Engineering properties of horse gram (Macrotyloma uniflorum) varieties as a function of moisture content and structure of grain

Abstract

The study was conducted to determine the engineering properties of horse gram varieties namely GPM-6, PAYIUR-2, and BHK as a function of moisture content in range of 10–30%. The average length, width, thickness, geometric mean diameter, arithmetic mean diameter, thousand kernel weight, sphericity, porosity and angle of repose ranged from 5.43 to 6.53 mm, 3.96 to 4.48 mm, 2.21 to 2.99 mm, 3.62 to 4.41 mm, 3.86 to 4.64 mm, 30.32 to 49.11 g, 63.56 to 72.66%, 35.20 to 38.76% and 22.72° to 29.86° respectively as the moisture content of the grain increased from 10.08 to 29.98%. The bulk density and true density of the grain decreased from initial range of 810–901 to 734–801 kg m⁻³ and 1250–1426 to 1168–1308 kg m⁻³. The volume, porosity and terminal velocity of the grain increased linearly with increase in moisture content. The coefficient of friction also showed positive correlation for all surface materials, the highest increase was found for plywood in all varieties of horse gram seeds. Dehulling properties of the grain found to be significantly affected by the change in moisture content. The overall dehulling ranged from 53.44 to 61.21% for GPM-6 and 55.58 to 61.06% for PAYIUR-2 variety of horse gram. Textural properties of the grains were also found to be significantly affected by the change in moisture content from 10 to 30%. The data generated in this study will be highly useful in optimization of post-harvest processing operations as well as to design and develop related processing equipment for horse gram.

Introduction

The cultivation of horse gram is well known around the world. Horse gram (Macrotyloma uniflorum) is cultivated throughout the world for its nutritional as well as medicinal values. The horse gram on an average contains 25% protein, 1% fat and 57% carbohydrates (Gopalan et al. 2012). Horse gram in India is consumed in its various forms like germinated, dried and conservative of horse gram. It is also used in ayurvedic as well as home remedies for curing problems of kidney stones, liver malfunctioning, cold, cough etc. and it is also used as animal feed. It is mixed with vegetative part of plant and other animal diet which render the diet rich in protein as well as other required nutrients. With increased awareness horse gram has made its way in culinary culture as well. In its different forms horse gram is used in preparation like idli, dosa and different kind of curries in both northern as well as southern belt of India.

In order to design equipment for harvesting, handling, separating, sorting and processing of horse gram it becomes necessary to evaluate and observe the physical characteristics of this grain as a function of its moisture content. Recent studies has made great efforts towards evaluating basic physical properties of agricultural produce and found their practical importance and utility in machine structure, design and in control engineering (Amin et al. 2004). Recent development has improved and changed the handling and processing of agricultural produce through different techniques like mechanical, optical as well as electrical means. But, still there is much more left to know about the basic physical characteristics of agricultural produce, such basic information is not only important for food scientist or plant breeder but also for the engineers and other scientist who may find new ways of their use.

The size, shape and mechanical properties of grains are important in designing of harvesting, separating, sizing, and cleaning machines. Bulk density and porosity of grain plays important role in determining the requirements of storage system, structural load and designing of dryers and aerators as these properties will define the resistance towards air flow (Amin et al. 2004). Terminal velocity and coefficient of friction are important in development of transporting equipment like conveyors etc. (Dursan and Dursan 2005; Sacilik et al. 2003). Angle of repose is important for transportation and storage system.

Several investigators have studied the effect of moisture content on physical properties of various crops such as Altuntas and Yaldiz (2007) for faba beans, Cetin (2007) for barbunia beans, Amin et al. (2004) for lentil seeds and Baryeh (2002) for pearl millet.

However, limited information is available on physical properties of horse gram as a function of moisture content. Hence, an attempt has been made to investigate some moisture dependent physical characteristics of horse gram seeds namely, axial dimensions, gravimetric, geometric and dehulling properties of grain at different moisture content ranging from 10 to 30%.

