Mechanical Behavior of Walnut under Cracking Conditions

MATERIALS AND METHODS

Fresh harvested walnut fruits in September 2006, in the West Azerbaijan province, Iran were dried in the sunshine and were used for all the compression tests. They were visually inspected and those with damaged shell were eliminated. Remaining walnuts were sorted according to their size, by using micrometer with an accuracy of 0.01 mm and then nuts with 31-32 mm of geometric mean diameter were selected (the size of majority of the nuts in the sample). The moisture content of 9, 21 and 27% (w.b.) were used. These levels of moisture content were selected according to the maximum and the minimum moisture content of the walnuts in the local market. The moisture content of the walnut was determined using the method recommended by Braga et al. (1999). On the basis of this method, a chamber with temperature and relative humidity controls was used to obtain samples at different moisture contents. Samples with higher moisture content were obtained by rewetting process at 25°C (room temperature) and 95% relative humidity. During this process, the moisture level of nuts was measured several times until to reach the desired moisture content. The nuts with 21 and 27% of moisture levels absorbed 54 and 91 g of water, respectively. But samples with lower moisture content were simply obtained by drying in the sunshine. The moisture content of the walnut (taken from 10 nuts in three replicates) was determined using an oven, set at 105°C for 24 h (Koyuncu et al., 2004). Walnuts were compression loaded by an Instron test machine until the shell rupture was initiated (Braga et al., 1999). In this research, based on the previous studies (Khazaei et al.,2002; Koyuncu et al., 2004), three loading velocities of 50, 200 and 500 mm min^-1 were selected. The mechanical behaviors of walnuts were expressed in terms of maximum force required to fracture the shell, nut specific deformation, absorbed energy and power required to rupture the nut shell. The values of the rupture force, absorbed energy, required power and specific deformation were developed from each compression curve obtained from Instron test machine. The absorbed energy, as shown in Fig.1, was determined directly by measuring the area under the force-deformation curve (Braga et al., 1999; Koyuncu et al., 2004). This measurement was performed by applying a digital planimeter with an accuracy of ±0.2% (Güner et al., 2003).

Fig. 1:	Typical force-deformation curve for compressed walnut

The specific deformation, ε, was obtained from the following expression (Braga et al., 1999):

where, L_u and L_f are the un-deformed and deformed nut dimensions on the direction of the compression axis, in mm, respectively. The required power was also calculated as below (Khazaei et al., 2002):

A coordinate system describing the three major compression positions of walnut is shown in Fig. 2. The X-axis is the longitudinal axis through the hilum (length position), the Y-axis (width position) and the Z-axis is in the plane containing the suture line (suture position) (Braga et al., 1999).

Fig. 2:	Schematic drawing of the 3 axes for the walnut compression test

The geometric mean diameter of the nuts, d_m, in mm, was calculated by the following equation (Mohsenin. 1970; Güner et al., 2003; Koyuncu et al., 2004):

d_m = (LWT)^1/3

Four replicates were made for each test. Therefore, in this research, 108 walnuts were examined by Instron machine. At 3 loading positions, 3 loading velocities and 3 moisture content, a completely randomized block design (factorial scheme) was selected for these experiments. Additionally, in order to show the relationships among the parameters of the experiment, data were grouped based on the moisture content, loading velocity and loading position (Koyuncu et al., 2004).

RESULTS AND DISCUSSION

Effects of loading velocity and moisture content on the measured parameters: Cracking force, specific deformation, energy and power required to rupture walnut shell were dependent on moisture content and loading velocity (Fig 3a-d, Table 1).

Fig. 3:	Effects of loading velocity on the measured cracking force (a), specific deformation (b), energy (c) and power (d), at 3 different moisture levels. MC1, MC2 and MC3 are 9, 21 and 27% of moisture content, respectively

Table 1:	Comparison of mean values of walnut mechanical properties, affected by loading position, loading velocity and moisture content (Dunken test)
a: Values assigned with different letter(s) are significantly different (α = 0.05)

Fig. 4:	Effects of loading velocity on the measured cracking force (a), specific deformation (b), energy (c) and power (d), at 3 different loading positions, namely; DX, DY and DZ as walnut length direction, width direction and suture direction, respectively

With increasing the loading velocity, the cracking force decreased at 9% moisture content, whereas at 21 and 27% of moisture content, it increased in the beginning, but decreased with further increase in loading velocity (Fig. 3a). The highest cracking force (797N) was recorded at 21% moisture content and 200 mm min^-1 loading velocity, when the load was applied in the X direction (not shown in the Fig. 3a). Analysis of variance indicated no significant differences among the 3 moisture levels which agree with the results of the study on Macadamia nut (Braga et al., 1999).

Specific deformation was linearly dependent on loading velocity at all 3 moisture levels. Specific deformation also increased with increasing moisture levels; however, the comparison of mean values indicated that this increase was not significant between the two higher moisture levels (Fig. 3b, Table 1). Oloso and Clarke (1993) in cashew nut and Güner et al. (2003) in hazelnut reported similar results.

The required energy to fracture walnut shell increased as the loading velocity was raised up to 200 mm min^-1, but with further increase, this change was not significant (Fig. 3c). Moisture content of the shell affected the rupture energy significantly which corresponds with the findings of Altuntaşand Yįldįz (2007) for faba bean grains. However, in our experiment, at moisture levels higher than 21%, this effect was non-significant. Similar to the required cracking force, here again, the highest required energy to rupture the walnut shell, 952 mJ, occurred at 21% of moisture content, 200 mm min^-1 loading velocity and at X-direction.

Figure 3d indicates that the required power to fracture walnut shell increases significantly as loading velocity increases, regardless of the shell moisture content. Khazaei et al. (2002) reported similar results for almond. The required power also increased as the moisture content increased up to 21%, but did not change later on. The highest power (1.94 W) was calculated at 500 mm min^-1 of loading velocity, 21% of moisture content when the force was applied at the X direction.

Effects of loading velocity and loading position on the measured parameters: Cracking force, specific deformation, energy and power required to rupture walnut shell were dependent on loading velocity and loading position (Fig. 4a-d, Table 1).

Maximum and minimum rupture force occurred at length (DX) and suture line directions (DZ), respectively (Fig. 4a, Table 1). Dursun (1997) also found the minimum walnut rupture force in the direction of suture line, but the maximum rupture force occurred in the width position.

Figure 4b shows that the specific deformation was linearly dependent of loading velocity at all 3 loading positions. However, loading position had no significant effect on the specific deformation of the walnut shell. The results correspond with the findings of Koyuncu et al. (2004). However, the highest specific deformation of the walnut shell before rupture (0.1 mm^-1) was evident at 21% of moisture content, 500 mm min^-1 of loading velocity, when the force was applied in the direction of suture line.

Loading at length direction, (DX), required the highest rupture energy (Fig. 4c) which corresponds with the results of Vursavuşand Özgüven (2004) for apricot pit. The required rupture power was also the highest when the loading was applied in the length direction and decreased by changing the loading position toward the Y and Z directions, respectively (Fig. 4d).

CONCLUSION

The following physical findings, related to the shelled walnut in West Azerbaijan, are important and must be considered for design and construction of a walnut cracker: