which objectively estimate fresh fruit ripeness. Studman (2001) reviewed the operations in the postharvest industry, where computers and electronic technologies have had a huge impact. Garcia-Ramos et al. (2005) reviewed state-of-the-art non-destructive fruit firmness sensors.
Numerous methods to determine firmness of fruit nondestructively have been proposed and most of them are acoustic methods (Schotte et al., 1999, Pearson, 2000; Diezma-Iglesias et al., 2006; Valente et al., 2009; Zdunek et al., 2011). Even though it is quick, accurate and nondestructive, the acoustic method has not been generally introduced in practice. As another nondestructive method, ultrasonic techniques have gained increasing research interest for evaluating the quality of agricultural produce owing to the availability of high speed data acquisition and digital signal processing technology (Mizrach et al., 1999; Mizrach, 2000; Kim et al., 2004; Kim et al., 2009). The ultrasonic technique is very practical for nondestructive measurement of the mechanical properties of materials, but most ultrasonic transducers used in previous research on fruit were not suitable because they were designed for industrial usage. Hence in some research, the fruit needed to be sliced uniformly to contact the surface of the ultrasonic transducers. Recently, an ultrasonic transducer for fruit has been successfully developed (Zaki Dizaji et al., 2009).
Between fruits, firmness of peaches due to limits the storage life of them after harvest is very important (Diezma-Iglesias et al., 2006). Physical properties such as size, color, flavor, sugar content and texture or firmness are commonly used to assess ripeness. Most of these factors, excluding size and color, can only be evaluated on a sample basis which is then representative of the entire lot of marketable fruit. Flavor and firmness or texture attributes are subjectively judged by inspectors. As such, these individuals must possess considerable testing experience and knowledge of cultivars. Because of the large range of ripeness within a lot of fruit considered ready for marketing, many fruit lack the desired quality attributes which may lead to unripe or overripe, undesirable product, at retail. The ability to nondestructively sort individual fruit for firmness would improve product uniformity and quality as well as enhance a distributor's marketing strategy.
Therefore, the objective of the present study was to apply the ultrasonic technique to peach fruits, and to analyze the ultrasonic attenuation measurements in conjunction with internal quality analysis such as the firmness, dry weight percentage, pH and sugar content of the fruits, in order to test the possible use of the ultrasonic method as a nondestructive technique for determination of peach ripeness.
2. Material and methods
2.1. Fruit samples and preparation
Iranian peaches (Core-apart Tabrizi Variety) were purchased at the market place. Twenty five kilograms peaches of mentioned variety were randomly taken and transported to the lab. At first, all peaches samples were inspected to ensure that fruit were uniform, undamaged and not attacked by pests and finally 60 samples were selected for experiments. Then, samples on the basis of ripening terms (color, aroma and appearance) divided to two groups (ripe and unripe) and also to determine the effect of fruit size, each group was classified to two categories (large and small). The selected physical properties of studied samples are represented in Table 1. At last, the samples were stored at low temperature (4°C in a refrigerator). Before starting the tests, the required quantities of fruits were taken out of the refrigerator and allowed to be warmed to room temperature for approximately 2 h (Gupta and Das, 2000; Ozarslan, 2002; Khodabakhshian et al., 2012). The ultrasonic measurements were conducted on all samples, after allowing them to reach room temperature. After the ultrasonic measurements, destructive tests of the selected peach samples were conducted.
Mean values and standard deviation of selected physical properties of tested peach fruits.
Parameter Ripe Unripe
Large Small Large Small
Mass (g) 164.7±11.1 135.8±12.4 145.9±12.6 103.1±12.6
Geometric mean diameter (mm) 58.3±4.5 64.5±2.4 62.7±2.4 55.0±24
Sphericity (%) 96.5±2.1 94.8±5.1 97.2±1.5 95.6±4.3
Density (g/cm^3) 1.42±0.23 1.10±0.26 1.94±0.42 0.88±0.25
2.2. Mechanical aspects measurement
After the ultrasonic measurements, compression tests were conducted using a universal testing machine (Model H5KS, Tinius Olsen Company) controlled by a personal computer (Fig.1). The accuracy of measurements for force and deformation was ±0.001 N and 0.001 mm, respectively. The parallel plates were used to compress the peach fruits, one plate was fixed and the other plate was moved to the fruit between the two plates. The compression test results, calculations and graphs were generated automatically. Special software was developed in Visual Basic to control the UTM and to calculate the firmness of the fruit. The loading rate of the crosshead was fixed at 25 mm/min, which was within the range of loading rate (2.5-30 mm/min) specified by the ASAE Standard S368.4 DEC00 (ASAE STANDARDS, 2004). As the compression began and progressed, a load-deformation curve was plotted automatically in relation to the response of each fruit to compression. As firmness parameters, apparent elastic modulus and rupture point were obtained from the equatorial region of the peach using a crosshead controlled by a universal testing machine.
