Cracking characteristics and kernel extraction quality of hazelnuts: Effects of compression speed and positions

ABSTRACT

Force, energy, and specific deformation required for initial rupture and kernel extraction quality of hazelnuts were investigated as functions of shell thickness and geometric mean diameter at different compression positions (length, width, and thickness) and speeds (0.5, 1.0, 1.5, and 2.0 mm/s). For this purpose, two groups of nuts of hazelnut (Corylus avellana L.) cv. Tombul were compressed with a universal testing machine. The lowest rupture force values were obtained from along the width direction, while nuts compressed at length position had higher quality scores (5.00 and 4.89) compared with other positions in both groups. The force for cracking increased with increasing test speeds, and reached to 214 N at the speed of 2 mm/s in the first group (larger than 16 mm diameter) samples. Less energy was required to crack nuts at the speeds of 0.5 and 1.0 mm/s compared with 1.5 and 2.0 mm/s compression speeds in both sample groups. Generally, rupture force and energy for cracking increased linearly by increasing shell thickness. The energy required to initiate cracking increased as the size of hazelnuts increased for all three axes. Results revealed that there was no relationship between kernel extraction quality and geometric mean diameter. Experimental results from compression tests indicated that cracking nuts at the width position required less force and yielded good kernel extraction quality at the speed lower than 1 mm/s.

KEYWORDS:

• Hazelnut
• Cracking
• Energy
• Kernel extraction quality
• Compression speed

Introduction

Hazelnut (Corylus avellana L.) is a perennial shrub grown in the Black Sea region of northern Turkey.^[1] Turkey is the main hazelnut producer of the world with approximately 75% of world production,^[2] and it exports 75% of its total hazelnut production.^[3] Hazelnuts are one of the most nutritious nuts that contain valuable amounts of nutrients, among which lipids predominate.^[4] The main fatty acid of the hazelnut oil is oleic acid (73.6−82.6%) followed by linoleic (9.8−16.6%), palmitic (4.1−6.8%), and stearic (1.6−3.7%) acids.^[5–7] Oleic acid, monounsaturated fatty acids, has a recognized beneficial effect on human health. As a fatty food, damaged hazelnuts are easily susceptible to rancidity during shelf life. Hazelnuts also contain significant quantities of protein, dietary fibber, mineral elements, and vitamins (especially E), and constitute one of the most important raw materials for the pastry and chocolate industry.^[4,8,9]

Understanding fracture properties of crop seeds play a crucial role in improvement of postharvest handling and processing.^[10] Since the edible part of hazelnut is kernel, which can be affected by cracking, its extraction from the shell is the most critical and delicate process. The shelling operation leads to damaged kernels due to the inappropriate cracking forces, speeds, and directions applied to the hazelnut during the shelling. Damaging the kernels during the shelling process greatly reduces the storage and market value of hazelnuts.^[3,11] Similarly, it was indicated that the mechanical force applied to the hazelnut, rotational speed of the cracker, and sizing grades, and physical characteristics of variety such as shape, size, thickness, and shell texture are the main factors affecting the kernel extraction quality of hazelnut.^[12–15] Hazelnut larger than 16 cm diameter had different cracking behaviour, as compared with smaller ones.^[12]

In several previous researches, generally, some physical properties and mechanical behaviour of hazelnut under compression as a function of moisture content, variety, and compression load direction were investigated.^{[1,3,8,9,12–14,16]} However, there is not much literature reported regarding cracking speed and kernel extraction quality of hazelnut during shelling process. Giacosa et al.^[16] tested three different speeds (0.2, 1.0, and 10.0 mm/s) and indicated that the third value (10.0 mm/s) was considered as very fast speed condition with no direct reference in hazelnuts literature.

