Effect of Moisture Contents and Compression Axes on Physical and Mechanical Properties of Pistachio Kernel

Abstract

Selected mechanical properties of pistachio kernel of Ohadi cultivar, such as deformation, deformation ratio, rupture force, rupture strength, toughness, and hardness, were measured as a function of moisture content (at 5 levels of 38.5, 31.0, 24.0, 17.0, and 6.0% wet basis) and compression axis (at three loading positions of x, y, and z). Physical properties of the pistachio kernel, such as length, width, thickness, geometric mean diameter, sphericity, and surface area, were also determined. The results showed that the physical properties decreased linearly by decreasing moisture content. However, all mechanical properties increased with decreasing moisture content, except deformation and deformation ratio, which decreased with decreasing moisture content. The highest rupture force, deformation ratio, and toughness in all moisture content levels were obtained for pistachio kernel loaded along the z-axis. While for rupture strength, deformation, and hardness, the highest values were determined along the x-axis. The highest force required to initiate kernel rupture was 38.5 N at z-axis for 6% moisture content and the lowest 30.2 N at x-axis for 38.5% moisture content.

Keywords:

• Pistachio
• Kernel
• Mechanical properties
• Compression
• Hardness

INTRODUCTION

Pistachio nut (Pistacia vera L.) is one of the most important export products of Iran. Global production of pistachio nuts was around 521,921 Mt in 2007. This product is mostly cultivated in Iran, Turkey, and the USA. According to statistical data from the Food & Agriculture Organization (FAO), Iran produced about 230,000 MT of pistachio nuts in 2007, which represented approximately 43.82% of the world's pistachio production.[1]

The physical properties of pistachio nut and its kernel are important to know in order to design the equipment for processing, transportation, sorting, separation, and storing. Designing such equipment without taking these into consideration may yield lower process performance and higher product loss. The major moisture-dependent physical properties of biological materials are shape, size, mass, bulk density, true density, and static friction coefficient against various surfaces.[2]

Mechanical properties of the pistachio nut and kernel are the key parameters in the design of mechanical harvesting, peeling, cracking, pressing, grinding, roasting, and drying machines. In more countries, harvesting, peeling, and de-hulling of pistachio nuts are mostly done manually, which increases time and cost for processing.[3] Mechanical properties, such as rupture force; hardness; and energy used for rupturing fruit, nut, and kernel, are useful information in designing the decupling or nut shelling machine and oil extractor. The rupture force indicates the minimum force required for de-hulling the fruit or shelling the nut and to extract the oil from the kernel. The deformation at rupture point can be used for the determination of the gap size between the surface to compress the fruit or nut for de-hulling or shelling.[4]

In recent years, physical and mechanical properties have been studied for various crops, such as almond nut and kernel;[5–7] caper seed;[8] cherry laurel fruits;[9] cucurbit seed;[10] cumin seed;[11] faba bean;[12] Filbert nut;[13] hazel nut;[14,15] groundnut kernel;[16] macadamia nut;[17] orecanut kernel;[18] pin nuts;^[19] Shea nut;[20] unsplit pistachio nut;[21] and walnut.^[3,22] Dursun found that the compression position influenced the amount of force applied to crack walnut and other nuts.[23] Furthermore, this researcher reported that the cracking position had an important effect for nuts. Koyuncu et al. found that while energy for initial rupturing of walnut increased linearly with increase in shell thickness except for length position, it decreased linearly with an increase in geometric mean diameter.[22]

Based on literature review, the most important factors that affect mechanical properties are moisture content, variety, and loading orientation. Since there is no published work on mechanical properties of pistachio nut and its kernel relating to loading position and moisture content, the objectives of this study were (i) to determine some physical properties (principal dimensions, geometric mean diameter, sphericity, and surface area) of Ohadi variety of pistachio kernel as a function of moisture content in the range of 6–38.5% wet basis (w.b.), (ii) to measure the mechanical properties (rupture force, rupture strength, deformation at rupture point, deformation ratio, toughness, and hardness) as affected by moisture content and three compression axes (x, y, z), and (iii) to model the physico-mechanical properties of pistachio kernel as a function of moisture content.

MATERIALS AND METHODS

O'hadi, Momtaz, Badami, Akbari, and Kalle-Ghuchi are the main varieties for cultivating and trading in Iran and among them, the first one, O'hadi, was selected for this research work. This cultivar was obtained from FeizAbad city, Khorasan province, Iran, during the summer season (July to September) in the year 2007. FeizAbad is located on the northeast part of Iran at 35° 01′ N latitude and 58° E longitude. This city is the main area of pistachio cultivation in Iran, which produces the earliest maturing pistachio varieties with high quality and low microbial level.