Materials and methods

Materials

Horse gram varieties GPM-6 and BHK were procured from University of Horticulture Sciences, Bhagalkut, Karnataka, India and PAYIUR-2 variety was procured from Tamil Nadu Agricultural University, Coimbatore, TamilNadu, India. Five kg of sample was collected from each of the varieties and manually cleaned to remove dirt, dust, broken, infested seeds, immature seeds and any other foreign material. The initial moisture content of grain was brought to 10.08, 10.18 and 10.12 for GPM-6, PAIYUR-2 and BHK varieties respectively. The samples of desired moisture content were prepared by adding the required amount of distilled water which is calculated using the following relation (Saclik et al. 2003).

$$Q=\frac{W\left(M_f-M_i\right)}{\left(100-M_f\right)}$$

where Q is the quantity of moisture content required; W is weight of the grains; M_f is final moisture content of grain; M_i is initial moisture content of grain. The samples were packed in separate low density polyethylene bags (thickness: 75µ) and kept at 5 °C in a cold storage chamber for a week to achieve uniform distribution of moisture content throughout the seeds. The required amount of samples were then taken out from the cold storage and allowed to equilibrate at room temperature for about 2 h for experiment preparation (Ozarslan 2002).

All the axial dimensions, gravimetric, geometric properties of seeds were assessed at moisture level of 10, 15, 22 and 30% db with 10 replications at each moisture contents.

Methods

Moisture

The initial moisture content of seeds was determined by oven drying method. Seeds were oven dried at 105 ± 1 °C for 24 h (Suthar and Das 1996).

$$\%Moisture=\frac{W_1-W_2}{W_1}\times 100$$

where W₁ is the initial weight of seeds; W₂ is the final weight of seeds.

Axial dimensions

In order to measure dimensions, 100 seeds were randomly selected. Three principal dimensions of physical properties namely length, width and thickness were measured in three directions by using a digital screw gauge having a least count of 0.001 mm. The geometric mean diameter (D_g) was calculated according to Mohsenin (1980) as shown below:

$$D_g=\sqrt[3]{\text{LWT}}$$

where L is length (maximum diameter), W is width (intermediate diameter) and T (minimum diameter) is thickness of seeds, in mm.

$$\text{V}=\frac{\pi LWT}{6}$$

The arithmetic average diameter was determined as per EL-Fawal et al. (2009).

$$D_a=\frac{\left(L+W+T\right)}{3}$$

Sphericity (Φ) considered to be important in describing shape of the seeds, was determined as per relation given by Maduako and Faborode (1990).

$$\Phi =\frac{\sqrt[3]{\mathit{LWT}}}{L}\times 100$$

Gravimetric parameters

Bulk density is defined as the ratio of mass of sample to its total volume. Average bulk density of horse gram was determined by following the method of Singh and Goswami (1996). Graduated cylinder of 500 ml volume was filled with the seeds from the height of 15 cm at content rate and then weighing the content.

$${\rho }_b=\frac{W_s}{V_s}$$

where ρ_b, bulk density of seeds in kg m⁻³; W_s, weight of seeds in kg; V_s, volume of seeds in m³.

True density (ρ_t) is defined as the ratio of mass of seeds to its true volume (Deshpande et al. 1993). True density of seeds was determined by toluene displacement method. The volume of toluene displaced was determined by immersing a known seeds quantity of horse gram in measured volume of toluene. Toluene was used instead of water to prevent absorption and also to get benefit of low surface tension during measurement of true density (Ogut 1998).

$${\rho }_t=\frac{W_s}{V_s}$$

where ρ_t, true density of seeds in kg m⁻³; W_s, weight of seeds in kg; V_s, true volume of seeds in m³.

