Selected Geometric Characteristics, Density, and Mechanical Properties of Unsplit Pistachio Nut

Abstract

In this study, several physical properties of unsplit pistachio nut were determined as functions of moisture content. By increasing moisture content from 5 to 20% all geometric characteristics increased linearly. Also, apparent and bulk density increased from 803 to 872 kg/m3 and from 458 to 575 kg/m3, respectively, while porosity decreased from 43% to 34%. Coefficient of static friction of the sample nuts on galvanized iron, plywood, and rubber surfaces increased linearly with moisture content. Nut splitting force decreased from 142 to 21 N and energy increased from 20 to 52 mJ with increasing moisture content from 5 to 20%.

Keywords:

• Pistachio nut
• Physical and mechanical properties
• Splitting force
• Coefficient of static friction

INTRODUCTION

Pistachio nut (Pistacia vera L.) is cultivated in the Middle East, United States and Mediterranean countries. It contributes substantially to the agricultural exports of some of these countries. Iran produces about 229,660 tonnes of pistachio nuts annually and 139,740 tonnes were exported in 2005.[1] There are several varieties of pistachio, but only the Pistacia vera fruit is sufficiently large to be acceptable to consumers as edible nuts.[2] Pistachio is usually consumed as raw, salted, or roasted nut. The kernels are a good source of fat (50–60%) and contain unsaturated fatty acids (linoleic, linolenic, and oleic acid), essential for human diet.[3,4]

Unsplit (closed shell) pistachio is not favored since pistachio nuts are used as snack food. Manual as well as mechanical apparatuses have been developed for splitting pistachios. Also, some unsplit pistachio kernels have a desirable deep green color, so it is preferred to crack the shell and remove the kernel safely for use in the confectionery and pastry industries.[5] During processing of pistachio nuts, unsplit pistachios should be separated. The physical and mechanical properties of unsplit pistachio nuts are important in designing the equipment for processing, sorting, and separation of unsplit nuts.

Geometric characteristics, density and mechanical properties of biological materials are dependent on moisture content. Many researchers have studied these moisture-dependent properties for various biological materials such as terebinth fruits,[6] hazelnuts,[7] almond nut and kernel,[8] apricot pit and its kernel,[9] and gram.[10] There are some reports on physical properties of pistachio nut and kernel, for example Hsu et al.[11] studied some geometrical properties of Kerman pistachio cultivar, and Pearson et al.[12] investigated some physical properties for unhulled pistachio nuts. Recently, studies have been conducted on physical properties of dried O'hadi nut and its kernel,[13] and on geometrical, gravimetrical, and frictional properties of five major varieties of Iranian pistachio nuts and kernels as a function of moisture content.[14–16] All these studies have been conducted on “split” pistachio nuts. Literature review indicated that investigation on the physical and mechanical properties of unsplit pistachio nuts are rare. These properties are required for design of processing equipment and apparatus such as splitting or shelling machines for unsplit pistachio nuts. Therefore, the objective of this study was to investigate the geometric characteristics, density and mechanical properties of unsplit pistachio nuts as a function of moisture content.

MATERIALS AND METHODS

Sample Preparation

O'hadi (also called Fandoghi) is the major commercial pistachio variety in Iran. Unsplit fruits of this variety were selected randomly for the experiment. This cultivar was obtained from Rafsanjan city of Kerman province during the summer of 2006. Moisture content of the samples was determined by oven method at 103 ± 2°C until a constant weight was reached.[17] The initial moisture content of the nuts was 4.10% (wet basis). To obtain samples with higher moisture content, a calculated quantity of distilled water (Eq. 1) was added and the sample was packed in a sealed plastic bag. To achieve equilibrium moisture content throughout the product, samples were kept in a refrigerator (5°C) for 10 days.

(1)

Before starting the experiment, the required quantities of sample were kept in the lab and allowed to warm up to room temperature.[6,13] Experiments for determination of the geometric, density and frictional characteristics were carried out in five replications and those for mechanical properties in three replications. These properties were measured at four moisture content levels (5, 10, 15, and 20% w.b.). This range of moisture content was selected because pistachio processing operations as well as storage occur from 5 to 20% (wet basis).

