Instrumental Textural Properties of Mango (cv Nam Doc Mai) at Commercial Harvesting Time

Abstract

Mango fruit (Mangifera indica L., cv Nam Doc Mai number 4) of three different sizes, were evaluated for their instrumental texture properties, in accordance with the exporter requirements at commercial harvesting time. The size classification of mangoes was determined by the mass of the fruit. The large size weighed more than 351 g, the medium size 330–350 g, and the small size 260–329 g. The results of deformation at a force of 20 N, energy of absorption from a compression test and the average hardness from puncture tests varied for the different sizes. The large size showed firmer and more elastic in relation to the compression force, as well as the hardest and most rigid in response to the puncture force. The peel and flesh strengths of large, medium, and small sizes at the commercial harvesting date did not differ with bio-yield force, which indicated that the strength of the flesh under the peel was very close to the rupture force, which indicated the strength of peel. Examples of the applications of these properties for postharvest handling are described.

Keywords:

• Mango
• Texture
• Mechanical properties
• Postharvest handling
• Objective test

INTRODUCTION

Thai Nam Dok Mai mangoes are a famous export variety and are consumed when ripe, they are therefore susceptible to different kinds of damage both during and after the harvest. The damage during harvesting is caused mainly by the mangoes falling accidentally from the hands of workers or from harvesting tools on to the ground. Postharvest damage is caused by compressive load packaging, as well as vibration in the packages during transportation to packing houses, markets, and export ports. This type of damage results in a major loss of quality in the fruit.[1] Farmers can estimate the maturity of mango fruit by calculating the age of the fruit after the flower have bloomed and by observing their size and shape. Nam Dok Mai mangoes are harvested following a flowering period 100–110 days. In Thailand, the sorting of mangoes for export is done manually at packing houses. Experienced workers visually examine the mango fruit individually. The sorting speed is approximately 20–25 fruit per minute. The parameters used for determining classification are those physical properties such as size, shape and peel appearance. The sizes are classified by weight and confirmed by electronic balance when questionable. The weight ranges for the size-classification groups are variable since they are specified by the exporters and importers. Generally, shape is determined once during the harvest before the mangoes arrive at the packing house. Peel appearance is determined by hue, uniformity of color, and the presence of defects or bruising. In order to kill the eggs and larvae of the oriental fruit fly, vapor heat treatment is obligatory, especially when exporting the mangoes to Japan. The heat treatment process is performed in a hot vapor of 96% relative humidity, which creates a temperature inside the fruit of 47°C, for 20 min.[2,3] The internal temperature is then reduced by cool air or cool water. Mangoes for export are also sometimes treated with calcium carbide (CaC₂) for one night prior to the vapor heat treatment in order to accelerate ripening.

The instrumental textural properties of intact mangoes are associated with the stage of maturity at which mangoes were studied.[4–6 According to Sirisomboon et al.[4] the firmness of intact mangoes at three different maturity levels 60, 70, and 80% of full ripeness, which were classified by farmers through sensory, was significantly different (p < 0.05). This observation indicated that the three different maturity stages could be classified by firmness. However, the technique utilized was unable to detect the difference between 60 and 70% of full ripeness, or between 70 and 80% of full ripeness, which are distinctions needed by growers in order to determine fruit maturity. This complication may however have been due to the interference of non-uniform sizes of the fruit. Compression and puncture tests are the classical methods used to evaluate the instrumental texture properties of fruit and vegetables. The compression test indicates the mechanical response of the whole fruit, while the puncture test indicates the approximate strength of the peel and the flesh at the puncture point. These two tests are also effective in testing most of the categories of damage to fruit and vegetables. These tests provide useful data for engineers, which can utilized in the design of postharvest handling machines and equipment for fruit and vegetables such as sorting, grading, and packing machines, as well as conveying equipment and storage systems. This study aimed to assess the instrumental textural properties of mangoes in relation to the postharvest handling techniques of mangoes for export. This study further evaluated the effect of fruit size had on the properties through use of plate compression and puncture tests. This research will provide useful data and information for engineers in the design of postharvest handling machines and harvesting equipment for fruit and vegetables such as sorting, grading, cutting, and packing machines, as well as conveying equipment and storage systems.