Porosity of seeds was determined using true density (ρ_t) and bulk density (ρ_b) according to the relation given by Mohsenin (1980).

$$Porosity=\frac{{\rho }_t-{\rho }_b}{{\rho }_t}$$

Thousand kernelsweight or thousand grain weight was measured by means of analytical balance reading to an accuracy of 0.001 g (Ozarslan 2002).

Geometric parameters

The angle of repose was investigated as described by Markowski et al. (2010). Cone was formed and the angle of repose was calculated according to the formula:

$$A_r=tan\left(\frac{2H_c}{D_c}\right)$$

where A_r is the angle of repose in degree, H_c height of cone and D_c is diameter of cone formed with grain.

The static coefficient of friction (μ) for horse gram was determined against plywood, glass and stainless steel. The seeds sample was placed over the surface to be tested; angle of inclination (α) was gradually increased to cause the grains move downward. The angle of inclination was read from the scale on movement of grain and friction coefficient was calculated according to the formula given by (Sharma et al. 2011):

$$\mu =tan\left(\alpha \right)$$

where μ is the coefficient of friction and α is the angle of inclination (°).

Theoretical terminal velocity of grain was determined by using the relation given by Gorial and O’Callaghan (1990). For this, the diameter of equivalent sphere and shape factor was calculated using following relation.

$$V_t^2=\frac{4g{D}_e{\rho }_t(6Z/\pi )}{3{\rho }_{a}0.44}$$

$$Z=\frac{\pi }{6}{\left(\frac{D_e}{D_g}\right)}^3\Phi$$

where V_t, the theoretical terminal velocity in ms⁻¹; ρ_a, true density of air in kg m⁻³; g, gravitational acceleration in ms⁻²; ρ_t, true density of seed; Z, shape factor.

The experimental air velocity was determined by using a vertical air tunnel with plexi-glass tube. Twenty seeds from each variety of horse gram were selected randomly for determining their terminal velocity. The seeds were placed on mesh screen in vertical tube. The grains were made to float by adjusting the speed of motor. The air velocity at which seeds just got suspended was measured using anemometer, having a least count of 0.1 ms⁻¹.

Dehulling and percentage of fines

Dehulling properties of the grain were determined by following the method reported by Figueiredo et al. (2011). Dehulling ability was calculated on percentage basis. The relationship between the hull percentage extracted mechanically (H mechanical) and the total hull content (H manual), expressed as follows on dry weight basis:

$$\text{D}.\text{a}\left(\%\right)=\frac{H_{\mathit{mechanical}}}{H_{\mathit{manual}}}\times 100$$

Mechanical dehulling of the grains was achieved by using pilot dehulling equipment which was based on centrifugal process. For all selected varieties, the output of dehuller was passed through standard sieve and the fractions were distributed in followingtwo categories; Fines and rest (whole not dehulled; whole dehulled; partially dehulled seeds and brokens seeds). The hull and fines percentage extracted mechanically were calculated by following relation:

$${\text{H}}_{\text{mechanical}}=\frac{Totalhullcontent}{Toatalhullextract\left(g\right)+Rest\left(g\right)+Fines\left(g\right)}\times 100$$

Similarly the fines percentage was calculated using the ratio of total mass of fines to sum of mass of fines, total hull extract and rest.

Texture analysis

Textural properties of horse gram varieties, with respective moisture content were evaluated using Texture Analyzer (Model XT2i; Stable Micro Systems Ltd., Surrey, UK) by following the method of Wani et al. (2015).

Statistical analysis

The experiments were performed in triplicates and data presented in mean ± standard error. The results were analysed using analysis of variance (ANOVA) (SPSS, 2002). The Duncan multiple range test was used to separate the means and accepted at the level of p ≤ 0.05.

