Selected Physical Properties of Peanuts

Selected physical properties of peanuts, which are effective in case of mobility or immobility were examined. For this aim, firstly, the geometrical shape of peanuts was defined and the specific mass and friction coefficient, which are quantitative measures of the inertia and frictional resistance that determine the ability of movement, were obtained. A geometrical shape consisting of a cylinder and two hemispheres at the ends has found quantitative appropriateness of at most 91% and at least 34% for the varieties in Turkish Standards. A variation of 0.37–0.58 g cm⁻³ for the solid density and that of 0.21–0.28 g cm⁻³ for the bulk density of hulled peanut have been found. The solid densities of kernel and shell have been determined in the range of 0.88–0.93 g cm⁻³ and 0.27–0.30 g cm⁻³, respectively. Their average bulk densities have been observed within 0.54–0.59 g cm⁻³ and 0.066–0.077 g cm⁻³ intervals, respectively. The angle of repose and internal friction angle measured by two methods have been in close agreement—around a 29° value. The friction coefficient has been found to be influenced by the materials in contact to a great extent, ranging from 0.23 for kernel on sheet metal up to 0.76 for hulled peanuts on an iron grate perpendicular to flow.

Keywords:

• Physical properties
• Geometric model
• Specific mass
• Friction coefficient
• Peanut

INTRODUCTION

Peanuts (Arachis hypogaea L.) are one of the important oil crops in the world. Its seeds are a rich source of edible oil (43–55%) and protein (25–28%). About two thirds of the world production is crushed for oil, and the remaining one third is consumed as food. Turkey has produced about 0.4% of the world’s peanuts with an annual production of about 80 thousands tons in 30 thousand ha areas, all of this production is consumed domestically. Since the harvest and post-harvest technology of peanuts are not developed sufficiently in Turkey, the production costs are quite high. This obstructs using peanuts in the oil industry. Therefore, it is generally used in the food industry. Because of all these factors, there has been an urgent need to design and develop machinery used in processing peanuts industrially, which, in turn requires knowledge of their physical properties.

A rational approach to the design of peanut processing machinery, equipment and facilities will involve a theoretical basis, laying down the mathematical and mechanical foundations that will enable the coupling of the physical properties of the agricultural product with the characteristics of the machinery, equipment, facilities, and so forth. The physical properties do not only constitute the basic engineering data required for machine and equipment design, but they also aid the selection of suitable methods for obtaining those data.[1]

The fundamental properties of peanuts under consideration will be their geometrical shape and the related dimensions, specific mass and friction coefficients of hulled peanuts, kernels and shells. Specific mass and friction coefficient depend primarily on the geometrical shape and the relevant dimensions. It is necessary to know specific mass for a number of good reasons. First of all, specific mass is an essential parameter for the storage, handling and processing of the product. For instance, the methods of processing like sieving, cleaning, and separating into several components may be based upon the differences of specific masses. Data on specific mass are also required for the dimensioning of warehouses, feeding units, as well as for determining capacities and mass flow rates in several processing units, for estimating critical or terminal velocities, and for evaluating inertia of the product.

There are also very good reasons why the frictional properties must be known. To name some examples: estimation of power requirements for the transportation of the product; realization of the velocity and the sieving control; calculating the lateral pressure in a silo wall will necessitate the data on frictional properties including friction coefficients; internal friction angle and angle of repose. Such data will not only affect the shapes and dimensions of storage, flow characteristics in the handling, and methods of processing units, but also overall costs.

