EFFECT OF STORAGE DURATION ON MECHANICAL PROPERTIES OF BELLO EGGPLANT FRUIT UNDER QUASI COMPRESSION LOADING

The mechanical properties of eggplant fruit (cv. Bello) harvested at physiological maturity stage were evaluated in three storage periods (3d, 6d and 9d). These mechanical parameters (rupture force, rupture energy and deformation at rupture point) were measured under quasi compression loading, using the Universal Testing Machine (Testometric model). The fruit’s toughness and rupture power were calculated from the data obtained from the rupture energy and deformation at rupture point. Results obtained showed that mechanical properties of the Bello eggplant fruit exhibited strong dependence on the storage period. The results showed that as the Bello fruit stored longer, its rupture force and rupture energy decreased from 812 N to 411 N, and 5.58 Nm to 3.11 Nm respectively. While the rupture power decreased from 1.095 W to 0.353 W. On the contrary, the toughness and deformation at rupture increased from 0.270 mJ/mm3 to 0.403 mJ/mm3, and 16.99 mm to 25.22mm respectively during the 9 days storage period. The knowledge of the mechanical properties of fruits is important for their harvest and post-harvest operations, therefore, information obtained from this study will be useful in the design and development of machines for the mechanization of eggplant production.


Introduction
Eggplant (Solanum aethiopicum L.) is one of the indigenous crops planted in Africa countries. The consumption of eggplant is steadily increasing because of greater awareness of the health beneficial effects associated with increased consumption of fruits and vegetables ). Eggplant's fruits and leaves are highly valued constituents of the Nigerian foods and indigenous medicines; and are commonly consumed almost on daily basis by both rural and urban families (Tindal, 1965). The fruits are either eaten raw or fried as ingredient of soups and vegetable sauces. There are several varieties of African eggplant, displaying different traits like corolla diameter, petiole length, leaf blade width, plant branching, fruit shape and colour (Osei et al., 2010). The consumption of eggplant is steadily increasing because of greater awareness of the health beneficial effects associated with increased consumption of fruits and vegetables  From literature review so far, information related to the effect of storage period on mechanical properties of locally cultivated eggplant fruit is lacking. Therefore, the specific objectives of this study were to: (1) measure the mechanical properties (rupture force, rupture energy, deformation at rupture point, toughness and rupture power) of Bello eggplant fruit under compression test; and (2) evaluate the effect of storage time on the mechanical properties of Bello eggplant fruit.

Collection and Sampling
The Bello eggplant fruits used for this research were harvested from the research farm of Delta State Polytechnic, Ozoro, Nigeria, at full maturity stage ( Figure 1). The harvested eggplant fruits were selected on the basis of uniformity of size and shape, washed with chlorine solution (concentration of 100 parts per million) to eliminate microbial growth and other infections during storage (Eboibi and Uguru, 2017). The washed and dried Bello fruits were packed in woven basket and stored at ambient temperature of 24ºC ± 5˚C and 65 -75 % humidity for 9 days. The mechanical properties of the Bello fruits were determined at intervals of three days (0, 3, 6 and 9 d).

Mechanical Properties Determination
The mechanical test of the Bello eggplant fruit was done using a Universal Testing Machine (Testometric model), equipped with a 500 N loading cell and integrator. During the test, each fruit

Statistical Analysis
All data obtained from this study were subjected to statistical analysis using the using the Statistical Package for Social Statistics (SPSS version 20.0) and Duncan's Multiple Range Test (DMRT) was used to compare the mean at 95 % confidence level.

Results and Discussions
Analysis of variance (ANOVA) result of the data obtained from this study showed that for the storage duration had significant effect (P ≤0.05) of all the mechanical parameters investigated ( Table 1). The plots (charts) of the results of the mechanical test of the Bello fruit are presented in Figures 2 to 6. From the plots, it can be clearly seen that the all parameters investigated, except

Rupture Force
As shown in Figure 2, the storage period had significant effect on the fruit rupture force. The rupture force decreased significantly (P ≤0.05) with increasing storage duration from 0 d to 6 d, but further increase in storage duration (from 6 d to 9 d) had no significantly effect on the rupture force. The maximum and minimum values of the Bello fruit rupture force were 812 N and 411 N, obtained at the harvest day and 9 days of storage respectively. In similar trend, Singh and Reddy, (2006) reported that increasing storage duration of orange fruit, the force and energy needed to cut fruit decreased. Rupture force of fruits is an essential parameter in the design and development of their crushing and other processing machines.