Results and discussion

General characteristics of horse gram varieties

The general characteristics of horse gram are important factors which need to be analysed as they will affect the textural as well dehulling properties of the grain. The general characteristics of varieties are given in Table 1. The parameters are analysed at initial moisture content of 10.08–10.18%. The fat content of the varieties did not vary significantly (p > 0.05) whereas the protein content and the hull to grain ratio of varieties did varied significantly (p < 0.05). The protein and hull to grain ratio of the varieties were in range of 21–24% and 8.12–8.68% respectively. The PAYIUR-2 variety of horse gram found to have higher protein content whereas, BHK had the higher hull to grain ratio followed by PAYIUR-2 and GPM-6. The hull percentage was found to vary in all three varieties of grain. The hull percentage of grains before moisture treatment was 8.12, 8.32 and 8.68% for GPM-6, PAYIUR-2 and BHK varieties respectively.

Table 1.

General characteristics of Horse gram

Variety	Moisture content (%)	Hull colour	Oil content	Protein content	Hull percentage
GPM-6	10.08a	Dark Reddish Brown	0.95a	21.8a	8.12a
PAYIUR-2	10.18a	Reddish Brown	0.94a	24.57b	8.32a
BHK	10.12a	Muddy brown	0.94a	24.07b	8.68b

Within the rows, mean ± standard error, mean values with the same lowercase letter superscripts are not significantly different (p ≤ 0.05)

Axial dimension and hull percentage of grain

The mean linear dimensions of 100 seeds measured at moisture content of 10% db namely length, width, thickness, geometric mean and arithmetic mean diameter are 6.01, 4.08, 2.28, 3.82 and 4.12 mm for GPM-6, 5.43, 3.96, 2.21, 3.62 and 3.87 mm for BHKand 5.94, 4.07, 2.25, 3.79 and 4.08 mm for PAYIUR-2 respectively. The frequency distribution curve for the mean value of dimension has shown a normal distribution trend. About 82% of GPM-6, 85% of BHK and 84% of PAYIUR-2 seeds has length in the range of 5.8–6.2 mm, 5.2–5.7 mm and 5.5–6.0 mm respectively. In case of width 88% of GPM-6, PAYIUR-2 and 91% of BHKseeds were in the range of 4.0–4.5 mm. Around 91%, 89% and 92% of GPM-6, BHK and PAYIUR-2 seeds respectively were found to have a thickness of 2.0–2.5 mm. It is evident from Table 2 that the dimensions of seeds and its geometric mean diameter got increased on absorption of moisture and the dimensional changes does occur in grain which is represented by the positive correlation between moisture content and the axial dimensions of grain. The increase in axial dimensions was in the range of 8–10% for length, 7.5–10% for width and 31–40.2% for thickness, 15–18% for Dg, 12–14% for Da. Maximum expansion of seeds was found along the thickness and minimum along the seeds width. BHK variety of horse gram showed maximum level of thickness among the varieties studied. However more increase in length and width was observed for PAYIUR-2 followed by GPM-6. Similar results have been reported by other researchers in previous studies (Deshpande et al. 1993; Altuntas and Yildiz 2007; Cetin 2007).

Table 2.

Regression equation as a function of moisture content with their respective coefficient of determination for given axial parameters

Parameter	Variety	Regression equation	R2
Length	GPM-6	5.832 + 0.172Mc	0.99
	PAYIUR-2	5.759 + 0.183Mc	0.99
	BHK	5.259 + 0.151Mc	0.99
Width	GPM-6	3.952 + 0.109Mc	0.99
	PAYIUR-2	3.910 + 0.140Mc	0.99
	BHK	3.825 + 0.113Mc	0.98
Thickness	GPM-6	1.986 + 0.264Mc	0.99
	PAYIUR-2	2.025 + 0.233Mc	0.99
	BHK	1.986 + 0.274Mc	0.98
Geometric diameter	GPM-6	3.599 + 0.207Mc	0.99
	PAYIUR-2	3.586 + 0.206Mc	0.99
	BHK	3.436 + 0.206Mc	0.99
Arithmetic diameter	GPM-6	3.925 + 0.182Mc	0.99
	PAYIUR-2	3.898 + 0.185Mc	0.99
	BHK	3.693 + 0.179Mc	0.99