Since the majority of harvest, threshing, and shelling of hazelnuts are still carried out by manually operated apparatus and machines in Turkey,^[14] more new researches on the mechanical behaviour and kernel extraction quality of hazelnuts as function of size, shell texture, compression direction, and speeds are needed. Experimental results will contribute to the design of new shelling machines as well as the storage and marketing of nuts. The objective of this research was to determine the effects of the compression position, compression speed, geometric mean diameter, and shell thickness of the nuts on the force, specific deformation, and energy required to achieve (1) rupture of the nut shell, and (2) optimum kernel extraction quality.

Materials and methods

The nuts of hazelnuts (C. avellana L.) cv. Tombul that is top variety of Turkey were used for the compression tests in this research. Harvested nuts were dried in a drying room having an air conditioner system operating at the temperature of 30 ± 1°C and relative humidity of 50%.^[17] The resulting shell moisture content was 11% wet basis (w.b.) with a standard deviation of 0.5. The moisture contents of the shells (taken from 25 nuts) were determined using an oven set at 105 ± 1°C for 24 h (three replicates). The dried nuts were stored at 0°C and at 60–65% relative humidity in plastic bags (moisture tight) during the tests. Before the cracking test, nuts were visually inspected and those with damaged shells were discarded. Nuts were divided in two groups to determine effect of volume of nuts on cracking characteristic. Özdemir and Özilgen^[12] stated that hazelnuts larger than 16 mm diameter had different cracking behaviour as compared with smaller ones. In this research, nuts larger than 16.08 mm geometric mean diameter were named as first group (FG). The others smaller than 16 08 mm were tested as second group (SG) material. The geometric mean diameter d_m (in mm) of the fruits was calculated by following equation^[11]:

(1)

where L is the length, W is the width, and T is thickness (in mm). A universal testing machine (Universal texture analyser, Lloyd Instruments equipped with 1 kN load cell) was used to compress the nuts. The hazelnuts fruits were tested at 20 ± 1°C temperature along the compression directions x, y, and z, as defined by Güner et al.^[3] A coordinate system describing the three major compression positions of hazelnuts is shown in Fig. 1. In this study, nuts were compressed between two parallel plates at four speeds (0.5, 1.0, 1.5, and 2.0 mm/s) based on the preliminary tests.^{[13,16,18–20]}

Figure 1. Representation of the three axial forces (F_x, F_y, and F_z) and three major perpendicular dimensions of hazelnut.

Nuts were compression loaded until 30% of the maximum compression force was reached. Based on our preliminary study,^[20] 30% of the maximum force was chosen as an optimum point to stop the tests. This permitted to study of kernel extraction quality as a function of geometric mean diameter, shell thickness, compression positions, and speeds. Correspondingly, Borges and Peleg^[21] applied a compression load to nuts to about 35% deformation. This deformation was chosen as a criterion for stopping the tests. In this study, nuts were compressed between two parallel plates based on the preliminary tests.

The cracking characteristics of hazelnuts were expressed in terms of maximum force required to fracture the shell, energy required to deform the nut shell to rupture, specific deformation, and kernel extraction quality. With the use of a computer software program, force as a function of deformation was graphically recorded during the experiments. Figure 2 shows a typical force–deformation curve for compressed hazelnuts. The energy was determined directly from the chart by measuring the area under the force–deformation curves. The specific deformation was obtained through the expression.^[22]

(2)

Figure 2. Typical compressive force deformation relationship of hazelnuts cv. Tombul.

where L₁ is the un-deformed nut dimension (in mm), and L₂ is the deformed nut dimension (in mm) in the direction of the compression axis. Kernel extraction quality was classified into grades as defined in Table 1.

Table 1. Evaluation of kernel extraction quality.