Rupture Energy and Power
The plot in Figures 3 and 4 showed the rupture energy and power of Bello eggplant. The carts showed that rupture energy and power of the Bello eggplant fruit, decreased with increase in storage duration for 0d to 9d. The declined in the rupture energy of the Bello fruit during storage could be attributed to increase of evaporation, transpiration and metabolic activity of fruits during storage at ambient temperature, which will be made the fruit texture softer due to the cellular water loss and disintegration the cell wall structure composition (Brusewitz et al., 1989). It is clear from Figures 3 and 4 that with increasing storage duration from 0 d to 9 d, the rupture energy and power decreased significantly (P ≤0.05) from 5.58 Nm to 3.11 Nm (46.75 decrease); while the power decreased significantly (P ≤0.05) from 1.095 W to 0.353 W (76.6% decrease).. These results indicated that the rigidity of the cellular structures of the Bello fruit diminished over 9 days of storage due to loss of moisture through transpiration and enzymatic changes and thus the fruit structure became softer as storage duration progresses (Cen et al., 2013).    Figure 5 showed the variation in the deformation of Bello fruit at rupture point during storage period. As seen in the chart in Figure 5, the deformation of the Bello fruit increased with increased in storage period. The deformation increased from 16.99 mm to 25.22mm within the 9 days storage period. This may be attributed to the declined in the rigidity of the cellular structure of the Bello fruit. Similar results were recorded by Ashtiani et al., (2016) for Siah-e-Mashhad eggplant fruit, where the deformation increased from 11.42 to 14.82 mm within 9 storage days. According to Zdunek et al., (2010) during storage the maximum force needed to rupture fruits decreased, but its position on the force-displacement curve increased, therefore the stress decreased and the strain increased by time passing. The decrease in stiffness of the tissue by the effect of the storage time depends largely on the extent to which the structural changes occur in the cell wall constituents (Pérez-López et al., 2014). The maturity stage of the fruit at harvest time, due to its influence on the speed at which occurs the loss in the form of parenchyma cells that give overall rigidity to the tissue (Kays and Paull, 2004;Redgwell and Fischer, 2002). Figure 5: Effects of storage duration on the deformation at rupture of Bello eggplant fruit Columns with the same common letters means that they are not significant different at (P ≤0.05).

Fruit Toughness
The chart presented in Figure 6 showed that storage period had significant effect (P ≤0.05) on Bello fruit toughness. Bello fruit toughness increased significantly between 3 and 6 days of storage, however, there was no significant difference between storage 0 d and 9 d. The maximum and minimum values of Bello fruit toughness were 0.270 and 0.403 mJ/mm 3 for 0d and 9d storage period. From the plot in Figure 6, it can be seen that toughness decreases after 6d of storage period. This could be attributed to the deterioration of the Bello fruits, leading to the breakdown of the fruit epicrap strength. Therefore, the inconsistency in the fruit toughness during may be attributed to weaken of the fruit epicarp after storage day 6, caused by enzymatic reactions. Apart from the increased in the firmness of the fruit epicarp during the initial storage period, another factor that could be responsible for the increased in the fruit toughness is the shrinkage in the fruit size and mass during the storage at the high temperature. According to Fricke and Wright, (2016) small seeds usually had the highest tissue densities across all crops species; therefore, seed toughness strongly decreases as its mass increases. In a research they reported that smaller seeds had higher specific toughness (2340 times greater) than larger seeds. Similar results were recorded by Ashtiani et al., (2016) for Siah-e-Mashhad eggplant fruit, where the fruit's pulp toughness increased from 0.113 Nm to 0.146 Nm during 9 storage days. But on the contrary Arendse et al., 2014 reported 53.56 % decreased in the toughness of pomegranate fruits stored for 5 months in cold storage condition. Figure 6: Effects of storage duration on the toughness of Bello eggplant fruit Columns with the same common letters means that they are not significant different at (P ≤0.05).

Conclusion
In this study, some mechanical properties of Bello eggplant fruit were assessed over the course of a 9-day storage period. The results obtained from the mechanical tests demonstrated that mechanical parameters of Bello fruit are highly dependent on storage duration. Apart from the fruit toughness and deformation at rupture, the three remaining mechanical parameters (rupture force, rupture energy and power) decreased monotonically with the increasing of storage time. The rupture force decreased from 812 N and 411 N; rupture energy declined from 5.58 Nm to 3.11 Nm (46.75 decrease); while the fruit toughness increased from 0.270 mJ/mm 3 and 0.403 mJ/mm 3 . The findings of this study are useful in designing of eggplants' processing and packaging systems.