- Email: [email protected]
A new method for measuring impact related bruises in fruits
Postharvest Biology and Technology 110 (2015) 131–139
Contents lists available at ScienceDirect
Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
A new method for measuring impact related bruises in fruits Zbigniew Stropek* , Krzysztof Gołacki Department of Mechanical Engineering and Automatics, University of Life Sciences in Lublin, Głe˛boka 28, 20-612 Lublin, Poland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 March 2015 Received in revised form 7 July 2015 Accepted 10 July 2015 Available online 3 August 2015
This paper presents a new method for the direct determination of apple deformation under impact loading conditions that uses a high speed camera. The separately mounted camera was not susceptible to impact oscillations, which allowed for more accurate measurements of displacement. The application of two independent measuring systems, a high speed camera and a force sensor, enabled the quantitative assessment of phenomena occurring during and after the impact of apples against a rigid, flat plate. For the apples with a mass between 170 and 180 g, bruising started at impact velocities of 0.5 m s1. Permanent deformation and maximum stress were the best parameters determining the damage under impact loading conditions. The experiment confirmed the importance of the critical stress criterion as regards the whole fruit under impact loading conditions. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Impact Apple High speed camera Bruise
1. Introduction An increasing demand for fresh fruit and vegetables requires mechanical handling, which results in mechanical damage during harvest, transport, sorting, packing and grading. Mechanical damage causes product quality deterioration which results in economic losses. To minimize these losses, some research has been carried out to evaluate the effect of impact and quasi-static loading conditions during harvest and postharvest handling of fruit, vegetables and other materials of biological origin on mechanical damage (Shirvani et al., 2014; Stropek et al., 2014; Li and Thomas, 2014). The most common impact methods are drop tests (Ragni and Berardinelli, 2001; Menesatti and Paglia, 2001; Lewis et al., 2007; Celik et al., 2011; Ozbek et al., 2014; Shafie et al., 2015) and pendulum tests (Yen and Wan, 2003; Opara et al., 2007; Ahmadi et al., 2010; Polat et al., 2012; Stropek and Gołacki, 2013; Abedi and Ahmadi, 2014). The research was focused on detection of different kinds of damages: bruise, puncture, rupture, cracking and abrasion. Bruise is the most frequent symptom of mechanical damage (Opara and Pathare, 2014). It is due to the action of excessive external force on fruit surface during the impact against a rigid plate or fruit against fruit. Impacts have been measured and the bruise size was quantitatively assessed by several authors (Bollen et al., 1999; Kabas, 2010). Another aspect of the research was the determination of bruise size dependence on mechanical parameters such as peak force, impact energy, absorbed energy, impact velocity, drop height (Brusewitz and Bartsch, 1989; Kitthawee et al., 2011; Boydas
* Corresponding author. E-mail address: [email protected] (Z. Stropek). http://dx.doi.org/10.1016/j.postharvbio.2015.07.005 0925-5214/ ã 2015 Elsevier B.V. All rights reserved.
et al., 2014). Moreover, the factors influencing the bruise damage were determined. The bruise incidence and its size depend on numerous factors, including ripeness, harvest date, temperature, irrigation, weather (Van Zeebroeck et al., 2007). Bruise susceptibility is also affected by macroscopic factors such as size and shape (mass, curvature radius), firmness and turgor (Opara, 2007) as well as microscopic ones which are cell wall strength, elasticity and cell shape (Devaux et al., 2005; Vanstreels et al., 2005; Alamar et al., 2008). The most frequent results of fruit impact measurements are force-time and displacement-time curves. The determination of the force-time curve during impact is not a difficult task, whereas that of the displacement-time curve gives some difficulty. The traditional method consists in measuring acceleration in time and double integration while calculating the initial velocity from the energy conservation law (Fluck and Ahmed, 1973; Lichtensteiger et al., 1988; Jaren and Garcia-Pardo, 2002). However, the double integration results in significant errors that are a result of inaccurate assumptions about integration constants and integration time (Fluck and Ahmed, 1973; Musiol and Harty, 1991). Van Zeebroeck et al. (2003) found that the largest errors occurred at the end of the impact and the final values of velocity and displacement were lower compared to the experimentally determined ones. To avoid this an attempt was made to determine displacementtime courses by direct measurements. Jarimopas et al. (1990) used an electro-optical displacement follower to determine deformation of whole apples under impact loading conditions. Bajema et al. (1998) installed an angular displacement transducer on the pendulum shaft to measure the hammer position during the impact in the cylindrical apple samples. A similar solution was proposed by Van Zeebroeck et al. (2003) who used a pendulum
132
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
impact device with the incremental optical encoder fixed to the axis to measure displacement in time. Tijskens et al. (2003) determined displacement-time courses by means of computer simulations using the DEM technique. The above mentioned measuring systems were permanently fixed to a measuring device in a mechanical way. As a consequence, mechanical oscillations may occur during the impact originating from the measuring system. The authors applied a high speed camera to eliminate the effect of mechanical oscillations on the measurement of displacement-time courses during the impact and thus to determine them more precisely. The use of two independent measuring systems, a high speed camera and a piezoelectric force sensor, enabled precise recording of three characteristics—the time shift between the peak force response and peak deformation, the elastic deformation at relaxation, and the third one permanent deformation. The aim of this research was the determination of force response-time and displacement-time curves as well as of parameters describing the course and effects of such impacts, such as permanent deformation, elastic relaxation after the impact and time difference between the peak force response and peak deformation during the impact. The maximum stress value during the impacts was also calculated. Subsequently, the mechanical parameters were related to the bruise appearance. 2. Materials and methods 2.1. Research material The research was carried out on ‘Rubin’, ‘Florina’ and ‘Freedom’ apple cultivars. Fresh apples stored no longer than two weeks after picking up were used in the examination. To eliminate the influence of mass and shape (curvature radius) on bruise size, all fruit were selected to have a mass between 170–180 g and a maximum diameter of 75–80 mm. One of the criteria of fruit selection was their almost spherical shape. This eliminated a large distribution of bruise surface area size. The second important criterion of apple cultivars selection was their firmness. The ‘Florina’ cultivar is considered to be hard, ‘Rubin’ firm and ‘Freedom’ soft. The apples were dropped from six different heights to obtain impact velocities equal to 0.25, 0.5, 0.75, 1, 1.25, 1.5 m s1, which corresponded to drop heights of 3, 13, 29, 50, 80, 115 mm respectively. For each drop height 10 repetitions were made, resulting in testing 60 apples of each cultivar. In total 180 fruit were subjected to measurements. 2.2. Apple flesh firmness measurements To make an appropriate choice of apple cultivars, preliminary research was carried out to determine firmness by means of the manual Magness-Taylor penetrometer. The firmness measurement consisted in determining the maximum force required for punching apple flesh with a bar of 11.1 mm diameter 8 mm deep at constant velocity. Apple skin and a thin flesh layer were removed using a special knife at half the distance between the stem and the calyx perpendicular to the largest apple diameter. The removed skin area had a shape of a circle with a 15–20 mm diameter. To ensure linearity of displacement and constant slope angle of the head, the penetrometer was mounted on a universal stand for drills. To eliminate temperature effects, apples were kept at room temperature for 12 h before examination. Measurements were made on 30 apples from each cultivar of the same mass and size as in the case of impact tests. To account for differences in flesh firmness within a single fruit (blush and non-blush sides), 5 replicate measurements were made for each apple. The accuracy of firmness measurements was 1 N. Based on this, ‘Florina’, ‘Rubin’