MATERIAL AND METHODS

Material

Mango fruit (Mangifera indica L., cv Nam Doc Mai number 4) from a commercial orchard in Chachengsao Province, Eastern Thailand, were transported one hour by car to the laboratory at King Mongkut's Institute of Technology Ladkrabang, Bangkok, Thailand for the experiments. The mango maturity was 100–110 days following flowering. The mangoes were classified into three different size groups according to exporter standards using an electronic balance. The large size weighed more than 351 g, the medium size 330–350 g and the small size 260–329 g. There were 20 fruit for each size category.

Methods

Whole mango fruit were subjected to plate compression and puncture tests using the methodology modified from Sirisomboon et al.[7] as follows. The flat plate compression test was carried out through the use of a texture analyzer (TA-XT2i, Stable Micro Systems, UK) with a load cell (capacity: 50 kg_f; Stable Micro Systems, UK). Each fruit was aligned in its most stable position horizontally from the stem end to the apex on a smooth surfaced glass holder. This technique prevented slipping and deformation on only one side of fruit was assumed. A 75 mm diameter flat aluminum circular plate (P75, Stable Micro Systems, UK) was used to compress the fruit at a deformation speed of 0.2 mm/s. The fruit was compressed up to the maximum force (20 N), and the plate was kept in position, while a relaxation test was performed. The maximum force (the initial compression force for the relaxation test) was determined by a trial and error method. The mango fruits were subjected to proposed forces and the relaxation curves were observed. The force was not too high, and therefore, resulted in non-ruptured fruit and a good shape of relaxation curve was selected. For the relaxation test, the plate was kept at maximum force for a 30-second relaxation time, the decline in the force was subsequently recorded at 30 seconds. The return speed of the test was equal to the compressed speed. The compression force-deformation profiles obtained were then used to calculate the fruit's textural properties, including deformation at maximum force, initial firmness (calculated at 2 N), average firmness (calculated at 20 N), energy of absorption, relaxation force, relaxation ratio and recovery ratio (degree of elasticity). Figure 1 illustrates a typical force-deformation curve obtained from a plate compression test through parameter calculating formulae.

Figure 1 Typical force-deformation curve obtained from the plate compression relaxation test and recovery test. Deformation (mm) at maximum force (ΔF) = ΔD; maximum force (N) = ΔF; ; ; ; energy of absorption (mJ) = AreaOAC-AreaBCD; .

Puncture tests were performed on the samples using the same texture analyzer. The side of the fruit not tested by the plate compression test, which was the side 180° to the direction of compression test, was placed on a glass holder in the same orientation as in the plate compression test. A stainless steel plunger with a flat end diameter of 2 mm was attached to the load cell and used to penetrate the fruit at a deformation speed of 0.2 mm/s, and a depth of up to 15 mm. The puncture force-deformation profiles obtained were used to calculate the textural properties, including initial hardness (calculated at 2 N), average hardness (calculated at the rupture force), deformation at the rupture point, rupture force, toughness, apparent modulus of elasticity, penetrating force in the flesh and the penetrating energy in the flesh. The penetrating force and the penetrating energy in the flesh were calculated at a 10 mm distance (from 5 to 15 mm depth). The bio-yield point was additionally observed. Figure 2 illustrates a typical force-deformation curve obtained from a puncture test through the parameter calculating formulae. Table 1 illustrates the interpretation of instrumental texture properties obtained from flat plate compression and puncture tests.

Figure 2 Typical force-deformation curve obtained from the puncture test. Rupture force (N) = F_r; deformation at the rupture point (mm) = D_r; toughness (mJ) (energy absorbed before rupture point) = area under the curve from the origin to point (0, D_r); ; ; ; bioyield force = F_b (N); penetration force in the flesh (the average of measured forces after rupture point) (N); penetrating energy in the flesh (mJ) = dotted area obtained by the sum of the areas under the curve.

Table 1 The interpretation of instrumental texture properties obtained from the flat plate compression test and puncture test