There are many studies focusing on determining physical properties of plant material.[2–7] Ogunjimi et al.[8] studied the physical properties of locust bean seeds. They reported that the bulk density varied between 538.02 and 565.30 kg m⁻³. They also found that the friction coefficient on wood was 0.43, and the angle of repose 20.32°. Baryeh and Mangope[9] investigated the physical properties of QP–38 variety pigeon pea. They observed that the bulk density was 0.7 g mm⁻³, the angle of repose was 17°, and the friction coefficient was 0.28 for plywood, 0.23 for galvanized steel, and 0.18 for aluminum at 5% moisture content (w.b). Sahoo and Srivastava[10] studied the physical properties of Okra seed as a function of moisture content, they emphasized that the size and friction coefficient increased as the moisture content increased. Vilche et al.[11] determined the angle of repose and friction coefficient of Quinoa seed. They found that the angle of repose and the friction coefficient varied from 18 to 25° and from 0.14 to 0.27 respectively, on wood and galvanized iron surface in the moisture range from 4.6 to 25.8% dry basis. Chowdhury et al.[12] determined some physical properties of grams at different moisture contents ranging from 10.83% to 31.20% dry basis. Results indicated that the length, width, and thickness increased linearly when the moisture content increased. Also, the angle of repose increase from 27.03 to 33.27° with the increase of moisture content as the bulk density decreased from 787.31 kg m⁻³ to 712.61 kg m⁻³. They emphasized that the friction coefficient for grams increased with moisture content. The highest friction coefficient was found over plywood and the lowest with a glass sheet.

A few researchers studied only the physical properties of peanut kernels. Agrawal et al.[13] emphasized the shapes of the hulled peanuts, which have one or two kernels were described as ellipsoid, double ellipsoid, and cassinoid. Olajide and Igbeka[14] determined the size of peanuts by measuring their principal axial dimensions. The average major intermediate and minor diameters of kernels were found to be 8.54, 3.55, and 6.93 mm, respectively. It was also observed that the angle of repose was 17°. Kaleemullah[15] also investigated the variations of these dimensions with moisture content.

So far, the researchers have focused only on peanut kernels in their studies. However, the raw peanut has potentially a valuable hull, kernel, and shell which can be subjected to industrial processing and economical evaluation. Thus, it is a necessity that as much as possible data on the physical properties of peanuts as a whole product should be provided for an effective utilization in the area of design and development. Nevertheless, such data has been insufficient so far. The fundamental contribution of this article, therefore, is to fill in this gap. Specific objectives can be summarized as classifying the peanuts dimensionally, defining a geometrical model, and determining the specific masses and friction coefficients of hulled peanuts, kernels and shells.

MATERIALS AND METHODS

The selection of the peanut material to be used in the experiments was based on the guidelines of the Turkish Standards No:310[16]. According to this document, the varieties grown in Turkey are standardized with respect to the geographical regions in which they are produced. Test material was chosen from a random set of peanuts in such a way that the experimental samples satisfied the criteria of being cream colored, big, undamaged, and well-matured, excluding extra constraints pertinent to a specific purpose. The average moisture content of a peanut was about 5% (d.b), as measured by oven drying at a temperature of 105°C for 24 hours.

Dimensional Classification and Defining Geometrical Model

Measurements and observations have shown that from among the basic dimensions, the width and thickness of the peanut exhibit only small differences. Therefore, width and thickness are reduced to a single parameter, which will also provide convenience for the theoretical approaches, thus making it possible to represent peanuts geometrically with length and diameter parameters. Either of width or thickness, whichever is larger, is to be defined as the diameter.

The basis on which dimensional classification is to be based is set up by calculating the average dimension () and the associated standard deviation (σ _x ). Then, small, medium, and large size peanuts are so defined that their specific X dimension satisfies the following inequalities:

In this research, a geometrical model, which is sufficient to describe the peanut, was formed. It is considered as being composed of a cylinder of finite length in the middle and two hemispheres of the same cylinder radius in the ends (Fig. 1). One advantage of this model is that it applies to both shell and kernel. The volume of this model, V_m was calculated by Eq. (1)

(1)

Figure 1. The hulled peanut geometry: L, length; R, radius; d, diameter.

where, L, d are the length and the diameter of the peanut, respectively. In order to facilitate classifying, a correlation was established between length and diameter by this model. Eq. (1) was utilized to test the appropriateness of the selected geometric model on a volumetric basis.

