Mechanical Properties of Melon Measured by Compression, Shear, and Cutting Modes

Abstract

Mechanical properties of three common varieties of melon were measured. These are toughness, rupture force, shear strength, maximum shearing force, and cutting force. The role of peel (%) on each property was also calculated as the relative contribution of peel to unpeeled produce. The shear strength of peel was statistically found similar (p > 0.05) for all varieties. The same result was also revealed for unpeeled produce. Using rupture force was not recommended for peeling watermelon, because of close values of this property for its peel and unpeeled case. The required energy for peeling all three varieties of melon was determined to be 500 N mm. Peeling melons using cutter tools could not be recommended.

Keywords:

• Mechanical properties
• Melon
• Toughness
• Rupture force
• Shear strength

INTRODUCTION

Mechanical properties of fruits and vegetables can be applied to improve the efficiency of processing equipment including peelers. Generally, the efficiency of a mechanical peeler depends on the influence of different forces on machine performance. Useful forces, such as rupture and cutting, are purposely applied for peeling; whereas undesirable forces, such as impact and compression, may reduce the effects of the former forces. The undesirable loads can be also the main reason for common problems such as bruising.[1] Knowing the mechanical properties of agricultural products would help designers to apply forces properly.

Compression (force–deformation) testing is employed to study the mechanical behaviour of fruits and vegetables for different reasons: (i) Force-deformation features of the produce beyond elastic limit can be used to simulate the occurrence of bruising. Sadrnia et al.[2] predicted internal bruising in two varieties of watermelon using nonlinear models. (ii) Force-deformation characteristics of the produce within elastic limit may also be used to predict sensory texture attributes such as the degree of ripeness. Harker et al.[3,4] showed that if the compression force of two apples varies up to 5 N, then their textures would be different. (iii) Mechanical properties of fruits and vegetables in different aspects including peel, flesh, and unpeeled produce can be identified by using compression testing.[5–13] Emadi et al.[12] determined mechanical properties including rupture force, toughness, cutting force, shear force and strength which are effective in the design of a mechanical peeler for pumpkin varieties. Also Ohwovoriole et al.[13] investigated mechanical properties of cassava tuber to be used in the design of a mechanical peeler. The properties were Poisson's ratio, shear stress, peeling stress, cutting force and rupture stress for both peeled and unpeeled cassava tubers. Tensile testing can also be applied for the same purposes. Recent studies have been conducted to show the accuracy of tensile testing in comparison with compression testing.[14] Applying tensile testing is not as common as the compression testing because of some shortcomings.[15] Difficulties in the holding of the peel samples during the test[16] and the creation of premature tensile failure during peel sample preparation[17,18] are some common limitations. Although some researchers are uncertain about the reliability of compression test results,[19,20] and some are introducing new methods (wedge test[21]; twist ties[22]; constrained disc compression[23]; single-edge notched bend test[24,20]), the method of compression testing is still trustworthy and widespread.[25]

Compression testing of peels can be conducted either directly or indirectly. The latter uses the difference of experimental results between unpeeled (overall) and peeled produce.[5,8,10] These methods have been criticised by several researchers (e.g.[18,5]) because of the likely errors. They identified the increase in the effective area of compression during the puncture of the peel as a reason.

One of the publications of Harker et al.,[26] which is related to the mechanical properties of melon focuses on the comparison of instrumental (using puncture, shear, and tensile strength) and sensory measurements of tissue strength and juiciness for some fruits and vegetables including watermelon and muskmelon. They studied only the cellular basis of the flesh to identify characteristics which affect sensory texture attributes of hardness and juiciness. The attempts to find any useful published data on mechanical properties of melon varieties, to be used as a database for the design of a mechanical peeler, were unsuccessful. The current study was conducted on three common varieties of melon, namely cantaloupe melon, honeydew melon, and watermelon aiming to investigate selected mechanical properties which will form a database for designing peeling equipment.

MATERIALS AND METHOD

Materials

Cantaloupe melon, honeydew melon, and watermelon, as the most common varieties of melon (Cucurbitaceae family), were randomly chosen from different local farms around Brisbane (Queensland, Australia). All melons were ripe (ready to eat), defect-free, and different sizes. They were prepared for tests by keeping them under controlled laboratory conditions for at least 24 and at most 48 hours prior to tests according to standard S 368.4.[27] Temperatures of 20–25°C and relative humidity of 50–55% were maintained as the main environmental parameters. Some small slices from different positions of unpeeled produce were cut, gently peeled and then the peel thickness was measured with a vernier calliper. The average peel thickness was 4.30, 2.40, and 9.60 mm for cantaloupe melon, honeydew melon, and watermelon, respectively.