Classification	Definition	Grade
Very good	No damage	5
Good	Little damage	4
Mean	A small fracture occurred at the edge of the fruit	3
Bad	Divided into 2–3 parts	2
Very bad	Crushed	1

To determine the average shell thickness and dimensions of each nut, a sample of 60 nuts was randomly selected and measured by digital micrometre with an accuracy of 0.01 mm. To obtain the mass, each nut was weighted on a balance reading to 0.001 g. A completely randomized design was selected for the experiment. The differences among means were compared by Duncan’s multiple range test (probability P = 0.05). Six replicates of 10 hazelnuts were compressed for cracking tests. Additionally, in order to show the relationship among the parameters (shell thickness, geometric mean diameter, force, energy, specific deformation, and kernel extraction quality), data were grouped based on the shell thickness and geometric mean diameter.^[23]

Results and discussion

The minimum, maximum, mean, and standard deviation of nut length, width, thickness, weight, geometric mean diameter, volume, and shell thickness are presented in Table 2.

Table 2. Some physical properties of the Tombul hazelnut variety (at 11% w.b. moisture content of shell).

	1. Group				2. Group
	Mean	Minimum	Maximum	Standard deviation	Mean	Minimum	Maximum	Standard deviation
Length (mm)	17.77	14.98	20.20	0.98	16.63	14.00	18.89	0.93
Width (mm)	16.49	14.87	18.47	0.72	15.30	12.75	17.51	0.94
Thickness (mm)	15.87	13.97	18.41	0.68	14.66	11.91	15.96	0.75
Weight (g)	1.76	0.92	2.44	0.24	1.44	0.66	1.80	0.22
Geometric mean diameter (mm)	16.68	16.08	17.69	0.41	15.49	13.55	16.08	0.51
Volume (cm^3)	2.03	1.60	2.70	0.20	1.63	1.10	2.10	0.21
Shell thickness (mm)	0.89	0.66	1.21	0.12	0.84	0.59	1.11	0.10

Effects of compression positions on force, energy, specific deformation, and kernel extraction quality of hazelnuts

Table 3 shows the forces, energies, specific deformations, and kernel extraction qualities obtained during cracking at different compression positions. As can be seen from Table 3, compression positions did not affect, statistically, rupture force, energy, deformation, and kernel extraction quality of hazelnuts. While the rupture force and energies of SG fruits were lower compared with FG nuts, the specific deformations of SG were found to be higher than those of FG. Even if compression positions did not affect statistically cracking characteristics of hazelnuts, in fact there were differences in mechanical behaviour according to loading directions as reported in previous studies carried out on some crops.^[1,24,25] In the present study, the values of force required to crack nut shell ranged from 145 to 211 N but, the lowest values of force were obtained from along the width in both groups. Similarly, Valentini et al.^[13] found that the lowest rupture forces were generally obtained from hazelnuts compressed on y-axis (width). On the other hand, Razavi and Edalatian^[24] reported that the rupture force of pistachio kernel was higher along the z-axis (thickness) depending on the larger contact area between kernel and compression plates compared with other axes. Özdemir and Özilgen^[12] stated that hazelnuts larger than 16 mm in diameter had different cracking behaviour, as compared with smaller ones.

Table 3. Effects of compression positions on force, energy, specific deformation, and kernel extraction quality of hazelnuts.

	Force (N)		Energy (J)		Deformation (%)		Kernel extraction quality (grade)
Direction	FG	SG	FG	SG	FG	SG	FG	SG
Fx (length)	211	179	0.100	0.082	6.26	6.79	5.00	4.89
Fy (width)	178	145	0.103	0.084	6.60	6.67	5.00	4.78
Fz (thickness)	189^ns	180^ns	0.087^ns	0.086^ns	5.70^ns	7.34^ns	4.61^ns	4.83^ns

ns: Not significant.

FG: first group; SG: second group.

Hazelnuts cracked at the length position had higher quality scores (5.00 and 4.89) compared with other positions in both groups. The lowest kernel extraction quality was obtained from FG fruits cracked at the thickness position. Likewise, cracking nuts along the length axes gave better results for kernel extraction quality in some previous researches.^[20,22,26] In this research, FG hazelnuts (average width = 16.49 mm) generally gave better kernel extraction scores than those of SG fruits (average width = 15.30 mm). Özdemir and Özilgen^[12] found that 3.3% damaged nuts were produced when the diameter of the nuts were larger than 16 mm, and 15.9% damaged nuts were produced when the diameter of the nuts was less than 16 mm. Because kernel extraction quality is of great importance for processing and consumption of nuts,^[20,27] this parameter should be taken into consideration for design and development of a cracking machine.

