Determination of engineering properties of onion crop required for designing an onion harvester

Abstract

The knowledge of the engineering properties of the onion bulb is a prerequisite to designing machinery for harvesting and post-harvest operations. The agronomical, mechanical, and biometric properties of the onion crop were measured for the Pusa Red variety at the harvesting stage. Onion (Allium cepa L.) seedlings of Pusa Red variety were grown at 150-mm row-to-row and 100-mm plant-to-plant spacing in sandy loam soil. The onion bulbs were obtained within 69.15 ± 7.17 mm below the ground surface with a coefficient of variation (CV) of 10.37%. The average length of onion plants was found to be 236.5 ± 35.28 mm with a CV of 14.91%. The cutting force required to cut the leaves from the onion was in the range of 45.42–105.87 N. The pulling force required to dig out the bulbs at the mean depth of 70 mm was found to be 32.33 ± 3.05 N. The shape of Pusa Red onion bulbs was found to be of prolate shape. The average values of polar diameter, equatorial diameter, and mass for the large onion bulbs were found to be 57.83 ± 5.26 mm, 46.88 ± 3.29 mm, and 65.68 ± 4.22 g, respectively, as compared to 47.44 ± 2.46 mm, 43.81 ± 3.33 mm, and 47.51 ± 7.07 g, for small onion bulbs. The average value of bulk density for the large and small onion bulbs was found to be 403 and 501.33 kg m⁻³, respectively. The mean value of angle of repose for small-size bulbs was obtained at 29.2°. A linear relationship was found to exist between polar and equatorial diameters as well as the mass of the bulb and polar/equatorial diameter for both large and small size onion bulbs.

Keywords:

• Onion harvester
• cutting force
• uprooting force
• biometric properties
• equatorial
• polar diameter

1. Introduction

Onion (Allium cepa L.) is one of the most consumed and commercial bulbous vegetable crops in the world. Globally, India is the second-largest producer of onion followed by China. The total production of onion in India is 22.07 MT with a yield of 16.78 t ha⁻¹ (from a cultivation area of 1.28 Mha), which is much lesser compared to the yields of the USA (56.40 t ha⁻¹, from a cultivation area of 0.05 Mha) and Iran (37.95 t ha⁻¹, from a cultivation area of 0.06 Mha) (Anon, 2018a; Kumawat & Raheman, 2022). In India, Maharashtra ranks first in onion production with a share of 38.09% followed by Madhya Pradesh. However, at present, it is being cultivated in all the states of the country. The national scenario shows that the production of onion has increased from 4.40 MT in 1994–1995 to 26.15 MT in 2019–2020. The area under onion is increasing from time to time mainly due to its high profitability per unit area (Olani and Fikare, 2010). Per capita onion consumption reached the highest to 11.9 kg in 2017, whereas it was lowest as 4.09 kg in 1964 in the world. But in India, it was highest as 14.7 kg in 2017, and it was the lowest as 2.25 kg in 1961 (Anon., 2018b). The Directorate of Onion and Garlic Research, Pune, has projected that the population of India will become around 1.7 billion by 2050 with no possibility of variations in the cultivable land. To fulfill the requirement of this ever-increasing population, keeping per capita consumption, export, processing, and losses at an existing rate (consumption, i.e., 7.83 kg/person/year, export 9%, processing 6.75% and losses 30%; base year 2011–2012), the requirement of onion will be 24.62 million tons in 2050. Efforts can be made to reduce losses up to 20%, increase export up to 25%, and processing up to 15% by 2050. With these targets, the production of onion has to be increased from 22.07 MT to 33.39 MT with a productivity of 30.72 t ha⁻¹ (Anon., 2021a). It was reported that various improved crop production practices, particularly mechanized cultivation, quantitatively and qualitatively can enhance the crop yield (Clarke, 1997; Srivastava, 1999; Nair, 2002).

The most common varieties of onion in India are Agrifound Dark Red, Agrifound Light Red, NHRDF Red, Agrifound White, Agrifound Rose, Agrifound Red, Pusa Ratnar, Pusa Red, and Pusa White Round. Depending upon the variety of onion crops, onion plants become ready to harvest after 70–90 days of transplanting. At this stage, the tops of the plants start to turn yellow from green, and they finally collapse from slightly above the bulbs. Once 50% of the tops fell, the crop is left for 1 week and then harvested. This period is considered the most suitable time for harvesting onion bulbs in the Rabi season. However, the leafy tops do not collapse during the Kharif season, so the bulbs are harvested when red pigmentation starts to appear on bulbs and the leaves turn slightly yellow. There are two crop cycles of onion in India: the first season crop is harvested from November to January and the second season crop is harvested from January to May. It is very important to harvest the crop at the right time as early harvest leads to sprouting in the bulb and any delay causes secondary root formation (Anon. 2019a; Kumawat & Raheman, 2022).

For the cultivation of the onion crop, first, onion seeds are grown in a nursery bed (3 × 2 m). The onion seeds take about 6–8 weeks to attain a seedling height of 150–200 mm and develop a pea-size bulb of 8 mm neck diameter, and then, these seedlings are transferred manually from the nursery to the main field. The seedlings are planted with a row-to-row spacing of 150 mm and plant-to-plant spacing of 100 mm for better growth (Anon., 2019b).

Agronomical, mechanical, and biometric properties of onion are highly important in order to design onion harvesting machines and also for other post-harvest operations such as cleaning, grading, and sorting (Bahnasawy et al., 2004; Khura et al., 2010). Khura et al. (2010) determined some engineering properties of the Pusa white round onion variety related to onion digger. It was reported that the root zone of 95% of onion bulbs was found within 70 mm depth from the ground surface. The equatorial diameter, polar diameter and average density for large bulbs were obtained as 64.68 mm, 53.20 mm, and 290 kg m⁻³, respectively. Dabhi and Patel (2017) determined some physical and mechanical properties of Tejla Red onion variety. It was reported that the shape may be considered oval to spherical. The mean bulk density was obtained as 548 kg m⁻³. Some physical and engineering properties such as equatorial and polar diameter, the weight of onion bulbs, and bulk density were determined by many researchers (Nieuwhof et al., 1973, for the onion variety of Primodoro and Rijnsburg; Abdel-Ghaffar & Hindey, 1984, for Abo-Fatla variety Egyptian onion; Eweida et al., 1986, for Giza 6 Mohassan onion; Maw et al., 1996, for Granex-Grano onion; Rani & Srivastava, 2006, for Agrifound Dark Red, Pusa Red, and NP-53 onion variety; Khura et al., 2010, for Pusa white round; Ghaffari et al., 2013, for Azarshahr red onion, Kashan white onion, and Isfahan yellow onion (Iranian varieties of onion); Shoba et al. (2017), for Ballari red, Arka kalyan, Satara (local variety), and Kalasa (local variety); Devojee et al. (2021). The knowledge of size, density, friction angle, angle of repose, and crushing strength of onions is very much required to develop the grading system (Chandrasekar & Viswanathan, 1999; Gosh, 1969). Some physical and mechanical properties of the Granex Grano onion variety such as mass, surface area, volume, density, and the overall mean equatorial and polar diameters were reported as 98 g, 111 cm², 95 cm³, 1.1 g cm⁻³, 6.2, and 4.2 cm, respectively (Maw et al., 1996). They also reported that the crushing and puncture forces for this variety of onions were 26.4 N and 25.0 N, respectively. Bahnasawy et al. (2004) studied the physical and mechanical properties of three different varieties of onion bulbs, viz. Giza 6 (white), Beheri (red), and Giza 20 (yellow). The equatorial and polar diameters were found to be in the range of 51.20 ± 3.30 to 62.0 ± 1.5 mm for all three varieties with a coefficient of variation (CV) of 11–25%. The shape of Beheri and Giza 20 onion bulb variety was spherical, and Giza 6 was Oval in shape. The density ranged from 1.04 ± 0.09 to 1.11 ± 0.15 g cm⁻³ with a CV of 8.04–13.5%. Rani and Srivastava (2006) determined some physical and mechanical properties of three different onion varieties, viz. Agrifound Dark Red, Pusa Red, and NP-53. Pusa Red variety was considered bigger and denser than the other two varieties. The density was reported as 270 kg m⁻³ for Pusa Red onion bulb. The texture analyser was used to measure cutting shear force requires for cutting. The average shear force required to cut the neck was found to be 16.23, 17.67, and 18 kgf for Agrifound Dark Red, Pusa Red, and NP-5, respectively.

It is difficult to design suitable machinery for carrying out harvesting operations, as there is very limited information available about the physical and mechanical properties of onions. Hence, a study was carried out to determine the engineering properties of onion bulbs at the harvesting stage, which are required for designing an onion harvester. Onion harvester majorly comprises four such units, viz. topping unit for cutting the matured onion leaves, digging unit for digging out the onion bulbs, conveying unit for conveying onion bulbs to the field and separating unit for the separation of soil and other unwanted materials from the bulbs. The engineering properties involve polar diameter, equatorial diameter, bulk density, mass of the bulb, depth of onion bulb, length of leaves, moisture content of leaves, neck diameter, shape factor, angle of repose, and cutting force.

2. Materials and methods

Knowledge of agronomical, mechanical, and biometric properties of onion plants is a prerequisite to designing a cutting unit for onion leaves as well as a digging cum conveying unit to dig out the topped onion bulbs. Therefore, the onion crop (Pusa Red) was planted on 22 December 2019 in the farm of Agricultural and Food Engineering Department, IIT Kharagpur, where the soil was sandy clay loam as shown in Figure 1. Eight beds each of size 10 × 0.6 m were prepared to transplant onion seedling with 150-mm row-to-row and 100-mm plant-to-plant spacing. It became ready to harvest after 90–100 days of transplanting.

Figure 1. Onion crop plantation at IIT Kharagpur.

2.1. Determination of soil properties

2.1.1. Soil bulk density

A core sampler of diameter 50 mm and length 150 mm was used to measure the bulk density of soil (Figure 2). It was pushed vertically into the soil at 10 random places. The edge of the sampler was sharpened at 30º bevel angle, which enables it to penetrate into the soil easily. The soil retained in the sampler was collected and weighted for calculating the bulk density (ASA, 1965).

Figure 2. Measurement of bulk density using core sampler.

2.1.2. Soil moisture content

A digital soil moisture meter (Rupson Industries) was used to measure the soil moisture content at 10 different places as shown in Figure 3. It consisted of a probe that was inserted up to 6 cm of soil depth. The value of saturation moisture content was noted down from the display.

Figure 3. The measurement of moisture content using a digital soil moisture meter.

2.1.3. Cone index measurement

The strength of soil was determined at 10 different places using tractor mounted hydraulically operated soil cone penetrometer by following ASABE Standards S313.3 (ASAE, 2001). The hydraulically operated soil cone penetrometer with a cone angle of 30º and base area of 323 mm² was operated at a speed of 25–30 mm s⁻¹ up to the onion bulb depth to measure the cone index (CI) of the soil. An S-type load cell of 1000 kg capacity and a rotary potentiometer with rack and pinion arrangement were used to measure the force required to push the cone penetrometer and its displacement, respectively, as shown in Figure 4. The output of the load cell and the potentiometer were fed to the DAS system (HBM QuantamX DATUM- M425) for the measurement of CI at different soil depths.

Figure 4. The measurement of cone index using tractor mounted cone penetrometer.

1. S-type load cell, 2. Cone penetrometer, 3. Hydraulic system, 4. Rotary potentiometer, 5. Hydraulic cylinder, and 6. Laptop and Data acquisition system

2.2. Determination of agronomical properties

The agronomical properties of onion crops that are suitable for Indian conditions were adopted from the standard source of onion producers (Anon. 2019).

2.2.1. Determination of moisture content

At the harvesting stage, onion bulbs with leaves were collected randomly at three different places. Leaves were detached from bulbs and weighed. Then, leaves were retained in the oven at 105 ± 5°C for 24 h (Khura et al., 2010). The following formula was used to compute the moisture content (dry weight basis):

$\mathrm{M}.\mathrm{C}.=\frac{{\mathrm{W}}_1-{\mathrm{W}}_2}{{\mathrm{W}}_2}\times 100$

where MC = percent moisture content, W₁ = weight of the wet soil sample (g), and W₂ = weight of dry soil sample (g)

2.2.2. Average depth of the bulb

The depth at which bulbs are found below the ground surface is known as the depth of the bulb. The depth of the bulb was measured at 10 random places in the plot. A scale was kept stationary on the horizontal plane, and another scale was in a vertical position. The reading of depth from the vertical scale was noted down for different bulbs. A scale has the least count of 0.1 mm.

2.2.3. Neck diameter above the bulb

A digital Vernier Caliper having the least count of 0.01 mm was used to measure the neck diameter above the crown of the bulb at the harvesting stage. Twenty bulb samples were chosen randomly to measure the neck diameter.

2.3. Determination of mechanical properties

Breaking strength of onion leaves is important to design the topping unit of the onion harvester as it helps to decide the cutting force required to detach the onion leaves above the bulb.

2.3.1. Measurement of the cutting force

The mature onion leaves are to be cut 20 mm above the crown of the onion bulbs. The cutting force required to cut the leaves from the onion bulbs was measured using a universal testing machine (Instron/UTM-6800 series). First, the neck of the onion plant sample having different diameters was fixed on the platform of the UTM as shown in Figure 5. The lower plate of the UTM was kept stationary, while the upper jaw approached downward on which the blade was mounted. The upper jaw applied the force required to detach the leaves from the onion bulb. The cutting operation was performed at two different speeds, viz. 200 and 300 mm per minute. The experiment was performed with five different samples having different stem diameters with three replications of each sample.

Figure 5. The experimental setup for measuring cutting force using UTM.

2.3.2. Measurement of the uprooting force for onion bulb

Traditionally, the field is irrigated 2–3 days prior to harvesting to maintain the moisture content around 12% for easiness in pulling of bulbs, and these bulbs are pulled out manually by holding the neck of the bulb (Kumawat et al., 2020). Hence, it is very important to know the uprooting force required to dig out the onion bulbs. A setup was developed to determine the uprooting force of the onion bulb as shown in Figure 6. It comprised a digital weighing scale (generic/weight capacity 45 kg), clamp, frame, data acquisition system (HBM QuantamX DATUM- M425), 12 V battery, laptop, dc to ac converter, operating switch and a 300-mm stroke length linear actuator. Before conducting the experiment, the properties of the soil were measured as described in Section 2.1. During the measurement of uprooting force, the onion bulb was held with a clamp 30 mm above the crown of the bulb. One end of the weighing scale was attached to the clamp, and another was fixed with the linear actuator. The linear actuator was operated at a speed of 30 mm s⁻¹ using an operating switch until the bulb was totally pulled out from the ground. A 12 V DC battery was used to supply power to linear actuator through DC to AC converter. The data of uprooting force were collected using data acquisition system and stored in the laptop. The random 10 different onion bulbs were chosen for the same procedure.

Figure 6. The determination of uprooting force in the actual field condition.

1. Digital weighing scale, 2. Clamp, 3. Frame, 4. Data acquisition system, 5. Battery, 6. Laptop, 7. DC to AC converter, 8. Operating switch, and 9. Linear actuator.

2.4. Determination of biometric properties of onion bulb

Biometric properties of onion bulbs involve polar diameter, equatorial diameter, average mass, shape factor, angle of repose, and bulk density of bulbs. Biometric properties are used to design the conveying unit as well as the storage unit. A randomly selected hundred onion bulb samples were taken to measure the biometric properties.

2.4.1. Polar diameter

The polar diameter is defined as the distance between the crown of the bulb to the bottom part of the bulb from where roots germinate as shown in Figure 7. It was measured with the help of a digital Vernier Caliper of 0.01 mm least count as shown in Figure 8a.

Figure 7. The position of polar and equatorial diameters (source: Gomathy et al., 2017).

Figure 8. The determination of biometric properties of onion bulb. 1. Polar diameter, 2. Equatorial diameter, and 3. Mass.

2.4.2. Equatorial diameter

The equatorial diameter is the extreme breadth of an onion bulb measured perpendicular to the polar diameter as shown in Figure 7. It was determined with the help of a digital Vernier Caliper of 0.01 mm least count as shown in Figure 8b.

2.4.3. Bulb mass

The mass of each onion bulb was measured using the digital weighing machine (SF-400) with a capacity of 5000 g as shown in Figure 8c.

2.4.4. Bulk density

Random samples of large and small onion bulbs were separately put in 0.10 × 0.10 × 0.10 m³ of the box without compaction and then weighed. The ratio of the total weight of the bulb to the volume of the box is defined as bulk density (Gautam et al., 2021).

2.4.5. Angle of repose

The angle of repose is measured between a horizontal base and a platform carefully rotated about a horizontal axis just enough to make the onion bulb roll (Sahay & Singh, 1994). Five random small bulb samples were chosen in order to determine the angle of repose. It was measured using the tilting-top drafting table method as shown in Figure 9. The bulb was placed at the centre of the platform, keeping it stable for determining its angle of repose. Then, the platform was inclined gradually with the help of a turning handle at a very low speed until the onion starts rolling. The position of the platform at this stage was measured in angles using the protractor. The same procedure was followed for all the samples (Buyanov & Voronyuk, 1985).

Figure 9. The measurement of angle of repose.

2.4.6. Shape and shape factor

The ratio of the equatorial diameter to the polar diameter is defined as the shape factor (Maw et al., 1996; Rani & Srivastava, 2006). The shape factor for different shapes of onions is given in Table 1.

Table 1. Values of shape factor for different shapes

S. No.	Shape factor	Shape
1.	<1	Prolate
2.	1	Spherical
3.	>1	Oblate
Source: Maw et al., 1996; Rani & Srivastava (2006).

3. Results and discussion

3.1. Soil properties

The properties of the soil at the harvesting stage, i.e. soil bulk density, moisture content and cone index, were measured. The values of bulk density, moisture content and cone index were found to be 1523 ± 0.04 kg m⁻³, 10.58 ± 0.84% and 825 ± 22 kPa, respectively.

3.2. Agronomical properties

Agronomical properties of the onion crop (Pusa Red) were measured with a measuring scale as given in Table 2. The depth of the onion bulb was measured at five different places, and the value was 69.15 ± 7.17 mm; this information helps in deciding the working depth and design of the onion digging/harvesting unit. Similar results were also reported by Rani and Srivastava (2006) for Pusa Red variety and Khura et al. (2010) for Pusa white round onion variety. The height of onion leaves at the harvesting stage was found to be 236.5 ± 35.28 mm with a CV of 14.91%. The number of leaves per onion plant was obtained in the range of 6–9. The neck diameter of the onion bulb above the crown was in the range of 4.5–11 mm.

Table 2. Agronomical properties of onion crops

S. No.	Parameters	Values
1.	Crop variety	Pusa Red
2.	Row-to-row spacing, mm	150
3.	Plant-to-plant spacing, mm	100
4.	Age after transplanting at the time of harvesting, days	90–100
5.	Average depth of onion bulb, mm	69.15±7.17
6.	Average diameter of the neck above the crown, mm	8.53±1.83
7.	Average moisture content of leaves (at harvesting stage), %db	11.8

3.3. Mechanical properties

3.3.1. Cutting force

A test was conducted to determine the force required to cut the onion leaves from the bulbs using UTM as given in Table 3. The cutting force required to cut the leaves from the onion was in the range of 45.42–105.87 N; the minimum cutting force of 45.42 N was required for a neck diameter of 10.5 mm and cutting speed of 300 mm min⁻¹, while the maximum cutting force 105.87 N was required for a neck diameter of 12 mm and cutting speed of 200 mm min⁻¹. Compression load and the cutting energy decreased with an increase in speed, and it increased with an increase in neck diameter (Figure 10). This could be due to the loading rate because with an increment in loading rate, the cutting force and the cutting energy decreased. Analogous findings were also reported by Hassan-Beygi et al. (2010) for saffron flower; Igathinathane et al. (2010) for corn stalk; Dange et al. (2011) for Pigeon pea stems; Heidari et al. (2012) for Lilium Stalk; Esgici et al. (2018) for rice stem; Pekitkan et al. (2020) for paddy stem.

Figure 10. Cutting force vs. compressive displacement grap.

Table 3. Force required to cut onion leaves above bulbs

Sample	Speed (mm/min)	Neck diameter (mm)	Maximum cutting force (N)	Cutting energy at maximum load (mJ)
1	200	8.5	89.53	208.43
2		9.0	92.83	221.70
3		12.0	105.87	258.45
4	300	11.2	57.46	185.96
5		10.5	45.42	75.67

3.3.2. Uprooting force

The value of uprooting force was found to be 32.33 ± 3.05 N. The graph between time and uprooting force is shown in Figure 11.

Figure 11. A graph between time and uprooting force.

3.3.3. Biometric properties

Biometric properties such as polar diameter, equatorial diameter, mass, and bulk density of onion bulbs are summarized in Table 4. The values of polar diameter for large and small onion bulbs were found to be 57.83 ± 5.26 mm and 47.44 ± 2.46 mm, with the CV of 9.10 and 5.19%, respectively. However, the values of equatorial diameter for large and small onion bulbs were found to be 46.88 ± 3.29 mm and 43.81 ± 3.33 mm, with the CV of 7.03 and 7.60%, respectively. From Figure 12 (a and b), it can be seen that the linear relationship was found between equatorial and polar diameter for both large and small bulbs, and it can be expressed by the following equations:

Figure 12. The relationship between D_e and D_p for large and small onion bulbs.

Table 4. Biometric properties of onion bulb

S. No.	Particulars	Values
		Large onion bulbs			Small onion bulbs
		Mean	S.D.	C.V. (%)	Mean	S.D.	C.V. (%)
1.	Polar diameter, mm	57.83	5.26	9.10	47.44	2.46	5.19
2.	Equatorial diameter, mm	46.88	3.29	7.03	43.81	3.33	7.60
3.	Mass of each bulb, g	65.68	4.22	6.42	47.51	7.07	14.89
4.	Shape factor	0.81	0.03	4.17	0.92	0.03	3.43
5.	Bulk density, kg m^−3	403	10.53	2.61	501.33	13.01	2.59
6.	Angle of repose, degree	-	-	-	29.2	0.83	2.86

S.D. = standard deviation; C.V. = coefficient of variation.

(1) $\mathrm{For}\mathrm{large}\mathrm{bulbs}:{\mathrm{D}}_{\mathrm{p}}=\mathit{\hspace{0.17em}}1.42{\mathrm{D}}_{\mathrm{e}}-9.04,\mathit{\hspace{0.17em}}\left({\mathrm{R}}^2=0.80\right)$(1)

(2) $\mathrm{For}\mathrm{small}\mathrm{bulbs}:{\mathrm{D}}_{\mathrm{p}}=\mathit{\hspace{0.17em}}1.07{\mathrm{D}}_{\mathrm{e}}+0.41,\mathit{\hspace{0.17em}}\left({\mathrm{R}}^2=0.97\right)$(2)

The mass of large and small onion bulbs was obtained at 65.68 ± 4.22 g and 47.51 ± 7.07 g, with the CV of 6.42% and 14.89%, respectively. The relationship between equatorial and polar diameters with the mass of the bulb for both the large and small size bulb is shown in Figure 13 (a and b). From Figure 13 (a and b), it can be observed that the polar diameter was higher than the equatorial diameter for both large and small size bulbs at the harvesting stage. Equations (3(3) $\mathrm{m}=\mathit{\hspace{0.17em}}0.77\mathit{\hspace{0.17em}}{\mathrm{D}}_{\mathrm{p}}+21.10,\mathit{\hspace{0.17em}}\left({\mathrm{R}}^2=0.92\right)$(3) and 4(4) $\mathrm{m}=\mathit{\hspace{0.17em}}1.04{\mathrm{D}}_{\mathrm{e}}-16.87,\mathit{\hspace{0.17em}}\left({\mathrm{R}}^2=0.66\right)$(4) ) showed the linear relationship between polar diameter with mass and equatorial diameter with mass for large bulbs.

Figure 13. The graph between D_p, D_e with the mass of bulb for large and small bulbs.

(3) $\mathrm{m}=\mathit{\hspace{0.17em}}0.77\mathit{\hspace{0.17em}}{\mathrm{D}}_{\mathrm{p}}+21.10,\mathit{\hspace{0.17em}}\left({\mathrm{R}}^2=0.92\right)$(3)

(4) $\mathrm{m}=\mathit{\hspace{0.17em}}1.04{\mathrm{D}}_{\mathrm{e}}-16.87,\mathit{\hspace{0.17em}}\left({\mathrm{R}}^2=0.66\right)$(4)

These findings are in agreement with the results reported by Rani and Srivastava (2006) for Agrifound Dark Red, Pusa Red, and NP-53 onion variety. The mean shape factor for both large and small onion bulbs was found to be less than one. Therefore, the shape of large and small onion bulbs was considered as prolate. The value of bulk density for large and small onion bulbs was obtained at 403 and 501.33 kg m⁻³, respectively. The average value of angle of repose for small-size bulbs was obtained at 29.2°. It was observed that the angle of repose increased with an increase in the mass of the bulb. Shape factor was also found to be an influencing factor to affect the angle of repose.

3.4. Influence of agronomical, mechanical, and biometric properties for designing an onion harvester

The neck diameter of the bulb above the crown, length of onion leaves, number of leaves per plant, and moisture content of leaves are important factors to be considered for the design of the topping unit for trimming onion leaves at the harvesting time. The diameter or length of the cutter of the topping unit could be decided based on the spacing of onion bulbs between row to row and plant to plant. The moisture content is also an influencing factor in deciding the optimum cutting speed of the topper. The compressive force required to trim the onion leaves could be used to decide the size of the power unit to carry out the cutting operation.

The capacity of the conveyor could be decided based on the bulk density and weight of the bulb. The equatorial diameter, polar diameter, and shape factor could be used to decide the spacing between the bars fitted to the conveyor. The angle of repose could be used to decide the slope of conveying chain.

4. Conclusions

The agronomical, mechanical, and biometric properties of the Pusa Red onion crop variety were determined. Agronomical properties of the onion crop (Pusa Red) such as the average height of onion leaves at the harvesting stage, the number of leaves per plant, and the depth of onion bulbs were found to be 236.5 ± 35.28 mm, 6–9, and 69.15 ± 7.17 mm, respectively. The biometric properties such as polar diameter, equatorial diameter, mass, and bulk density of onion bulbs were measured to be 57.83 ± 5.26 mm, 46.88 ± 3.29 mm, 65.68 ± 4.22 g, and 403 kg m⁻³, respectively, for large onion bulbs as compared to 47.44 ± 2.46 mm, 43.81 ± 3.33 mm, 47.51 ± 7.07 g, and 501.33 kg m⁻³ for small onion bulbs. The minimum compressive cutting force required to cut the leaves from the onion bulbs at a cutting speed of 300 mm min⁻¹ was found to be 45.42 N for a neck diameter of 10.5 mm, whereas the maximum cutting force was found to be 105.87 N for a neck diameter of 12 mm and cutting speed of 200 mm min⁻¹. The force required for pulling out the onion bulbs from the ground was obtained as 32.33 ± 3.05 N. By knowing the engineering properties of onion bulbs at the harvesting stage, the design of onion harvester for topping, digging and conveying can be suitably designed.

Disclosure statement

No potential conflict of interest was reported by the authors.