The Specific Mass and Model Appropriateness

The filling method was used in order to obtain the specific mass of peanuts and to quantify model appropriateness. For this aim, a transparent cylindrical cup with a measurement scale on it and wooden sawdust as a filling material were used. In the tests, the volumes of the filling material and certain diameters and numbers of peanuts were measured and recorded. By using these data, the length and volume of the peanuts were figured out via size correlation. Then, the solid density, bulk density, and mass per item were calculated. Here, the mass per item was calculated by dividing the relevant total mass by the number of items in the tests. The volume of hulled peanuts is equal to the difference between the volume of the filling material and the total volume. In this case, the appropriateness of the model can be obtained by dividing the difference between the calculated and experimental volumes by the calculated volume of the model on a percentage basis by Eq. (2). The solid, ρ _s and bulk, ρ _b densities were computed by Eqs. (3) and (4), respectively:

(2)

(3)

(4)

where, App% is the percentage of the appropriateness; M is the total mass of peanuts in the cup; N is the number of peanuts in the cup, and V_sd , V_s are the volumes of the sawdust and solid peanuts in the cup, respectively. App% is a quantity signifying how fit the chosen geometrical model is with respect to the real one as far as volumes are concerned. The correlation between App% and ρ _s , ρ _b is clear in view of the fact that all depend on volumes.

Friction Coefficient

In the research, the friction coefficients of hulled peanuts, kernels, and shells were found separately. To this end, three different methods were used: parallel wall method, shear box method, and inclined surface method.[17,18]

Parallel wall method

In this method, a rectangular prism in dimensions of 30 × 19 × 13 cm, with a movable wall, was used (Fig. 2). In order to eliminate wall friction, wall surfaces were made from plexiglas. After filling the box with peanuts, the movable wall was pulled carefully. So, the peanuts flowed along the horizontal line in the volume, and the angle (φ) between static bulk main line and horizontal plane was measured. This angle, referred to as angle of repose, was regarded as an index of the internal coefficient of friction.

Figure 2. The parallel wall experimental set: φ, angle of repose.

Shear box method

In this method, classified peanuts were placed inside the shear box which has 6 × 6 cm² filling area. The box depth is 4.1 cm and shearing surface is 2.5 cm above the box floor (Fig. 3). Metal pieces that represented roughness perpendicular to the shearing direction were inserted at the bottom and upper surfaces of the box. Comparators were used for measuring the quantities of shearing force and the horizontal displacement. The constant sliding velocity (1–2 mm min⁻¹) was achieved by the gears mounted to the bottom surface of the box.

Figure 3. The shear box.

The internal friction angle of hulled peanut is found out by determining the slope of the shearing stress vs. normal stress plot. Normal and shearing stresses were calculated by Eqs. (5) and (6):

(5)

(6)

where σ is the normal stress; τ is the shearing stress; F₁ is the normal force (0, 88, 177 N); F₂ is the shearing force; A₁ is the surface area of the shear box before movement, and A₂ is the surface area of the shear box after movement. Shearing force used in Eq. (6) is that shearing force that shows stability against changing horizontal displacement determined by drawing graphs of shearing force versus horizontal displacement.

Inclined surface method

In this method, the friction coefficients between the hulled peanuts, kernels, shells, and the materials that are used commonly in the peanut post-harvest processing such as sheet iron, galvanized sheet iron, chrome sheet, iron grate, wood, and wire screen were determined. Apart from these, the internal friction coefficient of hulled peanuts were obtained. As seen in Fig. 4, a group of bound material was placed on an inclined surface by which the inclination angle was increased gradually. At the moment of the movement, the angle was read from the scale. The friction coefficient was calculated by evaluating the tangent of the angle.[19]

Figure 4. Determination of friction coefficient by the inclined surface method: θ, inclination angle; μ, friction coefficient.

RESULTS AND DISCUSSION

Dimensional Classification and Defining Geometrical Models

Based on the average and standard deviation of the samples in the sets, hulled peanuts and kernels have been classified according to their diameter and length (Table 1). A review of Table 1 reveals that the lower and upper limits of a certain peanut variety increase roughly simultaneously. For instance, while medium size hulled peanut diameters remain within 13.2 and 17.7 mm values, the corresponding values for the Silifke variety get larger at the same time to 15.0 and 19.7 mm values. Similarly, the lengths of hulled peanuts of the Anamur and Silifke varieties, exhibit the same kind of relationship as their diameters, i.e., they change from 24.9–38.7 mm in Anamur to 31.0–44.7 mm in Silifke. Similar observations can be made from Table 1 about the kernel diameters and lengths. One conclusion is that a correlation between the diameter and length can be expected both for the hulled peanut and kernel. This brings in the question of correlation between fundamental dimensions. Finally, linear correlations between the diameter and the length of those have been established in each type, with their corresponding coefficients of correlation.

