A Preliminary Study on Classification of Mango Maturity by Compression Test

Abstract

A preliminary study toward the design of a firmness tester suitable for mango maturity classification was conducted through an experiment consisting of two parts: (1) probe selection, followed by (2) evaluation of the selected probe in mango maturity classification using a texture analyzer. This resulted in a technique based on the firmness of the fruit measured by a compression test with a maximum force of 3N using a 5mm diameter spherical stainless steel probe. This technique demonstrated the possibility of classifying the maturity of mangoes into two different stages, i.e., 60% and 80% of full ripeness. However, it could not detect the difference between 60% and 70% of full ripeness or between 70% and 80% of full ripeness.

Keywords:

• Mango
• Maturity
• Firmness tester
• Compression test

INTRODUCTION

The measurement of maturity is of paramount importance for harvesting good quality mangoes.[1] To ensure the supply of high-quality fruit, it is important to select fruit with the proper degree of maturation.[2] It is well known that when an immature mango is harvested, it may not develop a full flavor, aroma and taste, even with ethylene or acetylene treatment.[2,3] Conversely, an over-ripe fruit will decay rapidly after harvesting, resulting in reduced shelf life with greater susceptibility to disease.[2,3] The harvesting date of mangoes is decided according to the maturity period which is counted from the day of anthesis or the day of fruit budding. Nowadays, in the harvesting process, especially in developing countries where the majority of mangoes are produced, the maturity of the mangoes is classified by experienced farmers through sensory evaluation and then the mangoes are harvested. Highly experienced farmers can harvest more properly matured fruits while less experienced farmers get more improperly matured fruits. However, high quality mangoes, especially for export, are demanded by consumers. Low quality mangoes will deter customers from buying mangoes again. Therefore, an instrument for classifying the stages of maturity of mangoes is needed to replace the sensory evaluation method of classification whose success depends largely on the knowledge and experience of the individual farmer.

Destructive instrumental methods have been used for measuring the texture properties of fruits and vegetables.[4,5,6,7] There are some research reports on texture's relation to the maturity of fruits and vegetables in which the texture properties were measured using this traditional method.[7,8] Maturity indicators of mangoes are softening of the flesh, decrease in acidity, increase in sugar content, soluble solids and total solids, and increase in carotenoid pigments.[9] At present, these properties can be measured destructively using laborious techniques. A number of researchers have made efforts in the development of nondestructive methods to evaluate these properties. According to Jha et al.,[3] there have been studies on mangoes using near-infrared (NIR) spectroscopy,[10,11,12,13,14,15] visible spectral analysis,[16,17] and acoustic and ultrasound techniques.[2,18,19,20,21] However, these techniques are costly and difficult to implement in mango orchards. Jha et al.[3] developed a method for predicting a maturity index nondestructively using a portable colorimeter.

Softening in flesh changes the fruit's mechanical properties, i.e., the texture of the fruit. Softening of the flesh then can be indicated by mechanical properties or textural properties, which can be measured by a texture analyzer or a universal testing machine in the laboratory. A hand-held unit that can measure the textural properties of mangoes on the tree quickly and nondestructively would be desirable. This paper reports on a preliminary study toward the design of a firmness tester for the classification of mango maturity.

MATERIALS AND METHODS

The experiment was composed of two parts: (1) probe selection, followed by (2) evaluation of the selected probe in mango maturity classification using a texture analyzer.

Probe Selection

Samples

Mangoes (Mangifera indica L. variety Namdokmai) at 3 different stages of maturity (120 fruits per stage) were collected from a commercial mango orchard in Angtong Province in the central part of Thailand. These different maturity stages, including 60, 70, and 80% of full ripeness, were classified by farmers through sensory evaluation.

Methods

A texture analyzer (TA-XT2i, Stable Micro Systems, London, UK) with 4 probes including a 5, 10, and 15 mm diameter spherical stainless steel probe and a 75-mm diameter circular flat aluminum plate probe were used to measure the firmness of the mango fruits (Fig. 1). The different radii of curvature of the probes were selected in relation to the size of the human thumb: smaller, nearly the same, bigger and infinite radius (flat). Thirty mango fruits at each stage of maturity were subjected to a compression test (at a maximum force of 3 N and a probe speed of 0.2 mm/s) using a texture analyzer with each probe. The maximum force of 3 N was selected because it was observed that no bioyield point appeared during the test; hence, it is a non-destructive test. Experiments were conducted for all 4 probes to compare the performances of these probes. Each probe was compressed on the most convex part (the cheek) of one side of the fruit. The purpose was to test at only one point so as to facilitate measurement if the firmness tester is developed in the future. The firmness, the slope of the force and the deformation curve of each fruit was calculated by Texture Expert version 1.22 software (Stable Micro Systems, London, UK). To select the best probe for classifying mango maturity, the firmness values from each probe and each maturity level were analysed for the coefficient of variation (CV), which is the ratio of the standard deviation to the mean, and for statistical variance using the F-test for variances between two groups of samples. Duncan's test (p < 0.05) was applied to test the difference between the means of the firmness values tested by the best probe among the different stages of maturity.