Geometric Characteristics and Density Measurement

One hundred unsplit pistachio nuts were randomly selected at each moisture level. The three principal dimensions, namely length, width, and thickness were measured for each pistachio nut, using an electronic digital caliper (CD-6″CS, Mitutoyo, Japan) with ±0.01 mm accuracy. As shown in Fig. 1, length, width, and thickness were defined as the distance from the calyx end to the stem (L), the maximum diameter (W), and the distance from the highest point to the lowest point (T), respectively, of unsplit pistachio nut when placed on a horizontal plate.[13] The mass of each nut was determined using an electronic balance (GF-600, A&D, Japan) with an accuracy of ±0.01 g. Geometric mean diameter (D_g ) and degree of sphericity (φ) of the samples were calculated using Eqs. 2 and 3:[18]

(2)

(3)

Figure 1 Characteristic dimensions of unsplit pistachio nut. [13]

Surface area of unsplit pistachio nuts was found by analogy with a sphere of the same geometric mean diameter using Eq. 4:[14]

(4)

Bulk density (ρ _b ) was calculated from the mass and volume of the cylindrical container of 30 mm diameter and 60 mm height, which was filled with unsplit pistachio nuts. After filling the container, excess nuts were removed by passing a stick across the top surface using five zigzag motions, without any compaction. Apparent density (ρ _u ) of closed shell nuts is defined as the ratio of the mass of the sample to its volume determined by the fluid displacement technique. In this test, Toluene (C₇H₈) was used instead of water because it is absorbed by pistachio to a lesser extent.[6,1 6] All these tests were carried out using whole completely closed shell pistachio nuts. Sample porosity (ε) was calculated from the bulk and apparent densities using the following relationship:[18]

(5)

Mechanical Properties Measurement

Coefficient of static friction of the samples was determined against frictional surfaces of galvanized iron, plywood, and rubber. A wooden topless and bottomless box was filled with the sample and placed on an adjustable tilting plate. In order to avoid touching the surface, the sample container was raised slightly (5–10 mm). Using a screw device, slope of the test surface was increased gradually until the box just started to slide down and the angle of tilt was read from a graduated scale.[13,16] Coefficient of static friction (μ) was determined as the tangent of the slope angle.[18]

To determine the splitting force of unsplit pistachio nuts, a material testing machine (H50 K-S, Hounsfield, England) was used. The sample was placed between two parallel plates and compression force was exerted along the thickness (suture) of the sample to split the shell. The experiments were carried out at a deformation rate of 24 mm/min for all moisture content levels. Energy absorbed by the sample at splitting point was determined by calculating the area under the force–deformation curve.

RESULTS AND DISCUSSION

Dimensions and Size Distribution of Nuts

Table 1 shows the average and standard deviation of geometric characteristics of the unsplit pistachio nuts at various moisture contents. The dimensions show a trend towards a normal distribution (Fig. 2). As shown in this diagram at 5% moisture content, values of length, width and thickness of about 80% of all pistachio nuts were found to be in the range of 17.79–18.01 mm, 11.52–11.8 mm and 10.98–11.18 mm, respectively. The average values of the geometric mean diameter increased from 13.18–13.89 mm and sphericity increased from 73.68–73.95%, respectively, with moisture content increasing from 5 to 20% (wet basis) Razavi, et al.[14] have reported values for the sphericity of split pistachio which are close to the results of this investigation.

Table 1 Geometric characteristics of the unsplit pistachio nuts at various moisture contents

Physical properties	Moisture content (%w.b.)
	5	10	15	20
Length, mm	17.9 ± 0.09	18.23 ± 0.07	18.46 ± 0.13	18.73 ± 0.11
Width, mm	11.66 ± 0.12	11.87 ± 0.09	12.03 ± 0.12	12.28 ± 0.14
Thickness, mm	11.08 ± 0.08	11.33 ± 0.12	11.49 ± 0.09	11.64 ± 0.10
Geometric mean diameter, mm	13.18 ± 0.13	13.44 ± 0.16	13.62 ± 0.14	13.89 ± 0.17
Sphericity, %	73.68 ± 0.42	73.77 ± 0.52	73.83 ± 0.38	73.95 ± 0.58
Mass, g	1.013 ± 0.095	1.142 ± 0.113	1.235 ± 0.089	1.357 ± 0.106

Figure 2 Frequency distribution curves of nut dimension samples at 5% moisture content: (⋄) length, (□) width, and (Δ) thickness.

Bulk and Apparent Density

Bulk and apparent densities of closed shell pistachio nuts varied from 458–575 kg/m³ and 803–872 kg/m³, respectively, as a function of moisture content (Fig. 3). Razavi et al.[16] also found the values of bulk and apparent densities of O'hadi nuts at different moisture levels close to the results of this study. This result indicated that apparent and bulk densities increased linearly with increasing moisture content:

(6)

(7)

Figure 3 Effect of moisture content on: (a) - density: (⋄) apparent density; (□) bulk density, (b) - porosity, and (c) - surface area of unsplit pistachio nut.