and ‘Freedom’ apple cultivars were selected for impact studies. The firmness differed between cultivars in a statistically significant way (Table 1). 2.3. Measuring device The pendulum principle was used in this measuring stand and an apple was the striking object. The pendulum consisted of a pair of supported fishing lines each 1 m long to which a plastic plate with two tangs was fixed. An apple was fixed to the pendulum. The force sensor was screwed into a sliding case clamped to a thick steel plate fixed permanently to a concrete wall. Moreover, a light titanium plate was screwed to the force sensor. Its diameter was a little larger than the fruit bruise area (Fig. 1). The sliding case and clamp-joint made it possible to place the fruit (fixed to the pendulum) into a position vertical to the plate at the impact moment. In this way a perpendicular direction of the impact force to the impact surface was attained. The device was also equipped with control screws, which allowed positioning of a girder (pendulum rotation axis) so that the impact force direction could pass the fruit mass centre. The drop height was determined by means of a scale with the plotted quantities corresponding to the specified free fall values. The force during the impact was measured by means of a piezoelectric force sensor, model 231110 with a sensitivity of 2.27 mV/N and a measurement range of 2200 N (Technical Manual, 2013). The sensor incorporated a small piezoelectric crystal that was deformed to a very small extent during the impact. Therefore the influence of both the inertia force and of voltage signal delay generated by the deformed piezoelectric crystal on the measurement results could be neglected. At any time the voltage value generated by the piezoelectric crystal was proportional to the applied force. Hence, the error resulting from a dynamic character of apple force response measurement was insignificant and was treated as random. 2.4. Measuring apparatus Two systems were used for measuring impact. To determine the force response course in time, an LMS SCADAS recorder of Siemens company integrated with the LMS Test.Xpress software for data acquisition and analysiswas used. The force response recording frequency was 10 kHz and the measurement was released by means of the trigger after exceeding 0.5 N. The apple impact against a rigid plate was analysed by means of Phanton Miro M320S digital high speed camera of Vision Research company and a lens with the constant focal length of 50 mm. The impact course was recorded by means of the Phantom Camera Control (PCC-2) software at the resolution of 1024 768 pixels and velocity of 3300 frames/s. The measurement errors during image recording with a high speed camera may result from a deviation from perpendicularity of the optical axis of the camera towards the apple motion plane, inappropriate image focus of the observed object, conversion of the apple image dimensions from pixels into length unit (in this case the scale factor amounted to 0.125 mm/pixel). The camera was placed on a special head enabling regulation along three planes with an accuracy of 1. The large contrast of the
Table 1 Firmness of the tested apple cultivars. Cultivar
Firmness (N)
Standard deviation
Standard error
Freedom Rubin Florina
52 64 75
3.2 3.5 5.8
0.7 0.7 1.2
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
133
Then the apples were cut along the vertical plane through the apple centre where the bruise was observed. The bruise depth d and bruise diameter D were measured using a calliper with an accuracy of 0.1 mm. This allowed to determine the force response-time and deformation-time curves, as shown in Fig. 2. The combination of these two curves enabled determination of the following quantities: time dt between the peak force response and peak deformation, permanent deformation xperm, and elastic deformation with apple tissue relaxation as a result of the impact xrel. 2.6. Statistics
Fig. 1. High speed camera picture showing an apple just before the impact into a flat plate. The piezoelectric force sensor is visible, as well as a sliding case to the left of the plate.
apple image against the surrounding background was obtained by a led panel lighting of the object. The absolute measurement error of the apple centre mass displacement was at most 0.125 mm. The influence of the camera position and image contrast on the total measurement error of apple displacement was neglected. To determine the apple displacement-time curvethere was employed the Tema Motion Version 3.8 software (Image Systems, Linköping, Sweden) was used which allowed to analyse apple motion in a recorded image. This software also allowed to measure the apple contact diameter in a vertical plane passing the apple contact plane centre in any impact time point. 2.5. Carrying out measurements and preliminary results processing The mass of each apple with an accuracy of 0.2 g and the apple diameter with the accuracy of 0.1 mm were measured in each test. The apple diameter was measured in the vertical plane along which the impact took place. The apple was fixed to the pendulum by means of two metal grippers. Using control screws, the apple was oriented such that its impact axis passed the sensor axis. In this way the central collision conditions were satisfied. After dropping from a given height and recording the impact course with a high speed camera and a force sensor, the apples were left for 24 h at room temperature to allow browning of the bruised tissue.
The results were statistically analysed with Statistica 5.5 (Statsoft, Tulsa, Oklahoma). The statistical significance of differences between the mean values of studied quantities was determined based on one-way analysis (ANOVA). Tukey’s significance test on the significance level of 0.05 was applied. 3. Results and discussion 3.1. Permanent deformation, elastic deformation at relaxation and time difference between the peak force response and peak deformation The use of two independent measuring systems such as the high speed camera and the piezoelectric force sensor enabled quantitative assessment of three characteristics. The first one included appearance of peak force response and peak deformation in different time points. Under quasi-static loading conditions, peak force response and peak deformation were observed simultaneously, whereas under impact loading conditions the peak force response proceeded peak deformation. Fig. 2. shows the time difference between the occurrence of the peak force response and peak deformation. The positive dt value indicates that the peak force response proceeded the peak deformation. The dt value varied in a small range from 0.1–0.3 ms for the impact velocities under investigation. The mean dt values for apple cultivars were not statistically different either. The literature lacks quantitative analysis of the dt difference. However, there are extensive reports indicating that the peak of the force response and that of displacement were shifted in time (Fluck and Ahmed, 1973; Lichtensteiger et al., 1988; Jaren and Garcia-Pardo, 2002; Tijskens
1.8
180 Force response
160
Deformation
1.5
Force response (N)
140 120
1.2
100 0.9 80 60
Deformation (mm)
dt
0.6 xrel
40 0.3 20
xperm
0
0 0
0.6
1.1
1.6
2.2
2.7
3.3
3.8
4.3
Time (ms) Fig. 2. Typical force response-time and deformation-time curves during the impact of an apple against a rigid, flat plate at the impact velocity 1 m s1.
134
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
et al., 2003). The time shift between the peak force response and peak deformation can result from stresses, which under impact conditions, propagate in the form of a wave. Bajema et al. (1998) determined the shock wave speed in the cylindrical samples of the apple tissue which was 48 m s1. Therefore, deformation velocities were much lower than the stress wave speed. The second characteristic was the elastic deformation at relaxation after the impact. While performing impact tests, the impact time calculated from the force sensor was found to be lower than the values obtained using a high speed camera. When the force sensor indicated zero value of the force response, a high speed camera still showed the contact of the apple with the plate. This was due to the elastic relaxation of the apple after the impact. When the impact force reached zero, the deformation value could be determined for the same time point. Then by subtracting this value from the permanent deformation, the elastic deformation after the impact was obtained as a result of its relaxation. The mean value of the elastic deformation varied from 0.1–0.3 mm for both tested cultivars and impact velocities. The differences between the mean values of the elastic deformation for each cultivar and the tested range of impact velocities were not statistically significant (Fig. 3). Similar results were also observed by Tijskens et al. (2003) who impacted apples with a spherical aluminium impactor attached to a pendulum. Observing the zero value of contact force of apple with the pendulum, they found that the contact was over before the apple surface relaxed. However, they did not assess this phenomenon quantitatively. The third characteristic was the permanent deformation. Fig. 4 shows the relationship between permanent deformation and impact velocity. It was found that the permanent deformation increased with increasing impact velocity. Statistically significant differences between the mean values of permanent deformation for a given impact velocity for the tested cultivars were not found. However, the ‘Florina’ hard cultivar had lower values of permanent deformation than the ‘Freedom’ soft cultivar for each impact velocity. Fig. 4 shows statistically significant differences between the mean values of permanent deformation in the case of impact velocities 0.25 m s1 and 0.5 m s1 for each cultivar. The apples were not bruised at the impact velocity 0.25 m s1 but at 0.5 m s1. Thus this quantity could be used to assess apple bruise at the very beginning.
