Mechanical properties of tomato exocarp, mesocarp and locular gel tissues

Journal of Food Engineering 111 (2012) 82–91

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Mechanical properties of tomato exocarp, mesocarp and locular gel tissues Zhiguo Li a,⇑, Pingping Li b, Hongling Yang c, Jizhan Liu b, Yunfeng Xu b a

School of Mechanics and Power Engineering, Henan Polytechnic University, 454003 Jiaozuo, China Institute of Agricultural Engineering, Jiangsu University, 212013 Zhenjiang, China c Library, Henan Polytechnic University, 454003 Jiaozuo, China b

a r t i c l e

i n f o

Article history: Received 23 September 2011 Received in revised form 8 January 2012 Accepted 28 January 2012 Available online 6 February 2012 Keywords: Tomato Exocarp Mesocarp Locular gel Mechanical properties Finite element analysis

a b s t r a c t In order to accurately predict the internal stress distribution and damage region of tomato fruit subjected to an external force using finite element method, the tomato fruit can be regarded as a multibody system which consists of exocarp, mesocarp and locular gel tissues. In the paper, these tissues’ mechanical properties of two tomato varieties were determined using different types of test. Results showed the tension failure stress and elastic modulus of exocarp tissue varied from 0.421 to 0.582 MPa and 4.601 to 9.59 MPa respectively; the compression failure stress and elastic modulus of mesocarp tissue varied from 0.122 to 0.229 MPa and 0.726 to 0.846 MPa respectively; the compression failure stress and elastic modulus of locular gel tissue varied from 0.012 to 0.016 MPa and 0.048 to 0.124 MPa respectively; the failure stress and elastic modulus values of outer layer tissue of tomato fruit were higher than those of inner layer tissue. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction Today the tomato is one of the world’s most popular fruits, and more than 130 million tons of tomatoes were produced each year in the world since 2008 according to FAOSTAT. However, the mechanical damage of fruit usually occurs as a result of careless handling at mechanical harvest, package and transport. Recently much research has focused on the regions of mechanical damage of tomato fruit such as mechanical damage mechanisms (Linden and Baerdemaeker, 2005; Linden et al., 2008), mechanical damage detection methods (Linden et al., 2003, 2006a), factors that affect fruit mechanical damage (Allende et al., 2004; Desmet and Lammertyn, 2004), determination of fruit damage susceptibility and physiological changes after fruit mechanical damage (Idah and Yisa, 2007; Linden et al., 2006b). The external damage can be easily seen on the surface of fruit, while the internal damage is very difficult to be detected externally. With the development of finite element techniques, the prediction of internal damage region of fruits will be achieved using finite element analysis as soon as the mechanical properties of fruit tissues are obtained (Dintwa et al., 2011; Hernández and Bellés, 2007; Kabas et al., 2008; Nguyen et al., 2007; Sadrnia et al., 2008). According to the anatomical characteristics of tomato fruit, tomato fruit can be considered as a multibody system which consists of exocarp, mesocarp and locular gel tissues. Therefore, the mechanical properties of tomato exocarp, ⇑ Corresponding author. Tel./fax: +86 391 3987511. E-mail address: [email protected] (Z. Li).

mesocarp and locular gel tissues were determined in this paper, and the obtained data can be used to create finite element model by which the internal damage region of tomato fruit subjected to an external force will be predicted. Currently, there are no national standard test methods available for determining the mechanical properties of fruit and vegetable. According to the published literature, the test methods can be summarized as follows: (1) the whole fruit, which is assumed as a homogeneous spheroid, is compressed using a texture analyzer. The mechanical parameters (e.g. modulus of elasticity) are calculated from the Hertz Theory’s equations using the obtained force–displacement data. By using this method, some researchers determined the mechanical parameters of tomato (Arora and Kumar, 2005; Kabas and Ozmerzi, 2008; Li et al., 2006; Liu et al., 2008), apple (Yurtlu and Erdogan, 2005), olive (Kilickan and Guner, 2008), apricot (Haciseferogullari et al., 2007), orange fruits (Pallottino et al., 2011), and so on. (2) The whole fruit is regarded as a multibody system, and its each body’s tissue is assumed to be a homogeneous material. Subsequently the body’s tissue is made into standard specimens using a sampler for mechanical testing according to the ASTM D5379, E8, E9 and E290 standards. At last the mechanical parameters of fruit tissues are obtained. According to this method, some researchers determined the mechanical properties of isolated tomato cuticle (Allende et al., 2004; Bargel and Neinhuis, 2005; Gloria et al., 2007; Matas et al., 2004, 2005), tomato exocarp tissue (Gładyszewska et al., 2011; Hetzroni et al., 2011; Rajabipour et al., 2004), single tomato suspension cells (Blewett et al., 2000; Wang et al., 2006), orange tissue