The samples were manually peeled and cleaned to remove all foreign matters as well as immature and broken nuts. The nuts were cracked and the kernels separated from the shells by hand. The initial moisture content of each pistachio kernel was determined using an oven method at 103 ± 2°C until a constant weight was reached.[24] In this research project, to vary the moisture content of the pistachio kernel, the pre-determined quantity of samples was dried down in an oven at 80°C until the desired moisture contents according to the following formula:[25]

(1)

where B is final mass of sample after drying in kg; A is initial mass of sample in kg; a is initial moisture content of sample in % (w.b.); and b is desired moisture content of sample in % (w.b.). In order to determine the dimensions of pistachio kernel at different moisture levels, 30 individual kernels were randomly selected and the three principle dimensions namely, length (L, mm), width (W, mm), and thickness (T, mm) were measured using an electronic micrometer (model QLR digit IP 54, QRL Group Company, China) with an accuracy of 0.001 mm. The geometric mean diameter (D_g ) was calculated by the following relationship:[2]

(2)

The criterion used to describe the shape of the pistachio kernel was the sphericity. Thus, the sphericity (φ, %) of samples at each level of moisture content was calculated by following Eq. (2):

(3)

The surface area of pistachio kernel was found by analogy with a sphere of same geometric mean diameter. In obtaining the surface area (S, mm²) of the samples, the equation given by McCabe et al. was used as:[26]

(4)

Compression tests were performed on pistachio kernels to determine the mechanical properties at five moisture contents ranging from initial moisture content, 38.5% (w.b.) to 6% (w.b.), and three compression axes (x, y, z). The mechanical properties of pistachio kernel were measured using a texture analyzer (QTS Texture analyzer, CNS Farnell, Essex, UK) with a cylindrical probe (20 mm, platen) to an accuracy of ±0.2 g (Fig. 1). The operational conditions of texture analyzer were compression test, 5 mm target value, 20 g trigger point, one cycle, and 50 mm/min cross-head speed. Thirty kernels were randomly selected and tested at each moisture level and along the three major perpendicular axes, and the average values are reported in this article. The individual kernel was placed on the base plate and pressed by the moving probe at the preset condition until cracking occurred, as is denoted by a rupture point in the force-deformation curve. As soon as the target point (5 mm) was detected, the loading was automatically stopped (Fig. 2).

Figure 1 QTS-texture analyzer device used in this study (color figure available online).

Figure 2 Typical force-deformation curve for compressed pistachio kernel (color figure available online).

Rupture (cracking) forces (F_R , N) and deformation at the rupture point (D_R , mm) were measured through three compression axes, namely x, y, and z, for the sample to achieve the initial rupture. The x-axis (force F_x ) is defined as the loading axis through the length dimension, while the y-axis (force F_y ) is the transverse axis containing the minor dimension (width) at right angles to the x-axis, and z-axis (force F_z ) is the transverse axis containing the minimum dimension, namely, thickness as shown in Fig. 3.[15] Rupture strength (S_R , N mm⁻²) was calculated for three axes according to following formula:[2]

(5)

Figure 3 Representation of the three axes and three perpendicular dimensions of pistachio kernel.

where A is the cross section area in mm², which is calculated for each compression axis as follows:[2]

(6)

Deformation ratio or compression strain (ϵ, mm mm⁻¹) at the rupture point was calculated for each loading axis according to the following formula:[15]

(7)

Toughness (P, mJ mm⁻³) in this study is regarded as the energy absorbed by the kernel to the point of rupture per unit volume of the kernel which was calculated using the following relationship:[17,20]

(8)

where E_a is the energy absorbed in mJ and V is the volume of pistachio kernel tested in mm³, which is obtained by analogy with a triaxial ellipsoid using the following equation:[2]

(9)

In this study, hardness (Q, N mm⁻¹) is defined as the ratio of compressive force (F_C , N) to deformation at the target value (D_C , 5 mm) of pistachio kernel (Fig. 2). This was calculated using the following relationship:[20]

(10)

The data recorded in each test condition were statistically analyzed using the factorial (two factors) randomized complete block design to study the effects of five moisture contents, three compression axes and their interactions on mechanical properties of pistachio kernel. While, the randomized block design was used to determine the effect of moisture content on physical properties. In addition, least square difference (LSD) tests were used to compare the means for both physical and mechanical properties. To evaluate the regression models describing physico-mechanical properties of pistachio kernel as a function of moisture content, three statistical parameters including R ² (coefficient of determination), χ² (Chi-squared), and SE (standard error) were determined. Statistical analyses and modeling were performed by MSTATC (MSTATC director, Michigan State University, version 1.42, MI, USA) and Minitab (version 13.20, Minitab Inc., State College, PA, USA) software, respectively.