Table 1 Size classification of hulled peanuts and kernels

		Size, mm
Peanut variety		Small	Medium	Large
	Hulled peanut	d < 13.2	13.2 ≤ d ≤ 17.7	d > 17.7
Anamur		L < 24.9	24.9 ≤ L ≤ 38.7	L > 38.7
	Kernel	d < 7.5	7.5 ≤ d ≤ 11.1	d > 11.1
		L < 16.0	16.0 ≤ L ≤ 21.0	L > 21.0
	Hulled peanut	d < 14.8	14.8 ≤ d ≤ 18.4	d > 18.4
Antalya		L < 31.8	31.8 ≤ L ≤ 42.5	L > 42.5
	Kernel	d < 8.0	8.0 ≤ d ≤ 11.6	d > 11.6
		L < 17.9	17.9 ≤ L ≤ 21.9	L > 21.9
	Hulled peanut	d < 13.8	13.8 ≤ d ≤ 18.1	d > 18.1
Osmaniye		L < 25.6	25.6 ≤ L ≤ 39.1	L > 39.1
	Kernel	d < 7.5	7.5 ≤ d ≤ 11.2	d > 11.2
		L < 15.1	15.1 ≤ L ≤ 21.7	L > 21.7
	Hulled peanut	d < 15.0	15.0 ≤ d ≤ 19.7	d > 19.7
Silifke		L < 31.0	31.0 ≤ L ≤ 44.7	L > 44.7
	Kernel	d < 8.1	8.1 ≤ d ≤ 11.2	d > 11.2
		L < 16.8	16.8 ≤ L ≤ 22.0	L > 22.0

Table 2 demonstrates that although clearly there is a dependence of length on diameter, this dependence is linear only to the extent displayed by the magnitude of the coefficient of correlation. However, even with such values of coefficients of correlation, a characterization of peanut geometry is possible by only one parameter, namely diameter. The advantages working with one parameter will override the errors resulting from an assumption of such a limited correlation. It will greatly simplify the theoretical work based on an elaborate model.

Table 2 Length-diameter correlations for hulled peanuts and kernels

Peanut variety		Length (L) and diameter (d) correlation	Coefficient of correlation (R^2)
Anamur	Hulled peanut, mm	L = 17.554d + 4.699	0.32
	Kernel, mm	L = 0.8732d + 9.8704	0.26
Antalya	Hulled peanut, mm	L = 14.905d + 12.477	0.25
	Kernel, mm	L = 0.7673d + 11.5620	0.23
Osmaniye	Hulled peanut, mm	L = 13.892d + 3.022	0.33
	Kernel, mm	L = 0.8937d + 9.4071	0.28
Silifke	Hulled peanut, mm	L = 16.480d + 9.098	0.33
	Kernel, mm	L = 9.1505d + 7.6113	0.40

Finally, the relative magnitudes of error, defined as (100-App%), resulting from approximating the peanut shape by a regular geometric model are presented in Table 3. As can be seen from the table, the quantitative suitability of the model ranges from 56% for the Silifke type, through 73% for the Antalya type to 77% for the Anamur and Osmaniye types. Moreover, appropriateness of the model increases as the size of the peanut gets larger. In this sense, the model volumes of the large size of Anamur and Antalya varieties indicate as little as 10% and 9% deviations, respectively from the volumes of the actual peanut shape.

Table 3 Extent of deviation of peanut shapes from the model

	Error (%)
Peanut variety	Small	Medium	Large	Mixed
Anamur	51	33	10	23
Antalya	58	33	9	27
Osmaniye	66	31	13	23
Silifke	64	39	29	44

The Specific Mass

The specific mass which is one of the effective parameters quantifying the ability of a material to move was investigated under the concepts of solid and bulk densities. As has already been implied by Eqs. (3) and (4), explicitly solid density was defined as the ratio between actual peanut mass and volume, and bulk density was described as the ratio between the total mass of a given group and the total volume occupied by this group. The specific mass results are presented in Tables 4 and 5. A review of figures in Tables 4 and 5 shows that as the size of hulled peanut gets larger, the solid density decreases. On the other hand, the bulk density under the same conditions, remains within small variations. Moreover, the bulk density of a hulled peanut is approximately half of the solid density whereas a peanut kernel has, on the average, a bulk density two thirds of its solid density. A considerable difference between the solid and bulk densities of a shell is a good indication for compressibility.

