Mass-Volume-Area Related and Mechanical Properties of Soybean as a Function of Moisture and Variety

Abstract

Selected physical properties of three varieties of soybean were determined within a moisture content range of 6.25 to 11.60% dry basis. The seed length, width and thickness for the three varieties increased with the increase in moisture content while the sphericity and roundness of the three varieties increased within the range of 43.0 to 72.3% and 45.5 to 75.9% respectively. True density, bulk density and porosity decreased with increase in moisture content within the range of 1203 to 964 kgm⁻³, 809 to 740 kgm⁻³ and 0.35 to 0.22. The coefficient of friction also decreased with increase in moisture and the highest and lowest value was 23.7 and 17.3 on plywood while that on glass was 19.8 and 11.6, respectively. The angle of repose and terminal velocity increased within the range of 10.2° to 15.3° and 10.10° to 12.60 m/s for the varieties. The compressive force however decreased and the highest and lowest value was 12.85 and 3.5 N respectively for the three varieties.

Keywords:

• Moisture content
• Variety
• Physical property
• Terminal velocity
• Compressive force

INTRODUCTION

Soybean is one of the most important legumes and oilseed crop on a worldwide basis. It has about 40% protein and 18% oil and it is used as food for humans, raw materials for many industries and supplement for livestock feed. Soybean is a dual purpose plan with its residue from oil extraction representing more than 40% of the utilisation value of the plant. It has a high virtually unrivaled protein content of about 30%, a total sugar content of 10% and ash content of about 5%.[1] Soybean is an excellent dietary source of essential poly unsaturated fatty acids essential to diets of human and animals. It has a low free fatty acid value and is useful in reducing susceptibility to cardiovascular diseases and heart ailments caused or aggravated by excessive intake of cholesterol. Soybean has dominated world oilseed production followed by cottonseed, peanut and sunflower and it accounts for one third of the world's total vegetable oil production.[2] Soybean oil accounts for 20–25% of the total world fats and oil production and 30–35% of the total edible vegetable oil production.[2,3] Soybean oil is highly digestible, has a low iodine value and can be used for manufacture of industrial goods, medicinal purposes or for cooking oils. The major domestic food uses of soybean oil (90%) are cooking and salad oils, shortening and margarine and less than 10% is used for non-food products. Soybean oil and meal are the most produced, traded and utilized oil and meal in the world.[3] The residual cake is rich in protein and mineral salts and it is valuable as fertilizers for soils, feeds for livestock and flour for animals.

The four stages which influence the extraction/expression of oil are seed preparation, cooking, seed pressing and separation of solids from expressed oil. In order to design and develop methods and equipment for soybean oil expression in the tropics, knowledge of physical properties and the effect of moisture content on these properties are necessary. This study is carried out to determine the physical properties of three varieties of soybean seeds within a moisture content range of 6.25 to 11.76% dry basis.

Physical properties of biological materials especially oilseeds are prerequisites for the design of equipment for handling, dehulling, expression etc. These physical properties include shape, size, bulk density, apparent density, angle of repose, porosity and mass. A number of studies have been carried out by researchers on different grains and oilseeds. Studies have been carried out on the physical properties of neem nut and that of pumpkin seeds and kernels.[4,5] Some physical properties of oilbean seed, lentil seeds, squash seeds, gram and that of chillies such as size, sphericity, roundness, angle of repose, coefficient of friction, etc. were also determined among others.[6–10] Most of the authors indicated an increase in majority of the physical properties with an increase in moisture content. However some of the properties i.e. the kernel and bulk density decreased with increase in moisture content in some cases as reported for soybeans, for gram, for sunflower seeds for squash seeds and for neem nut.[4,5,11–13]

A study on physical properties of soybean at different moisture content levels has been carried out, however, a number of physical properties, namely angle of repose, static coefficient of friction, and terminal velocity were not considered in the study.[11] Therefore this study was carried out to determine these physical properties since they are however important in the design of equipment for the processing of soybean oil.

MATERIALS AND METHODS

Soybeans of three different varieties were obtained from the seed store of the International Institute of Tropical Agriculture, Ibadan. The grains were cleaned and all foreign materials, broken and immature seeds were removed. The moisture content was determined using a moisture meter. The grain samples were conditioned and the required moisture content was obtained by adding calculated amounts of distilled water to the grains. The samples were sealed in separate polyethylene bags and kept at 3°C in a refrigerator for at least a week to allow for moisture distribution. Before performing the tests, the required amount of soybeans is removed from the refrigerator and allowed to equilibrate to room temperature. The quantity of water added was obtained using the following equation.

where Q is the mass of water added in kg, A is the initial mass of sample in kg, a is the initial moisture content in % d.b. and b is the final moisture content in % d.b. The physical properties were determined at three levels of moisture content between 6.25 to 11.76% d.b. and the test used are in accordance with those used by Desphande et al.[11] to provide an adequate basis for comparison. Ten samples of weight 15 g were randomly taken from the seeds and the grains were randomly selected from each sample. The three linear dimensions i.e., length, L; weight, W; and thickness, T of the selected grains were measured using a micrometer screw gauge of accuracy 0.01 mm, and the values are means of 20 replications.

