ABSTRACT
The physical properties of agricultural materials are functional in solving many glitches associated with machine design during handling and mechanical processing. Physical properties of five groundnut varieties (“Obolo”, “Yenyawoso”, “CRI Nkatie”, “Agbeyeyie” and “Pion”) and their relations to the design of food processing equipment were studied. Obolo variety recorded the maximum axial dimensions, 1000 mass grain, angle of repose, unit volume, and porosity. However, the values of bulk and true densities for Obolo variety were minimal compared with the other four varieties. Data for the angle of repose for the groundnut varieties were 17.74° (Yenyawoso), 18.02° (Pion), 18.73° (Agbeyeyie), 18.71° (Cri-Nkatie), and 18.89° (Obolo). The porosity of the kernels ranged from 21.97 to 24.54%. The mean greatest porosity was found in Obolo (24.54%), followed by Yenyawoso (24.38%), while Agbeyeyie recorded the least mean porosity of 21.97%. The coefficient of friction was greater for the galvanized steel surface than the other experimental surfaces for all the groundnut varieties studied. Analysis of variance (ANOVA) revealed varietal differences among some means of the physical attributes at p < 0.05. Except for the angle of repose, the geometric, gravimetric and frictional properties showed some significant differences at p < 0.05. Obolo variety was statistically different compared with the other four varieties for all the parameters studied. In selecting or designing equipment for processing, Obolo variety will require separate equipment different from that of the other four varieties. Additionally, the study provides pertinent data for use in the selection and designing of machines for processing groundnut kernels.
Key words: Postharvest processing equipment, geometric mean diameter, bulk density, frictional properties, groundnut kernels, angle of repose.
The rate of population growth in the world coupled with demand for quality and safety of food materials require appropriate postharvest machineries for handling, processing, preservation and storage. An empirical data on engineering properties of agricultural biological materials will provide a suitable basis for designing and selection of the right equipment for the various postharvest operations to ensure that the processed biological material is of quality and safe for consumption. Data on engineering properties such as thermal, physical, mechanical, sensory, frictional, electromagnetic and aerodynamic properties, among others are vital for designing and selection of equipment for mechanical handling and processing of any agricultural biological materials. Among these properties, physical properties of biological materials are of great significance (Ofori et al.., 2019).
The knowledge in physical property provides a basis for better option during designing and selection of appropriate equipment for mechanical handling and processing of agricultural products. A determining factor for selection and designing of equipment for processing is linked-up with the physical attributes of the product. Among these attributes include, 1000 grain mass, axial size, volume, mass per unit volume, porosity, angle of repose, and the frictional properties. The importance of these physical attributes are valuable in choosing sieve separators, aeration, heating, estimating cooling, determining power requirement during size reduction process, displaying of grain drying, and scale draw of preharvest and postharvest equipment (Khan et al., 2019). The use of equipment for sorting, grading, cleaning and processing into different items all require data on the physical attributes of the biological materials (Stroshine and Hamann, 1995). Equipment and operating variable for size reduction, receptacle containers, grain silos, grain hopper dimensions, and holding facilities all depends on the engineering properties of biological materials of which physical attributes form great significance (Khan et al., 2019; Serpil and Servet, 2006).
Experiential knowledge on data abounds with physical properties of biological materials for bambara groundnut (Baryeh, 2001), tiger nut (Abano and Amoah, 2011), Moringa oleifera seeds (Aviara et al., 2013) and inter alia. Physical properties of several other local varieties of groundnut pods and kernel have been examined in different countries (Krishnappa et al., 2017; Firouzi et al., 2009; Olajide and Igbeka, 2003; Akcali et al., 2006). Data on anatomical structure of grain is critical for conducting movement of heat during processes such as drying, cooling, freezing and thawing. The rate of drying of biological materials is dependent on the nature of the exposed surface as well as the volume of the material; the more exposed of the surface and lesser volume, the quicker the rate of dispersion of water from the material. Density, size and shape of biological materials are product determinants for estimating terminal velocity in from product with different varieties. Equipment design for processing will be affected if consideration to the physical properties is compromised (Bala, 2017). Safety and quality of processed material is paramount to consumers, hence, appropriate design of equipment for processing are key. Designing of efficient processing equipment significantly depends on the kernel physical properties. Frequent equipment reinvention without consideration to empirical data of the biological material in question contributes to environmental pollution which affects the climate. The operational effectiveness and output to input ratio of processing equipment such as seed cleaners, grading, sorting, conveyors, and seed metering mechanism systems are dependent on the physical attributes of the biological material, and the data on these properties must be carried out and characterized appropriately.