Occurrence of permanent deformation called the residual displacement was already found by Fluck and Ahmed (1973). Lu and Wang (2007) observed increasing permanent deformation with the drop height of the apple, while determining the bruise limit for the ‘Gala’ cultivar. They obtained similar values of permanent deformation for comparable impact velocities, although the tested drop height was much larger. From the zero value of permanent deformation they found that the critical value of the drop height at which no bruise occurred, was 4 cm, which corresponded to an impact velocity 0.88 m s1. This value was correlated to the mass of tested apples (140–150 g). The mass had a significant effect on the bruise appearance. The larger the fruit mass, the lower the drop height at which the bruise took place (Lang, 1994; Stropek and Gołacki, 2007). 3.2. Comparison of the results of bruise measurements after discoloration and obtained by a high speed camera The bruise sizes were determined based on two parameters: the bruise depth d and the bruise diameter D. The above mentioned quantities were measured in the vertical plane going through the apple centre and parallel to the impact direction. Fig. 5 shows the relationship between the bruise depth and the impact velocity whereas Fig. 6 shows the relationship between the bruise diameter and the impact velocity. The bruise depth and the bruise diameter increased with the increasing impact velocity for each tested cultivar. Large values of standard deviation of the bruise depth and bruise diameter at the impact velocity 0.5 m s1 resulted from the fact that one group of apples was bruised whereas another was not. This indicated that an impact velocity 0.5 m s1 for the apples of 170–180 g mass was the critical velocity and its corresponding drop height, 13 mm, was the bruise threshold. Similar results were observed in previous research (Stropek and Gołacki, 2010). They determined the energy restitution coefficient for three apple cultivars and found the limit of the apple bruise for the impact velocity to be close to 0.44 m s1. Using the IHMI technique (increasing height multiple impact) Bajema and Hyde (1998) obtained the bruise threshold for the apples at a drop height 16 mm. The use of a high speed camera and Tema Motion software allowed to determine both the relationships between the peak
Fig. 3. Influence of the impact velocity on the elastic deformation developed as a result of apple tissue relaxation after the impact.
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
135
Fig. 4. Influence of the impact velocity on the permanent deformation for three apple cultivars.
deformation and the impact velocity (Fig. 7), as well as between the contact diameter and the impact velocity (Fig. 8). Both figures show that with an increase of impact velocity, peak deformation and contact diameter increased as well. Similar peak deformation was found earlier. Dintwa et al. (2008) applied the FEM technique to analyse the impact of ‘Jonagold’ against a rigid, flat plate. At an impact velocity 0.5 m s1 they obtained a peak deformation of around 1.1 mm. From Figs. 5 and 7 it can be seen that smaller values of peak deformation resulted in larger values of bruise depth for each impact velocity for all cultivars. For instance, at an impact velocity of 1.5 m s1, the peak deformation of 2 mm resulted in the bruise
depth of 6 mm. At the lowest impact velocity 0.25 m s1, the peak deformation 0.5 mm did not cause visible bruising of tested samples (Fig. 7). The mean values of the contact diameter were also larger than those of the bruise diameter for each impact velocity (Figs. 6 and 8). The larger the impact velocity, the smaller the difference between the contact diameter and bruise diameter. Yuwana and Duprat (1998) found that, for all the apples they tested, the bruise diameter was on the average 6.44% lower than its corresponding contact diameter. As follows from Fig. 8 for an impact velocity 0.25 m s1 at which the apples were not bruised, the mean values of the contact diameter was about 11.5 mm for all cultivars. Assuming a round
Fig. 5. Relationship between the bruise depth and the impact velocity for three apple cultivars.
136
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
Fig. 6. Relationship between the bruise diameter and the impact velocity for three apple cultivars.
shape of the contact surface area, a value around 1 cm2 was obtained which is consistent with the current standards applied in most postharvest handling systems that allow the bruise surface area to be not larger than 1 cm2 (Bollen et al., 2001). According to the United States Standards for Grades of Apples (NN, 2002), a bruise depth lower than 3.2 mm does not cause quality deterioration of the apple. Fig. 5 shows that the value of the bruise depth in the experiment developed at the impact velocity above 0.75 m s1. This corresponded to a bruise diameter of around 15 mm (Fig. 6) and a bruise surface area of around 1.8 cm2 which was much larger than the limit of 1 cm2.
3.3. Determining the maximum stress A round shape of the apple contact surface A on a rigid, flat plate was assumed and its value at the time point corresponding to the peak force response was calculated from A¼
pd2cont 4
(1)
where dcont is the contact diameter at peak force response (mm). The maximum stress s max was calculated as
s max ¼
F max A
Fig. 7. Relationship between the peak deformation and the impact velocity for three apple cultivars.
(2)
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
137
Fig. 8. Relationship between the contact diameter at the point of the peak deformation and the impact velocity for three apple cultivars.
Comparing the contact surface area and peak force response, a linear relationship was obtained (Fig. 9). This indicates a constant value of the maximum stress for all impact velocities. Fig. 10 shows statistically significant differences of the mean values of the maximum stress between the lowest impact velocity of 0.25 m s1 and other impact velocities for all tested cultivars. It should be mentioned that an impact velocity 0.5 m s1 was the limit beyond which the bruise developed. It seems that the maximum stress was a good parameter for estimation of bruise appearance. The constant value of maximum stress s max for the impact velocities from 0.5 m s1 to 1.5 m s1 showed that apple tissue damage was caused by exceeding the specific stress value. Hence the experiment confirmed the importance of the critical
stress criterion as regards the whole fruits tested under the impact loading conditions. Therefore the obtained mean values of stress could be interpreted as the maximum values sustained by the apple tissue. For impact velocities 0.5–1.5 m s1, the differences between the mean values of the maximum stress were not statistically significant. The ‘Florina’ hard and ‘Freedom’ soft cultivars had the maximum stress values about 0.4 MPa and 0.35 MPa respectively. According to the authors, the mean value of maximum stress, for the tested apple cultivars was in the range of 0.4–0.5 MPa (Stropek and Gołacki, 2013). Fluck and Ahmed (1973) pointed out that the increase of the peak force response resulted from the increasing drop height and the mass dropped onto a fruit. The above confirmed the hypothesis that the peak force and
Fig. 9. Relationship between the maximum force response and the contact surface area for three apple cultivars.
138
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
Fig. 10. Relationship between the maximum stress and the impact velocity.
resulting internal stress were critical factors for damage to occur during impact. 4. Conclusions The application of a high speed camera allowed to eliminate mechanical oscillations coming from the measuring device which affected the accuracy of the force response-time and displacement-time curves during the impact. The displacement-time curves were determined in a direct way without double integration of the acceleration-time course associated with significant calculation errors. The research showed that the best parameters describing the damage beginning under impact loading conditions were permanent deformation and maximum stress. The constant value of the maximum stress in all tested impact velocities causing the bruise confirmed the importance of the critical stress criterion as regards the whole fruit under impact loading conditions. An impact velocity 0.5 m s1 was the critical velocity for apples of 170– 180 g at which the bruise appeared. Impact velocities lower than 0.5 m s1 resulted in the bruise surface area about 1 cm2, which corresponds to the current standards for apple grading. The deformation which was caused by the elastic relaxation of apples after the impact, varied from 0.1 to 0.3 mm for three cultivars for all tested impact velocities. References Abedi, G., Ahmadi, E., 2014. Bruise susceptibilities of Golden Delicious apples as affected by mechanical impact and fruit properties. J. Agr Sci. 152, 439–447. Ahmadi, E., Ghassemzadeh, H.R., Sadeghi, M., Moghaddam, M., Neshat, S.Z., 2010. The effect of impact and fruit properities on the bruising of peach. J. Food Eng. 97, 110–117. Alamar, M.C., Vanstreels, E., Oey, M.L., Molto, E., Nicolai, B.M., 2008. Micromechanical behaviour of apple tissue in tensile and compression test: storage conditions and cultivar effect. J. Food Eng. 86, 324–333. Bajema, R.W., Hyde, G.M., 1998. Instrumented pendulum for impact characterization of whole fruit and vegetable specimens. Trans. ASAE 41 (5), 1399–1405. Bajema, R.W., Hyde, G.M., Peterson, K., 1998. Instrumentation design for dynamic axial compression of cylindrical tissue samples. Trans. ASAE 41 (3), 747–754. Bollen, A.F., Cox, N.R., Dela Rue, B.T., Painter, D.J., 2001. A descriptor for damage susceptibility of population of produce. J. Agric. Eng Res. 78 (4), 391–395. Bollen, A.F., Nguyen, H.X., Dela Rue, B.T., 1999. Comparison of methods for estimating the bruise volume of apples. J. Agric. Eng. Res. 74, 325–330.