Within the rows, mean ± standard error, mean values with the same lowercase letter superscripts are not significantly different (p ≤ 0.05)

Gravimetric properties

The results pertaining to the gravimetric analysis are shown in Table 3. Thousand kernels weight is an important factor in determining the soundness of grain before subjecting it to the processing. The thousand kernels weight m₁₀₀₀got increased from 32.98 to 47.72 g for GPM-6, 32.56 to 49.11 g for PAYIUR-2-2 and 30.32 to 44.72 g for BHK seeds as the moisture content of the seeds increased from 10 to 30% db. An increase of 44.69, 50.82 and 47.49% in thousand kernel weight was observed for GPM-6, PAYIUR-2 and BHK variety respectively. The greater thousand kernel weight of PAYIUR-2 variety can be attributed to its higher water uptake ratio and swelling capacity (Vashishth et al. 2017). Similar results were also reported by Ozarslan (2002) for cotton seeds, Sacilik et al. (2003) for hemp seeds and Aydin et al. (2002) for Turkish mahaleb.

Table 3.

Regression equation as a function of moisture content with their respective coefficient of determination for given gravimetric and geometric properties

Parameter	Variety	Regression equation	R2
Bulk density	GPM-6	847.56 − 3.8326Mc	0.9978
	PAYIUR-2	912.24 − 4.1881Mc	0.9992
	BHK	950.60 − 5.0291Mc	0.999
True density	GPM-6	1290.3 − 4.1922Mc	0.9873
	PAYIUR-2	1391.4 − 4.5312Mc	0.9946
	BHK	1488.3 − 6.0040Mc	0.9983
Porosity	GPM-6	34.200 + 0.0960Mc	0.9833
	PAYIUR-2	34.318 + 0.1005Mc	0.9744
	BHK	36.004 + 0.9460Mc	0.9731
Thousand kernels weight	GPM-6	25.660 + 0.7402Mc	0.9994
	PAYIUR-2	24.253 + 0.8382Mc	0.9979
	BHK	23.495 + 0.7269Mc	0.9887
Sphercity	GPM-6	61.632 + 0.1994Mc	0.9968
	PAYIUR-2	62.177 + 0.1897Mc	0.9601
	BHK	64.269 + 0.2948Mc	0.9481
Volume	GPM-6	21.249 + 0.8004Mc	0.9933
	PAYIUR-2	20.573 + 0.8122Mc	0.9965
	BHK	18.166 + 0.6439Mc	0.9984
Terminal velocity	GPM-6	7.8802 + 0.0465Mc	0.9829
	PAYIUR-2	8.1181 + 0.0484Mc	0.9917
	BHK	8.0103 + 0.0447Mc	0.9989
Angle of repose	GPM-6	23.061 + 0.2232Mc	0.9964
	PAYIUR-2	21.465 + 0.2800Mc	0.9998
	BHK	20.296 + 0.2406Mc	0.9997

Within the rows, mean ± standard error, mean values with the same lowercase letter superscripts are not significantly different (p ≤ 0.05)

Bulk and true density of grain are considered as one of the major factors for determining the storage parameters of the grain, which in turn will result in the economically feasible storage of grain. The value of the bulk density for the horse gram seeds at different moisture content got decreased from 810 to 734 kg m⁻³ for GPM-6, 901 to 801 kg m⁻³ for BHKand 871 to 787 kg m⁻³ for PAYIUR-2 with the change in moisture content from 10 to 30% db. An increase of 9.38, 11.09 and 9.61% in bulk density was observed for GPM-6, PAYIUR-2 and BHK variety respectively. The true density of horse gram seeds varied from 1250 to 1168 kg m⁻³ for GPM-6, 1426 to 1308 kg m⁻³ for BHK and 1348 to 1258 kg m⁻³ for PAYIUR-2 when moisture level of seeds were varied from 10 to 30% db. An increase of 6.56, 8.27 and 6.67% in thousand kernel weight was observed for GPM-6, PAYIUR-2 and BHK variety respectively. A negative correlation was found between the moisture contentbulk density and true density, which is expressed in regression equation (Table 3). Similar variation in bulk density and true density was also observed by Chowdhury et al. (2001) for gram, Karababa (2006) for popcorn kernels and Deshpande et al. (1993) for soybean.

The porosity of seeds was obtained from bulk density and true density of seeds. Porosity is dependent on the bulk and true density of grain, this dependency is due to the shape and size of the grain which in turn decides the behavior of grain during processing mainly during the handling of grains with the help of air i.e. drying, winnowing, aeration etc. Therefore it is important to determine the behavior of grain against the air which will define resistance of grain towards air flow and their movement. Porosity of seeds was found to increase from 35.20 to 37.16% for GPM-6, 36.82 to 38.76% for BHKand 35.38 to 37.44% for PAYIUR-2 with increase in moisture content from level of 10–30% db. The change in the values of porosity is well defined and supported by the change in bulk and true density. Positive correlation was found between moisture content and porosity, which expressed by regression equation (Table 3) The values obtained were found lower than the values reported for faba beans (Altuntas and Yildiz 2007), barbunia beans (Cetin 2007), millet (Baryeh 2002), peas (Yalcin et al. 2007) and chick pea Konak et al. (2002). Similar trends of change in porosity were also reported by Aydin et al. (2002) for Turkish Mahleb and for coffee (Chandrasekhar and Viswanathan 1999).

Geometric parameters

The results illustrating geometric analysis are presented in Table 3. Sphericity of the grains indicates the shape and outer structure of grains which define its resistance against air and surface in contact during it movement in processing equipment during processing like aeration and drying. The sphericity of horse gram at moisture content of 10% db was highest for BHK 66.67% followed by PAYIUR-2 and GPM-6 with sphericity of 63.80% and 63.56%. The higher sphericity of BHKwas well supported by its dimensions values. Sphericity of grains observed to increase from 63.56 to 67.53% for GPM-6, 66.67 to 72.66% for BHK and 63.8 to 67.59% for PAYIUR-2 with change in moisture content of seeds from 10 to 30% db. Similar trends of increase in sphericity with increase in moisture content were reported by Aydin et al. (2002) for almond nut kernel, (Ozarslan 2002) for cotton, (Sacilik et al. 2003) for hemp seeds and Cetin (2007) for corn seeds.

Volume of the horse gram seeds increased from 29.26 to 44.86 mm⁻³, 24.87 to 37.64 mm⁻³ and 28.47 to 44.57 mm⁻³for GPM-6, BHK and PAYIUR-2 respectively with increase in the moisture content from 10 to 30% db. The results obtained were found in conformitywith data reported by Baryeh (2002) for millet, Yalcin et al. (2007) for peas, Karababa (2006) for popcorn kernels and Amin et al. (2004) for lentil seeds.

Terminal velocity of grain is the air velocity required to keep the grain in its fluid motion, which is quite critical parameter during the processing of grain as, by keeping the grain in their fluid motion each grain, can be uniformly treated and the movement of grains can be achieved in an organized manner. At the moisture content of 10% db terminal velocity of horse gram seeds were 8.32, 8.45 and 8.57 ms⁻¹ for GPM-6, BHK and PAYIUR-2 respectively. The terminal velocity of horse gram grains found to increase linearly with increase in moisture content form 10 to 30% db. The increase in terminal velocity of grain was 8.32 to 9.23 ms⁻¹ for GPM-6, 8.45 to 9.34 ms⁻¹for BHKand 8.57 to 9.54 ms⁻¹ for PAYIUR-2. The higher terminal velocity of PAYIUR-2 variety can be attributed to the increase in thousand kernel weight during moisture treatment and that of BHK can be attributed to its higher sphericity which reduces the surface area of contact between grain and air hence higher air velocity is required to bring the grain in its fluidized motion (Vashishth et al. 2017). The linear increase interminal velocity of seeds was also reported by Cetin (2007), Gupta and Das (1997), Sacilik et al. (2003), Singh and Goswami (1996) and Suthar and Das (1996).

