PICKING FORCE OF SAFFRON FLOWER AND SHEAR STRENGTH OF SAFFRON STALK

Seyed Reza Hassan-Beygi¹, Hadi Vale Ghozhdi¹, Javad Khazaei^2
1 Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran
² Department of Mechanical Engineering of Biosystems, College of Aboureihan, University of Tehran, Tehran, Iran

ABSTRACT

The shear strength and shear energy per unit area as well as force and energy for picking of flowers are important parameters to design and develop harvesting mechanisms. In this study shear strength and shear energy per unit area of saffron (Crocus sativus L.) stalk were determined as a function of bevel angle and shear velocity. Also, picking force and energy of saffron flower were determined as affected by tension velocity and age of plant. The experiments were conducted on samples selected from fields of Kashmar. The results showed that with an increase in bevel angle of cutting blade from 17° to 24°, the shear strength and shear energy per stalk area increased significantly from 0.13 to 0.19 N/mm² and 0.305 to 0.443 mJ/mm², respectively. With increasing cutting rate from 20 to 200 mm/min the average values of shear strength and shear energy per stalk area decreased significantly in the range of 0.179 to 0.158 N/mm² and 0.467 to 0.340 mJ/mm², respectively. Further increase in cutting velocity in the range of 200 to 500 mm/min was not decreased shear strength and shear energy significantly. The average values of picking force, tensile strength and energy per unit area were increased from 0.339 to 0.459 N, 0.169 to 0.229 MPa and 0.473 to 1.914 mJ/mm², respectively when tension rate increased from 50 to 500 mm/min. The age of plant had not significant effect on picking force and energy and tensile strength. The obtained data was useful in designing and development of saffron harvesting mechanisms.

Key words: Saffron, harvesting, tensile strength, shear strength, picking force.

INTRODUCTION

Saffron (Crocus sativus L.) is a perennial spice species of Iridaceae family. This plant is the most precious spice in the world. At present, saffron plant cultivates in Greece, Central Asia, India, Iran, Italy, Morocco, Spain, Switzerland and Turkey. However, Iran and Spain are known as the main saffron producers in the world [10]. The saffron flower has three stigmas, which are the most important economic part of the plant and known as saffron. Saffron has general ingredients such as carbohydrates, proteins, minerals and vitamins. Furthermore, the saffron has special ingredients that determine quality of its [10]. The world's total production of dried saffron is estimated around 300 tons per year [7]. Iran produces more than 90 percent world's total production of saffron. More than 92 percent of Iranian saffron cultivates in Khorasan province with about 210 tons annual production. Unique agrological and eco-physiological characteristics of saffron along with aroma, flavor and yellow dye attributes were reserved a special place for this plant in pharmaceutical, food and textile industries [3,10]. The novel use of saffron in recent years has been associated with cancer cure [2]. Saffron's processing is included harvesting of flower from field, separating stigma from flower, drying of stigma, packaging and exposition. The flower harvesting is one of the main stages of saffron's processing, which is performed by hand at present. Harvesting of flower by hand cause lack of local labors in saffron cultivation areas at harvesting time. Furthermore, increases production costs as well as infection of stigma.

Shear strength and shear energy per unit stalk area of plants are important in designing and developing cutting mechanisms. As well, force and energy for picking flowers from stalk are useful in developing head striper and flower picking equipments. Many studies have been conducted to determine the shear properties of plants. Prasada and Gupta [17] studies showed that with increasing cutting rate from 200 to 1000 mm/min the shear strength of maize stalk decreased from 3.63 to 2.10 MPa. The average values of shear strength and shear energy of grasses were reported 16 MPa and 12 mJ/mm², respectively by McRandal and McNulty [13]. Kushwaha et al. [12] investigations revealed that the average value of shear strength of wheat straw was in the range of 8.6 to 13.0 MPa. Persson [16] believes that the bevel angle of blade to be effective on force and energy of cutting process of agricultural materials. When there was not problem of stalk holding versus cutting blade, Persson recommends using smooth blade for cutting of grasses due to the lower force and energy requirement. Khazaei et al. [11] reported that with increasing cutting rate from 20 to 200 mm/min the shear strength of pyrethrum stalk decreased. The maximum values of shear force and energy for cutting of hemp were 243 N and 2.1 J, respectively [6]. Ince et al. [9] reported that the maximum shear stress and specific shear energy of sunflower stalk were 1.07 MPa and 10.08 mJ/mm², respectively. Chattopadhay and Pandey [4] reported that the shear strength of sorghum stalk decreased from 3.74 to 1.94 MPa at the forage stage and decreased from 4.68 to 2.20 MPa at the seed stage when the cutting rate was increased from 10 to 100 mm/min at 30° knife bevel angle.

The required force and energy for picking flower from stems as well as binding force of plants leaf were also determined by previous researchers. The force required to detach the rice kernel from the branch for seven varieties cultivated in Italy were in the range of 1.29 to 2.37 N [18]. Khazaei et al. [11] were reported that with an increase in picking velocity the tensile force and energy of pyrethrum flower increased but the tensile strength and tensile energy per unit area of decreased. Hashemifard-Dehkordi and Chegini [8] investigation showed that with increasing tension rate the required tensile force and energy for picking Rosa flower leaf increased in the range of 5.98 to 9.99 N and 9.42 to 14.45 mJ, respectively. The average values of tensile force and energy for picking flower of chrysanthemum flower from stalk increased from 5.46 to 7.37 N and 10.62 to 15.74 mJ, respectively when tensile velocity increased from 10 to 500 mm/min [5]. Nazari Galedar et al. [15] investigations showed that the tensile strength of alfalfa stem increased in the range of 9.24 to 32.79 MPa with increase in tension rate from 5 to 20 mm/min.

Literature survey showed that there was no detailed study concerning the shear strength and shear energy of saffron stalk as well as picking force and energy of saffron flower. So in this study the shear strength and shear energy per unit stalk area of saffron were determined as function of cutting rate and bevel angles of blade. The picking force and energy of saffron flower were also determined as function of tensile rate and age of saffron plant. The obtained data would be useful in designing and developing harvesting equipments of saffron.

MATERIALS AND METHODS

The samples were considered for this study selected from different fields of Kashmar city in Khorasan province, east of Iran, on Nov., 2008. The experiments were done at physical properties laboratory, research institute of agricultural engineering and natural resource of Khorasan province, Mashhad, Iran. The moisture content of the sample was determined by using air oven method. The oven temperature was set at 105±3°C for 24 hours [1]. The shear force of stalk and picking force of flower were measured by using a texture analyzer machine (Farnel QTS25 model), which equipped with 250 N load cell with an accuracy of ±0.001 N.

To measure picking force of the flower a sample holder was made (Fig. 1). The tension test was carried out on saffron flower which was connected to stalk and corm. The upper part of sample holder was connected to moving jaw of the texture analyzer. The stalk of flower was laid on the sample holder groove (Fig. 1). The saffron corm was fixed at stationary jaw of the texture analyzer. The flower was extended due to upward motion of the moving jaw of the machine and the flower was separated from stalk at the weakest point. The effect of tension rate at four levels of 50, 100, 200 and 500 mm/min and plant age at three levels of 3, 4 and 5 years were studied on force and required energy for picking using two factors completely randomized design. Each experiment was replicated 5 times.

The tensile force-displacement curve was recorded in a computer during the tension process. The maximum value of force in the force-displacement curve was considered as picking force of the saffron flower. The required energy for picking of flower was calculated from area under force-displacement curve. Diameter of the stalk at rupture point was measured by a caliper with accuracy of ±0.01 mm for calculation of cross section area of rupture. The tensile stress (MPa) and tensile energy per unit area (mJ/mm²) of stalk were calculated based on the picking force and energy divided by the cross section area of rupture [5,11,14].

To measure shear force of the stalk a special device (Fig. 2) was made according to Khazaei et al. [11] recommendations. The device was fixed at stationary jaw of the texture analyzer. Steel cutting blade of reciprocating mower was used as cutting blade in this device. The cutting blade was connected to moving jaw of the texture analyzer. A distance between the cutting blade and ledger plate was fixed at 1 mm. The saffron stem was laid on the device so that the cutting of stalk was performed 15 mm lower than receptacle of flower. The stalk was cut due to downward motion of the moving jaw of the test machine. The shear force-displacement curve was recorded in a computer during the cutting process. The maximum value of force in the force-displacement curve was considered as shear force of the saffron stalk. The shear energy of the stalk was calculated from area under force-displacement curve. Diameter of the stalk at cutting point was measured by a caliper with accuracy of ±0.01 mm to calculate cutting cross section area. The shear stress (MPa) and specific shear energy (mJ/mm²) of stalk were calculated, respectively based on the shear force and energy divided by the cutting cross section area [4,6,14]. The effect of cutting rate at three levels of 20, 200 and 500 mm/min and bevel angle of cutting blade at three levels of 17, 20 and 24 degree were investigated on the shear force and energy by using two factors completely randomized design. Each experiment was replicated 5 times.

RESULTS AND DISCUSSION

The initial moisture content of the samples was 89.8% (w.b.). The effects of tension rate on the tensile strength and tensile energy per unit area of saffron stalk for three, four and five years old plants are shown in Figs. 3 and 4, respectively. As shown in Fig. 3 with an increase in tension rate from 50 to 100 mm/min the tensile strength was increased but further increase in loading rate in the range of 100 to 500 mm/min was not increased the tensile strength considerably. It is clear from this figure that the tensile strength of saffron stalk was not changed considerably for various ages of the plants. Fig. 4 shows that the tensile energy per unit area of saffron stalk was increased with increasing rate of loading. It is also clear from this figure that the tensile energy was not changed considerably for various ages of the plants. The results of analysis of variance (ANOVA) showed that the effect of tension rate was significant (P < 0.01) and that of plant age was not significant on the picking force, tensile strength and tensile energy.

The results of Duncan's multiple range tests to compare mean values of tension rate effect on the picking force, tensile strength and tensile energy per unit area are given in Table 1. As shown in this table, with increasing tension rate from 50 to 100 mm/min the required picking force and the tensile strength increased significantly (P < 0.01) in the ranges of 0.339 to 0.433 N and 0.169 to 0.229 MPa, respectively. However, the required force for picking and the tensile strength were not increased significantly with further increase in rate of loading from 100 to 500 mm/min. as the results, with increasing tension rate from 50 to 500 mm/min the required tensile strength per unit area of stalk increased significantly from 0.473 to 1.914 mJ/mm². The effect of tension rate was also reported by pervious researchers. For example Khazaei et al. [11] reported that with increasing tension rate from 5 to 500 mm/min the required energy per unit area of pyrethrum flower increased significantly (P < 0.01) from 2.6 to 4.9 mJ/mm². As well, the tensile strength of pyrethrum increased significantly from 1.2 to 1.6 MPa with increasing tension velocity in the range of 5 to 200 mm/min. Hashemifard Dehkorki and Chegini [8] research work revealed that with an increase in tension rate from 10 to 500 mm/min the average values of force and energy required to detach leaf from Rosa flower increased in the ranges of 5.975 to 9.998 N and 9.415 to 14.447 mJ, respectively. The results of Chegini et al. [5] investigations showed that with increasing tension rate from 10 to 500 mm/min the average values of picking force and energy for chrysanthemum flower increased from 5.46 to 7.37 N and 10.62 to 15.74 mJ, respectively. Nazari Galedar et al. [15] reported that the tensile strength of alfalfa stem increased in the range of 9.24 to 32.79 MPa when tension rate increased from 5 to 20 mm/min.