RESULTS AND DISCUSSION

Physical Properties

In this study, the effect of moisture content on the physical properties of pistachio kernel were found to be significant for length, width, thickness, mean diameter, sphericity, and surface area (p < 0.01). The mean value and standard deviation of all physical properties of pistachio kernel of Ohadi variety at five moisture content levels are summarized in Table 1. As it is seen, the length, width, thickness, geometric mean diameter, surface area, and sphericity of the kernels decreased with a decrease in moisture content. These results can be explained with contraction of the kernel as a result of decreasing the moisture. Similar results have been reported by Altuntas and Yildiz[12] for faba bean grains; Aktas et al.[6] for almond nut; Dursun and Dursun[8] for caper seed; Kaleemullah and Gunasekar[18] for arecanut kernel; Maghsoudi et al.[21] for unsplit pistachio nut; Razavi et al.[24] for pistachio nuts and kernels; Abalone[28] for amaranth seeds; Bart-Plange and Baryeh[29] for cocoa bean, etc. They showed that physical properties increase numerically with an increase in moisture content.

Table 1 Some physical properties of pistachio kernel in different moisture contents (%, w.b.).*

Moisture content (%, w.b.)	Length (mm)	Width (mm)	Thickness (mm)	Dg (mm)	Surface area (mm^2)	Sphericity (%)
38.5	16.25 [0.05]^a	10.40 [0.04]^a	9.76 [0.08]^a	11.78 [0.03]^a	436.24 [2.64]^a	72.50 [0.19]^a
31	15.96 [0.02]^b	10.29 [0.07]^a	9.38 [0.37]^b	11.51 [0.16]^b	416.56 [12.40]^b	72.13 [0.94]^b
24	15.93 [0.05]^b	10.07 [0.03]^b	9.40 [0.03]^b	11.44 [0.03]^b	410.90 [2.41]^b	71.82 [0.54]^b
17	15.82 [0.16]^b	9.40 [0.06]^c	9.21 [0.17]^b	11.07 [0.10]^c	385.30 [7.28]^c	69.98 [0.48]^c
6	15.60 [0.04]^d	9.11 [0.05]^d	8.96 [0.21]^c	10.81 [0.09]^d	366.31 [4.15]^d	69.10 [0.61]^d
*Means on the same column with different letters are significantly different (P < 0.01) using LSD test. Standard deviations are in brackets.

Table 2 shows the relationships obtained for physical properties of pistachio kernel as a function of moisture content. In all cases, high correlation (R ² > 0.9) was observed between physical properties and moisture content. During moisture desorption, the pistachio kernel contracted in length, width, and thickness linearly. The positive linear relationship of dimensions with moisture content was also observed by Abalone[28] Baryeh,[30] Kashaninejad et al.[31] and Milani et al.[10] for amaranth seeds, millet, pistachio nut, and cucurbit seeds, respectively.

Table 2 Regression equations obtained for physical properties of pistachio kernel (O'hadi variety) in the moisture content range of 6–38.5%.*

Property	Equation	R ^2	χ^2	SE
Length (mm)	L = 0.018 Mc  + 15.490	0.937	1	0.065
Width (mm)	W = 0.043 Mc  + 8.851	0.917	0.999	0.179
Thickness (mm)	T = 0.022 Mc  + 8.826	0.913	0.999	0.094
Geometric mean diameter (mm)	Dg = 0.029 Mc  + 10.630	0.961	0..999	0.082
Surface area (mm^2)	S = 2.145 Mc  + 353.400	0.966	0.993	5.724
Sphericity (%)	φ = 0.112 Mc  + 68.510	0.903	0.999	0.506
*R ^2: Determination coefficient; χ^2: Chi-squared; SE: Standard error.