Boydas, M.G., Ozbek, I.Y., Kara, M., 2014. An efficient laser sensor system for apple impact bruise volume estimation. Postharvest Biol. Technol. 89, 49–55. Brusewitz, G.H., Bartsch, J.A., 1989. Impact parameters related to post harvest bruising of apples. Trans. ASAE 32 (3), 953–957. Celik, H.K., Rennie, A.E.W., Akinci, I., 2011. Deformation behaviour simulation of an apple under drop case by finite element method. J. Food Eng. 104, 293–298. Devaux, M.F., Barakat, A., Robert, P., Bouchet, B., Guillon, F., Navez, B., Lahaye, M., 2005. Mechanical breakdown and cell wall strusture of mealy tomato pericarp tissue. Postharvest Biol. Technol. 37, 209–221. Dintwa, E., Van Zeebroeck, M., Ramon, H., Tijskens, E., 2008. Finite element method of the dynamic collision of apple fruit. Postharvest Biol. Technol. 49, 260–276. Fluck, R.C., Ahmed, E.M., 1973. Impact testing of fruits and vegetables. Trans. ASAE 16 (4), 660–666. Jaren, C., Garcia-Pardo, E., 2002. Using non-destructive impact testing for sorting fruits. J. Food Eng. 53, 89–95. Jarimopas, B., Sarig, Y., Peiper, U.M., Manor, G., 1990. Instrumentation for measuring the response of apples subjected to impact loading. Comput. Electron. Agric 5, 255–260. Kabas, O., 2010. Methods of measuring bruise volume of pear (Pyrus Communis L.). Int. J. Food Prop. 13, 1178–1186. Kitthawee, U., Pathaveerat, S., Srirungruang, T., Slaughter, D., 2011. Mechanical bruising of young coconut. Biosyst. Eng. 109, 211–219. Lang, Z., 1994. The influence of mass and velocity on the maximum allowable impact energy of apples. J. Agric. Eng. Res. 57, 213–216. Lewis, R., Yoxall, A., Canty, L.A., Reina Romo, E., 2007. Development of engineering design tools to help reduce apple bruising. J. Food Eng. 83, 356–365. Li, Z., Thomas, C., 2014. Quantitative evaluation of mechanical damage of fresh fruits. Trends Food Sci. Technol. 35, 138–150. Lichtensteiger, M.J., Holmes, R.G., Hamdy, M.Y., Blaisdell, J.L., 1988. Impact parameters of spherical viscoelastic objects and tomatoes. Trans. ASAE 31 (2), 595–602. Lu, L.X., Wang, Z.W., 2007. Dropping bruise fragility and bruise boundary of apple fruit. Trans. ASABE 50 (4), 1323–1329. Menesatti, P., Paglia, G., 2001. Development of drop damage index of fruit resistance to damage. J. Agric. Eng Res. 80 (1), 53–64. Musiol, C., Harty, D., 1991. The use of numerical integration for the estimation of displacement from an accelerometer signal. Application note 12. nCode International Ltd. NN, 2002. United States Standards for Grades of apples. United States Department of Agriculture. Opara, U.L., 2007. Bruise susceptibilities of ‘Gala’ apples as affected by orchard management practices and harvest date. Postharvest Biol. Technol. 43, 47–54. Opara, U.L., Al-Ghafri, A., Agzoun, H., Al-Issai, J., Al-Jabri, F., 2007. Design and development of a new device for measuring susceptibility to impact damage of fresh produce. N. Z. J. Crop Hortic. Sci. 35, 245–251. Opara, U.L., Pathare, P.B., 2014. Bruise damage measurement and analysis of fresh horticultural produce-A review. Postharvest Biol. Technol. 91, 9–24. Ozbek, I.Y., Boydas, M.G., Kara, M., Demir, B., 2014. Low cost measurement setup based on a piezoelectric microphone for estimating apple bruising using Shannon entropy. Postharvest Biol. Technol. 98, 23–29. Polat, R., Aktas, T., Ikinci, A., 2012. Selected mechanical properties and bruise susceptibility of nectarine fruit. International J. Food Prop 15, 1369–1380.
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139 Ragni, L., Berardinelli, A., 2001. Mechanical behaviour of apples and damage during sorting and packaging. J. Agric. Eng Res. 78 (3), 273–279. Shafie, M.M., Rajabipour, A., Castro-Garcia, S., Jimenez-Jimenez, F., Mobli, H., 2015. Effect of fruit properties on pomegranate bruising. Int. J Food Prop. 18, 1837– 1846. Shirvani, M., Ghanbarian, D., Ghasemi-Varnamkhasti, M., 2014. Measurement and evaluation of the apparent modulus of elasticity of apple based on Hooke’s, Hertz’s and Boussinesq’s theories. Measurement 54, 133–139. Stropek, Z., Gołacki, K., 2007. Relation between mass and drop height at impact causing bruising of apple. Polish J. Food Nut. Sci. 57 (2), 165–167. Stropek, Z., Gołacki, K., 2010. Determining the coefficient of restitution of apples at different impact velocities. Teka Kom. Mot. Energ. Roln. PAN 10, 417–424. Stropek, Z., Gołacki, K., 2013. The effect of drop height on bruising of selected apple cultivars. Postharvest Biol. Technol. 85, 167–172. Stropek, Z., Gołacki, K., Kołodziej, P., Gładyszewska, B., Samociuk, W., Rejak, A., 2014. Effect of polyvinyl alcohol and keratin on stress relaxation course in thermoplastic starch. Przem. Chem. 93, 364–367.
139
Technical Manual, 2013. ISOTRON Force Sensor—ENDEVCO model 2311–10. Tijskens, E., Ramon, H., De Baerdemaeker, J., 2003. Discrete element modelling for process simulation in agriculture. J. Sound Vib. 266, 493–514. Vanstreels, E., Alamar, M.C., Verlinden, B.E., Enninghorst, A., Loodts, J.K.A., Tijskens, E., Ramon, H., Nicolai, B.M., 2005. Micromechanical behaviour of onion epidermal tissue. Postharvest Biol. Technol. 37, 163–173. Van Zeebroeck, M., Tijskens, E., Van Liedekerke, P., Deli, V., De Baerdemaeker, J., Ramon, H., 2003. Determination of the dynamical behaviour of biological materials during impact using a pendulum device. J. Sound Vibr. 266, 465–480. Van Zeebroeck, M., Van linden, V., Darius, P., De Ketelaere, B., Ramon, H., Tijskens, E., 2007. The effect of fruit factors on the bruise susceptibility of apples. Postharvest Biol. Technol. 46, 10–19. Yen, M., Wan, Y., 2003. Determination of textural indices of guiava fruit using discriminate analysis by impact force. Trans. ASAE 46 (4), 1161–1166. Yuwana, Y., Duprat, F., 1998. Prediction of apple bruising based on the instantaneous impact shear stress and energy absorbed. Int. Agrophys. 12, 133–140.