0260-8774/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2012.01.023

Z. Li et al. / Journal of Food Engineering 111 (2012) 82–91

(Singh and Reddy, 2006), apple tissue (Alamar et al., 2008), watermelon tissues (Sadrnia et al., 2008), and so on. To sum up, some of the articles included in the above literature showed the mechanical properties of partial tissues of tomato fruit. However, the mechanical properties of three portions (exocarp, mesocarp and locular gel tissues) of tomato fruit such as modulus of elasticity, failure stress, and failure strain, need to be obtained at the same time for a single tomato fruit when the finite element model will be created to predict the internal stress and damage region of tomato fruit using finite element method; besides, there is a dearth of information available about whether the variety has a significant effect on the mechanical properties of three portions for a single tomato fruit at a specific ripening stage or not. Therefore, the objective of this paper is to determine the mechanical properties of three portions (exocarp, mesocarp and locular gel tissues) of two tomato varieties, and further provide essential data for damage prediction and quality assessment of tomato fruit.

2. Materials and methods 2.1. Materials The experiments were conducted in May 2010 at the School of Food and Biological Engineering of Jiangsu University. Fruits of two tomato varieties, namely Fenguan906 and Jinguang28, were used for this study. The tomato fruits were hand-harvested at the light red ripening stage according to the US Department of Agriculture standards (USDA, 1991) from Yangzhou Vegetable Research Institute. Subsequently, these fruits were transported to the laboratory and inspected again to ensure that they were not damaged and not infected by worms. The fruit’s surface was cleaned manually and dried. The anatomical characteristics of tomato fruit are presented in Fig. 1. Firstly, each tomato fruit was cut into quarters with a sharp knife along the stem/stylar axis, and the pericarp and locular gel tissues were separated from each quarter. Subsequently, the pericarp and locular gel tissues were made into standard rectangular blocks using a slicer and parallel razor blades to ensure constant dimensions (length and width) and smooth surfaces respectively, and the needed locular gel tissue samples would be prepared. At last, the endocarp and mesocarp tissues were removed immediately from the rectangular pericarp using a slicer respectively, and the needed mesocarp tissue samples would be prepared. The needed exocarp tissue samples were prepared by removing the adhering parenchyma tissue from the remaining tissues boiled in hot water for 1 min with a razor blade. The rectangular block

83

sample is helpful to be applied force in parallel to cell columns distribution. The determination of mechanical properties of tomato exocarp, mesocarp and locular gel tissues were performed by means of a TA-TX2 Texture Analyzer (Texture Technologies Corp., NY, USA) within 24 h at room temperature (25 ± 1 °C: 55–57% RH). 2.2. Methods The mechanical properties of tomato exocarp tissue were determined by one-dimensional tension testing and shear testing respectively; the mechanical properties of tomato mesocarp tissue were determined by one-dimensional compression testing, tension testing, shear testing and bend testing respectively; the mechanical properties of tomato locular gel tissue were determined by one-dimensional compression testing and shear testing respectively. The analyzer was calibrated with a 5 kg weight before the first test, and the test speed was set to 0.1 mm/s (quasistatic loading). The samples were labeled before test and its thickness d were measured with an electronic digital caliper to an accuracy of 0.01 mm. In total, 160 tomato tissue samples (10 samples  2 varieties  8 test series) were loaded in the test. The reported values of all mechanical parameters are means for 10 replications. 2.2.1. Compression testing The dimensions (length  width) of the prepared tomato mesocarp tissue and locular gel tissue specimens were 10 mm  5 mm and 16 mm  8 mm for compression testing respectively. After prepared, put it at the center of base plate, and the specimen was compressed along its length direction by P50 parallel plate probe (Fig. 2). The compressibility level was set to 60%, and the force–deformation data were recorded by computer in real time. Subsequently, the mechanical parameters of tomato mesocarp and locular gel tissues such as compression elastic modulus, failure stress, failure strain and failure energy were calculated using the following equations (Alamar et al., 2008; Masoudi et al., 2007):

rc ¼

F c max F c max ¼ Ac wd

ð1Þ

ec ¼

DL L

ð2Þ

Ec ¼

rc 2e0:5rc

Erec ¼

Z

ð3Þ

ec

rde

ð4Þ

0

where rc is the failure stress during compression testing (MPa), Fcmax is the elastic peak force (N), Ac is the specimen’s cross sectional area (mm2), w is the specimen’s cross sectional width (mm), d is the specimen’s cross sectional thickness (mm), ec is the failure strain during compression testing (%), DL is the length different before and after test (mm), L is the initial length of specimens (mm), Ec is the compression elastic modulus (MPa), e0:5rc is the strain at 50% of the yielding point stress on the stress–strain curves (%), Erec is the failure energy during compression testing (kJ/m3), r is the normal stress during compression (MPa), e is the normal strain during compression (%).