The degree of sphericity (N), was determined using the equation from Moshesin (14)

The roundness was determined by placing each seed on a sheet of graph paper and the edges carefully traced with a sharp pencil. The largest inscribed circle and smallest circumscribing circle were constructed for each trace. The area of the smallest circumscribing circle (A_c ) was calculated while the largest projected area (A_p ) of each trace was measured. The roundness (R) was computed[13] as

One thousand grain mass was measured using a Mettler PH electronic balance reading to 0.01 g. The grain volume was determined by the liquid displacement method.[16] Samples of twenty seeds were fully submerged after being weighed and coated with a very thin layer of epoxy resin adhesive (Araldite) in order to avoid water absorption during the experiment. The volume of liquid displaced was measured as the volume of the samples. The adhesive is insoluble in water and increase in grain weight due to adhesive coating was negligible (less than 1.5%) and there was no change in weight of grain while submerged. The volume and mass of the submerged samples were used to determine the apparent density. The Bulk density was determined using the standard test weight procedure by filling a standard size container with grains from a special height at a constant rate.[17] The excess grain was removed with a strike-off stick. The measurements of these properties are means of 10 replications. The bulk porosity of the bulk grain was determined from the apparent and bulk density[14] using the following equation

where P_t is the porosity in %; ρ_b is the bulk density in kg/m³ and ρ_t is the true density in kg/m³.

The values of all density characteristics are means of 10 replications. The dynamic angle of repose was determined as follows a plywood box of dimensions 450 × 450 × 450 mm³ with a removable front panel was filled with grains and the front panel was quickly removed allowing the seeds to flow and assume its natural slope.[18] The emptying or dynamic angle of repose was calculated from the measurements of the vertical depth and radius of spread of the samples. Measurements of these properties were replicated 10 times. A galvanised iron cylinder was placed on an adjustable tilting plate, faced with test surface and filled with soybeans. The cylinder was raised slightly so as not to touch the surface. The structural surface with the box resting on its surface was inclined gradually with a screw device till the box just started to slide down and the angle of tilt was read from a graduated scale. The tangent of this angle is the static coefficient of friction for the surface. The terminal velocities were measured using a wind tunnel similar to the one reported by Omobuwajo et al.[15]

For each test, a small sample was dropped into the air stream from the top of the air column and the air was blown to suspend the material in the air stream. The air velocity near the location of the grain suspension was measured to give the terminal velocity. The compressive strength of soybean was measured using the Avery Tensiometer machine and the line of action of the force was along the minor axis of the seed.

RESULTS AND DISCUSSION

A summary of the average values for all the parameters measured are shown in Table 1. The mean values for the seed length (L), width (W) and thickness (T) of soybean measured at different moisture contents in the range of 6.25–11.60% d.b. for the three varieties are shown in Table 1. The values of the dimensions for the three varieties increased as the moisture content increased.

Table 1 Some selected properties of soybean seeds.

Variety	A			B			C
Moisture content,% d.b.	6.25	8.90	11.60	6.25	8.90	11.60	6.25	8.90	11.60
Length, mm	4.32 (0.19)*	4.47 (0.12)	5.21 (0.24)	4.85 (0.27)	5.29 (0.34)	5.90 (0.25)	4.23 (0.20)	4.47 (0.31)	4.89 (0.26)
Width, mm	4.30 (0.27)	4.42 (0.33)	5.19 (0.26)	4.82 (0.21)	5.02 (0.17)	5.23 (0.36)	4.20 (0.19)	4.44 (0.23)	4.87 (0.32)
Thickness, mm	3.20 (0.12)	3.46 (0.20)	4.93 (0.22)	3.02 (0.30)	3.46 (0.19)	4.02 (0.33)	3.24 (0.27)	3.74 (0.29)	4.14 (0.30)
Volume, cm^3	1.10 (0.11)	1.12 (0.19)	1.28 (0.11)	1.06 (0.17)	1.36 (0.18)	1.52 (0.13)	0.92 (0.10)	1.13 (0.12)	1.27 (0.15)
Mass, g	0.12 (0.011)	0.13 (0.017)	0.15 (0.011)	0.14 (0.016)	0.16 (0.011)	0.18 (0.015)	0.11 (0.014)	0.12 (0.012)	0.14 (0.018)
Roundness, %	52.2	57.5	68.8	45.5	64.1	75.9	57.9	60.5	71.4
Sphericity, %	47.2	52.7	68.5	50	65.7	72.3	43	54	62.2
Apparent density, kgm^−3	1203	1161	1062	1191	1142	980	1162	1106	964
Bulk density, kgm^−3	776	764	752	809	786	765	764	752	740
Porosity, decimal	0.35	0.34	0.29	0.32	0.31	0.22	0.34	0.32	0.23
Coefficient of friction (plywood)	19.3	18.4	17.3	21	19.2	17.1	23.7	22.3	21
Coefficient of friction (glass)	15.3	14.7	14.2	15.6	13.7	11.6	19.8	18.5	17.2
Angle of repose, deg	10.8	12	12.9	10.2	10.7	13.4	11.3	11.8	15.3
Terminal velocity, m/s	11.55	11.2	10.42	11.75	11.14	10.1	12.6	12.26	11.9
Compressive force, N	9.65	7.77	7.11	12.85	8.86	7.8	5.3	3.95	3.5