Fig. 1. Universal test machine used in the compression test.
2.3. Experimental set-up and ultrasonic tests
To measure the ultrasonic velocity and attenuation for the whole fruit, tests were carried out using the through-transmission mode of the ultrasonic measurement setup that was design and developed by Zaki Dizaji et al. (2009). The system consisted of a set of fabricated pulser/receiver units with ultrasonic transducers, control software and a data acquisition system, as shown in Fig. 2. The system semi-automatically determined the two important ultrasonic parameters of wave velocity and attenuation coefficient by signal processing. The range of frequency, diameter of the ultrasonic transducer and excitation voltage were 20-50 kHz, 10 mm and less than 600 v, respectively. All experiments were done at 20° C.
Two main parameters those are associated with ultrasonic measurements of agricultural products include ultrasonic velocity and attenuation. The ultrasonic transmission velocity was calculated from the measurements of time of flight and thickness of sample using the following equation (Kim et al., 2009):
where V is the ultrasonic velocity of the fruit (m/s), d the thickness of the sample (m), and t the time of flight (s). The thickness of the peach was measured with a digital vernier caliper (Diamond, China) with an accuracy of ±0.02 mm. The attenuation of the ultrasonic signal from the measurement of signal amplitudes was calculated using the following equation (Cartz, 1998):
where A and A0 are transmitted and received ultrasonic signal amplitudes, respectively, and α is the apparent attenuation of the product (dB/mm).
For best passing of ultrasound waves between the probes and skin of the fruit, a special ultrasound gel was used. After processing in signal processing unit, all transmitted and received ultrasound waves was transmitted to TNM Oscilloscope software.
2.4. Chemical properties measurements
Tissue samples were cut from each fruit separately, from the area close to the region in which the acoustic readings had been taken, and were macerated with a commercial juice extractor, filtered and centrifuged (10000g for 10 min) (Mizrach, 2004). The supernatant juice was used for the determination of sugar content with a manual refractometer (TYM Model, China), and expressed as % Brix in the juice. Dry weight percentage of samples (Between 3-5 g) was done by weighting (MC12200s Model, Sartarius Company) and drying at 105 °C in a forced-air oven for 4 h and reweighed (Mizrach et al., 1999). pH value of peach fruits was determined by a pH meter (3020 Model, GenWay Company).
3. Results and discussion
3.1. Values of Chemical, Mechanical and ultrasonic parameters of peach fruit
A summary of the result of some determined mechanical parameters of studied variety of peach fruit by means of a steel probe having an 8 mm diameter are shown in Table 2. Centre Technique Interprofessionel des Fruits et Legumes, (CTIFL) considers that unhulled peaches are best for eating at firmness below 17 N (Moras, 1995). Many researchers revealed that best ripening time for unhulled peaches is firmness between 10 until 15 N (Crisosto, 1996; Golias et al., 2003; Lleo et al., 2009; Yurtlu., 2012). As it can be found from Table 2, the obtained mechanical properties of unhulled peach fruits in two groups (ripe and unripe) agree with these results. The values of ultrasonic parameters (such as ultrasonic wave velocity and attenuation) on the basis of internal quality of peaches fruits for both studied groups and size categories are also shown in Table 2. As it can be found from this Table 2, the changes in the ultrasonic velocity and attenuation were dependent on the internal quality, so it was concluded that the determination of internal quality of perches using ultrasonic velocity and attenuation will be possible. This justification also was revealed for mango, avocado, tomato, apple fruits by Mizarch et al., 1997; Mizarch et al., 1997; Verlinden et al., 2004; Kim et al., 2009. The obtained relations between these parameters and internal quality of studied variety of peach fruit are represented in the next part.
Mean values and standard deviation of selected Chemical, Mechanical and ultrasonic parameters of tested peach fruits.