Fig. 1. Anatomical characteristics of tomato fruit.

2.2.2. Tension testing The dimensions (length  width) of the prepared tomato exocarp tissue and mesocarp tissue specimens were 40 mm  3 mm for tension testing respectively. After prepared, the specimen was tensioned along its length direction by A/TG probe (Fig. 3). The tension displacement was set to 10 mm. The distance between the upper and lower probes was measured before testing and the

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Z. Li et al. / Journal of Food Engineering 111 (2012) 82–91

Fig. 2. Compression testing. (a) The mesocarp specimen was compressed; (b) the locular gel specimen was compressed.

Fig. 3. Tension testing. (a) The exocarp specimen was tensioned; (b) the mesocarp specimen was tensioned.

force–deformation data were recorded by computer. Subsequently, the mechanical parameters of tomato exocarp and mesocarp tissues such as tension elastic modulus, failure stress, failure strain and failure energy were calculated using the following equations (Singh and Reddy, 2006):

rl ¼

F l max F l max ¼ Al wd

ð5Þ

el ¼

DL L

ð6Þ

El ¼

rl 2e0:5rl

2.2.3. Shear testing The dimensions (length  width) of the prepared tomato exocarp tissue and mesocarp tissue specimens were 40 mm  5 mm for shear testing respectively, and the dimensions of locular gel tissue specimens were 40 mm  10 mm. After prepared, the specimen was sheared along its thickness direction by HDP/BS probe until it fractured (Fig. 4), and the force–displacement data were recorded. Subsequently, the shear strength of tomato exocarp, mesocarp and locular gel tissues were calculated using the following equation (Abdul Khalil et al., 2010; Fidelibus et al., 2002):

ss ¼ ð7Þ

F s max F s max ¼ As wd

ð9Þ

ð8Þ

where ss is the shear strength during shear testing (MPa), Fsmax is the rupture force in shear (N), As is the specimen’s cross sectional area (mm2), w is the specimen’s cross sectional width (mm), d is the specimen’s cross sectional thickness (mm).

where rl is the failure stress during tension testing (MPa), Flmax is the elastic peak force (N), Al is the specimen’s cross sectional area (mm2), w is the specimen’s cross sectional width (mm), d is the specimen’s cross sectional thickness (mm), el is the failure strain during tension testing (%), DL is the length different before and after test (mm), L is the initial distance between the upper and lower probes (mm), El is the tension elastic modulus (MPa), e0:5rl is the strain at 50% of the yielding point stress on the stress–strain curves (%), Erel is the failure energy during tension testing (kJ/m3), r is the normal stress during tension (MPa), e is the normal strain during tension (%).

2.2.4. Bend testing The dimensions (length  width) of the prepared tomato mesocarp tissue specimens were 40 mm  5 mm for bend testing. After prepared, the specimen was loaded along its thickness direction based on three-point bending test by HDP/3PB probe until it fractured (Fig. 5). The span of specimen was set to 16 mm and the force–deflection data were recorded by computer. Subsequently, the bend mechanical parameters of tomato mesocarp tissues such as bend strength, elastic modulus, maximum deflection, distance from the neutral axis to the compression edge of specimen and distance from the neutral axis to the tensile edge of specimen were

Erel ¼

Z

ec

rde

0

Z. Li et al. / Journal of Food Engineering 111 (2012) 82–91

85

Fig. 4. Shear testing. (a) The exocarp specimen was sheared; (b) the mesocarp specimen was sheared; (c) the locular gel specimen was sheared.

Fig. 5. Three point bending test. (a) The mesocarp specimen was loaded; (b) the mesocarp specimen was fractured.

calculated using the following equations (Jawaid et al., 2011; Pitts et al., 2008):

rb ¼ Eb ¼

3F max c 2

2wd

ð10Þ

2.4. Statistic analysis Results were analyzed for statistical significance by variance analysis using the SAS9.1 software, a = 0.05. 3. Results

c3 k 3

4wd

ð11Þ

Ec C t ¼ Et C c

ð12Þ

Ct þ Cc ¼ d

ð13Þ

where rb is the bend strength (MPa), Fmax is the peak force (N), c is the span of specimen (mm), w is the specimen’s cross sectional width (mm), d is the specimen’s cross sectional thickness (mm), Eb is the bend elastic modulus (MPa), k is the slope of the initial straight-line portion of the force–deflection curve (N/mm), Ec is the compression elastic modulus (MPa), Et is the tension elastic modulus (MPa), Cc is the distance from the neutral axis to the compression edge of the specimen (mm), Ct is the distance from the neutral axis to the tensile edge of the specimen (mm). 2.3. Scanning electron microscope (SEM) The upper surfaces of tomato exocarp, mesocarp and locular gel specimens were scanned using a Leica Z16 APO scanning electron microscope. Chip magnification: Specimen-2.5.