Since conducting the test on the whole melon was difficult, circular shape samples of 80-mm diameter and 10-mm thick (flesh depth) were prepared from unpeeled produce, using a special borer (Fig. 1a). The borer was a drum with the edge sharpened at 30°, which was fabricated from stainless steel. The external diameter of the borer was fitted to the internal diameter of the unpeeled sample holder. The sample taken by the borer fitted directly into the unpeeled sample holder (Fig. 1b). This holder was made from stainless steel and could be installed directly on the Instron Universal Testing Machine (IUTM). The diameter was chosen to be at least ten times that of the probe to ensure that the unpeeled sample reflects the properties of the whole melon.

Figure 1. (a) The vegetable borer; (b) the holder of unpeeled sample and hemispherical end probe; (c) the holder of peel sample; (d) the flat end probe; (e) the cutting probe.

The peel samples were prepared from the equatorial part,[8,28] or the top and bottom of the whole melon. The samples were prepared with a diameter of about 30 mm, and the flesh was gently removed by means of scraping. Peel samples had a diameter of at least three times that of the probe to facilitate the holding of sample by the holder (Fig. 1c). The sample was kept by the holder while stretching during experiments. It was made from stainless steel with an external 54-mm diameter with ease of installation on IUTM. The flesh samples were prepared in circular shape with 30-mm diameter and 5-mm thickness. Middle layers of flesh close to peel were chosen for flesh samples. The IUTM model for all experiments was H5000M which was fabricated by Hounsfield Test Equipment, England.

Compression Test

Observing the behaviour of samples under the force-deformation test and the determination of rupture force and toughness for both peel and unpeeled samples were the objectives of the test. Tests were carried out according to the ASAE standard S 368.4.[27] A cylindrical probe, 8 mm in diameter, with a hemispherical end was used (Fig. 1b). As it was already indicated that there is no significant difference in the shape of the curves for different speeds of penetration between 5 to 25 mm/min,[29] the penetration speed of the probe was chosen to be 20 mm/min, as proposed in the ASAE standard. Experiments were carried out using the IUTM. The increasing compression force led to loosening the resistance of the sample and finally rupturing at rupture point. Applied forces in N and the resulting deformation in mm could be read off the machine and recorded on the connected computer. Toughness was calculated as the area under the force-deformation curve from the origin up to the rupture point as below[30–32]:

(1)

where: T is the toughness in N mm; F_r is rupture force in N; and D_r is deformation at rupture point. Every experiment was repeated 20 times for both cases (using different samples) and the average results were compared statistically.

Shear Strength Test

The shear strength of the produce indicates the degree to which the cells are held together. The shearing strength was investigated by shearing a plug from a sample. A flat end probe (Fig. 1d), 8 mm in diameter, was used to shear the peel by means of driving it through the samples using the IUTM. The penetration speed was 20 mm/min. The shear strength was calculated from the maximum load during a shear test and on the basis of original dimensions (the cross section) of the plug using the following formula[31]:

(2)

where: S is shear strength in N/mm²; F is shearing force in N; d is the diameter of the solid cylindrical probe with flat end in mm; and t is the thickness of the slice in mm. Tests for three different varieties of melon in the three cases, namely, peel, flesh, and unpeeled produce were carried out to determine the shear strength and maximum shearing force. Experiments were conducted randomly on samples of different sizes and from different parts of the melons. The tests were repeated 10 times for different cases (using different samples).

Cutting Force Test

This test was carried out to determine the necessary cutting force of the produce in three different cases namely unpeeled, peel and flesh. A stainless steel cutting probe (Fig. 1e) with sharpened edges positioned at 30° (included angle), and 1.5 mm thickness[13] was fabricated and used on the IUTM. The speed of loading was 20 mm/min. The test was repeated 10 times for different cases (using different samples) and the average results were calculated and statistically compared.