Contents lists available at ScienceDirect
Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
A new method for measuring impact related bruises in fruits Zbigniew Stropek* , Krzysztof Gołacki Department of Mechanical Engineering and Automatics, University of Life Sciences in Lublin, Głe˛boka 28, 20-612 Lublin, Poland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 March 2015 Received in revised form 7 July 2015 Accepted 10 July 2015 Available online 3 August 2015
This paper presents a new method for the direct determination of apple deformation under impact loading conditions that uses a high speed camera. The separately mounted camera was not susceptible to impact oscillations, which allowed for more accurate measurements of displacement. The application of two independent measuring systems, a high speed camera and a force sensor, enabled the quantitative assessment of phenomena occurring during and after the impact of apples against a rigid, flat plate. For the apples with a mass between 170 and 180 g, bruising started at impact velocities of 0.5 m s1. Permanent deformation and maximum stress were the best parameters determining the damage under impact loading conditions. The experiment confirmed the importance of the critical stress criterion as regards the whole fruit under impact loading conditions. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Impact Apple High speed camera Bruise
1. Introduction An increasing demand for fresh fruit and vegetables requires mechanical handling, which results in mechanical damage during harvest, transport, sorting, packing and grading. Mechanical damage causes product quality deterioration which results in economic losses. To minimize these losses, some research has been carried out to evaluate the effect of impact and quasi-static loading conditions during harvest and postharvest handling of fruit, vegetables and other materials of biological origin on mechanical damage (Shirvani et al., 2014; Stropek et al., 2014; Li and Thomas, 2014). The most common impact methods are drop tests (Ragni and Berardinelli, 2001; Menesatti and Paglia, 2001; Lewis et al., 2007; Celik et al., 2011; Ozbek et al., 2014; Shafie et al., 2015) and pendulum tests (Yen and Wan, 2003; Opara et al., 2007; Ahmadi et al., 2010; Polat et al., 2012; Stropek and Gołacki, 2013; Abedi and Ahmadi, 2014). The research was focused on detection of different kinds of damages: bruise, puncture, rupture, cracking and abrasion. Bruise is the most frequent symptom of mechanical damage (Opara and Pathare, 2014). It is due to the action of excessive external force on fruit surface during the impact against a rigid plate or fruit against fruit. Impacts have been measured and the bruise size was quantitatively assessed by several authors (Bollen et al., 1999; Kabas, 2010). Another aspect of the research was the determination of bruise size dependence on mechanical parameters such as peak force, impact energy, absorbed energy, impact velocity, drop height (Brusewitz and Bartsch, 1989; Kitthawee et al., 2011; Boydas
* Corresponding author. E-mail address: [email protected] (Z. Stropek). http://dx.doi.org/10.1016/j.postharvbio.2015.07.005 0925-5214/ ã 2015 Elsevier B.V. All rights reserved.
et al., 2014). Moreover, the factors influencing the bruise damage were determined. The bruise incidence and its size depend on numerous factors, including ripeness, harvest date, temperature, irrigation, weather (Van Zeebroeck et al., 2007). Bruise susceptibility is also affected by macroscopic factors such as size and shape (mass, curvature radius), firmness and turgor (Opara, 2007) as well as microscopic ones which are cell wall strength, elasticity and cell shape (Devaux et al., 2005; Vanstreels et al., 2005; Alamar et al., 2008). The most frequent results of fruit impact measurements are force-time and displacement-time curves. The determination of the force-time curve during impact is not a difficult task, whereas that of the displacement-time curve gives some difficulty. The traditional method consists in measuring acceleration in time and double integration while calculating the initial velocity from the energy conservation law (Fluck and Ahmed, 1973; Lichtensteiger et al., 1988; Jaren and Garcia-Pardo, 2002). However, the double integration results in significant errors that are a result of inaccurate assumptions about integration constants and integration time (Fluck and Ahmed, 1973; Musiol and Harty, 1991). Van Zeebroeck et al. (2003) found that the largest errors occurred at the end of the impact and the final values of velocity and displacement were lower compared to the experimentally determined ones. To avoid this an attempt was made to determine displacementtime courses by direct measurements. Jarimopas et al. (1990) used an electro-optical displacement follower to determine deformation of whole apples under impact loading conditions. Bajema et al. (1998) installed an angular displacement transducer on the pendulum shaft to measure the hammer position during the impact in the cylindrical apple samples. A similar solution was proposed by Van Zeebroeck et al. (2003) who used a pendulum
132
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
impact device with the incremental optical encoder fixed to the axis to measure displacement in time. Tijskens et al. (2003) determined displacement-time courses by means of computer simulations using the DEM technique. The above mentioned measuring systems were permanently fixed to a measuring device in a mechanical way. As a consequence, mechanical oscillations may occur during the impact originating from the measuring system. The authors applied a high speed camera to eliminate the effect of mechanical oscillations on the measurement of displacement-time courses during the impact and thus to determine them more precisely. The use of two independent measuring systems, a high speed camera and a piezoelectric force sensor, enabled precise recording of three characteristics—the time shift between the peak force response and peak deformation, the elastic deformation at relaxation, and the third one permanent deformation. The aim of this research was the determination of force response-time and displacement-time curves as well as of parameters describing the course and effects of such impacts, such as permanent deformation, elastic relaxation after the impact and time difference between the peak force response and peak deformation during the impact. The maximum stress value during the impacts was also calculated. Subsequently, the mechanical parameters were related to the bruise appearance. 2. Materials and methods 2.1. Research material The research was carried out on ‘Rubin’, ‘Florina’ and ‘Freedom’ apple cultivars. Fresh apples stored no longer than two weeks after picking up were used in the examination. To eliminate the influence of mass and shape (curvature radius) on bruise size, all fruit were selected to have a mass between 170–180 g and a maximum diameter of 75–80 mm. One of the criteria of fruit selection was their almost spherical shape. This eliminated a large distribution of bruise surface area size. The second important criterion of apple cultivars selection was their firmness. The ‘Florina’ cultivar is considered to be hard, ‘Rubin’ firm and ‘Freedom’ soft. The apples were dropped from six different heights to obtain impact velocities equal to 0.25, 0.5, 0.75, 1, 1.25, 1.5 m s1, which corresponded to drop heights of 3, 13, 29, 50, 80, 115 mm respectively. For each drop height 10 repetitions were made, resulting in testing 60 apples of each cultivar. In total 180 fruit were subjected to measurements. 2.2. Apple flesh firmness measurements To make an appropriate choice of apple cultivars, preliminary research was carried out to determine firmness by means of the manual Magness-Taylor penetrometer. The firmness measurement consisted in determining the maximum force required for punching apple flesh with a bar of 11.1 mm diameter 8 mm deep at constant velocity. Apple skin and a thin flesh layer were removed using a special knife at half the distance between the stem and the calyx perpendicular to the largest apple diameter. The removed skin area had a shape of a circle with a 15–20 mm diameter. To ensure linearity of displacement and constant slope angle of the head, the penetrometer was mounted on a universal stand for drills. To eliminate temperature effects, apples were kept at room temperature for 12 h before examination. Measurements were made on 30 apples from each cultivar of the same mass and size as in the case of impact tests. To account for differences in flesh firmness within a single fruit (blush and non-blush sides), 5 replicate measurements were made for each apple. The accuracy of firmness measurements was 1 N. Based on this, ‘Florina’, ‘Rubin’