Table 4 The solid and bulk densities of hulled peanuts

		Average specific mass (g cm^−3)
Peanut variety		Small-size	Medium-size	Large-size
Anamur	Solid	0.54	0.51	0.42
	Bulk	0.27	0.28	0.23
Antalya	Solid	0.50	0.48	0.37
	Bulk	0.23	0.27	0.21
Osmaniye	Solid	0.58	0.49	0.44
	Bulk	0.22	0.26	0.25
Silifke	Solid	0.52	0.49	0.47
	Bulk	0.26	0.27	0.23

Table 5 The solid and bulk densities of peanut kernels and shells

		Average specific mass (g cm^−3)
Peanut variety		Solid	Bulk
Anamur	Kernel	0.89	0.59
	Shell	0.30	0.068
Antalya	Kernel	0.88	0.58
	Shell	0.29	0.071
Osmaniye	Kernel	0.93	0.57
	Shell	0.27	0.066
Silifke	Kernel	0.92	0.54
	Shell	0.29	0.077

Another index of mass property is the average mass per item defined as the total mass of a given set of hulled peanuts divided by the number of hulled peanuts in that set. The results of measurements have been collected in Table 6. As is to be expected, the mass per item values become larger with increasing sizes. In a mixed set of varieties, the average mass per item turns out to be approximately 2 g obtained by totaling the last column of Table 6 and dividing by 4.

Table 6 The average mass per item

	Hulled peanut mass (g per item)
Peanut variety	Small	Medium	Large	Mixed
Anamur	0.85	1.71	2.80	1.84
Antalya	1.07	2.23	2.89	2.21
Osmaniye	0.70	1.83	3.01	1.97
Silifke	0.98	2.28	3.62	2.09

The significance of these figures in Table 6 is that they are needed in the calculation of critical velocity, which is a quantity determining the transportability of peanut by pneumatic means.

Friction Coefficient

The average angles of repose obtained by the parallel wall method are displayed in Table 7. As the size of hulled peanut increases, the angle of repose decreases somewhat for all varieties. These values together with other mass properties play important roles about defining the flow characteristics of bulk solids like peanut.[20]

Table 7 The average angles of repose of hulled peanut

	Average angle of repose (°)
Peanut variety	Small-size	Medium-size	Large-size
Anamur	31.0	28.0	26.5
Antalya	29.2	26.6	25.3
Osmaniye	30.0	29.0	26.0
Silifke	32.0	28.0	26.0

The internal friction angles for all varieties have been found in between 29°–29.7° in the method of shear box. Therefore, here, shearing force vs. displacement and shearing vs. normal stress curves are given only for Osmaniye variety (Fig. 5 and 6). Fig. 6 shows that although no load is applied along the normal direction to the peanut, the shear load on the peanut set appears to be non-zero. This seemingly unexpected result reveals that there are adhesive effects of peanut. Values of the internal friction angle are important for they affect the physical constant, lateral-to-vertical pressure ratio, as well as grain bin wall pressures.[21]

Figure 5. The force vs. displacement curve for Osmaniye variety: •, 0 N; ▪, 88 N; ▴, 177 N.

Figure 6. The shearing vs. normal stress curve for Osmaniye variety.

The results of the friction coefficients, which will directly and indirectly affect the design of the processing machine, have been tabulated in Table 8 according to the different contacting materials. It is observed from Table 6 that while the peanut has a strong interaction with the kind of the contacting material, on a percentage basis, the internal coefficient of friction showed a relatively small difference from variety to variety. On the other hand, the values associated with average angle of repose and internal friction angle are quite close to each other on the basis of medium-size peanuts. Within this context, there is a considerable difference between the values of friction coefficient associated with sheet metal and wire screen or iron grate. In all contacting materials, due to the surface roughness of shells, hulled peanuts, and kernels, the relevant friction coefficients are higher in the case of shells followed by hulled peanuts and kernels. The biggest friction coefficient has been observed in the iron grate perpendicular to the flow for all peanut modes. From the results in Table 8, it appears that friction coefficient for perpendicular flow to grate is very similar to that between peanuts with the inclined surface method. This similarity casts light to explain the differences of internal friction angle discovered by two different methods (i.e., shear box and inclined surface method), whereby the selected peanuts are regularly placed in the available box cavity in the shear box method, while the peanuts move on a wavy medium of peanuts tied to each other by tape in the inclined surface method (Fig. 4).