Angle of repose determines the behavior of grain during their storage in silos and in the construction of processing equipment like hoppers, silos, grain dispensers etc. The angle of repose for the horse gram seed was observed to be increase from 25.42° to 29.84° for GPM-6, 22.72° to 27.51° for BHK and 24.29° to 29.86° for PAYIUR-2with increase in the moisture content of seeds from 10 to 30% db. This increase in angle of repose may be due to increase in the adhesion force between the grains at higher moisture levels. Similar results regarding linear increase in angle of repose with change in moisture content has also been reported by Vilche et al. (2003) for quinoa seeds, Baryeh (2002) for millets and Karababa (2006) for popcorn kernels.

Static coefficient of friction

The experimental result for static coefficient of friction for horse gram seeds against three surface materials andat varying moisture level is shown in Table 4. The static coefficient of friction found to increase by 15.79% (Glass), 19.56% (SS) and 26.19% (Ply) for GPM-6, 16.67% (Glass), 17.78% (SS) and 24.39% (Ply) for BHK variety and 17.64% (Glass), 19.51% (SS) and 21.62% (Ply) for PAYIUR-2 variety against surface of glass, stainless steel and plywood respectively. Higher increase in coefficient of friction on ply was observed for GPM-6 which is followed by BHK whereas it was similar for GPM-6 and PAYIUR-2 for stainless steel. The increase in static coefficient of friction may be due to the increase in adhesion forces between seeds and material surface as well as between seeds itself with increase in the moisture content. The grain may have become rough on surface as a result of increase in moisture content therefore increasing the coefficient of friction (Baryeh 2002). The least change in static coefficient of friction was observed for glass which may be because of more smoother and polished surface when compared with other materials. Similar trend in results were also observed and reported by other investigators (Baryeh 2002; Gupta and Das 1998; Suthar and Das 1996; Amin et al. 2004; Karababa 2006; Altuntas and Yildiz 2007).

Table 4.

Regression coefficient and coefficient determination of Eq. for static coefficient of friction on various friction surfaces

Surface	Material	Regression equation	R2
GPM-6	Plywood	0.4312 + 0.0046Mc	0.9949
	Stainless Steel	0.3212 + 0.0046Mc	0.9949
	Glass	0.2537 + 0.0056Mc	0.9981
PAYIUR-2	Plywood	0.4012 + 0.0046Mc	0.9946
	Stainless Steel	0.3280 + 0.0041Mc	0.9975
	Glass	0.2603 + 0.0050Mc	0.9996
BHK	Plywood	0.3710 + 0.0044Mc	0.9702
	Stainless Steel	0.2880 + 0.0041Mc	0.9975
	Glass	0.2480 + 0.0041Mc	0.9975

Within the rows, mean ± standard error, mean values with the same lowercase letter superscripts are not significantly different (p ≤ 0.05)