Figure 1 Measurement of mango firmness by texture analyzer using 5-mm diameter spherical stainless steel probe.

Mango Maturity Classification by Texture Analyzer

Samples

Three hundred and sixty mangoes (Mangifera indica L. variety Namdokmai) at 3 different stages of maturity were collected from a commercial mango orchard in Angtong Province in the central part of Thailand.

Methods

Each of 360 fruits was measured for firmness and classified into 3 firmness categories including firmness of more than 13.0 N/mm, firmness between 13.0 and 11.6 N/mm and firmness of less than 11.6 N/mm. A texture analyzer with a 5-mm diameter spherical stainless steel probe was used to measure the firmness of the mango fruits. The firmness values in each category were calculated from the average of the mean values of firmness obtained from the 5mm diameter spherical stainless steel probe at two consecutive stages of maturity in the previous experiment. For example, the 13.0 N/mm was from (14.0 + 12.0)/2 (see Table 1).

Table 1 Statistics of firmness (N/mm) of mango fruits at different maturity levels tested with different probes

Maturity	Probe type	n	Firmness	SD	CV	Variance
60%	5 mm diameter spherical	30	14.0	0.9	0.1	0.8a
	10 mm diameter spherical	30	16.1	1.3	0.1	1.6b
	15 mm diameter spherical	30	16.3	1.9	0.1	3.5c
	75 mm diameter circular flat	30	26.8	4.4	0.2	19.4d
70%	5 mm diameter spherical	30	12.1	1.2	0.1	1.6a
	10 mm diameter spherical	30	14.8	1.9	0.1	3.6b
	15 mm diameter spherical	30	13.2	1.6	0.1	2.5ab
	75 mm diameter circular flat	30	20.4	3.5	0.2	12.4c
80%	5 mm diameter spherical	30	11.2	1.1	0.1	1.2a
	10 mm diameter spherical	30	12.3	1.7	0.1	2.8b
	15 mm diameter spherical	30	12.6	1.9	0.1	3.5b
	75 mm diameter circular flat	30	20.4	3.4	0.2	11.3c
a–d = variance separation; n = number of samples; SD = standard deviation; CV = coefficient of variation; a–d and variance separation among different probes at each maturity stage by F-test for variances between two kinds of probes (p < 0.05).

Eighty fruits of each firmness category were randomly sampled and were divided into 4 groups (20 fruits each). The first group was immediately measured for soluble solids, and the other three groups were brought to ripen by exposure to calcium carbide (CaC₂) at approximately 25°C, i.e., room temperature for 1, 2, and 3 days, respectively. The soluble solids content was measured for each day. The calcium carbide used was 10 g per 1 kg of mangoes, and it was weighed by a 0.001 g resolution electronic balance (Shimadzu, BX300, Japan). A 4 cm × 4 cm × 2.5 cm (approximately) thick piece of mango flesh at the part of the fruit that was subjected to the firmness test was extracted, scraped into a paste and squeezed to extract the juice for measuring soluble solids. The soluble solids content was measured by a digital refractometer (Atago, PR-32, Japan) and expressed in %Brix. The results obtained in the experiment were subjected to statistical calculations. The significance of the differences observed among mean values was assessed using Duncan's test. A probability of 5% was considered of significance in such tests.

RESULTS AND DISCUSSION

Probe Selection

The results of the F-test (p < 0.05) were as follows: At 60% maturity, there were significant differences in the variances of firmness measured among the different probes. At 70% maturity, there were significant differences between the flat plate probe and the spherical probes and between the 5-mm and 10-mm diameter spherical probes, but no significant difference among the 15-mm, 5-mm and 10-mm diameter spherical probes. At 80% maturity, there were significant differences between the flat plate probe and the spherical probes and between the 5-mm, 10-mm, and 15-mm diameter spherical stainless steel probes. But there was no significant difference between the 15-mm and 10-mm diameter spherical probes. Table 1 shows the statistics of firmness (N/mm) of mango fruits at different stages of maturity tested with different probes. It is obvious that the 5-mm diameter spherical stainless steel probe provided the best performance due to its minimum value of standard deviation, coefficient of variation and variance. This may be due to having the smallest contact area between the probe and mango surface. According to the Duncan's test the means of the firmness values tested by the probe at different stages of maturity were significantly different (p < 0.05). This indicated that the three different maturity stages (60, 70, and 80% of full ripeness) could be classified by firmness. The decrease in firmness corresponded with the results of Jha et al.[1] which measured firmness in terms of the maximum force recorded by the probe while passing through the fruit. Jha et al.[1] indicated that after the mango fruit attained the stage of maturity, the firmness decreased.