Porosity

Porosity decreased linearly by increasing moisture content from 5–20% (wet basis) as shown in Fig. 3. The magnitude of the variation in porosity depends on the variation of bulk and apparent densities only [Eq. (5)]. Equation (8) shows the relationship between porosity and moisture content:

(8)

This form of porosity change for unsplit pistachio nut is similar to that for split pistachio nut as found by Kashaninejad et al. [13]

Surface Area

Surface area of unsplit pistachio nut increased about 11.1%, while moisture content of pistachio nut increased from 5 to 20% (Fig. 3). Similar trend was reported for split pistachio nuts.[14] The surface area was found to have the following linear relationship with moisture content:

(9)

Static Coefficient of Friction

Static coefficient of friction for unsplit pistachio nuts against frictional surfaces at different moisture contents is presented in Fig. 4. At all moisture contents, the static coefficient of friction was highest on rubber surface (0.43–0.59) and lowest on galvanized iron surface (0.36–0.48). Static coefficient of friction increased linearly as moisture content of pistachio nuts increased. The relationships between these coefficients against various surfaces and moisture contents of pistachio nuts are given in Table 2. Static coefficient of friction increased with moisture content due to increasing cohesive force between the nut and the frictional surface. Therefore, the surface of the sample became stickier when moisture content of the nuts increased. Similar trends have been reported for apricot pit, split pistachio nut, sunflower seed and peanut samples.[9, 13, 16, 19, 20]

Figure 4 Effect of moisture content on: (a) - static coefficient of friction on (⋄) Galvanized iron, (□) Plywood and (Δ) Rubber surfaces, (b) - splitting force, and (c)- splitting energy of unsplit pistachio nuts.

Table 2 Relationships between static coefficient of friction and moisture content of unsplit pistachio nuts against different surfaces

Galvanized iron	μ g = 0.007Mc + 0.30	R ^2 = 0.98
Plywood	μ p = 0.009Mc + 0.33	R ^2 = 0.97
Rubber	μ R = 0.014Mc + 0.36	R ^2 = 0.98

Splitting Force and Energy

The results show (Fig. 4) that there is a strong relationship between moisture content and splitting force in the range of moisture content investigated (5 to 20% w.b.). At lower moisture contents, greater force is needed to split the pistachio nuts. The small rupturing forces at higher moisture content might be due to the fact that the nut's shell tends to be soft at higher moisture contents. The results are similar to those reported by Wang,[21] for pear, Braga et al.,[22] for macadamia nut and Guner et al.,[23] for hazelnuts. Thus for using unsplit pistachio nuts in splitting or shelling machines, nuts should be rewetted before exertion of force, so that they would split properly with lower force. The relationship between the splitting force and moisture content of the unsplit pistachio nut can be represented by the following correlation:

(10)

As presented in Fig. 4, by increasing moisture content, splitting energy generally increased. The results are similar to those reported by Oloso and Clarke, ^[25] for cashews and Guner et al.,[24] for hazelnuts. The relationship between the splitting energy and moisture content of unsplit pistachio nuts can be represented by following Eq. 11:

(11)

CONCLUSION

Results of this study showed that by increasing the moisture content from 5–20% (wet basis), the average length, width, and thickness of unsplit pistachio nuts increased from 17.90–18.73, 11.66–12.28, and 11.08–11.64 mm, respectively. Also, the average geometric mean diameter and sphericity increased from 13.18–13.89 mm, and 73.68–73.95%, respectively. Pistachio unit mass and surface area increased from 1.013–1.357 g and 545–606 mm², respectively, when moisture content increased from 5–20% (wet basis). Bulk density and apparent density of unsplit pistachio nut varied linearly from 458–575 kg/m³ and from 803–872 kg/m³, respectively, while porosity decreased from 43–34% as the moisture content increased from 5–20% (wet basis). The static coefficient of friction for unsplit pistachio nuts increased linearly with moisture content irrespective of surface employed. It was highest for rubber, followed by plywood and galvanized iron. The splitting force was highly dependent on moisture content and increased in a quadratically. Generally, the shell became softer at higher moisture contents and required less force to split. Splitting force and energy increased from 21–142 N and 20–52 mJ, respectively, as moisture content increased.

NOMENCLATURE

Dg	=	Geometric mean diameter (mm)
E	=	Splitting energy (mJ)
F	=	Splitting force (N)
L	=	Length (mm)
M1	=	Final moisture content (%w.b.)
M2	=	Initial moisture content (%w.b.)
Mc	=	Moisture content (%w.b.)
S	=	Surface area (mm2)
T	=	Width (mm)
W1	=	Mass of sample (g)
W2	=	Mass of distilled water (g)
α	=	Angle of tilt (deg)
ε	=	Porosity (%)
μ	=	Static coefficient of friction
ρ b	=	Bulk density (kg/m3)
ρ u	=	Apparent density (kg/m3)
φ	=	Sphericity (%)

ACKNOWLEDGMENTS

The authors would like to thank Tarbiat Modares University for financial support of this research.