The effects of cutting rate on the shear strength and shear energy per unit area of saffron stalk for 17, 20 and 24 degree bevel angle of cutting blade are shown in Figs. 5 and 6, respectively. As depicted from these figures, with increasing cutting rate the shear strength and specific shear energy were decreased for all of the bevel angle levels. The results of analysis of variance (ANOVA) also showed that the effect of the cutting rate on both the shear strength and energy were significant. The results of Duncan's multiple range tests to compare mean values of cutting rate effect on the shear strength and specific shear energy are given in Table 2. As shown in this table, with increasing cutting velocity from 20 to 200 mm/min the shear strength and the specific shear energy decreased significantly (P < 0.01) in the ranges of 0.179 to 0.158 MPa and 0.467 to 0.340 mJ/mm², respectively. Further increase in cutting velocity in the range of 200 to 500 mm/min was not increased significantly the shear strength and energy. The reason for decreasing shear strength and shear energy with an increase in cutting rate could be contributed to this phenomenon that at low cutting rate the stalk was compressed against the blade. However, at higher cutting rate elastic wall of cells was not enough time to transmit the shear force to viscous fluid within the cells so the stalk cut with lower force. Similar results were reported by previous researchers for example Prasada and Gupta [17] reported that the shear strength of maize stalk was decreased with increasing rate of loading from 200 to 1000 mm/min. The investigations of Chattopadhyay and Pandey [4] showed that with increasing cutting rate from 10 to 100 mm/min the shear strength of sorghum stalk decreased from 3.74 to 1.94 MPa. Khazaei et al. [11] research work showed that with increasing cutting velocity from 20 to 200 mm/min the shear strength of pyrethrum stalk decreased from 2.4 to 2.1 MPa. They also reported that the shear energy of the pyrethrum stalk decreased from 3.23 to 2.76 mJ/mm² with increasing cutting velocity from 200 to 500 mm/min. Chegini et al. [5] reported that the shear energy of chrysanthemum flower decreased from 1.977 to 1.433 MPa when cutting velocity increased in the range of 10 to 500 mm/min.

The effects of bevel angle of blade on the shear strength and shear energy per unit area of saffron stalk at different cutting rate are shown in Figs. 7 and 8, respectively. As depicted from these figures, with increasing bevel angle of blade the shear strength and specific shear energy were increased for all of the loading rates. The results of analysis of variance (ANOVA) also showed that the bevel angle of cutting blade were significant on the shear strength and specific shear energy. The results of Duncan's multiple range tests to compare mean values of bevel angle effect on the shear strength and specific energy are given in Table 3.

As shown in this table, with increasing the bevel angle of blade from 17 to 20 degree the shear strength and the specific shear energy were not increased significantly (P > 0.01). However, with further increasing of the bevel angle from 20 to 24 degree the shear strength and energy per unit area of the stalk increased in the ranges of 0.156 to 0.190 MPa and 0.388 to 0.443 mJ/mm², respectively. The reason for increasing the shear strength and energy with an increase in bevel angle could be contributed to this phenomenon that with increasing the bevel angle, the blade more willing to compress and squash the stalk instead of cutting it, so the shear resistance and energy will be increased. In this connection Persson [16] believes that the required energy to compress and squash the stalk may be in amount of 40 to 60 percent of total energy. Similar results were found by other researchers for example Chattopadhyay and Pandey [4] studies revealed that with increasing bevel angle of blade from 30 to 70 degree the shear strength of sorghum stalk in the stages of grass and seed increased in the ranges of 3.74 to 8.18 MPa and 4.68 to 9.02 MPa, respectively. Khazaei et al. [11] investigations also showed that with increasing bevel angle in the range of 15 to 19 degree the shear strength and energy of pyrethrum flower stalk were not increased significantly, while further rising of bevel angle in the range of 19 to 27 degree the shear strength and energy per unit area increased in the ranges of 1.86 to 3.14 MPa and 2.30 to 4.21 mJ/mm², respectively.

CONCLUSIONS

1. The average values of the picking force, tensile strength and energy were 0.418 N, 0.209 MPa and 1.144 mJ/mm², respectively.

2. The picking force of the saffron flower as well as the tensile strength and energy per unit area of the saffron stalk were increased due to tension rate increasing. So the tension rate of 50 mm/min was recommended for picking the saffron flower. The age of saffron plant had not significant effect on the picking force, tensile strength and required energy.

3. Increasing the cutting velocity in the range of 20 to 200 mm/min reduce the shear energy consumption and shear strength of the stalk while further rising rate of cutting were not decreased the shear strength and energy.

4. Increasing the bevel angle of cutting blade increased the shear strength and energy per unit area so the bevel angle of 17 degree was recommended. The average values of the shear strength and required energy were 0.159 MPa and 0.379 mJ/mm², respectively.

5. The obtained data of the shear strength and shear energy per unit area is useful in developing cutting mechanisms of saffron. As well, the force and required energy for picking flower from stem and tensile strength are necessary to develop picking mechanisms for saffron flower harvesting.

ACKNOWLEDGEMENTS

The authors would like to express their appreciation to University of Tehran for full support of the project. As well, the authors express special thanks to Dr. Saeedirad for cooperation in this study.

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Accepted for print: 5.02.2010

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