and ‘Freedom’ apple cultivars were selected for impact studies. The firmness differed between cultivars in a statistically significant way (Table 1). 2.3. Measuring device The pendulum principle was used in this measuring stand and an apple was the striking object. The pendulum consisted of a pair of supported fishing lines each 1 m long to which a plastic plate with two tangs was fixed. An apple was fixed to the pendulum. The force sensor was screwed into a sliding case clamped to a thick steel plate fixed permanently to a concrete wall. Moreover, a light titanium plate was screwed to the force sensor. Its diameter was a little larger than the fruit bruise area (Fig. 1). The sliding case and clamp-joint made it possible to place the fruit (fixed to the pendulum) into a position vertical to the plate at the impact moment. In this way a perpendicular direction of the impact force to the impact surface was attained. The device was also equipped with control screws, which allowed positioning of a girder (pendulum rotation axis) so that the impact force direction could pass the fruit mass centre. The drop height was determined by means of a scale with the plotted quantities corresponding to the specified free fall values. The force during the impact was measured by means of a piezoelectric force sensor, model 231110 with a sensitivity of 2.27 mV/N and a measurement range of 2200 N (Technical Manual, 2013). The sensor incorporated a small piezoelectric crystal that was deformed to a very small extent during the impact. Therefore the influence of both the inertia force and of voltage signal delay generated by the deformed piezoelectric crystal on the measurement results could be neglected. At any time the voltage value generated by the piezoelectric crystal was proportional to the applied force. Hence, the error resulting from a dynamic character of apple force response measurement was insignificant and was treated as random. 2.4. Measuring apparatus Two systems were used for measuring impact. To determine the force response course in time, an LMS SCADAS recorder of Siemens company integrated with the LMS Test.Xpress software for data acquisition and analysiswas used. The force response recording frequency was 10 kHz and the measurement was released by means of the trigger after exceeding 0.5 N. The apple impact against a rigid plate was analysed by means of Phanton Miro M320S digital high speed camera of Vision Research company and a lens with the constant focal length of 50 mm. The impact course was recorded by means of the Phantom Camera Control (PCC-2) software at the resolution of 1024 768 pixels and velocity of 3300 frames/s. The measurement errors during image recording with a high speed camera may result from a deviation from perpendicularity of the optical axis of the camera towards the apple motion plane, inappropriate image focus of the observed object, conversion of the apple image dimensions from pixels into length unit (in this case the scale factor amounted to 0.125 mm/pixel). The camera was placed on a special head enabling regulation along three planes with an accuracy of 1. The large contrast of the
Table 1 Firmness of the tested apple cultivars. Cultivar
Firmness (N)
Standard deviation
Standard error
Freedom Rubin Florina
52 64 75
3.2 3.5 5.8
0.7 0.7 1.2
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
133
Then the apples were cut along the vertical plane through the apple centre where the bruise was observed. The bruise depth d and bruise diameter D were measured using a calliper with an accuracy of 0.1 mm. This allowed to determine the force response-time and deformation-time curves, as shown in Fig. 2. The combination of these two curves enabled determination of the following quantities: time dt between the peak force response and peak deformation, permanent deformation xperm, and elastic deformation with apple tissue relaxation as a result of the impact xrel. 2.6. Statistics
Fig. 1. High speed camera picture showing an apple just before the impact into a flat plate. The piezoelectric force sensor is visible, as well as a sliding case to the left of the plate.
apple image against the surrounding background was obtained by a led panel lighting of the object. The absolute measurement error of the apple centre mass displacement was at most 0.125 mm. The influence of the camera position and image contrast on the total measurement error of apple displacement was neglected. To determine the apple displacement-time curvethere was employed the Tema Motion Version 3.8 software (Image Systems, Linköping, Sweden) was used which allowed to analyse apple motion in a recorded image. This software also allowed to measure the apple contact diameter in a vertical plane passing the apple contact plane centre in any impact time point. 2.5. Carrying out measurements and preliminary results processing The mass of each apple with an accuracy of 0.2 g and the apple diameter with the accuracy of 0.1 mm were measured in each test. The apple diameter was measured in the vertical plane along which the impact took place. The apple was fixed to the pendulum by means of two metal grippers. Using control screws, the apple was oriented such that its impact axis passed the sensor axis. In this way the central collision conditions were satisfied. After dropping from a given height and recording the impact course with a high speed camera and a force sensor, the apples were left for 24 h at room temperature to allow browning of the bruised tissue.
The results were statistically analysed with Statistica 5.5 (Statsoft, Tulsa, Oklahoma). The statistical significance of differences between the mean values of studied quantities was determined based on one-way analysis (ANOVA). Tukey’s significance test on the significance level of 0.05 was applied. 3. Results and discussion 3.1. Permanent deformation, elastic deformation at relaxation and time difference between the peak force response and peak deformation The use of two independent measuring systems such as the high speed camera and the piezoelectric force sensor enabled quantitative assessment of three characteristics. The first one included appearance of peak force response and peak deformation in different time points. Under quasi-static loading conditions, peak force response and peak deformation were observed simultaneously, whereas under impact loading conditions the peak force response proceeded peak deformation. Fig. 2. shows the time difference between the occurrence of the peak force response and peak deformation. The positive dt value indicates that the peak force response proceeded the peak deformation. The dt value varied in a small range from 0.1–0.3 ms for the impact velocities under investigation. The mean dt values for apple cultivars were not statistically different either. The literature lacks quantitative analysis of the dt difference. However, there are extensive reports indicating that the peak of the force response and that of displacement were shifted in time (Fluck and Ahmed, 1973; Lichtensteiger et al., 1988; Jaren and Garcia-Pardo, 2002; Tijskens
1.8
180 Force response
160
Deformation
1.5
Force response (N)
140 120
1.2
100 0.9 80 60
Deformation (mm)
dt
0.6 xrel
40 0.3 20
xperm
0
0 0
0.6
1.1
1.6
2.2
2.7
3.3
3.8
4.3
Time (ms) Fig. 2. Typical force response-time and deformation-time curves during the impact of an apple against a rigid, flat plate at the impact velocity 1 m s1.
134
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
et al., 2003). The time shift between the peak force response and peak deformation can result from stresses, which under impact conditions, propagate in the form of a wave. Bajema et al. (1998) determined the shock wave speed in the cylindrical samples of the apple tissue which was 48 m s1. Therefore, deformation velocities were much lower than the stress wave speed. The second characteristic was the elastic deformation at relaxation after the impact. While performing impact tests, the impact time calculated from the force sensor was found to be lower than the values obtained using a high speed camera. When the force sensor indicated zero value of the force response, a high speed camera still showed the contact of the apple with the plate. This was due to the elastic relaxation of the apple after the impact. When the impact force reached zero, the deformation value could be determined for the same time point. Then by subtracting this value from the permanent deformation, the elastic deformation after the impact was obtained as a result of its relaxation. The mean value of the elastic deformation varied from 0.1–0.3 mm for both tested cultivars and impact velocities. The differences between the mean values of the elastic deformation for each cultivar and the tested range of impact velocities were not statistically significant (Fig. 3). Similar results were also observed by Tijskens et al. (2003) who impacted apples with a spherical aluminium impactor attached to a pendulum. Observing the zero value of contact force of apple with the pendulum, they found that the contact was over before the apple surface relaxed. However, they did not assess this phenomenon quantitatively. The third characteristic was the permanent deformation. Fig. 4 shows the relationship between permanent deformation and impact velocity. It was found that the permanent deformation increased with increasing impact velocity. Statistically significant differences between the mean values of permanent deformation for a given impact velocity for the tested cultivars were not found. However, the ‘Florina’ hard cultivar had lower values of permanent deformation than the ‘Freedom’ soft cultivar for each impact velocity. Fig. 4 shows statistically significant differences between the mean values of permanent deformation in the case of impact velocities 0.25 m s1 and 0.5 m s1 for each cultivar. The apples were not bruised at the impact velocity 0.25 m s1 but at 0.5 m s1. Thus this quantity could be used to assess apple bruise at the very beginning.