With groundnut production and consumption cutting across all the sixteen regions of Ghana, empirical data on the kernels are momentous in selecting or designing machine for processing and storage. The need to investigate these common elite varieties of groundnut kernels recently released by the Crop Research Institute (CRI), Fumesua, Ghana, to assist in making well informed choices cannot be overemphasized. This research offers appropriate data on geometric, gravimetric and frictional properties of the selected groundnut varieties as a basis for designing and selection of equipment for processing and storage. This study therefore, seeks to determine the geometric, gravimetric and frictional attributes of these groundnut varieties and their correlation to the designing of mechanical equipment for food handling, processing and storage.
Five groundnut varieties namely: “Obolo”, “Yenyawoso”, “CRI Nkatie”, “Agbeyeyie” and “Pion” (Figures 1 to 5) used for the study were obtained from the warehouse of Crop Research Institute (CRI), under the Council for Scientific and Industrial Research (CSIR) at Fumesua, Ghana. These samples were manually cleaned free from dirt, leaves, pest and weeds. A sample size of 2 kg each was used for the analysis. The storage moisture contents (MC) of the groundnut kernels when measured were 7.2, 7.9, 7.8, 7.9 and 7.8% wet basis (wb), respectively, for “Obolo”, “Yenyawoso”, “CRI Nkatie”, “Agbeyeyie” and “Pion”. The initial MC of the kernels was obtained by using the method essayed by Ofori et al. (2019) as in Equation 1.
% MCwb = × 100 (1)
where MCwb = percentage moisture content in wet basis, Mr = mass of water removed and Mi = initial mass of sample before drying.
A sample size of one hundred kernels were stochastically chosen from the 2 kg mass sample and the principal axial dimensions of the kernels were determined (Figure 6). From each sample size, three axial dominant dimensions, that is length (L), width (W) and thickness (T) were determined with a digital vernier caliper of precision of ±0.01 mm (Aviara et al., 2013). The geometric parameters [Arithmetic mean diameter (Da), geometric mean diameter (Dg), sphericity ( ), surface area (S) and aspect ratio (R)] were then estimated from the sample measured axial dimensions.
Equations 2, 3 and 4 were used respectively, to determine arithmetic mean diameter (Dam), geometric mean diameter (Dgm) and sphericity (Ø) (Aviara et al., 2013; Khan et al., 2019; Mohsenin, 1986; Vengaiah et al., 2015).
A method reported by Serpil and Servet (2006) was employed to obtain the angle of repose. A circular plate of known diameter was placed under an open-ended cylinder of size 150 mm in diameter and 220 mm in height. Grain samples were poured into the cylinder from a pre-determined height until it was full. The cylinder was carefully raised for kernels to form a cone on the circular plate. The angle of repose was calculated by the ratio of the height to the base radius of the heap formed.
The coefficient of static friction (μ) was assayed with three different structural surfaces, viz, glass, polished wood and galvanized steel. To measure the static friction, an open-ended PVC cylinder 110 mm in diameter and 90 mm in height was stuffed with kernel and placed on an inclined plane. The PVC cylinder was elevated about 2.5 mm off-contact from the slanted board. The slanting board was lifted bit by bit with the aid of an adjustable mechanism until the cylinder began to move. The angle of tilt was measured with a protractor. Measurement was repeated three times and for each repeat, the sample in the container was poured out and refilled with a fresh sample. Equation 11 was used to calculate for the static coefficient of friction (Vengaiah et al., 2015).