Table 8 The friction coefficients between hulled peanuts, kernels, shells, and different materials

Friction coefficients
Iron grate
Peanut variety		Iron sheet	Glvzd. sheet	Chrome sheet	Parallel	Perpendicular	Wood	Wire screen	Internal
	Hulled peanut	0.33	0.33	0.31	0.54	0.76	0.50	0.70	0.76
Anamur	Kernel	0.28	0.24	0.27	0.51	0.74	0.29	0.53	0.72
	Shell	0.35	0.38	0.35	0.51	0.72	0.40	0.76	0.76
	Hulled peanut	0.33	0.32	0.31	0.51	0.77	0.52	0.65	0.70
Antalya	Kernel	0.32	0.25	0.34	0.51	0.77	0.33	0.51	0.72
	Shell	0.36	0.38	0.33	0.53	0.74	0.41	0.71	0.78
	Hulled peanut	0.35	0.32	0.31	0.51	0.77	0.52	0.65	0.70
Osmaniye	Kernel	0.30	0.23	0.27	0.51	0.74	0.31	0.51	0.72
	Shell	0.37	0.37	0.35	0.53	0.72	0.42	0.76	0.79
	Hulled peanut	0.34	0.34	0.31	0.54	0.73	0.48	0.69	0.76
Silifke	Kernel	0.30	0.25	0.34	0.53	0.77	0.31	0.53	0.74
	Shell	0.36	0.37	0.32	0.57	0.74	0.40	0.72	0.74

The comparable values given for the friction coefficient between this study and those reported in Olajide and Igbeka[14] exhibit a difference mainly because the peanut material used in the measurements have significantly different geometric properties.

CONCLUSIONS

The major findings of the research conducted on peanut material defined in Turkish Standards No: 310[16] can be summarized as follows:

1. The appropriateness of the selected geometric model (a cylinder and two hemispheres) has been found between 34% and 91%.

2. The solid and bulk densities for hulled peanuts have been determined in the range of 0.37–0.58 g cm⁻³ and 0.21–0.28 g cm⁻³, respectively. Also, the average bulk and solid densities variations have been observed in the intervals 0.54–0.59 g cm⁻³ for kernel, 0.066–0.077 g cm⁻³ for shell, and 0.88–0.93 g cm⁻³ for kernel, 0.27–0.30 g cm⁻³ for shell, respectively.

3. The average mass per item for single hulled peanuts has been found to vary from 0.70 g to 3.62 g.

4. The parallel wall method has produced an interval of variation of 25.3°–32° for the angle of repose, and the shear box method has led to a variation of 29°–29.7° for the average internal friction coefficient.

5. The maximum and minimum values for the friction coefficients of the hulled peanut have been determined between 0.31–0.77, those of the kernel between 0.23–0.77, those of the shell between 0.32–0.76 on different materials.

NOMENCLATURE

L	=	Length (mm)
d	=	Diameter (mm)
Vm	=	Volume of the model (mm3)
Vsd	=	Volume of the sawdust (mm3)
N	=	Number of peanuts
M	=	Mass (g)
ρ s	=	Solid density (g cm−3)
ρ b	=	Bulk density (g cm−3)
σ	=	Normal stress (N mm−2)
τ	=	Shearing stress (N mm−2)
F1	=	Normal force (N)
F2	=	Shearing force (N)
A1	=	Surface area of the shear box before movement (mm2)
A2	=	Surface area of the shear box after movement (mm2)
	=	average dimension
σ x	=	standard deviation

Notes

16. Turkish Institute of Standards (TIS). Peanut. No:310, Ankara, Turkey, 1972