Effects of compression speed on force, energy, specific deformation, and kernel extraction quality of hazelnuts

The maximum force, energy, specific deformation, and kernel extraction quality values at different compression speeds are presented in Table 4. Cracking speeds statistically influenced rupture force and kernel extraction quality of nuts in both sample groups. While the effects of compression speeds on specific deformation of SG was significant, the energy required for cracking of nuts was not affected by test speeds (P < 0.05). The force for cracking generally increased with increasing test speeds, and reached to 214 N at the speed of 2 mm/s in FG samples. The maximum rupture force was 187 N for SG nuts at the speed of 1.5 mm/s. The lowest rupture force (145 N) was obtained when the 0.5 mm/s speed was applied to nuts. Giacosa et al.^[16] indicated that the rupture forces for hazelnut cracking were higher at the speed of 10 mm/s than those of lower compression speed (0.5 mm/s) for length and thickness directions. They also found that the use of different test speeds in mechanical test for hazelnut gave different results influencing the correlation between instrumental measurements and sensory judgments. Similar findings were also obtained from previous researches carried out by Gurhan et al.,^[28] Kılıckan and Güner,^[29] and Altuntaş and Erkol.^[30] They indicated that rupture forces of the apricot, olive, and walnut fruits increased as loading velocity increased for some compression axes.

Table 4. Effects of compression speed on force, energy, specific deformation, and kernel extraction quality of hazelnuts.

	Force (N)		Energy (J)		Deformation (%)		Kernel extraction quality (grade)
Velocity (mm/s)	FG	SG	FG	SG	FG	SG	FG	SG
0.5	177b*	145b	0.103^ns	0.084^ns	6.60^ns	6.67b	5.00a	4.78ab
1.0	165b	162ab	0.102	0.095	7.20	7.51b	4.83a	5.00a
1.5	189ab	187a	0.137	0.132	9.43	9.23ab	4.21b	4.14b
2.0	214a	160ab	0.126	0.125	7.85	11.5a	3.96b	3.13c

*Means in columns followed by the same letters are not significantly different.

ns: Not significant.

FG: first group; SG: second group.

Even if compression speeds did not affect statistically energy for rupture, change in these energy values was evident according to plate speeds. Less energy was required to crack nuts at the speeds of 0.5 and 1.0 mm/s compared with 1.5 and 2.0 mm/s compression speeds in both sample groups. The maximum cracking energy values (0.137 and 0.132 J) were found when 1.5 mm/s speed was applied to hazelnuts. These results are in accordance with those obtained in previous researches with walnuts.^[30,31] The rupture energy values were greater at the higher compression speed of 1.5 mm/s than those of the other speeds (0.5 and 1.0 mm/s) tested in walnuts. The highest rupture energies were obtained at 1.5 mm/s compression speed with 170.8 and 241.4 mJ for Yalova-1 and Yalova-3 varieties, respectively.^[30]

As can be seen from Table 4, specific deformation generally increased with increasing compression speeds; however, the comparison of mean values indicated that this increase was not significant in FG samples. The specific deformation of hazelnuts increased from 6.60% to 9.43% for FG and 6.67% to 11.50% for SG as the compression speed increased from 0.5 to 1.5–2.0 mm/s, respectively. In a previous study,^[16] the maximum specific deformation (17.10%) for hazelnut was obtained at a compression speed of 10 mm/s, while the lowest value (15.40%) was at a deformation rate of 0.2 mm/s for compression along the thickness. Similar trends were also observed by Altuntaş and Erkol^[30] for walnut and Kılıckan and Güner^[29] for olive fruit and pit in three loading directions.