Mechanical Properties

Rupture force

The statistical analysis revealed that effects of compression axis and moisture content and their interaction on rupture force of pistachio kernels were found to be highly significant (P < 0.01). As shown in Table 3, rupture force of pistachio kernel increased linearly as the moisture content decreased from 38.5% to 6% (w.b.). The reason for this trend is that when the kernel desorbed water, it became harder and this was responsible for the increase in rupture force with moisture content. Braga et al.,[17] Singh and Goswami,[11] Aydin,[14] Olaniyan and Oje,[20] and Calisir and Aydin[9] found a similar trend for macadamia nut, cumin seed, hazel nut, Shea nut, and cherry laurel fruits, respectively.

Table 3 Regression equations obtained for mechanical properties of pistachio kernel (O'hadi variety) in the moisture content range of 6–38.5%.*

Property	Equation	R ^2	χ^2	SE
Rupture force (N)	FR  = −0.146 Mc  + 38.51	0.916	0.999	0.612
Rupture strength (N mm^−2)	SR  = −0.003 Mc  + 0.447	0.950	0.999	0.009
Deformation (mm)	DR  = 0.030 Mc  + 1.696	0.996	1.00	0.025
Deformation ratio (mm mm^−1)	ϵ R  = 0.001 Mc  + 0.178	0.994	0.999	0.001
Toughness (mJ mm^−3)	P = −0.001 Mc  + 0.084	0.962	0.999	0.003
Hardness (N mm^−1)	Q = −0.227 Mc  + 20.21	0.991	0.999	0.297
*R ^2: Determination coefficient; χ^2: Chi-squared; SE: Standard error.

From Table 4, it can be seen that the rupture force was higher along the z-axis. This could be because the area of contact between the kernel and compression plates was larger along the z-axis than those along the x- and y-axes. Aydin[14] showed that the highest rupture force was obtained for hazelnuts loaded along the z-axis (F_z ), while those loaded along the x-axis (F_x ) required the least force to rupture. However, Ozgüven and Versavus,[19] Aktas et al.,[6] and Altuntas and Ozkan[3] reported that the highest rupture force was on the y-axis for pine nuts, some selected almond cultivars, and walnut varieties, respectively. On the other hand, Altuntas et al.[7] determined the greatest and least ruptured force of three almond cultivars along x and z loading axes, respectively.

Table 4 Effect of loading position on some mechanical properties of pistachio kernel (O'hadi variety) in the moisture content range of 6–38.5%.*

	Compression axis
Property	Vertical (x)	Horizontal (y)	Thickness (z)
Rupture force (N)	33.17 [2.10]^c	34.65 [0.97]^b	36.63 [2.22]^a
Rupture strength (N mm^−2)	0.44 [0.06]^a	0.30 [0.02]^b	0.28 [0.03]^c
Deformation (mm)	3.22 [0.28]^a	2.70 [0.30]^b	2.54 [0.36]^c
Deformation ratio (mm mm^−1)	0.14 [0.01]^c	0.26 [0.03]^b	0.29 [0.03]^a
Toughness (mJ mm^−3)	0.038 [0.013]^b	0.043 [0.015]^b	0.047 [0.018]a
Hardness (N mm^−1)	14.64 [2.41]^a	13.95 [2.16]^b	13.39 [2.03]^b
*In each row, means with the same letters are not significantly different at 0.01 level of significance using LSD test. Standard deviations are in brackets.

Rupture strength

The statistical analysis showed that all factors had significant effect on the rupture strength of pistachio kernels (P < 0.01). As shown in Table 3, rupture strength increased linearly with increase of moisture content. Aydin[14] and Gezer et al.[32] obtained similar results for hazelnut and apricot pit and their kernels, respectively. As Table 4 shows, the rupture strength was higher along the x-axis. The reason is that according to rupture strength formula (rupture force/area), the area of contact between the kernel and compression plates was minimum along the x-axis, thus the rupture strength became maximum along this axis. Similar results reported for the rupture strength of almond nut and kernel.[5] However, Gezer et al.^[32] found that the rupture strength was the biggest through length in apricot pit and through thickness in apricot kernel.

Deformation and deformation ratio

The results of the statistical analysis revealed that deformation and deformation ratio of pistachio kernel were significantly affected by all the factors studied (P < 0.01). As shown in Table 3, deformation and deformation ratio required to rupture the pistachio kernel was decreased linearly as moisture content decreased from 38.5 to 6% (w.b). This trend attributed to the fact that at higher moisture contents, kernels were softer and tended to flatten easily under load and, thus, subjected to greater deformation. Olaniyan and Oje[20] reported a similar trend for Shea nut. Singh and Goswami[11] also showed that the deformation for cumin seed increased linearly with the increasing of moisture content.