μ = tan θ (11)
where μ is the coefficient of static friction and θ is the angle of tilt of table.
Data analysis
Analysis of variance (ANOVA) was performed on the data of the physical attributes measured and computed using SPC for Excel v5 (trial) hosted on Microsoft Excel 2016 at 5% significance level. A trial of least significance difference (LSD) was performed on the means of the physical attributes measured for the five varieties of the groundnut kernels.
Results of the principal dimensions for the five groundnut kernels are depicted in Table 1. Figures 7 to 11 depict the variations of the kernel three axial magnitudes (length, width and thickness). The degree of variations in the minimum and maximum widths and thicknesses among the kernels studied were minimal as shown in Figures 7 to 11. However, differences among kernels minimum and maximum lengths were clearly evident, of which the highest value was found in Obolo variety (8 mm), followed by Agbeyeyie (7.5 mm) and the lowest was recorded in Yenyawoso and pion at 4.5 mm each. Grading to obtain uniform sizes is imperative for planting with the use of a metering mechanism. Obolo variety had the highest minimal and maximal range of sizes 15.00 to 23.00 mm, 7.47 to 11.67 mm and 8.90 to 11.95 mm, respectively, for the length, width and thickness. The lowest minimal and maximal range of sizes for length, width and thickness were registered for Yenyawoso variety which were, respectively at 9.37 to 15.42 mm, 6.16 to 9.27 mm and 6.14 to 10.11 mm. These seed dimensions are useful for selection or designing sieve apertures in the segregation compartment of machine for shelling (Maduako and Hamann, 2005). The obtained data in Table 1 shows that among the five groundnut varieties studied, Obolo variety recorded the largest sizes in terms of length, width and thickness. However, the remaining four varieties were ostensibly similar in axial dimensions. Empirical data on axial dimensions of any biological material is imperative with the reason that equipment for processing of these bio-materials is dependent on the physical attributes (Davies, 2010).
Hence, in designing sieves for cleaning, Obolo variety will have different sieve-size due to its immense axial dimensions distinct from that of the other four varieties.
Geometric and arithmetic mean diameters
The geometric and arithmetic mean diameters for the kernels had averages of 12.28 and 12.92 mm, 9.25 and 9.47 mm, 9.62 and 9.94 mm, 9.52 and 9.71 mm, and 9.50 and 9.71 mm, respectively, for the Obolo, Yenyawoso, CRI-Nkatie, Agbeyeyie and Pion varieties as presented in Table 1. Geometric mean diameters and the arithmetic mean diameters for the varieties studied are in variance with those of Manipintar, Local I and Local II (Muhammad et al., 2015) as well as RMP-9, ICGV and RMP-12 groundnut varieties (Vengaiah et al., 2015). This variation may be due to varietal differences. Having the requisite data on these properties is significant in the determination of the clearance within the concave openings of groundnut decorticating and separating machines. Also, these attributes are important in screening out solids to remove foreign materials as well as designing equipment for grading.
Sphericity
Sphericity depicts the shape of a biological material in variance to sphere. The mean values of sphericity were 65.21% (Obolo), 75.26% (Yenyawoso), 70.79% (CRI-Nkatie), 76.93% (Agbeyeyie) and 75.55% (Pion) for the kernels as presented in Table 1. Agbeyeyie variety had the highest mean sphericity while the least mean sphericity was found in Obolo. A sphericity value of a biomaterial between 50 and 100% is an indication of the ability of that material to slide on the surface in contact with it (Muhammad et al., 2015). These values obtained for sphericity indicate that all the groundnut kernels have the propensity to roll on its axis. A spiral separator cleaner can be used for cleaning and removal of immature and shrivelled kernels due to the rolling ability of the kernels. A similar trend was found by Muhammed et al. (2015) for Local II and Manipintar with sphericity of 64.20 and 78.24%, respectively.
Surface area
Surface area is vital for quantifying the rate of heat, water and gas transfer during processing such as drying and roasting of kernels. The larger the surface area of the material, the higher the exposure of kernel to the heat source and the greater the heat absorption and desorption during processing that involves heating (drying or roasting) and cooling of kernels (Bala, 2017). From Table 1, the surface areas of the kernels were 269.31, 284.13, 285.43, 291.50 and 474.98 mm2 for Yenyawoso, Pion, Agbeyeyie, Cri-Nkatie and Obolo, respectively. Odesanya et al. (2015) found surface areas of 149 and 97 mm2, espectively, for Samnut 22 and Ex-Dakar. This variation may be due to varietal differences in the products investigated.
Thousand (1000) seed mass
The mean values for the 1000 grain mass of the groundnut kernels were 843.70, 416.66, 420.30, 429.30 and 452.10 g for Obolo, Yenyawoso, Cri-Nkatie, Agbeyeyie and Pion, respectively (Table 2). Among the five varieties, Obolo had the highest mean of thousand grain mass of 843.70 g while Yenyawoso had the least value of 416.66 g. The thousand grain mass is substantial in calculating the size of grain holder units (hoppers) and shelling compartments of machines for processing. The 1000 grain mass property is also useful for estimating machine stability during operations such as size reduction and planting (Muhammad et al., 2015).
True and bulk densities
The minimum bulk density of the groundnut kernels was found in Obolo variety (758.05 kg/m3) and the maximum was found in Agbeyeyie variety (799.71 kg/m3). Also, Obolo variety recorded the minimum true density of 1004.43 kg/m3 while the maximum was recorded in Pion variety (1033.67 kg/m3) as shown in Table 2. The results obtained attest to that recorded by Muhammad et al. (2015) for bulk density range of 0.55 to 0.82 g/cm3 and true density from 0.87 to 1.08 g/cm3 for Local II and Manipintar, respectively. It also concurs with that found by Maduako and Hamman (2005). These properties are useful tools for evaluating maximum load that seed separators can resist without breaking down during groundnut shelling. The particle and the bulk densities are valuable for estimating the aero and hydrodynamic separation of groundnut kernels from foreign material and imposed pressures during design of silo bottoms.
Porosity
Table 2 depicts the porosity of the groundnut kernels. This attribute provides significant information when developing equipment for material handling such as drying, storage, aeration and ventilation. It is also a useful determinant property for estimating material transport in pneumatic conveyors. The porosity for the five kernel varieties ranged from 21.97 to 24.54%. Obolo variety recorded the highest mean porosity of 24.54%, followed by Yenyawoso with 24.38%. Agbeyeyie recorded the least mean porosity of 21.97%. The experimental data obtained concurs with that obtained by (Firouzi et al., 2009) for Local I, Manipintar and Local II with porosities of 24.70, 28.89 and 37.00%, respectively. Boukouvalas et al. (2006) indicated that porosity is among the single most significant properties by which the shape of food materials can be described. Porosity is a critical parameter among others for equipment design.
Angle of repose
Data obtained for the angle of repose were 17.74° (Yenyawoso), 18.02° (Pion), 18.73° (Agbeyeyie), 18.71° (Cri-Nkatie) and 18.89° (Obolo) as in Table 3. These values are closely related to that of Maduako and Hamman (2005), who obtained 20.03° for ICGV-SM-93523, 20.05° for RMP-9 and 20.8° for RMP-12 groundnut seeds, but lower than that obtained by Muhammad et al. (2017). As depicted in Table 3, the angle of repose for kernels of smaller axial dimensions was less than that for bigger seeds. The study unraveled that smaller seeds have smaller and smoother surface area as compared to larger seeds which have larger and rougher surface area, hence flowability is reduced with decreasing angle. This property is valuable for determination of optimum sides for planting machine seed hoppers, silos and storage containers to allow easy sliding of materials (El-Fawal et al., 2009). Also, this attribute is significant for conveyor width analysis and bottoms of storage equipment (Galedar et al., 2008).