3.1. Compression mechanical properties The force–deformation curves of tomato mesocarp and locular gel tissues during compression are presented in Fig. 6. The initial linear portion OA is the elastic deformation region of tissue, and the plastic deformation takes place after the yield point A. The means and standard errors of compression elastic modulus Ec, failure stress rc, failure strain ec and failure energy Erec of tomato mesocarp and locular gel tissues are shown in Table 1. Statistical analysis showed that the variety and tissue material had a significant effect (P < 0.05) on the compression failure stress, elastic modulus and failure energy of tomato fruit tissues respectively. The average compression failure stress, elastic modulus and failure energy of mesocarp tissue were higher than those of locular gel tissue in two tomato varieties; the corresponding ratio for Fenguan906 tomato were 10.17:1, 5.85:1 and 3.3:1 respectively, and for Jinguang28 tomato were 14.31:1, 17.63:1 and 7.61:1 respectively. The average compression failure stress, failure strain, elastic modulus and failure energy of mesocarp tissue from Jinguang28 tomato fruits were higher than those from Fenguan906 tomato fruits, and the corresponding ratio for mesocarp tissue were 1.88:1, 1.31:1, 1.17:1 and 1.95:1 respectively. The failure stress and strain of locular gel

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Fig. 6. Force–deformation curves during compression. (a) The force–deformation curve of mesocarp tissue; (b) the force–deformation curve of locular gel tissue.

Table 1 Compression mechanical parameters of tomato mesocarp and locular gel tissues. Variety

a

Tissue material

Mechanical parameters

rc (MPa)a

ec (%)a

Ec (MPa)a

Erec (mJ)a

Fenguan906

Mesocarp Locular gel

0.122 ± 0.03 0.012 ± 0.007

25.758 ± 1.654 20.184 ± 5.269

0.726 ± 0.142 0.124 ± 0.074

8.033 ± 1.724 2.433 ± 1.578

Jinguang28

Mesocarp Locular gel

0.229 ± 0.101 0.016 ± 0.005

33.853 ± 4.022 34.613 ± 11.843

0.846 ± 0.058 0.048 ± 0.021

15.671 ± 4.725 2.058 ± 1.386

rc – compression failure stress, ec – failure strain, Ec – compression elastic modulus, Erec – failure energy.

tissue from Jinguang28 tomato fruits were higher than those from Fenguan906 tomato fruits, but it was contrary for compression elastic modulus and failure energy; the ratios of corresponding mechanical parameters for the locular gel tissue of two tomato varieties were 1.33:1, 1.71:1, 0.39:1 and 0.85:1 respectively. Up to now, relatively little information is available on the compression mechanical properties of tomato mesocarp and locular gel tissues, so it is difficult to make comparisons. There are some papers as related to the compression mechanical properties of apple and watermelon fruit tissues. Masoudi et al. (2007) noted the variety has a significant effect on the compression failure stress, failure strain, elastic modulus and failure energy of apple flesh tissue, and the compression failure stress, failure strain, elastic modulus and failure energy vary from 0.13 to 0.34 MPa, 7.7% to 13%, 1.53 to 2.84 MPa and 5 to 19 mJ, respectively (Masoudi et al., 2007). Alamar et al. (2008) reported the variety has a significant effect on the compression failure stress and strain of apple flesh tissue but has no significant effect on the compression elastic modulus, and the compression failure stress, failure strain and elastic modulus of apple flesh vary from 0.25 to 0.4 MPa, 23.76% to 28.26% and 0.35 to 2.01 MPa, respectively (Alamar et al., 2008). Sadrnia et al. (2008) showed the variety and tissue material have a significant effect on the compression failure stress, failure strain and elastic modulus of watermelon flesh and white rind tissues respectively; the compression failure stress, failure strain and elastic modulus of red flesh vary from 0.022 to 0.045 MPa, 4.6% to 10.6% and 0.33 to 0.63 MPa, respectively, and those of white rind vary from 0.19 to 0.34 MPa, 16% to 35% and 0.73 to 1.39 MPa, respectively (Sadrnia et al., 2008). In contrast to the obtained compression mechanical parameters of tomato tissues, similar results were that the variety always has a close relationship with the fruit’s mechanical param-