As seen in Table 4, the kernels that were loaded in x-axis and z-axis experienced higher deformation and deformation ratio, respectively. The reason for this is that, when the nut samples were compressed in x-axis, they absorbed more energy before reaching the rupture point because of the smaller area contact between kernel and probe, and consequently experienced greater deformation. On the other hand, deformation ratio was maximum along z-axis because of the smaller dimension in the force direction. The results obtained are in agreement with Braga et al.[17] for macadamia nut and Altuntas and Ozkan[3] for walnut. Singh and Goswami[11] and Olaniyan and Oje[20] reported the highest deformation for cumin seed and Shea nut in a horizontal position, respectively, while Altuntas et al.[7] found that the y axis yielded the highest deformation ratio for three almond cultivars studied.

Toughness

Statistical analysis showed that the effect of moisture content and compression axis on the toughness of pistachio kernels were significant at 0.01 probability level, while moisture content by compression axis interaction on toughness was not significant statistically (P < 0.01). It is seen from Table 3 that toughness to rupture the pistachio kernel increases linearly when moisture content decreases. The increase in toughness or energy absorbed per unit volume with decreasing moisture content of the kernel may be due to hardening of the kernel at lower moisture contents. The results are in agreement with Singh and Goswami[11] who reported a decrease in energy absorbed by cumin seed with increasing in moisture content. Aktas et al.[6] obtained a similar trend for some almond cultivars, whereas Maghsoudi et al.[21] showed that greater energy is needed to split the pistachio nut at higher moisture content.

The response of toughness to compression axis is presented in Table 4. It can be found that the toughness in the z-axis was higher than the toughness in other axes. This was so because when the kernel samples were compressed in z-axis, the area of individual kernel in contact with the compression plates of the texture analyzer was larger than the contact area in other axes. Thus, energy absorbed by individual kernel before rupture was more in z-axis and this resulted in higher toughness value. Olaniyan and Oje[20] showed that toughness of Shea nut in horizontal loading position was higher than the toughness in the vertical loading position. Singh and Goswami[11] reported that cumin seed loaded in the vertical orientation required more energy for rupture than that in the horizontal orientation. Altuntas and Ozkan[3] found that the highest nut rupture energy among the three walnut varieties occurred for Kaman loaded on the y-axis. Aktas et al.[6] showed that the highest toughness for some selected almond cultivars occurred along y-axis.

Hardness

From the statistical analysis, it was concluded all factors (exception for interaction) had significant effect on the hardness of pistachio kernel (P < 0.01). From Table 3, it can be seen that the hardness of pistachio kernel at target deformation increased linearly to a maximum value when moisture content was decreased from 38.5 to 6% (w.b). The reason for this trend is that as the kernel desorbed moisture, it became structurally hard and hardness under this condition was increased. Olaniyan and Oje[20] reported a similar trend for hardness of Shea nut under compression tests in moisture range of 6.81–62.9% (d.b.). Aktas et al.[6] showed that the hardness of some selected almond cultivars decreased to a minimum value when moisture content increased to a maximum value.

As shown in Table 4, the maximum value for hardness was obtained for the x-axis, because when the kernel was compressed in the x-axis, the area in contact with the compression plates of texture analyzer was smaller compared with the other axes. Thus, the kernel experienced just slight deformation before rupture and, hence, the ratio of force to deformation at the target point (hardness) was higher in x-axis. Aktas et al.[6] reported that maximum hardness occurred in y-axis for some selected almond cultivars. Olaniyan and Oje[20] showed that hardness in vertical loading position (x-axis) was higher than horizontal loading position for Shea nut.

CONCLUSION

The physical properties of pistachio kernel showed positive linear behavior with moisture content. All mechanical properties (with exception of deformation and deformation ratio) decreased with increasing moisture content. The lowest rupture force, deformation ratio and toughness were obtained for x-axis in all moisture levels. However, for rupture strength, deformation and hardness the lowest values were obtained along the z-axis. Thus, cracking operation should be made along the x-axis at highest moisture content in order to decrease the energy requirement for kernel breakage. The maximum rupture force was 38.5 N, rupture strength 0.44 N/mm², toughness 0.0839 mJ/mm³, and hardness 20.177 N/mm at 6% moisture content. Maximum deformation and deformation ratio was obtained 2.85 N and 0.25 mm/mm, respectively at 38.5% moisture content.