Estimation of Relative Contribution

Comparison of identical properties obtained by different researchers is extremely difficult because of non-standard experimental conditions. The relative contribution of peel (%) is defined as the ratio of peel to the unpeeled value of a property, which can be used to identify the role of peel to that property. Moreover it may be used as a criterion to compare the melon varieties with other vegetables considering a specific property. The relative contribution (%) of each property was calculated by dividing the value of peel property to the value of its unpeeled property and multiplying by 100.

Statistical Analysis

One-way analysis of variance (ANOVA) with post-hoc comparisons by using SPSS software was used for the analysis of variance. Variations of mean values of each property for three varieties of melon were determined by the least significant difference (LSD) at the 5% level.

RESULTS AND DISCUSSION

Force-Deformation Curve

Force-deformation curves for peel and unpeeled melons were obtained directly from the compression testing. Typical curves obtained from compression testing on unpeeled and peel cases of cantaloupe melon by hemispherical end probe are shown in Fig. 2. The initial parts of the curves up to the rupture point (peak point) were nearly linear in both cases with different slopes. However, a more linearity and steeper slope can be seen for peel curve. The latter indicates a higher stiffness or rigidity of peel compared with an unpeeled cantaloupe melon. The inclination of the unpeeled curve towards convexity may attribute to the flexibility of the whole melon. After rupture point, the force fell to zero sharply for the peel case and a gradual reduction in the unpeeled case. The non-zero force after rupture point for the unpeeled curve was also reported for pumpkin.[12] This behaviour attributed to the frictional effect of the probe moving through the tissue. Both curves were smoothed at the peak points. Similar results and behaviours[2,26] were found for the shearing of watermelon flesh. This was attributed to either a change in the mechanical properties of the cell walls just before the tissue failure or a gradual progression of breakage of individual cells until rupture point.[26]

Figure 2. Force-deformation curves for cantaloupe melon: –––, peel; —–, unpeeled produce.

Rupture Force

The lowest rupture force of peel (91 N) and unpeeled case (100 N) belonged to the cantaloupe melon, while the highest rupture force of peel (175 N) and unpeeled case (183 N) was found for watermelon and honeydew melon, respectively (Fig. 3a). Cantaloupe melon was significantly (probability p < 0.05) different compared to the two other varieties in the mentioned properties. That could be because of the smaller thickness or less strength of cantaloupe melon peel compared to the other melons. Watermelon and honeydew melon were similar (p > 0.05) regarding the rupture force of peel and unpeeled cases. The test showed a considerable strength of honeydew melon (unpeeled and peel cases) in spite of lower peel thickness compared to the other varieties. The relative contribution of peel to the force required to puncture the produce was 82, 89, and 97% for honeydew melon, cantaloupe melon and watermelon, respectively. The corresponding increasing trend of this property was proportional to the increasing of the produce's peel thickness. The higher relative contribution of watermelon peel could be related either to the thicker peel or stronger tissue in comparison with the other melons. This property was not significantly (p > 0.05) different for cantaloupe melon and any of the two other varieties (Table 1). Honeydew melon and watermelon were significantly different (p < 0.05) in this respect. The relative contribution of peel for cantaloupe and honeydew melons is close to the reported values of the other fruits and vegetables. Thompson et al.,[18] reported 58 to 88% of this property using the puncture test on different fruits, including avocado, Bartlett pear, McIntosh apple and also vegetables such as eggplant, green bell pepper, slicing-type cucumber, zucchini squash. Grotte et al.[8] also obtained 65–70% for different varieties of apples. Furthermore, a close value of relative contribution (73%) was determined for Butternut variety of pumpkin.[12]

Figure 3. (a) Rupture force; (b) toughness:

, peel;

, unpeeled produce.

Table 1 Relative contribution (%) of peel to the different mechanical properties of unpeeled melon (Mean ± Standard Deviation)

Varieties	Rupture force, N	Toughness, N mm	Cutting force, N	Maximum shearing force, N	Shear strength N/mm^2
Cantaloupe melon	89 ± 06^a1a2	28 ± 14^b1	102 ± 17^c1	95 ± 08^d1	141 ± 30^e1
Honeydew melon	82 ± 16^a1	21 ± 18^b1	102 ± 25^c1	89 ± 17^d1	336 ± 86^e2
Watermelon	97 ± 02^a2	50 ± 15^b2	100 ± 14^c1	97 ± 08^d1	178 ± 43^e1
Note: Values with the same letter and number are not significantly different (probability p < 0.05) by least significant difference (LSD) (a1a2 is not different than a1 or a2).

