Physical properties of unshelled, shelled and kernel of velvet tamarind (Dialium guineense) fruit from Nigeria

Physical properties of agricultural products are the most important parameters to determine the optimum design of grading, conveying, processing, and packaging systems. This study was conducted to investigate some post-harvest physical properties of unshelled (black), shelled (yellow) and kernel of velvet tamarind (Dialium guineense) fruit. The physical properties studied include axial dimensions, geometric mean diameter (GMD), arithmetic mean diameter, square mean diameter, equivalent mean diameter, surface area, aspect ratio, specific surface area, sphericity, mass, volume, density, porosity, moisture content, angle of repose and coefficient of static friction on various surfaces. The mean length, width, and thickness of velvet tamarind fruit (unshelled) were found to be 17.4, 16, and 8.1 mm respectively; while for the kernel they were 7.7, 7.2, and 3.6 mm, respectively. The sphericity, porosity, bulk density, and angle of repose of tamarind fruit (unshelled) were 0.75, 12.5%, 843.7 kg/m3, and 31.5° respectively. The GMD, surface area, and aspect ratio were 13 mm, 46.3 x 10−5 m2, and 0.93 respectively for the unshelled *Corresponding author: F.U. Asoiro, Department of Agricultural and Bioresources Engineering, University of Nigeria, Nsukka, Nigeria E-mails: felix.asoiro@unn.edu.ng, felixasoiro@yahoo.com Reviewing editor: Fatih Yildiz, Middle East Technical University, Turkey Additional information is available at the end of the article ABOUT THE AUTHORS F.U. Asoiro is a lecturer in the Department of Agricultural and Bioresources Engineering, University of Nigeria. He is presently working towards a PhD degree. His research interests include crop storage, food and bioprocess engineering, characterization of indigenous crops for post harvest mechanization, renewable energy, production of biofuel from non-food feedstocks etc. S.L. Ezeoha, PhD, is a senior lecturer in the Department of Agricultural and Bioresources Engineering, University of Nigeria. His research interests include processing of local crops, food and bioprocess engineering, agricultural wastes management, and renewable energy (biomass and solar). C.B. Ugwu just bagged a BE degree under, F.U. Asoiro in the food and bioprocess option, Department of Agricultural and Bioresources Engineering, University of Nigeria. He is currently working towards a post graduate programme. G.I. Ezenne is a lecturer in the Department of Agricultural and Bioresources Engineering, University of Nigeria, Nsukka, and currently on a PhD program. Her research interests are in the area of Soil and Water Resources Engineering. PUBLIC INTEREST STATEMENT Face with present economic recession occasioned by the neglect of the agricultural sector by the previous governments, coupled with the current global fall in the crude oil prices, which is her major sources of foreign income, Nigeria is seriously investing in agricultural mechanization in other to boost its agricultural productivity. Data on the physical properties of velvet tamarind (Dialium guineense) fruit, in all its forms, is imperative for determining the design and development of grading, conveying, processing, storage and packaging systems for the post harvest processing of velvet tamarind (Dialium guineense) fruit, which is widely consumed and has huge socioeconomic potential to the country. Received: 28 September 2016 Accepted: 23 January 2017 First Published: 31 January 2017 Page 1 of 10 F.U. Asoiro © 2017 The Author(s). This open access article is distributed under a Creative Commons Attribution (CC-BY) 4.0 license.


PUBLIC INTEREST STATEMENT
Face with present economic recession occasioned by the neglect of the agricultural sector by the previous governments, coupled with the current global fall in the crude oil prices, which is her major sources of foreign income, Nigeria is seriously investing in agricultural mechanization in other to boost its agricultural productivity. Data on the physical properties of velvet tamarind (Dialium guineense) fruit, in all its forms, is imperative for determining the design and development of grading, conveying, processing, storage and packaging systems for the post harvest processing of velvet tamarind (Dialium guineense) fruit, which is widely consumed and has huge socioeconomic potential to the country.