Parameter Ripe Unripe
Large Small Large Small
Firmness of unhulled peaches (N) 8.78±9.41 6.22±10.88 16.47±6.6 16.01±12.02
Firmness of hulled peaches (N) 47.58±6.21 30.09±10 57.79±13.02 49.37±22.96
Ultrasonic velocity (m/s) 167.85±19.74 180.04±25.76 191.89±18.58 197.43±30.34
Attenuation (dB/mm) 1.89±0.24 1.66±0.12 1.84±0.22 1.59±0.15
pH (%) 4.25±0.057 4.19±0.071 4.22±0.035 4.10±0.042
Acidity (%) 0.47±0.05 0.40±0.02 0.41±0.01 0.36±0.03
Sugar content (%) 1.56±0.02 1.32±0.07 1.29±0.18 1.26±0.12
Soluble solid content (Brix) 15.25±0.82 12.46±1.4 14.19±1.39 11.29±1.37
Dry weight (%) 14.96±0.54 13.81±2.33 14.29±0.95 12.43±1.84
Table 2 also show mean values and standard deviation of selected chemical parameters of peach fruits such as pH, acidity, sugar content of the fruits, soluble solid content and Dry weight percentage. Soluble solid content is one of the main internal qualities of peach fruit. As it can be found from this Table, during the observed ripening terms (ripe and unripe) SSC of peach fruit increased from about 14% Brix at the unripe term to about 15% at unripe for large sizes. This trend was from 11% to 12.5% for small sizes. Similarly, this increase was reported by Marriot et al. (1981); Mizrach et al.(1999); Mizrach (2004), Lleo et al. (2009) and Moghimi et al. (2010) for banana, avocado, plum, peach and kiwifruit, respectively. Kader (1999) proposed minimum soluble solids content and titratable acidity for acceptable flavour quality of selected fruits in California. According to Kader (1999) the minimum soluble solids content for peach fruit should be above 10% Brix. However, the studied variety is an ordinary cultivar and levels of SSC in the middle of Europe or another continent may be lower or higher than studied variety. Soluble solids content of peach fruit was reported approximately at 10% Brix. Also, this value for peach fruit was found between 12-13% Brix by Lleo et al. (2009). Most of the soluble solids content is sugar. During ripening, the starch of peach is converted to sugar so SSC is increased. Increase of SSC is an important trait of hydrolysis of starch into soluble sugars such as glucose, sucrose and fructose (Kader, 1999). Acidity and pH are other main quality indicators of peach fruits. Variations in these parameters during ripening are shown in Table 2. As it is shown in Table 2, with beginning of ripening stage at the peach fruits acidity increased from 41% to 47% and 36% to 40% for large and small categories, respectively. The minimum titratable acidity for peach fruit was reported by Kader (1999) at 10%.
3.2. Obtained Relations between ultrasonic parameters and internal quality of peach fruits
The calculated attenuation of the ultrasonic waves passing through the peach fruits was plotted against DW content for ripening terms (ripe and unripe) and the large and small categories (Fig. 3). Data points in the figure are the value of 15 samples stored at room temperature and 85% RH for both studied categories and ripening terms. A similar growth in attenuation in the course of ripening terms was found in a previous application of the same method, which addressed the determination of apple firmness (Kim et al., 2009).
Fig. 3. Attenuation against DW content during ripening terms, for peach fruit
A non-linear regression procedure was used to relate variations in ultrasound attenuation and DW to ripening terms. A simple curve fitted to the experimental results defined constants for the curves. A polynomial function (second order equation) was fitted to the values to describe the model relating the variation of attenuation with DW for both studied categories and ripening terms and provided relatively good correlations (Table 3). There is a likeness between results of this paper for peach fruit and the finding of Mizrach et al. (1999); Mizrach (2004) and Kim et al. (2009) for avocado, plum and apple, respectively. In all of these works, the attenuation increased non-linearly with ripening terms.
Table 3- Regression models and coefficient of determination achieved for attenuation and ultrasonic velocity of studied variety of peach fruit as a function of DW.
Ripening term size attenuation R2 ultrasonic velocity R2
Ripe Large y = 0.0779x2 - 2.2134x + 17.512 0.88 y = -4.6004x2 + 115x - 519.33 0.91
Small y = -0.0002x2 + 0.0701x + 0.7241 0.97 y = -2.1533x2 + 36.214x + 91.479 0.97
Unripe Large y = 0.0467x2 - 1.1111x + 8.1599 0.94 y = 6.9584x2 - 222.23x + 1942.9 0.98
Small y = -0.0242x2 + 0.7881x - 4.6217 0.98 y = 13.415x2 - 393.29x + 3052.3 0.94
The experimental results of the ultrasonic velocity of peach fruit for the studied variety are shown in Figure 4. The results show a non-linearly decreasing trend of the ultrasonic velocity with for both studied categories and ripening terms. Since the ultrasonic velocity as ultrasound attenuation is highly correlated with DW (Table 3), it may be possible to estimate the internal quality of peach fruits by measurement of ultrasonic parameters.
Fig. 4. Ultrasonic velocity against DW content during ripening terms, for peach fruit.
Figure 5 showed a nonlinear increase of attenuation during the ripening terms for the peach fruits in conjunction with an increase of soluble solid content. These results are in agreement with previous findings that soluble solid content increased the ripening process (Marriot et al., 1981; Mizrach et al., 1999; Mizrach, 2004, Lleo et al., 2009 and Moghimi et al., 2010) and which simultaneously the attenuation increased with ripening terms (Mizrach et al., 1999; Mizrach, 2004 and Kim et al., 2009). By contrast, as it is depicted in figure 6 there is a nonlinear decrease of ultrasonic velocity with increasing soluble solid content during the same terms. Of course, Contreras et al. (1992) and Saggin and Coupland (2001) found that speed of sound increased with soluble solid content.