This same increase in linear dimensions was observed for cocoa bean when the length, width and thickness of the coca beans increased from 22.41 to 22.5 mm, 12.2 to 12.86 mm and 7.36 to 7.70 mm respectively for a moisture content increase from 5 to 24% d.b.[19] This same increase was also observed for soybeans, the length increased from 6.32 to 6.75 mm, width increased from 5.22 to 5.55 mm and thickness from 3.99 to 4.45 mm for an increase in moisture content from 8.7 to 25.0% d.b. while the length of millet was reported to increase from 3.522 to 4.163 mm, its width 2.735 to 3.211 mm and its thickness 2.180 to 2.788 mm [9,20] The three dimensions increased with increase in moisture content which indicates that the swelling of the seed is in all three dimensions. The seed dimensions indicates that soybean seed used in this study is larger than millet but smaller than cocoa beans[19,20] and the soybean used by Desphande et al.[11]

The mass (m) of the soybean varieties also increased within the moisture content range for varieties A, B and C. The seed is thus lighter than pumpkin seed which is 0.203 g but heavier than that of sunflower seed and that of karingda seed, which is 0.049 and 0.099 g respectively.[5,13,21]

The seed roundness and sphericity increased for varieties A, B and C respectively (Table 1). The sphericity and roundness of the soybean varieties is therefore less than that of gram, 74% and pidgeon pea, 82.2 and 81.8% [12,16] while the sphericity of sunflower seed, 57.0% and that of oilbean seeds, 60.5% falls within the range for the three varieties.[6,13] The sphericity and roundness of ackee seeds, 75.8 and 74.4%, however falls within the upper limit of the range while the roundness of oilbean seed, 40.0% is less than that of soybean seed.[6,11,15] This indicates that at higher moisture contents, the shape of the soybean tends towards a sphere and the corners of the seed become rounder while at low moisture contents the seed corners are shaper. Thus the higher the moisture content the more the tendency of the soybean to roll like gram and ackee seeds while at low moisture contents the soybean seeds would rather slide like oilbean seed.[6,11,12,15]

The apparent and bulk density (ρ_t , ρ_b ) however decreased with increase in moisture content of the three varieties (Table 1). The correlation between the apparent density (ρ_t ) and moisture content as well as that between the bulk density (ρ_b ) and moisture content of the three varieties are shown in Table 2

Table 2 Relationship for coefficient of friction of soybeans on selected surfaces.

	Variety A		Variety B		Variety C
Physical property	Equation	R^2	Equation	R^2	Equation	R^2
Apparent density, (ρt)	1377.29–26.39 M	0.95	1456.57–39.50 M	0.92	1407.77–37.06 M	0.94
Bulk density, (ρb)	804.00–4.49 M	1.0	859.99–8.22 M	1.0	792.00–4.49 M	1.0
Porosity, Pt	0.43–0.012 M	0.95	0.45–0.019 M	0.82	0.48–0.021 M	0.90
Coefficiant. of friction (plywood), μ	21.67–0.37 M	1.0	25.60–0.73 M	1.0	26.83–0.50 M	1.0
Coef. of friction (glass), μ	16.57–0.21 M	1.0	20.30–0.75 M	1.0	22.83–0.49 M	1.0
Angle of repose, Θ	8.40 + 0.39 M	0.99	6.09 + 0.60 M	0.87	6.12 + 0.75 M	0.85
Terminal velocity, Vt	12.94–0.21 M	0.96	13.75–0.31 M	0.98	13.42–0.13 M	1.0
Compressive force, Fc	12.40–0.47 M	0.93	18.24–0.94 M	0.90	7.25–0.34 M	0.92