Coefficient of static friction
The mean values for static coefficients of friction for the kernel varieties on the three structural surfaces viz polished wood, galvanized steel and glass were 0.42, 0.46, and 0.39 for Obolo, 0.34, 0.34, and 0.32 for Yenyawoso, 0.32, 0.35 and 0.32 for CRI-Nkatie, 0.33, 0.35, 0.32 for Agbeyeyie and 0.33, 0.34, and 0.32 for Pion, respectively (Table 3). It was observed that the highest static coefficient of friction was found with galvanized steel and the least was found with glass in all the varieties studied. Coefficient of static friction has effect on the nature of the surface in contact with the kernel. An increase in frictional resistance was associated with rough surfaces as compared with smooth and polished surfaces. This observation depicts that the smoother and more polished the structural surface, the lower the static coefficient of friction of the samples and vice versa. The values obtained for this property concurs with findings from Baryeh (2001), Davies (2009) and Muhammad et al. (2015) for bamabara groundnut, groundnut grains, and groundnut pods and kernels, respectively. The design dimension of hoppers, bunker silos and other bulk solid storage and handling structures are dependent on the static coefficient of friction. This property is a significant bench mark for calculating the angle of inclination in inclined grain transporting equipment like chutes (Gharibzahedi et al., 2010). Coefficient of friction is a dependent variable needed for selecting materials for fabrication and power required for transporting a given biological material.
Analysis of variance on varietal differences
Table 4 depicts a test of variance performed on the means of the physical attributes to examine the deviations among means of the kernel varieties. There were apparent significant differences at p ≥0.05 among the means of the physical attributes studied. The computed F Critical at 5% for all the parameters studied was 3.48. Except for the mean angle of repose; the properties for geometric, gravimetric, and frictional attributes were in variance. LSD was used to establish the veracity of differences among the means of the attributes as indicated in Tables 5 to 7. From Tables 5 to 7, the mean length, width, thickness, geometric mean diameter, sphericity and surface area of Obolo variety were statistically higher as compared to the other four varieties. The mean bulk density, true density and porosity of Yenyawoso, Cri-Nkatie, Agbeyeyie and Pion were statistically similar but differ from Obolo. Similarly, the mean coefficient of friction on the glass, polished wood and galvanized steel for Yenyawoso, CRI-Nkatie, Agbeyeyie and Pion were statistically similar but differ from Obolo. These differences are very pertinent when deciding on designing of mechanical equipment for handling, processing and storage.
The study carried out on five groundnut varieties, namely, Obolo, Yenyawoso, Cri-Nkatie, Agbeyeyie and Pion, respectively, at moisture contents (% wet basis) of 7.2, 7.9, 7.8, 7.8 and 7.8 revealed the following results:
(1) Among the groundnut varieties studied, Obolo had the maximum geometric dimensions and this is evident on its physical appearance.
(2) For the 1000 mass grain and porosity, Obolo variety recorded extremely high values; however its bulk density was the least among the varieties studied.
(3) The results obtained for frictional properties indicate that the static coefficient of friction varies with the property of the frictional surfaces of the material and this was maximum for galvanized steel, followed by polished wood and glass surface for all the five varieties studied on the structural surfaces.
(4) The means of some of the physical attributes for the groundnut varieties studied were statistically different.
(5) Statistically, Obolo variety had geometrical properties different from the remaining four varieties studied. The implication is that, to design or select an equipment for cleaning, winnowing, conveying, storage and grading, these four varieties (Yenyawoso, Cri-Nkatie, Agbeyeyie and Pion) may use a single equipment for the pre-harvest and post-harvest operations as compared with Obolo variety which will require separate equipment for processing.
The authors have not declared any conflict of interests.
The authors thank Dr. James Y. Asibuo, Head, Legumes Improvement Program and other Co-workers at the unit of the Council for Scientific and Industrial Research-Crops Research Institute (CSIR-CRI), for their assistance in providing samples of the groundnut kernels for the experiment. They are indebted to the various authors whose works were advertently used and duly referenced. They also appreciate the team of the editorial board and the innominate reviewers for their constructive criticism in helping shape this write-up.
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