eters; the compression failure stress, failure strain and elastic modulus of internal tissue were lower than those of the external tissue of fruit respectively and its average values are less than 0.5 MPa, 35% and 3 MPa respectively. Tomato, apple and watermelon fruits have slight differences in the compression failure stress, failure strain and elastic modulus of flesh tissues. 3.2. Tension mechanical properties The force–deformation curves of tomato exocarp and mesocarp tissues during tension are presented in Fig. 7, the linear portion OA is the elastic deformation region of tissue. The means and standard errors of tension elastic modulus El, failure stress rl, failure strain el and failure energy Erel of tomato exocarp and mesocarp tissues are shown in Table 2. Statistical analysis showed that the variety and tissue material had a significant effect (P < 0.05) on the tension failure stress, elastic modulus and failure energy of tomato fruit tissues respectively. The average tension failure stress and elastic modulus of mesocarp tissue were lower than those of exocarp tissue in two tomato varieties; the corresponding ratio for Fenguan906 tomato were 1:32.33 and 1:48.43 respectively, and for Jinguang28 tomato were 1:30.07 and 1:34.59 respectively. The average tension failure stress and elastic modulus of exocarp tissue from Jinguang28 tomato fruits were lower than those from Fenguan906 tomato fruits, and the corresponding ratio for exocarp tissue were 1:1.38 and 1:2.08 respectively. The failure stress, elastic modulus and failure energy of mesocarp tissue from Jinguang28 tomato fruits were lower than those from Fenguan906 tomato fruits, but it was contrary for tension failure strain; the ratios of mechanical parameters for the mesocarp tissue of two tomato varieties were 1:1.29, 2.48:1, 1:1.49 and 1:1.54 respectively.

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Fig. 7. Force–deformation curves during tension. (a) The force–deformation curve of exocarp tissue; (b) the force–deformation curve of mesocarp tissue.

Table 2 Tension mechanical parameters of tomato exocarp and mesocarp tissues. Variety

Tissue material

Mechanical parameters

rl (MPa)a

a

el (%)a

El (MPa)a

Erel (mJ)a

Fenguan906

Exocarp Mesocarp

0.582 ± 0.028 0.018 ± 0.012

6.929 ± 1.173 6.323 ± 2.568

9.59 ± 2.186 0.198 ± 0.033

0.347 ± 0.129 0.734 ± 0.724

Jinguang28

Exocarp Mesocarp

0.421 ± 0.144 0.014 ± 0.002

8.551 ± 1.401 15.653 ± 4.958

4.601 ± 1.419 0.133 ± 0.042

0.837 ± 0.326 0.476 ± 0.133

rl – tension failure stress, el – failure strain, El – tension elastic modulus, Erel – failure energy.

Recently, there are some papers as related to the tension mechanical properties of tomato exocarp tissues. Batal et al. (1970) reported the tension failure stress, failure strain and elastic modulus of ripe tomato skin vary from 0.9 to 1.7 MPa, 7.8% to 13.9% and 10 to 20 MPa, respectively (Batal et al., 1970). Vincent (1990) reported the tension failure stress, failure strain and elastic modulus of ripe tomato skin are 2.5 MPa, 23.5% and 600 MPa respectively (Vincent, 1990). Matas et al. (2004) reported the variety has a significant effect on the tension failure stress and elastic modulus of mature-green tomato skin tissue, and the failure stress and elastic modulus vary from 0.97 to 1.16 MPa and 27.1 to 43.5 MPa respectively (Matas et al., 2004). Bargel and Neinhuis (2005) reported the variety has a significant effect on the tension elastic modulus of tomato skin tissue at the ripening stage, and the failure stress, failure strain and elastic modulus of maturegreen tomato skin vary from 23 to 35 MPa, 12% to 15% and 173.8 to 229.3 MPa, respectively. The elastic modulus of tomato skin increases from immature to fully ripe fruits, while the failure stress and strain display a tendency to decrease (Bargel and Neinhuis, 2005). Hetzroni et al. (2011) noted the variety has a significant effect on the tension failure stress and elastic modulus of ripe tomato exocarp tissue, and the failure stress and elastic modulus vary from 5.5 to 10.1 MPa and 52 to 173 MPa respectively (Hetzroni et al., 2011). Gładyszewska et al. (2011) reported the tension failure stress and elastic modulus of initial ripening tomato skin were 0.49 MPa and 6.4 MPa respectively (Gładyszewska et al., 2011). By contrast, the showed tension mechanical parameter values of