Introduction
Velvet tamarind (Dialium guineense) is a tree of an average height of 30 m. Dialium guineense fruit is an important agricultural product in south-eastern Nigeria The fruits are widely sold in local markets in Nigeria and are consumed fresh by people of all ages as a snack. The fruits of the plant are chewed by most women in southeastern Nigeria to improve lactation and check genital infection (Akinpelu, Awotorebo, Agunbiade, Aiyegoro, & Okoh, 2011;Balogun, Oji, Besong, & Umahi, 2013;Nwosu, 2000). The processing method employed is still traditional. There is need, therefore, for a comprehensive study of its physical properties to enable rational development of appropriate technologies for its processing.
Physical properties of agricultural products are the most important parameters to determine the proper standard of design of grading, conveying, processing, and packaging systems (Tabatabaeefar & Rajabipour, 2005). Quality differences in fruits are often detected by difference in density. Mass, volume, and projected area are important in determining sizing systems. When fruits are transported hydraulically, the design fluid velocities are related to both density and shape. Physical characteristics of crops are essential parameters in utilization, development of processing methods, and design of equipment (Bagherpour, Minaei, & Khoshtaghaza, 2010). The shape and size of crops are relevant in the design and selection of appropriate cleaning equipment. Physical attributes of crops such as size, shape, density, are major consideration in designing hopper, drying and aeration systems, as these properties affect the resistance to air flow through the stored materials (Hosseinzadeh, Feyzollahzadeh, & Afkari, 2013).

Sample collection and preparations
Freshly harvested fruits of Dialium guineense were sourced from some local farmers at Ogige market, Nsukka local government area of Enugu state, Nigeria. The fruits were cleaned manually by hand to remove all foreign matter such as dust, dirt, pieces of stones, as well as broken and immature fruits. Some of the fruits were manually cracked, grouped and thereafter measured. Determination of the physical properties was carried out in the Food and Bioprocess Laboratory in the University of Nigeria, Nsukka, Nigeria. Figure 1 shows the unshelled, shelled and kernel of Dialium guineense fruit.

Determination of dimensions and shape
A digital vernier calliper (0-150 ± 0.01 mm) was used to measure the dimensions of the unshelled, shelled and kernel of Dialium guineense fruits as demonstrated by Mohsenin (1986). The GMD, AMD, EMD and SMD were determined using the expression by Asoegwu, Ohanyere, Kanu, and Iwueke (2006) given in Equations (1) to (4) where, D g is GMD (m), D a is AMD (m), D e is EMD (m), D s is SMD, L is length (m), W is width (m) and T is thickness (m).
The sphericity (S p ) and aspect ratio of the fruit were determined using the expressions by Mohsenin (1986) and Omobuwajo, Akande, and Sanni (1999) given in Equations (5) and (6).

Determination of surface area and specific surface area
The surface area and SSA were estimated by the relationship in Equations (7) and (8) given by Hacıseferoğulları, Gezer, Özcan, and MuratAsma (2007) and Subukola and Onwuka (2011).
where, S is surface area (m 2 ), S s is SSA (m 2 m −3 ), ℓ b is bulk density (kg m −3 ) and M s is solid mass (kg) of the fruit.

Moisture content
The moisture content of the fruit was determined according to ASAE (1983) recommended method for edible beans. This involves the oven drying of fruit samples at 105°C for 24 h. The samples were later weighed and recorded using an electronic digital weight balance (Metler Digital Electric Weighing Balance (Range: 0-30,000 g) ± 0.01 g; Model: ME-702718). The moisture content of the fruit (%) wet basis (wb) and dry basis (db) were calculated using Equations (9) and (10) where, MC wb is moisture content, wet basis (%); MC db is moisture content, dry basis (%); M i is initial mass of fruit sample (g) and M f is final mass of fruit sample (g).

Solid volume and bulk volume
The solid volume (SV) and bulk volume (BV) of the samples were determined by water displacement. In this process, individual and bulk samples were weighed and immersed in a measuring cylinder containing known volume of water thus leading to an increase (rise) in the water volume, the difference between the final level of water in the measuring cylinder and the initial level of water was the solid and BV of the unshelled, shelled and kernel of the fruit samples.

Solid density, bulk density, density ratio and porosity
Solid density (SD) is the ratio of solid mass of the fruit to its SV. Bulk density (BD) is the ratio of the mass of the bulk fruit to its BV. Density ratio is the ratio of SD to BD expressed as a percentage. Porosity is the ratio of free space between fruit to that of the total bulk fruit. It is expressed in percentage. The SD, BD, density ratio and porosity of unshelled, shelled and kernel of Dialium guineense fruit were calculated using Equations (11) to (14).