Occurrence of permanent deformation called the residual displacement was already found by Fluck and Ahmed (1973). Lu and Wang (2007) observed increasing permanent deformation with the drop height of the apple, while determining the bruise limit for the ‘Gala’ cultivar. They obtained similar values of permanent deformation for comparable impact velocities, although the tested drop height was much larger. From the zero value of permanent deformation they found that the critical value of the drop height at which no bruise occurred, was 4 cm, which corresponded to an impact velocity 0.88 m s1. This value was correlated to the mass of tested apples (140–150 g). The mass had a significant effect on the bruise appearance. The larger the fruit mass, the lower the drop height at which the bruise took place (Lang, 1994; Stropek and Gołacki, 2007). 3.2. Comparison of the results of bruise measurements after discoloration and obtained by a high speed camera The bruise sizes were determined based on two parameters: the bruise depth d and the bruise diameter D. The above mentioned quantities were measured in the vertical plane going through the apple centre and parallel to the impact direction. Fig. 5 shows the relationship between the bruise depth and the impact velocity whereas Fig. 6 shows the relationship between the bruise diameter and the impact velocity. The bruise depth and the bruise diameter increased with the increasing impact velocity for each tested cultivar. Large values of standard deviation of the bruise depth and bruise diameter at the impact velocity 0.5 m s1 resulted from the fact that one group of apples was bruised whereas another was not. This indicated that an impact velocity 0.5 m s1 for the apples of 170–180 g mass was the critical velocity and its corresponding drop height, 13 mm, was the bruise threshold. Similar results were observed in previous research (Stropek and Gołacki, 2010). They determined the energy restitution coefficient for three apple cultivars and found the limit of the apple bruise for the impact velocity to be close to 0.44 m s1. Using the IHMI technique (increasing height multiple impact) Bajema and Hyde (1998) obtained the bruise threshold for the apples at a drop height 16 mm. The use of a high speed camera and Tema Motion software allowed to determine both the relationships between the peak
Fig. 3. Influence of the impact velocity on the elastic deformation developed as a result of apple tissue relaxation after the impact.
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
135
Fig. 4. Influence of the impact velocity on the permanent deformation for three apple cultivars.
deformation and the impact velocity (Fig. 7), as well as between the contact diameter and the impact velocity (Fig. 8). Both figures show that with an increase of impact velocity, peak deformation and contact diameter increased as well. Similar peak deformation was found earlier. Dintwa et al. (2008) applied the FEM technique to analyse the impact of ‘Jonagold’ against a rigid, flat plate. At an impact velocity 0.5 m s1 they obtained a peak deformation of around 1.1 mm. From Figs. 5 and 7 it can be seen that smaller values of peak deformation resulted in larger values of bruise depth for each impact velocity for all cultivars. For instance, at an impact velocity of 1.5 m s1, the peak deformation of 2 mm resulted in the bruise
depth of 6 mm. At the lowest impact velocity 0.25 m s1, the peak deformation 0.5 mm did not cause visible bruising of tested samples (Fig. 7). The mean values of the contact diameter were also larger than those of the bruise diameter for each impact velocity (Figs. 6 and 8). The larger the impact velocity, the smaller the difference between the contact diameter and bruise diameter. Yuwana and Duprat (1998) found that, for all the apples they tested, the bruise diameter was on the average 6.44% lower than its corresponding contact diameter. As follows from Fig. 8 for an impact velocity 0.25 m s1 at which the apples were not bruised, the mean values of the contact diameter was about 11.5 mm for all cultivars. Assuming a round
Fig. 5. Relationship between the bruise depth and the impact velocity for three apple cultivars.
136
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
Fig. 6. Relationship between the bruise diameter and the impact velocity for three apple cultivars.
shape of the contact surface area, a value around 1 cm2 was obtained which is consistent with the current standards applied in most postharvest handling systems that allow the bruise surface area to be not larger than 1 cm2 (Bollen et al., 2001). According to the United States Standards for Grades of Apples (NN, 2002), a bruise depth lower than 3.2 mm does not cause quality deterioration of the apple. Fig. 5 shows that the value of the bruise depth in the experiment developed at the impact velocity above 0.75 m s1. This corresponded to a bruise diameter of around 15 mm (Fig. 6) and a bruise surface area of around 1.8 cm2 which was much larger than the limit of 1 cm2.
3.3. Determining the maximum stress A round shape of the apple contact surface A on a rigid, flat plate was assumed and its value at the time point corresponding to the peak force response was calculated from A¼
pd2cont 4
(1)
where dcont is the contact diameter at peak force response (mm). The maximum stress s max was calculated as
s max ¼
F max A
Fig. 7. Relationship between the peak deformation and the impact velocity for three apple cultivars.
(2)
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
137
Fig. 8. Relationship between the contact diameter at the point of the peak deformation and the impact velocity for three apple cultivars.
Comparing the contact surface area and peak force response, a linear relationship was obtained (Fig. 9). This indicates a constant value of the maximum stress for all impact velocities. Fig. 10 shows statistically significant differences of the mean values of the maximum stress between the lowest impact velocity of 0.25 m s1 and other impact velocities for all tested cultivars. It should be mentioned that an impact velocity 0.5 m s1 was the limit beyond which the bruise developed. It seems that the maximum stress was a good parameter for estimation of bruise appearance. The constant value of maximum stress s max for the impact velocities from 0.5 m s1 to 1.5 m s1 showed that apple tissue damage was caused by exceeding the specific stress value. Hence the experiment confirmed the importance of the critical
stress criterion as regards the whole fruits tested under the impact loading conditions. Therefore the obtained mean values of stress could be interpreted as the maximum values sustained by the apple tissue. For impact velocities 0.5–1.5 m s1, the differences between the mean values of the maximum stress were not statistically significant. The ‘Florina’ hard and ‘Freedom’ soft cultivars had the maximum stress values about 0.4 MPa and 0.35 MPa respectively. According to the authors, the mean value of maximum stress, for the tested apple cultivars was in the range of 0.4–0.5 MPa (Stropek and Gołacki, 2013). Fluck and Ahmed (1973) pointed out that the increase of the peak force response resulted from the increasing drop height and the mass dropped onto a fruit. The above confirmed the hypothesis that the peak force and
Fig. 9. Relationship between the maximum force response and the contact surface area for three apple cultivars.
138
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139
Fig. 10. Relationship between the maximum stress and the impact velocity.
resulting internal stress were critical factors for damage to occur during impact. 4. Conclusions The application of a high speed camera allowed to eliminate mechanical oscillations coming from the measuring device which affected the accuracy of the force response-time and displacement-time curves during the impact. The displacement-time curves were determined in a direct way without double integration of the acceleration-time course associated with significant calculation errors. The research showed that the best parameters describing the damage beginning under impact loading conditions were permanent deformation and maximum stress. The constant value of the maximum stress in all tested impact velocities causing the bruise confirmed the importance of the critical stress criterion as regards the whole fruit under impact loading conditions. An impact velocity 0.5 m s1 was the critical velocity for apples of 170– 180 g at which the bruise appeared. Impact velocities lower than 0.5 m s1 resulted in the bruise surface area about 1 cm2, which corresponds to the current standards for apple grading. The deformation which was caused by the elastic relaxation of apples after the impact, varied from 0.1 to 0.3 mm for three cultivars for all tested impact velocities. References Abedi, G., Ahmadi, E., 2014. Bruise susceptibilities of Golden Delicious apples as affected by mechanical impact and fruit properties. J. Agr Sci. 152, 439–447. Ahmadi, E., Ghassemzadeh, H.R., Sadeghi, M., Moghaddam, M., Neshat, S.Z., 2010. The effect of impact and fruit properities on the bruising of peach. J. Food Eng. 97, 110–117. Alamar, M.C., Vanstreels, E., Oey, M.L., Molto, E., Nicolai, B.M., 2008. Micromechanical behaviour of apple tissue in tensile and compression test: storage conditions and cultivar effect. J. Food Eng. 86, 324–333. Bajema, R.W., Hyde, G.M., 1998. Instrumented pendulum for impact characterization of whole fruit and vegetable specimens. Trans. ASAE 41 (5), 1399–1405. Bajema, R.W., Hyde, G.M., Peterson, K., 1998. Instrumentation design for dynamic axial compression of cylindrical tissue samples. Trans. ASAE 41 (3), 747–754. Bollen, A.F., Cox, N.R., Dela Rue, B.T., Painter, D.J., 2001. A descriptor for damage susceptibility of population of produce. J. Agric. Eng Res. 78 (4), 391–395. Bollen, A.F., Nguyen, H.X., Dela Rue, B.T., 1999. Comparison of methods for estimating the bruise volume of apples. J. Agric. Eng. Res. 74, 325–330.