Table 4 shows a decrease in the kernel extraction quality scores with increasing compression speeds regardless of sample groups. There is a high tendency for kernel extraction quality to decrease with increase in compression speeds for both nut groups. The lowest scores (3.96 and 3.13) were obtained from hazelnuts compressed with a plate speed of 2.0 mm/s, while lower cracking speeds (0.5 and 1.0 mm/s) gave the best (5.00) kernel extraction quality in FG and SG, respectively. Test speed affects mechanical and quality properties of some food products during cracking or processing. Giacosa et al.^[16] stated that the use of different test speeds in mechanical tests gave different results, and this aspect could influence the correlation between instrumental measurements and sensory judgments. Since cracked kernels are very crucial for postharvest life and marketing of nuts, the cracking velocity of hazelnuts should be considered during processing. Table 4 shows that higher speeds more than 1 mm/s are not advised for cracking tests of hazelnuts cv. Tombul in terms of kernel extraction quality. However, higher speeds could be investigated for cracking test of different hazelnut varieties to obtain detailed results on kernel extraction quality. It is thought that hazelnut varieties would give various quality score under different compression velocities.

Relationship between shell thickness and some cracking characteristics of hazelnuts

Figure 3 shows the forces required for the initial rupture as a function of shell thickness at three different loading positions in both sample groups. Shell thickness, which varies with the varieties and maturity stages, is an important factor which affects the pit and nut stiffness.^[32] In this research, while result of regression analysis yielded values for the coefficient of determination R² of 0.72, 0.74, 0.84 for length, width, and thickness positions, respectively, for FG, these values were 0.61, 0.70, and 0.52 for SG, respectively. Rupture forces of both groups at three compression positions increased linearly by increasing shell thickness. Regardless of compression position, the forces needed to crack FG were higher than those of SG. This can be due to the fact that first group had a relatively homogeneous sampling compared with second group in terms of shell thickness. Bostan^[33] reported that there was a positive relationship between stiffness and shell thickness in hazelnuts cv. Tombul. Similarly, Koyuncu et al.^[20] found that the rupture force required to crack walnuts increased linearly with increasing shell thickness for all compression directions. On the other hand, in a previous research^[26] the linear relationship between force and shell thickness with a low correlation coefficient from 0.006 to 0.417 for selected genotypes from different regions of Turkey was found. The low correlation coefficient might have resulted from non-homogeneous sampling of nuts which were obtained from many different genotypes.

Figure 3. Mean values of force, compressed at three positions (length, width, and thickness), as a function of shell thickness.

The effects of shell thickness on rupture energy of hazelnuts are shown in Fig. 4. It can be seen in Fig. 4 that by increasing shell thickness, the rupture energy for all combinations except for samples compressed along the thickness direction in SG increased. There was not obvious relationship (R² = 0.09) between shell thickness and rupture energy of nuts loaded along the thickness direction in SG, but other tests gave relatively clear relationships. The correlation coefficient values (R²) of these combinations changed from 0.53 to 0.78. Similar results were found by Koyuncu et al.^[20] in cracking tests of walnuts loaded along the width and thickness axis. Our results are also close to those reported by Braga et al.^[22] for macadamia nuts and Guner et al.^[34] for apricot seeds.

Figure 4. Mean values of energy, compressed at three positions (length, width, and thickness), as a function of shell thickness.

A linear relationship between specific deformation and shell thickness was determined for length and width positions for the SG hazelnuts with R² of 0.78 and 0.77, respectively. The results indicated that the increase in shell thickness led to increase in specific deformation for aforementioned directions. However, weak relationship was observed for the rest of experimental tests (Fig. 5). The results are in agreement with that reported by Koyuncu et al.^[20] Since, generally, the kernel extraction quality of all samples was good and close to each other for length, width, and thickness directions, there was not a relationship between kernel extraction quality and shell thickness (data were not shown).

Figure 5. Mean values of specific deformation, compressed at three positions (length, width, and thickness) as a function of shell thickness.