Solid mass, bulk mass and 1,000-unit mass
The solid mass, bulk mass and 1,000-unit-mass of the unshelled, shelled and kernel of Dialium guineense fruit were determined for randomly selected samples from a mass of 20 kg, weighed and recorded using a digital weight balance (Metler Digital Electric Weighing Balance (Range: 0-30,000 g) ± 0.01 g; Model: ME-702718).

Coefficient of static friction on various surfaces
The coefficient of static friction of the fruit was determined against six structural material surfaces namely: plywood, corrugated metal sheet, aluminum, asbestos, glass and plastic according to Shepherd and Bhardwaj (1986), Joshi, Das, and Mukherjee (1993) and Pliestic, Dobricevic, Filipovic, and Gospodaric (2006). The coefficient of static friction, μ was calculated from the expression in Equation (15).

Angle of repose
The angle of repose was determined using a hollow cylinder and then trigonometry rules according to Heidarbeigi, Ahmadi, Kheiralipour, and Tabatabaeefar (2008). The angle of repose is the angle with the horizontal at which the material will stand when piled (Paksoy & Aydin, 2004). It was calculated using the expression in Equation (16).
where, θ is angle of repose (°), H is height of the pile (cm) and D is diameter of the pile (cm).

Results and discussion
Results of the physical properties of unshelled, shelled and kernel of Dialium guineense fruit is shown in Table 1.

Dialium guineense fruit
Notes: N is number of replication; St. dev. is standard deviation of the mean; GMD is geometric mean diameter; AMD is arithmetic mean diameter; SMD is square mean diameter; EMD is equivalent mean diameter; SSA is specific surface area; SV is solid volume; BV is bulk volume; SD is solid density and BD is bulk density.   indicative of its pattern of behaviour in a flowing fluid such as air, as well as the ease of separating extraneous materials from the particle during cleaning by pneumatic means. The shape of the unshelled, shelled and kernel of the fruit was close to ellipsoid, with the kernel having a highest sphericity of 0.7594 than the unshelled (0.7520) and shelled (0.6132). Earlier, Çalışır, Özcan, Hacıseferoğulları, and Yıldız (2005) reported sphericity value ranges of 0.897-0.905 between moisture content values of 6.35-15.22% db for Turkey Okra. Jatropha fruit, nut and kernel gave sphericity values of 0.95, 0.64 and 0.68 respectively (Sirisomboon et al., 2007). Based on the result obtained in this study, Dialium guineense fruit has the tendency to roll better, when it has a particular orientation. This property is key when designing machine parts such as hoppers and dehulling equipment for seeds.

Surface area and SSA
The surface area of the unshelled Dialium guineense fruit was observed to be higher than that of the shelled fruit and kernel. The surface area of a biomaterial affects the velocity of air stream that can be used in other to separate the product from an unwanted material in pneumatic separator or to convey seed in pneumatic conveying. The SSA for the unshelled, shelled and kernel of the fruit were 830.14, 574.63 and 521.2 m 2 m −3 respectively. As a result of the large SSA of the product, the mass or energy transfer rate through the surfaces of the product will be large. Mass and energy transfer rate through the surfaces of the kernel might be slower than the rate for unshelled and shelled fruit.

Moisture content
The moisture content (dry basis) values for unshelled, shelled and kernel of Dailium guineense fruit was 20.321, 23.558 and 13.481% (db) respectively. Similarly, the moisture content values (wet basis) for the unshelled, shelled and kernel of the fruit were 16.878, 19.038 and 11.829% (wb) respectively. This result shows that the shelled has the highest moisture content, followed by the unshelled and the kernel, as a result of the fact that the shell of the fruit has the capacity to absorb moisture which is stored within. This indicates that bulk drying of the fruit should be carried out after shelling.

BD, SD, density ratio and porosity
The SD, BD and density ratio for the unshelled Dialium guineense fruit were 1,004.59, 843.73 kg m −3 and 119.046% respectively; 874.5, 780.99 kg m −3 and 112.991% respectively for the shelled; and 849.73, 672.6 kg m −3 and 127.404% respectively for the fruit kernel. Hacıseferoğulları et al. (2007) investigated and reported similar trend for solid and BD for different species of apricot fruits. The shelled fruit and kernel of Dialium guineense fruit were less dense than water (1,000 kg m −3 ). From the result of the SD, unshelled fruit is denser than water and had higher values than the shelled and the kernel. Shelled fruit (edible pulp) had relatively same values as the kernel. This makes it possible to separate a mixture of the shelled fruit and kernel from the unshelled Dialium guineense fruit and also from other products that are less dense than water. Separation of a mixture of the shelled fruit and kernel could not be done by blowing air or floating in water.
The porosity of the unshelled, shelled and kernel of the fruit were 12.471, 4.049 and 18.066% respectively. The porosity values were generally low, with the kernel having the highest porosity. Similarly, low range of values of porosity had earlier been reported for Azivash seed (Azadbakht & Pourbagher, 2015). During aeration or drying of Dialium guineense fruit, high powered fans and motor will be needed to pass cold or heated air through the pore spaces. Natural aeration or drying will be very difficult, slow and an unlikely choice. The result implies that aeration or drying of the bulk kernel and unshelled fruit would be easier than the shelled. Porosity values show how easily a stream of heated air for drying or aeration will pass through a pack of material. This will affect the rate of drying of the material. Porosity values are required in air and heat flow in agricultural materials. Fruit with low porosity value have less pore spaces and hence will dry very slowly.

Solid mass, bulk mass and 1,000-unit mass of seeds
The solid mass and bulk mass for the unshelled, shelled and kernel of fruit were 62.4 × 10 −5 and 0.0508 kg; 35.9 × 10 −5 and 0.0267 kg; and 15.7 × 10 −5 and 0.0142 kg respectively. Values for the 1,000-unit mass were 0.626, 0.3476 and 0.1458 kg for the unshelled, shelled and kernel of fruit respectively. These properties are necessary in the design of transportation, conveying and handling systems for the bulk handling of the fruit in their different forms.

Coefficient of static friction on various surfaces
For the unshelled Dialium guineense fruit, metal surface had the highest coefficient of static friction (0.503), followed by plywood (0.499), aluminium (0.426), asbestos (0.461), glass (0.419) and plastic (0.4). As for the shelled fruit, Plywood had the highest coefficient of static friction (0.569) followed by plastic (0.524), metal (0.513), aluminium (0.476), asbestos (0.473) and glass (0.42). Plastic had the highest mean coefficient of static friction (0.449) for the kernel, followed by metal (0.416), plywood and asbestos (0.408), glass (0.405) and aluminium (0.388). Surface roughness, as shown from the results, affected the coefficient of static friction, particularly for the unshelled and shelled fruit. The more the roughness of these material surfaces the higher the coefficient of static friction. Slippery surfaces, sphericity (shape) and hardness of the kernel enabled them to move easily on the surfaces tested. Hence, the relatively lower values of coefficient of static friction observed. Coefficient of static friction values determine how a pack of seeds or grain will flow in systems (surfaces). This is a design parameter needed in the design of agricultural machine hoppers and other conveying equipment.

Angle of repose
Using the filling method, the value for the angle of repose of unshelled, shelled and kernel of Dialium guineense fruit were 31.521°, 30.532° and 30.591° respectively. This property helps to determine the minimum slope of flow in self emptying bin and minimum slope of flow in a hopper. The unshelled fruit has the lowest flow ability compared to the shelled fruit and the kernel.

Conclusion
Several post harvest physical properties of the unshelled, shelled and kernel of the Dialium guinense fruit were investigated and reported. The mean length, width and thickness of unshelled, shelled and kernel of the fruit were 0.0174, 0.016 and 0.0081 m; 0.015, 0.01 and 0.0052 m; and 0.0077, 0.0072 and 0.0036 m respectively. The fruit kernel had the highest sphericity and aspect ratio values of 0.7594 and 0.94 respectively, when compared with the unshelled which had values of 0.7520 and 0.926 respectively; and the shelled which had values of 0.6132 and 0.674 respectively. The porosity of the unshelled (12.471%), shelled (4.049%) and kernel (18.066%) were observed to be generally low. Natural convection is not advisable during aeration or drying, based on this observation. Forced convection induced by with high-powered fan is required to achieve better drying for these purposes. Individual unshelled fruits are denser than water, consequently would easily sink in water. This makes separation of the unshelled fruit from the shelled, kernel and other contaminants less dense than water possible, during separation. The coefficient of static friction for the unshelled fruit was highest on metal (0.503) and lowest on plastic (0.4); highest on plywood (0.569) and lowest on glass (0.42); and highest on plastic (0.449) and lowest on glass (0.388) for the unshelled, shelled and kernel of the fruit respectively. Unshelled fruit had the highest angle of repose (31.521°), compared to the shelled fruit (30.532°) and kernel (30.591°).