The values for the apparent and bulk density of soybean are larger than that of squash seeds, which is 450 to 625 kgm⁻³ and 350 to 475 kgm⁻³ and that of cocoa beans which is 946 to 991 kgm⁻³ and 560 to 505 kgm⁻³ (8, 19). The apparent and bulk density for lentil seed, 1270 to 1212 kgm⁻³ and 832 to 768 kgm⁻³ and the apparent density for pigeon pea, 1305 to 1251 kgm⁻³ are larger than that of soybeans while the bulk density for pigeon pea, 806 to 745 kgm⁻³ and that for gram, 787.31 to 712.61 kgm⁻³ are similar to the bulk density of soybeans.[7,9,11,16] The apparent density decreased with increase in moisture content like that of karingda, gram and lentil seeds while the bulk density of cocoa beans, gram and sunflower seeds also decreased with increase in moisture content.[7,9,13,19,21] The apparent and bulk density for the soybean seeds reported by Desphande et al.[11] also decreased with increase in moisture content. The values gave good agreement with the ones obtained in this study. The porosity (P_t ) of the varieties decreased as moisture content increased for varieties A, B and C. The correlation between porosity and moisture content are shown in Table 2.

The value for apparent density shows that the seeds will sink in water along with lentil seeds and pigeon pea while squash seeds and cocoa beans will float. This property is useful during separation and transportation of seeds using hydrodynamic means. The porosity or percentage volume of voids is low and it may be due to the relatively low sphericity and roundness that gives a loose arrangement of seeds and thus a higher percentage of voids.

The coefficient of friction (μ) on plywood and glass decreased with increase in moisture content for varieties A, B and C. However the coefficient of friction on plywood was higher than that on glass for the three varieties. The higher coefficient of friction on plywood is evident because of the roughness of the plywood surface which will offer a higher resistance to flow/moment of the soybeans. These results agree with the observations for lentil, gram and that for karingda seeds where the coefficient of friction on smoother surfaces decreased compared to that on rough surfaces.[7,9,21] The coefficient of friction decreased with increase in moisture content because the seed corners become rounder as the moisture content increases. This causes the friction between the seed corners and the surfaces to reduce thereby reducing the friction experienced during flow of seeds. The equation for relationship between coefficient of friction and moisture content of the three varieties are shown in Table 2. The angle of repose (Θ) increased for varieties A, B and C. This angle of repose is lower than 24.8 to 27.78° for lentil seeds and 17° for oilbean seeds.[6,7] The variations of the angle of repose with moisture content are indicated in Table 2.

The terminal/suspension velocity increased with increase in moisture content (Table 1). The values are higher than that reported for karingda seeds, 4.5 to 6.5 m/s; for pumpkin seeds, 4.7 to 6.5 m/s; and 4.37 to 6.13 m/s for squash seeds.[5,8,21] The general trend of an increase in terminal velocity of soybean seeds with increase in moisture content is similar to those reported by the authors. This is because an increase in moisture content will increase the mass of seed and thus a higher air velocity will be required to suspend the seeds. The correlation between terminal velocity and moisture content are shown in Table 2. The compressive force decreased as the moisture content increased for varieties A, B and C respectively. This is because the seed becomes softer with increase in moisture content. This property is useful in unit operations during processing of the seeds which require application of mechanical forces e.g., size reduction and especially in predicting the behavior of the seed under the application of mechanical forces. The relationship between the compressive force and moisture content of the seed is shown in Table 2.

CONCLUSIONS

The average length, width and thickness of the soybean seed varieties increased with increase in moisture content. The mass, roundness, sphericity and terminal velocity of the soybean seeds also increased with increase in moisture content. The tendency of the soybean seed to roll increased with increase in moisture content. The average apparent and bulk density, porosity, coefficient of friction and compressive strength however decreased with increase in moisture content. The average density of the soybean seeds indicates that water can be used for its separation, cleaning or transportation processes. The average coefficient of friction of the seeds against plywood was higher than that on glass.

NOMENCLATURE

A	=	initial mass of sample, kg
a	=	initial moisture content of the sample, % d.b.
Ac	=	area of the smallest circumscribing circle of traced out seed, mm2
Ap	=	largest projected area of traced out seed, mm2
b	=	final moisture content of the sample, % d.b.
Fc	=	compressive force of seed, N
L	=	length of seed, mm
M	=	moisture content, % d.b.
m	=	hundred seed mass, g
Pt	=	porosity, %
Q	=	mass of water to be added, kg
R	=	roundness,%
R2	=	coefficient of determination
T	=	thickness of seed, mm
V	=	volume of 100 seeds
Vt	=	terminal velocity, m/s
W	=	width of seed, mm
&#1256	=	angle of repose, deg
ρb	=	bulk density
μ	=	static coefficient of friction, decimal
Φ	=	sphericity of seed, %
ρt	=	true density

Notes

*Values in brackets are standard deviations.

2. Beauregard, L.C. Soybean oil extraction and refining. American soybean Association Madrid, Spain. From Soybean, Food Production and Utilisation Workshop, Lagos, Nigeria, February 13‐15 1989, American Soybean Association: 1989.