tomato exocarp tissue have clear differences, and the determined data in this section were approximate to the reported values by Batal et al. The discordant results can be attributed to that the prepared samples have obvious differences in variety, ripening stage, loading rate, size, and shape. By far, not much is known about the tension mechanical parameters of tomato mesocarp tissue. Singh and Reddy (2006) noted the tension failure stress and elastic modulus of orange peel are 0.173 MPa and 1.57 MPa respectively (Singh and Reddy, 2006). Alamar et al. (2008) reported the tension failure stress, failure strain and elastic modulus of apple flesh vary from 0.22 to 0.24 MPa, 7.83% to 10.21% and 3.19 to 3.91 MPa respectively (Alamar et al., 2008). Sadrnia et al. (2008) reported the tension stress, failure strain and elastic modulus of watermelon green rind are 0.027 MPa, 5.2% and 0.536 MPa respectively (Sadrnia et al., 2008). By contrast, tomato, orange, apple and watermelon fruit have slight differences in the tension failure stress, failure strain and elastic modulus of flesh tissues. 3.3. Shear mechanical properties The force–displacement curves of tomato exocarp, mesocarp and locular gel tissues during shearing are presented in Fig. 8. These shear force vs probe displacement plot are nonlinear functional relationships, and the point A relates to the peak shear force as the sample is fractured. The means and standard errors of shear strength ss of tomato exocarp, mesocarp and locular gel tissues are shown in Table 3.

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Z. Li et al. / Journal of Food Engineering 111 (2012) 82–91

Fig. 8. Force–displacement curves during shearing. (a) The force–displacement curve of exocarp tissue; (b) the force–displacement curve of mesocarp tissue; (c) the force– displacement curve of locular gel tissue.

Table 3 Shear mechanical parameters of tomato exocarp, mesocarp and locular gel tissues.

a

Variety

Tissue material

ss (MPa)a

Fenguan906

Exocarp Mesocarp Locular gel

2.981 ± 1.033 0.073 ± 0.024 0.007 ± 0.005

Jinguang28

Exocarp Mesocarp Locular gel

2.053 ± 0.487 0.069 ± 0.03 0.022 ± 0.01

ss – shear strength.

Statistical analysis showed that the variety and tissue material had a significant effect (P < 0.05) on the shear strength of tomato fruit tissues respectively. The average shear strength values in two tomato varieties were the highest for tomato exocarp tissue and the lowest for locular gel tissue. The average shear strength of exocarp tissue from Fenguan906 tomato fruits was 45.2% higher than that from Jinguang28 tomato fruits, and the average shear strength of mesocarp tissue from Fenguan906 tomato fruits was 5.8% higher than that from Jinguang28 tomato fruits, whereas the average shear strength of locular gel tissue from Jinguang28 tomato fruits was 3.14 times higher than that from Fenguan906 tomato fruits. In recent years, research on the shear mechanical properties of tomato exocarp, mesocarp and locular gel tissues is scarce. Thompson (2001) reported the Poisson’s ratio of Counter tomato skin is 0.72 (Thompson, 2001). Kabas et al. (2008) reported the Poisson’s ratio of cherry tomato is 0.335 (Kabas et al., 2008). Gładyszewska et al. (2011) reported the Poisson’s ratios of Admiro tomato skin stored at 13 °C and 21 °C are 0.73 MPa and 0.46 MPa respectively (Gładyszewska et al., 2011). Additionally, Fidelibus et al. (2002) reported the shear strength, shear modulus and rupture force of Valencia orange peel are 0.53 MPa, 0.7 MPa and 139.5 N respectively (Fidelibus et al., 2002). Singh and Reddy (2006) reported the storage period has no significant effect on the peak shear force but has a significant effect on the shear energy; the peak shear force and shear energy of Nagpur Mandarin orange peel are 79.5 N and 240.7 J respectively (Singh and Reddy, 2006). Montero-Calderon et al. (2010) reported the peak shear force and shear energy of Gold pineapple flesh which vary from 6 to 8.7 N and 10.4 to 16.9 mJ respectively, have not significantly vary among fruit pieces from different sections of the fruit (Montero-Calderón et al., 2010). By contrast, tomato, orange and pineapple fruit have obvious differences in the shear mechanical properties of flesh tissues.

3.4. Bend mechanical properties The force–deflection curve of tomato mesocarp tissue during bending is presented in Fig. 9. The longitudinal axis and transverse axis indicate the applied force and deflection of test specimen at the loading position during bending respectively, and the point A relates to the applied peak force as the specimen is fractured. The means and standard errors of bend strength rb, elastic modulus Eb, maximum deflection D, distance from the neutral axis to the compression edge of the specimen Cc and distance from the neutral axis to the tensile edge of the specimen Ct of tomato mesocarp tissue are shown in Table 4. Statistical analysis showed that the variety had no significant effect (P < 0.05) on the bend strength and elastic modulus of tomato mesocarp tissues. The average bend strength and maximum deflection of mesocarp tissue from Jinguang28 tomato fruits were higher than those from Fenguan906 tomato fruits, and their corresponding ratio was 1.3:1 for bend strength and 1.33:1 for maximum deflection. The distance from the neutral axis to the compression edge of the mesocarp specimen from Jinguang28 tomato fruits was lower than that from Fenguan906 tomato fruits, but it was contrary for the distance from the neutral axis to the tensile edge of the specimen. The distance

Fig. 9. Force–deflection curve during bending.