Instrumental texture properties	Unit	Interpretation
Flat plate compression test
Deformation at maximum force	mm	Deformation of fruit at maximum force defined for the test.
Initial firmness	N/mm	Resistance to deformation under small load at the beginning of the test, in this case 2 N.
Average firmness	N/mm	Resistance to deformation under maximum load at the beginning of the test, in this case 20 N.
Energy of absorption	mJ (N mm)	Work or energy that fruit obtained to deform permanently.
Relaxation		Ability of the fruit to release its stress.
Relaxation force	N	The level the force is reduced to from the initial applied force (maximum force) during relaxation time.
Relaxation ratio	decimal	The amount that the stress is relaxed.
Recovery ratio (degree of elasticity)	decimal	Capability of the fruit to recover from deformation after removing the load, indicating the elasticity of the fruit.
Puncture test
Initial hardness	N/mm	Stiffness or rigidity of fruit.
Average hardness	N/mm	Resistance to deformation under load until the point of sudden fracture.
Deformation at rupture point	mm	Deformation of fruit from the initial state until rupture point.
Rupture force	N	The force necessary to penetrate the skin of the intact fruit.
Bioyield force	N	The force that breaks the initial cell under the skin when the skin is not yet broken.
Toughness	mJ (N mm)	Work or energy required to cause rupture through the skin.
Apparent modulus of elasticity	MPa (N/mm^2)	Stiffness or rigidity of fruit.
Penetrating force in the flesh	N	The force necessary to penetrate the flesh of the fruit.
Penetrating energy in the flesh	mJ (N mm)	Work or energy required to penetrate the flesh of the fruit to a specific depth.

Statistical Analysis

ANOVA of the data was performed for this experiment. Differences between the means of the data of different fruit sizes were compared by least significant difference (LSD). Differences at p ≤ 0.01 were considered significant.

RESULTS AND DISCUSSION

Figure 3 illustrates the average force-time curve for a plate compression test of mangoes of different sizes. The results indicate that in the average curves the maximum force was not at 20 N, due to the fact that the average was taken from all fruits of each size and the time taken to reach 20 N was not the same time for each fruit. Figure 4 illustrates an average force-deformation curve for a puncture test of mangoes at different sizes. Table 2 illustrates the instrumental texture properties of mango fruit. The results are described in the following sections.

Figure 3 Average force-time curve of the plate compression test of mango at different sizes. (Number of samples = 18, 20 and 20 fruit for large, medium and small sizes, respectively.)

Figure 4 Average force-deformation curve of the puncture test of mangoes at different sizes. (Number of samples = 20, 20 and 20 fruit for large, medium and small sizes, respectively.)

Table 2 Instrumental texture properties of mango fruit at different size for export

	Fruit size
Instrumental texture properties	Large	Medium	Small
By plate compression test
Initial firmness (N/mm)	12.9 ± 2.4b	11.2 ± 1.1a	10.1 ± 1.3a
Average firmness (N/mm)	19.3 ± 3.4b	16.9 ± 1.8a	15.1 ± 1.4a
Deformation at max force (mm)	1.1 ± 0.2a	1.2 ± 0.1b	1.3 ± 0.1c
Energy of absorption (mJ)	4.8 ± 0.8a	5.6 ± 0.8b	6.3 ± 0.6c
Relaxation force (N)	16.0 ± 0.2b	15.8 ± 0.2a	15.9 ± 0.1ab
Relaxation Ratio	0.20 ± 0.01b	0.21 ± 0.01a	0.21 ± 0.01a
Recovery ratio	0.74 ± 0.02b	0.72 ± 0.02a	0.71 ± 0.02a
By puncture test
Initial hardness (N/mm)	6.4 ± 1.2b	6.0 ± 0.6b	5.1 ± 0.7a
Average hardness (N/mm)	10.0 ± 1.0c	9.3 ± 0.8b	8.5 ± 0.7a
Deformation at rupture point (mm)	2.8 ± 0.3a	3.0 ± 0.2a	3.2 ± 0.3b
Rupture force (N)	27.6 ± 1.2a	27.6 ± 2.0a	27.5 ± 2.0a
Toughness (mJ)	36.1 ± 4.5a	39.0 ± 4.7ab	40.9 ± 4.8b
Apparent modulus of elasticity (MPa)	3.2±0.6b	3.00±0.3b	2.6±0.3a
Penetrating force in flesh (N)	10.6 ± 1.1a	10.8 ± 1.9a	9.9 ± 1.3a
Penetrating energy in flesh (mJ)	106 ± 11a	108 ± 19a	99 ± 13a
Data are mean values of 20 fruit, with the standard deviation. Values with different letters in each row are significantly different by least significant difference (LSD) at p = 0.01.