Boydas, M.G., Ozbek, I.Y., Kara, M., 2014. An efficient laser sensor system for apple impact bruise volume estimation. Postharvest Biol. Technol. 89, 49–55. Brusewitz, G.H., Bartsch, J.A., 1989. Impact parameters related to post harvest bruising of apples. Trans. ASAE 32 (3), 953–957. Celik, H.K., Rennie, A.E.W., Akinci, I., 2011. Deformation behaviour simulation of an apple under drop case by finite element method. J. Food Eng. 104, 293–298. Devaux, M.F., Barakat, A., Robert, P., Bouchet, B., Guillon, F., Navez, B., Lahaye, M., 2005. Mechanical breakdown and cell wall strusture of mealy tomato pericarp tissue. Postharvest Biol. Technol. 37, 209–221. Dintwa, E., Van Zeebroeck, M., Ramon, H., Tijskens, E., 2008. Finite element method of the dynamic collision of apple fruit. Postharvest Biol. Technol. 49, 260–276. Fluck, R.C., Ahmed, E.M., 1973. Impact testing of fruits and vegetables. Trans. ASAE 16 (4), 660–666. Jaren, C., Garcia-Pardo, E., 2002. Using non-destructive impact testing for sorting fruits. J. Food Eng. 53, 89–95. Jarimopas, B., Sarig, Y., Peiper, U.M., Manor, G., 1990. Instrumentation for measuring the response of apples subjected to impact loading. Comput. Electron. Agric 5, 255–260. Kabas, O., 2010. Methods of measuring bruise volume of pear (Pyrus Communis L.). Int. J. Food Prop. 13, 1178–1186. Kitthawee, U., Pathaveerat, S., Srirungruang, T., Slaughter, D., 2011. Mechanical bruising of young coconut. Biosyst. Eng. 109, 211–219. Lang, Z., 1994. The influence of mass and velocity on the maximum allowable impact energy of apples. J. Agric. Eng. Res. 57, 213–216. Lewis, R., Yoxall, A., Canty, L.A., Reina Romo, E., 2007. Development of engineering design tools to help reduce apple bruising. J. Food Eng. 83, 356–365. Li, Z., Thomas, C., 2014. Quantitative evaluation of mechanical damage of fresh fruits. Trends Food Sci. Technol. 35, 138–150. Lichtensteiger, M.J., Holmes, R.G., Hamdy, M.Y., Blaisdell, J.L., 1988. Impact parameters of spherical viscoelastic objects and tomatoes. Trans. ASAE 31 (2), 595–602. Lu, L.X., Wang, Z.W., 2007. Dropping bruise fragility and bruise boundary of apple fruit. Trans. ASABE 50 (4), 1323–1329. Menesatti, P., Paglia, G., 2001. Development of drop damage index of fruit resistance to damage. J. Agric. Eng Res. 80 (1), 53–64. Musiol, C., Harty, D., 1991. The use of numerical integration for the estimation of displacement from an accelerometer signal. Application note 12. nCode International Ltd. NN, 2002. United States Standards for Grades of apples. United States Department of Agriculture. Opara, U.L., 2007. Bruise susceptibilities of ‘Gala’ apples as affected by orchard management practices and harvest date. Postharvest Biol. Technol. 43, 47–54. Opara, U.L., Al-Ghafri, A., Agzoun, H., Al-Issai, J., Al-Jabri, F., 2007. Design and development of a new device for measuring susceptibility to impact damage of fresh produce. N. Z. J. Crop Hortic. Sci. 35, 245–251. Opara, U.L., Pathare, P.B., 2014. Bruise damage measurement and analysis of fresh horticultural produce-A review. Postharvest Biol. Technol. 91, 9–24. Ozbek, I.Y., Boydas, M.G., Kara, M., Demir, B., 2014. Low cost measurement setup based on a piezoelectric microphone for estimating apple bruising using Shannon entropy. Postharvest Biol. Technol. 98, 23–29. Polat, R., Aktas, T., Ikinci, A., 2012. Selected mechanical properties and bruise susceptibility of nectarine fruit. International J. Food Prop 15, 1369–1380.
Z. Stropek, K. Gołacki / Postharvest Biology and Technology 110 (2015) 131–139 Ragni, L., Berardinelli, A., 2001. Mechanical behaviour of apples and damage during sorting and packaging. J. Agric. Eng Res. 78 (3), 273–279. Shafie, M.M., Rajabipour, A., Castro-Garcia, S., Jimenez-Jimenez, F., Mobli, H., 2015. Effect of fruit properties on pomegranate bruising. Int. J Food Prop. 18, 1837– 1846. Shirvani, M., Ghanbarian, D., Ghasemi-Varnamkhasti, M., 2014. Measurement and evaluation of the apparent modulus of elasticity of apple based on Hooke’s, Hertz’s and Boussinesq’s theories. Measurement 54, 133–139. Stropek, Z., Gołacki, K., 2007. Relation between mass and drop height at impact causing bruising of apple. Polish J. Food Nut. Sci. 57 (2), 165–167. Stropek, Z., Gołacki, K., 2010. Determining the coefficient of restitution of apples at different impact velocities. Teka Kom. Mot. Energ. Roln. PAN 10, 417–424. Stropek, Z., Gołacki, K., 2013. The effect of drop height on bruising of selected apple cultivars. Postharvest Biol. Technol. 85, 167–172. Stropek, Z., Gołacki, K., Kołodziej, P., Gładyszewska, B., Samociuk, W., Rejak, A., 2014. Effect of polyvinyl alcohol and keratin on stress relaxation course in thermoplastic starch. Przem. Chem. 93, 364–367.
139
Technical Manual, 2013. ISOTRON Force Sensor—ENDEVCO model 2311–10. Tijskens, E., Ramon, H., De Baerdemaeker, J., 2003. Discrete element modelling for process simulation in agriculture. J. Sound Vib. 266, 493–514. Vanstreels, E., Alamar, M.C., Verlinden, B.E., Enninghorst, A., Loodts, J.K.A., Tijskens, E., Ramon, H., Nicolai, B.M., 2005. Micromechanical behaviour of onion epidermal tissue. Postharvest Biol. Technol. 37, 163–173. Van Zeebroeck, M., Tijskens, E., Van Liedekerke, P., Deli, V., De Baerdemaeker, J., Ramon, H., 2003. Determination of the dynamical behaviour of biological materials during impact using a pendulum device. J. Sound Vibr. 266, 465–480. Van Zeebroeck, M., Van linden, V., Darius, P., De Ketelaere, B., Ramon, H., Tijskens, E., 2007. The effect of fruit factors on the bruise susceptibility of apples. Postharvest Biol. Technol. 46, 10–19. Yen, M., Wan, Y., 2003. Determination of textural indices of guiava fruit using discriminate analysis by impact force. Trans. ASAE 46 (4), 1161–1166. Yuwana, Y., Duprat, F., 1998. Prediction of apple bruising based on the instantaneous impact shear stress and energy absorbed. Int. Agrophys. 12, 133–140.