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Z. Li et al. / Journal of Food Engineering 111 (2012) 82–91 Table 4 Bend mechanical parameters of tomato mesocarp tissue. Variety

Fenguan906 Jinguang28

Mechanical parameters

rb (MPa)a

Eb (MPa)a

D (mm)a

Cc (mm)a

Ct (mm)a

0.152 ± 0.026 0.198 ± 0.068

36.36 ± 4.79 36.30 ± 12.46

6.505 ± 1.991 8.627 ± 3.017

1.127 ± 0.192 0.758 ± 0.059

4.131 ± 0.706 4.823 ± 0.376

a

rb – bend strength, Eb – bend elastic modulus, D – maximum deflection, Cc – distance from the neutral axis to the compression edge of the specimen, Ct – distance from the neutral axis to the tensile edge of the specimen.

from the neutral axis to the compression edge of the mesocarp specimen was lower than the distance from the neutral axis to the tensile edge. This indicated the neutral axis was closer to the compressive side of the mesocarp specimen, and the compressive strain at the compression side of the mesocarp specimen was lower than the tensile strain at the tension side during bending. By far, research on the bend properties of tomato tissues is also scarce, Desmet and Lammertyn (2004) reported the variety has a significant effect on the bend elastic modulus of tomato stem, and the bend elastic modulus of stem varies from 76.48 MPa to 312.42 MPa (Desmet and Lammertyn, 2004). Other related literature such as Dobrzanskijr et al. (2000) reported the variety has no significant effect on the bend elastic modulus of apple flesh beam, and the bend elastic modulus of apple flesh and flesh with skin tissues vary from 1.04 MPa to 2.45 MPa and 2.7 MPa to 6.5 MPa respectively (Dobrzanskijr et al., 2000). Harker et al. (2006) reported the fracture toughness and fracture energy of apple flesh tissue vary from 1.3 kPa to 13.8 kPa and 3.5 to 83.7 mJ respectively (Harker et al., 2006). By contrast, tomato and apple fruit have obvious differences in the bend elastic modulus of flesh tissues. The obtained bending properties of fruit tissue can be used to characterize the behavior of structural element subjected to an external load which is perpendicular to the long axis of fruit; besides, the bending properties can also be applied to evaluate the tensile properties of fruit tissue as the sample preparation for tension test is difficult. 4. Discussions 4.1. Different mechanical properties between compression and tension As can be seen from the obtained mechanical properties of tomato mesocarp tissue, the failure stress and elastic modulus of mesocarp tissue in compression were higher than those in tension. In fact, many biomaterials have obvious differences in the mechanical properties between compression and tension, such as apple, orange and bone. The reason is that the failure modes of fruit tissues are different during compression and tension. It is known that the fruit is composed of cells and the cell consists of cell wall and protoplast; the cell wall is located outside the proto-

plast and provides the cell with structural support and protection. The cell wall has three layers, namely middle lamella, primary cell wall and secondary cell wall. The middle lamella, which glues the primary cell walls of adjacent fruit cells, is thin and rich in pectin; the primary cell wall is thin and rich in pectin, hemi-cellulose and cellulose; the secondary cell wall is thick and rich in cellulose. The cell walls of fruit tissue are rich in protopectin at the immature stage; the insoluble protopectin content of tissue cell walls gradually decreases with fruit ripening while the soluble pectin content increases, so the fruit tissue gradually become soft during ripening. The failure of fruit tissue at the half-ripe stage under compression loading always occurs mainly due to the cell wall rupture caused by great compression force, while the failure of tissue under tension loading occurs mainly due to the separation of adjacent cells caused by the breakdown of middle lamella of cell wall. Therefore, less applied force is needed to break tomato mesocarp tissue during tension testing than that during compression testing, and the higher failure stress and elastic modulus of tomato mesocarp tissue were obtained for compression testing when comparing to tension testing. 4.2. Effect of fruit tissue on its mechanical properties Comparing the mechanical properties of tomato exocarp, mesocarp and locular gel tissues, the failure stress and elastic modulus of mesocarp tissue were higher than those of locular gel tissue during compression for the same variety; the failure stress and elastic modulus of exocarp tissue were higher than those of mesocarp tissue during tension; the shear strength of exocarp tissue is the maximum and the minimum for locular gel tissue during shearing. In brief, the higher mechanical parameter value will be obtained for outer layer tissue of tomato fruit when comparing to inner layer tissue. These phenomena can be explained by the fact that the cells of exocarp tissue are small and their arrangement is compact and dense, the cell walls are thick; on the contrary, the cells of mesocarp and locular gel tissues are large and their arrangement is sparse and loose (Fig. 10). Therefore, less applied force is needed to break inner tissue of fruit than outer tissue, and the corresponding mechanical parameter of inner tissue is lower than that of

Fig. 10. Representative SEM micrographs of tomato exocarp, mesocarp and locular gel tissues. (a) The micrograph of exocarp tissue; (b) the micrograph of mesocarp tissue; (c) the micrograph of locular gel tissue.