Relationship between geometric mean diameter and some cracking characteristics of hazelnuts

The rupture force, rupture energy, and specific deformation of hazelnuts as a function of geometric mean diameter at different compression positions are presented in Figs. 6–8. Figure 6 shows an increase in the rupture force depending on geometric mean diameter at length and thickness directions for both groups. However, no relationship was observed for hazelnuts cracked along the width axis. The relationship was expressed by a linear equation with values for R² of 0.73, 0.78, and 0.60, 0.84 for length and thickness positions in FG and SG, respectively. Similar trends of increase have been reported by Borghei et al.,^[35] Khazaei et al.,^[36] and Altuntaş et al.^[37] They reported that the rupture force increased with increasing size of walnut and almonds. Kılıçkan and Güner^[29] also indicated that rupture forces of the olive pit and olive fruit increased as geometric mean diameter increased for all compression axes.

Figure 6. Mean values of rupture force, compressed at three positions (length, width, and thickness) as a function of geometric mean diameter.

Figure 7. Mean values of energy, compressed at three positions (length, width, and thickness) as a function of geometric mean diameter.

Figure 8. Mean values of specific deformation, compressed at three positions (length, width, and thickness) as a function of geometric mean diameter.

The energy required to initiate shell cracking increased as the size of hazelnuts increases for all three axes, and correlated positively with values of R² = 0.55, 0.60 for length, 0.68 and 0.61 for width, and 0.64, 0.59 for thickness directions. These results showed that greater rupture force and energies were necessary to crack the hazelnuts with higher size. Since the large size of nuts are preferred both for commercial production and breeding programs, these results should be considered when designing and developing a cracking machine. Results found in this research are in agreement with those of several authors: Braga et al.,^[22] Khazaei et al.,^[36] Kılıçkan and Güner^[29] for macadamia nuts, almond, and olive pit, respectively. On the other hand, Koyuncu et al.^[20] obtained a contrary result working with walnuts. This can be due to the difference of biological materials (different species and varieties).

Plots in Fig. 8 show that specific deformation values decreased when the nut size increased, except for the FG samples compressed along the thickness direction. The values of R² (positively correlated) were between 0.60 and 0.80. As can be seen in Figs. 6–8, specific deformation as a function of geometric mean diameter showed different pattern compared with rupture force and energy. The results are similar to that reported by Koyuncu et al.^[20] for walnuts. On the other hand, Kılıçkan and Güner^[29] indicated that specific deformation increased as the size increases, but they did not find any significant difference between the specific deformations and sample sizes for olive pit and for olive fruit. There was no relationship between kernel extraction quality and geometric mean diameter at all compression positions (data were not shown).

Conclusion

The loading positions did not affect statistically mechanical behaviour (force, energy, specific deformation) and kernel extraction qualities of hazelnuts, but some differences in cracking characteristics according to loading directions could be observed. Generally, the force and energy required for initiating nut rupture and kernel extraction quality of FG fruits at three positions were higher than those of SG nuts. The lowest rupture force values (178 and 145 N) for cracking were obtained at the width position, but hazelnuts cracked along the length axis gave the highest kernel extraction quality scores (5.00 and 4.89) in both sample groups. The rupture force, rupture energy, and specific deformation for hazelnut cracking were higher at the speeds of 1.5 and 2.00 mm/s compared with lower compression speed of 0.5 mm/s for all the directions. The best kernel extraction quality was also obtained from the lowest compression speed (0.5 mm/s) tests. Shell thickness and geometric mean diameter affected the rupture force, rupture energy, and specific deformation of samples, but kernel extraction quality during cracking of hazelnuts was not affected from these parameters. The rupture forces, energy, and specific deformation of both groups increased by increasing shell thickness at three compression positions except for SG samples compressed along the thickness direction. Similarly, the rupture forces and energy required to initiate shell cracking increased as the size of hazelnuts increases for all three axes except for width direction for force, and correlated positively. In contrast, generally specific deformation values decreased when the nut size increased. Among the three compression positions and four compression speeds, the best combination (for force and kernel extraction quality) was obtained from width position at the speed of lower than 1 mm/s.