Toughness

The highest value of peel toughness (436 N mm) was found for watermelon while the lowest value (180 N mm) was recorded for the cantaloupe melon (Fig. 3b). The peel toughness of honeydew melon (220 N mm) was similar (p > 0.05) to cantaloupe melon in spite of different peel thicknesses. It shows that the peel toughness depends more likely to the tissue rather than thickness. The peel of watermelon showed significant difference (p < 0.05) in comparison with the other varieties in this case. Although the toughness of unpeeled honeydew melon was 76 N mm higher than that of watermelon, no significant (p > 0.05) difference was observed between them. The unpeeled cantaloupe melon still showed the lowest toughness (603 N mm) and was different from the other varieties. The relative contribution of peel to the toughness of unpeeled produce increased from 21 to 50% for honeydew melon and watermelon respectively (Table 1). A low value of this property and high unpeeled toughness for honeydew melon shows a higher flexibility with adequate resistance to rupture as it was already resulted. While the relative contribution for cantaloupe and honeydew melons was statistically alike (p > 0.05), watermelon's peel exhibited a much higher value than others. This is related to its high peel thickness. This property was reported 45% for Golden Delicious apple at harvest time,[8] which increased up to 78% after 210 days of storage at 2°C. This value was also reported for pumpkin varieties[12] such as Jarrahdale (2.1%), Jap (1.8%), and Butternut (22%). The comparison of the above data for the different fruits and vegetables shows that the moisture content has a significant effect on the toughness value of produce.

Cutting Force

The minimum and maximum necessary cutting force of peel was 10 and 13 N for cantaloupe melon and honeydew melon, respectively (Fig. 4). The same results were also observed for the flesh and unpeeled produce with the same order. Harker et al.[26] stated that the reason for increasing the cutting force of tissue can be related to the size of cells, i.e., tissues containing small cells will have more cell walls, less juice and higher cutting forces than tissues with large cells. The peel cutting force of both cantaloupe and honeydew melons was higher than their unpeeled cases. The most compelling reason for this behaviour might be due to the existence of flesh for unpeeled cases. The flesh moisturises the unpeeled produce and keeps it softer compared to the peel case. Watermelon showed almost the equal values for the cutting force of peel and unpeeled cases, which may be attributed to its thicker peel. While the cutting force of peel for honeydew melon was statistically similar (p > 0.05) to the other two melons, cantaloupe melon and watermelon appeared to be markedly (p < 0.05) different. In the case of flesh, watermelon was similar to the others, but the difference of the cutting force of flesh between cantaloupe and honeydew melons was found to be significant (p < 0.05). The cutting force of the unpeeled cantaloupe melon was different (p < 0.05) from other melons but the honeydew melon and watermelon showed statistically similar values (Table 1). The relative contribution of peel to the cutting force for all varieties was almost 100%. This property was calculated for cassava tuber equal to 76%[13] and also determined for pumpkin varieties including Jarrahdale, Butternut, and Jap as 54, 85, and 85 percent, respectively.[12] It can be concluded that unlike the other vegetables, the cutting resistance of the whole melon is entirely dependent on the peel.

Figure 4. Cutting force:

, cantaloupe melon;

, honeydew melon;

, watermelon.

Maximum Shearing Force

The maximum shearing force increased from flesh to unpeeled cases for all three melons (Fig. 5). It can be concluded that the number of cell walls in tissue increases from the flesh to the unpeeled melon.[26] The mean of the maximum shearing force of peel was statistically alike (p > 0.05) for honeydew melon (131 N) and watermelon (135 N), while for cantaloupe melon (94 N) it was significantly (p < 0.05) different. For the unpeeled case, the lowest (100 N) and the highest (148 N) maximum shearing force belonged to cantaloupe and honeydew melons, respectively. Nevertheless, similar to the peel case, the honeydew melon and watermelon did not vary significantly. The relative contribution of peel to unpeeled maximum shearing force increased from 89 to 97% for honeydew melon and watermelon respectively (Table 1). That increasing trend was in order of increasing peel thickness and was not statistically (p > 0.05) a variety feature for three varieties. This property was also reported for Jap, Jarrahdale, and Butternut varieties of pumpkin as 43, 28, and 67%, respectively.[12]

Figure 5. Maximum shearing force:

, cantaloupe melon;

, honeydew melon;

, watermelon.

Shear Strength

Generally, the shear strength decreased from peel to flesh cases for all three melons (Fig. 6). They varied significantly (p < 0.05) in the shear strength of peel. The lowest (0.7 N/mm²) and the highest (2.25 N/mm²) shear strength of peel belonged to cantaloupe and honeydew melons respectively. The peel tissue of honeydew melon showed higher resistance (at least two and a half times) to shear compared with the other two melons. It was an important result because of low peel thickness. The possible reason can be attributed to the cellular tissue. The peel tissue of honeydew melon probably has smaller sizes of cells and a higher number of cell walls[26] or less voids,[14] compared to the other melons. This property was also studied for watermelon varieties[2] and reported as 0.27–1.23 and 0.26–1.14 N/mm² for Crimson sweet and Charleston grey varieties, respectively. All three melons showed statistically similar shear strength for the flesh and unpeeled cases. The range of variation was from 0.09 to 0.11 N/mm² for flesh and from 0.5 to 0.65 N/mm² for the unpeeled case. Sadrnia et al.[2] revealed lower values for red flesh shear strength of two watermelon varieties as 0.02 and 0.03 N/ mm² for Crimson sweet and Charleston grey, respectively. They concluded bruise will occur in the area where stresses are equal or more than the shear strength of the red flesh. The relative contribution of peel to unpeeled produce increased from 141 to 336% for cantaloupe and honeydew melons, respectively (Table 1). Although the values of shear forces showed high resistance of the studied melons, especially honeydew melon, it also confirmed that shear strength considerably depends on the peel. The property of relative contribution was also reported 91% for cassava tuber[13] and 153, 145, and 102 percent for Jarrahdale, Jap, and Butternut varieties, respectively.[12]

Figure 6. Shear strength:

, cantaloupe melon;

, honeydew melon;

, watermelon.

Application of Mechanical Properties

The investigation was carried out to identify useful mechanical properties of melon at the peeling stage. The identification of mechanical properties for different cases including peel, flesh, and unpeeled melon can be applied to remove peel aiming to reduce wasting flesh. The mean values of properties (ignoring deviations) are considered for recommendations because of the close values of peel and unpeeled cases. The comparison of rupture values showed that the determination of specific values of rupture force for the three studied melons is impossible. Peeling cantaloupe and honeydew melons with suitable tools using rupture technique can be possible. To keep the edible portions safe, the maximum value of rupture force is determined as 92 and 155 N for cantaloupe and honeydew melons, respectively. Applying rupture force for peeling watermelon cannot be suggested due to the rupture values of peel which are revealed to be almost equal or even bigger than in unpeeled cases. The application of rupture force with maximum 500 N mm energy consumption (Fig. 3b) is suggested for all three varieties. This value is chosen because of the existing differences between toughness values of peel and unpeeled cases. Limiting applied work, under the mentioned values, can protect the underlying good tissue and save peeling energy. Peeling melons using cutting forces especially in the radial direction is not recommended because of equal or even higher values of this property for peel compared to the unpeeled melon. Applying cutting forces may be attempted successfully by cutters with different design than that mentioned in this paper. The lower value of maximum necessary shearing force for peel compared to unpeeled cases showed the possibility of shearing peel of all three varieties without destroying the whole melon. Using peeling methods on the basis of shearing peel would be much safer for honeydew melon because of a significant difference between peel and unpeeled cases. While applying shearing forces higher than 95 N is not recommended for peeling of cantaloupe melon, the value of 135 N is recommended for both honeydew melon and watermelon. Although the higher values of shear strength for peel compared to the whole melon may be due to different sample thicknesses, this would remind the designer that melon strength and stability of shape are highly dependent on the peel.

CONCLUSION

Selected mechanical properties of three varieties of melon namely cantaloupe melon, honeydew melon, and watermelon were identified and statistically compared to form a useful database in design of a mechanical peeler. All three varieties were found alike with respect to the shear strength of both peel and unpeeled cases. Watermelon and honeydew melon were similar (p > 0.05) on the rupture force of peel and unpeeled cases. Applying rupture force for peeling of cantaloupe (92 N) and honeydew melons (155 N) was recommended. The required energy for peeling of all three melons was determined as 500 N mm. Peeling melon using cutter tools was not recommended owing to the fact that the cutting resistance of peel and the whole melon are similar.