Mango Maturity Classification by Texture Analyzer

Table 2 shows the soluble solids of mangoes at different ripening stages and different firmness categories classified by the 5-mm spherical probe. As described in the introduction, the soluble solids content is one of the indices of the maturity of mango fruits.[9] Therefore, if the mangoes, which were classified into different firmness categories by the probe, had corresponding differences in soluble solids content, then the maturity of mangoes could also be classified by firmness measurement using the texture analyzer with the probe. The soluble solids rates of increase within 4 days of ripening were 1.9, 1.9, and 2.0 °Brix/day for mangoes of the first, second, and third firmness categories, respectively. This indicated that the softer mangoes developed sugar in the fruit better than the harder ones, i.e., they ripened faster. At day 0 of the ripening stage, the soluble solids at different firmness categories were different but the difference was not statistically significant. The reason could be that the physiological change was very small at this stage. The results showed that at each ripening stage, the lower the firmness the higher the soluble solids. The soluble solids at day 1, 2 and 3 of the ripening stage were significantly different between different firmness categories. The first category (> 13.0 N) and the third category (<11.6 N) had different soluble solids but they were not different from the second category (13.0–11.6 N). This indicated that a texture analyzer with a 5-mm spherical probe could classify the maturity into two different stages, the 60% and 80% stages of full ripeness. This implies that the development of a nondestructive maturity tester suitable for mango maturity classification during harvesting time can not be done using this level of force (3 N) and/or speed of compression (0.2 mm/s) or might not be possible by using firmness as the only harvesting index. Most of the commercial firmness testers are destructive penetrometers, and hand-held which has operator-instrument interaction effects,[22] cannot be used online and have not been applied in mango maturity classification. Jha et al.[1] concluded in his work that the maturity of mango fruit has been correlated with size, sphericity, color and firmness in an attempt to fix it in a measurable unit. Still, there is no report on the development of such a unit for mango fruits. The nondestructive firmness tester using a texture analyzer involves a direct measurement of force and deformation which is different from the other nondestructive firmness testers such as acoustic firmness testers,[21,23,24] which use acoustic data such as transmission velocity, dominant and first frequencies, damping ratio, and firmness indices as parameters indirectly indicating firmness. It is obvious that further study is needed on a firmness tester that can work nondestructively online with high resolution in classifying the maturity of mangoes during harvesting time. The development of a firmness tester based on the direct measurement of force and deformation at a force level not higher than the bioyield force of the fruit, i.e., nondestructive measurement, may be more appropriate for a nondestructive system than other indirect measurements and additionally such a tester can be adapted to an online system and made independent of the effects of operator-instrument interaction.

Table 2 Soluble solids of mangoes at different ripening stages and different firmness categories

Ripening stage (Days)	Firmness category (N)	Soluble solids(%Brix)
0	>13.0	8.0 ± 2.2a
	13.0–11.6	8.1 ± 1.5a
	<11.6	9.0 ± 1.7a
1	>13.0	8.8 ± 1.8a
	13.0–11.6	10.0 ± 1.7b
	<11.6	10.0 ± 2.0b
2	>13.0	10.3 ± 2.1a
	13.0–11.6	10.7 ± 1.6ab
	<11.6	11.5 ± 1.7b
3	>13.0	15.5 ± 0.9a
	13.0–11.6	15.6 ± 1.5a
	<11.6	16.8 ± 1.5b
a–b = mean seperation. Data were the means of 20 fruits with the standard deviation a–b. Mean separation among firmness categories within ripening stage by Duncan's test (p < 0.05).

CONCLUSION

This study investigated a technique for classifying the maturity of mangoes based on the firmness of the fruit measured by a compression test with a maximum force of 3 N using a 5 mm diameter spherical stainless steel probe attached to a texture analyzer. The experiment showed the possibility of classifying the maturity of mangoes into two different stages, i.e., 60% and 80% of full ripeness. However, the technique could not detect the difference between 60% and 70% of full ripeness or between 70% and 80% of full ripeness, which are distinctions needed by growers to determine fruit maturity.