Compression Test

The properties measured by the compression test reflect the response of the whole fruit. The results indicate that, the deformation at a force of 20 N and the energy of absorption varied in different sizes of mangoes. Energy of absorption refers to the amount of energy that fruit obtained to deform permanently. The results show that small fruit gained more energy to permanently deform them than large fruits. This indicates that small fruit are more prone than large fruits. The initial firmness, which was measured at a 2 N compression force where nondestruction can be assumed, did not differ between the medium size and small size fruits. This indicates that large fruit can be nondestructively separated from medium and small fruits by applying a small force of 2 N. The results were the same for the average firmness and recovery ratio. The recovery ratio refers to the degree of elasticity of the mango fruit. These results indicate that the elasticity and firmness of medium and small size mangoes were the same, while the large size mangoes were firmer and more elastic. The relaxation ratio, which indicates the ability of the fruit to distribute stress from the loading point to other parts of the fruit, illustrates the viscoelastic characteristics of the fruit. Lower relaxation ratios indicate more elasticity, meaning the fruit will be less viscous. An ideal elastic body has a zero relaxation ratio,[8] therefore the stress cannot be relaxed. The relaxation ratio proved not to differ between sizes. Therefore, the recovery ratio proved to be a better indicator of the elasticity of the mango fruit than the relaxation ratio.

Puncture Test

The properties of the puncture test indicate the response of the peel and the flesh at a tested point. The average hardness was the only parameter that differed amongst the different sizes of mango fruit, although, this parameter was measured destructively at rupture point. The initial hardness and the apparent modulus of elasticity, which were assumed to be nondestructive, did not differ between the medium and small size fruit. The initial hardness and apparent modulus of elasticity indicates the stiffness and rigidity of the fruit.[9] Results indicated that the large mango fruit was the hardest and most rigid in response to the puncture force. The deformation at the rupture point of large and medium fruit was less than that of small fruit. The rupture force did not differ between sizes. The toughness, which refers to the energy needed to penetrate through the peel of the fruit, did not differ obviously between fruit sizes. There were no differences in rupture force, penetrating force in flesh and penetrating energy in flesh amongst the varying fruit sizes. The rupture force is the approximate resistant force of the peel to puncture force. The penetrating force in flesh refers to the average resistance of the flesh to the penetrating force. The penetrating energy in flesh refers to the energy required to penetrate the flesh to a required depth. The results indicated that the peel strength and texture of the flesh of large, medium, and small size mango fruit on the commercial harvesting date did not differ. The bio-yield point appeared in 5.3% of large size fruits, 10% of medium size fruit, and 0% of small size fruit. The bio-yield force occurred very close to the rupture force, indicating that the flesh and peel strengths were similar. The low percentage of bio-yield points for each size could have been because the bio-yield points were close to the rupture points, where at this stage of ripeness, the peel and flesh had similar strength.

Application for Postharvest Handling

The response of the whole fruit to the compression force indicated that smaller fruit deformed, and required more energy to deform permanently, than larger fruit. This indicates that small fruit are more prone to plate compression than large fruit. The large size mango was firmer and more elastic. Results further indicated that the smaller fruit were more prone to compression force than larger fruit. Therefore, postharvest handling of layers of fruit of different sizes during transportation may cause more damage to smaller fruit than larger fruit. The rupture force did not differ between fruit sizes. This implies that the cutting force for the processing of fruit peel and flesh, (not including the seed) amongst different fruit sizes is the same. The response of the peel and flesh to puncture force at one point indicated that the average hardness of larger fruit was greater than that of smaller fruit. The large mango fruit was the hardest and most rigid at the puncture point. This indicates that the small fruit required greater protection from force, such as cushioning and support, than the large fruit. The results also indicate that the flesh and peel strength are close to each other at this stage of ripeness. This indicates that the peel cannot protect the flesh from applied force. Therefore, a cushion, such as net foam, is required.

CONCLUSION

The instrumental texture properties of Nam Dok Mai mangoes of three different sizes at commercial harvesting time were studied intensively for the first time. The application of the properties measured is discussed in this paper. Significant new knowledge can be concluded as follows: the response of the whole fruit to the plate compression test indicated that the large size mango was firmer and more elastic than the medium and small size mangoes. The small fruit were more prone to plate compression damage than the large fruit. The recovery ratio had the ability to better illustrate the elasticity of the mango fruit than the relaxation ratio. The response of the peel and the flesh to the puncture test indicated that the large mango fruit was the hardest and most rigid in response to the puncture force. The bio-yield force indicated that the flesh and peel strength, were similar to each other at this stage of ripeness. The small fruit needed more protection from force, such as a cushion or support, than the large fruit. A cushion, such as net foam, is unavoidable as the strengths of the peel and flesh of mangoes at commercial harvesting time are very similar.