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outer tissue (exocarp vs locular gel). This was also observed by Matas et al. (2004), Stertz et al. (2005) and Rancic et al. (2010) (Matas et al., 2004; Rancic et al., 2010; Stertz et al., 2005). 4.3. Effect of variety on mechanical properties In this study, the mechanical parameters of tomato fruit tissues, such as compression failure stress and elastic modulus, tension failure stress and elastic modulus and shear strength, were significant differences between two varieties (Fenguan906 vs Jinguang28). The compression failure stress and elastic modulus of fruit tissue from Jinguang28 tomato fruit were higher than those from Fenguan906 tomato fruit. The reason is that the protopectin content of cell wall from Jinguang28 tomato fruit may be higher than that from Fenguan906 tomato fruit; the protopectin content is very closely associated with the mechanical properties of fruit cell wall, and the fruit tissue failure under compression loading occurs mainly due to cell wall rupture. Therefore, larger applied force per unit area (failure stress) is needed to break down the cell wall of Jinguang28 tomato fruit than that of Fenguan906 tomato fruit during compression; for the same reason, the elastic modulus of fruit tissues have obvious difference between two varieties. However, the tension failure stress and elastic modulus of fruit tissue from Jinguang28 tomato fruit were lower than those from Fenguan906 tomato fruit. The reason is that the soluble pectin content of middle lamella from Jinguang28 tomato fruit may be higher than that from Fenguan906 tomato fruit; the soluble pectin is a weak material in middle lamella of cell wall and the fruit tissue failure under tension loading occurs mainly due to the separation of tissue cells. Therefore, less applied force per unit area (failure stress) is needed to separate the cell wall of Jinguang28 tomato fruit than that of Fenguan906 tomato fruit during tension; for the same reason, the elastic modulus of fruit tissues have large difference between two varieties. The tissue under shear loading has similar failure mode to the tissue under tension loading according to the force bore by fruit tissue. Hence, for the same reason, the shear strength of fruit tissue from Jinguang28 tomato fruit was lower than that from Fenguan906 tomato fruit. To sum up, the cell walls of Jinguang28 tomato fruit tissue have higher protopectin and soluble pectin content than those of Fenguan906 tomato fruit tissue. The similar observations are showed by Devaux et al. (2005) and Ali and Abu-Goukh (2007) for tomato, Billy et al. (2008) for apple and Ozturk et al. (2009) for pear (Ali and Abu-Goukh, 2005; Billy et al., 2008; Devaux et al., 2005; Ozturk et al., 2009).

5. Conclusions In this research, the mechanical properties of tomato exocarp, mesocarp and locular gel tissues were determined during compression, tension, shear and bend testing respectively. The main results can be summarized as follows: (1) The tension failure stress and elastic modulus and shear strength of tomato exocarp tissue varied from 0.421 to 0.582 MPa, 4.601 to 9.59 MPa and 2.053 to 2.981 MPa, respectively; The compression failure stress and elastic modulus, tension failure stress and elastic modulus, shear strength and bend strength and elastic modulus of tomato mesocarp tissue varied from 0.122 to 0.229 MPa, 0.726 to 0.846 MPa, 0.014 to 0.018 MPa, 0.133 to 0.198 MPa, 0.069 to 0.073 MPa, 0.152 to 0.198 MPa and 36.3 to 36.36 MPa, respectively; the compression failure stress and elastic modulus and shear strength of tomato locular gel tissue varied from 0.012 to 0.016 MPa, 0.048 to 0.124 MPa and 0.007 to 0.022 MPa, respectively.

(2) The variety had a significant effect on the compression failure stress and elastic modulus, tension failure stress and elastic modulus and shear strength of tomato fruit tissues but had no significant effect on the bend strength and elastic modulus. The higher failure stress and elastic modulus values were obtained for outer layer tissue of tomato fruit when comparing to inner layer tissue. (3) The tomato fruit can be regarded as a multibody system, and the obtained mechanical parameters of tomato exocarp, mesocarp and locular gel tissues can be used to create multi-scale finite element model by which the internal damage region of tomato fruit subjected to an external force will be predicted. Additionally, the obtained data can also be used for quality assessment.

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