Dehulling properties of grain

In all the respective varieties of horse gram, the dehulling ability increased significantly with subsequent increase in moisture content. For the given moisture content, coefficient of dehulling varied from 5.56 to 15.57% for GPM-6, 5.53 to 15.56% for PAYIUR-2 and 5.58 to 15.45% for BHK respectively. It was found to increase with increase in moisture content for all three varieties studied. The correlation of dehulling parameters with moisture content is depicted with the help of regression equation in Table 5. Among the horse gram varieties studied BHK has shown the highest dehulling ability. Hence from the above results it can be stated that variety with the higher hull content and low seed size results in higher dehulling ability. Figueiredo et al. (2011) also observed and reported the similar results during in his research on sunflower seeds. The seeds size of the grains was found to be highest for the GPM-6 followed by PAYIUR-2 and BHK. The dehulling ability, degree of dehulling and coefficient of dehulling has shown the similar trends as they got increased with increase in moisture content of 10–30%. Whereas, yield of brokens for all studied varieties has shown the reverse trend as it decreased with respective increase in moisture content. Maximum positive effect of increase in moisture content on coefficient of dehulling and degree of dehulling was observed for GPM-6 and minimum was observed for PAIYUR-2 and subsequent reduction in brokens yield was observed to be highest for GPM-6 and minimum for PAIYUR-2 variety. Figueiredo et al. (2011) reported similar results for the effect of moisture content on yield of brokens and fine. Tranchino et al. (1984), also stated in their results that yield of broken or fines increases with decrease in moisture content. Hence the above result explains the fact that why it is important to study and measure the various characteristics of horse gram and its varieties before subjecting them to the dehulling process. As the mixture of different varieties can actually makes it difficult to calibrate the dehulling equipment and adjusting the critical setting of instrument to achieve the maximum efficiency and the most economical process as well. The hull percentage of the grain obtained during dehulling process also varied significantly during respective moisture treatments. The hull percentage of the grain found to be in range of 8.68–16.92, 9.87–18.77 and 8.12–15.57% for GPM-6, PAYIUR-2 and BHK varieties respectively. The increase in the hull percentage of the grain may be attributed to the increase in moisture content as it may help in loosening of the hull from the surface of grain, which in turn will increase the dehulling of grain and greater hull quantity. The increase in hull quantity is well supported by the increase in dehulling ability in the current research.

Table 5.

Regression equation as a function of moisture content with their respective coefficient of determination for dehulling properties

Parameter	Variety	Regression equation	R2
Degree of hulling	GPM-6	0.3092 + 0.5285Mc	0.9922
	PAYIUR-2	0.3708 + 0.5157Mc	0.9925
	BHK	0.3824 + 0.5101Mc	0.9911
Yield of brokens	GPM-6	2.3226 − 0.0176Mc	0.9348
	PAYIUR-2	2.0970 − 0.0083Mc	0.9450
	BHK	2.0564 − 0.0073Mc	0.9906
Coefficient of dehulling	GPM-6	64.030 + 0.2133Mc	0.9930
	PAYIUR-2	64.830 + 0.1642Mc	0.9110
	BHK	64.650 + 0.1778Mc	0.9732
Over all dehulling	GPM-6	50.660 + 0.3759Mc	0.9089
	PAYIUR-2	53.020 + 0.2780Mc	0.9784
	BHK	53.110 + 0.2986Mc	0.9732

Within the rows, mean ± standard error, mean values with the same lowercase letter superscripts are not significantly different (p ≤ 0.05)

Textural properties

Textural properties of the grains subjected to the respective moisture content are shown in Table 6. Textural quality of grains was found to be significantly affected by the moisture content. Change in moisture content affected all the major textural parameters like hardness, Springiness, cohesiveness etc. Hardness, which is maximum peak force achieved on the first compression cycle (first bite), found to vary significantly from 322.5 to 360.47 kg at 10% moisture content and 179.51 to 209.23 kg at 30% moisture content among the studied varieties. BHK showed the highest hardness of 360.47 kg and the lowest 322.50 kg was observed in GPM-6 at 10% moisture content. Hardness of all the varieties decreased significantly as the moisture content varied from 10 to 30%. The high protein content of varieties can be the reason for the higher hardness of horse gram cultivar as Juliano and Perez (1983) reported in their studies that rice cultivars with higher protein content requires more force for rupture of grain, due to the adhesiveness between the protein and carbohydrate matrix present in grain. The other textural parameters such as cohesiveness (ratio of the positive force areas under the first and second compressions), gumminess (product of hardness and cohesiveness) and chewiness (product of gumminess and springiness) also showed significant changes during the variation in moisture content. Gumminess and chewinesswere in the range of 19.24–79.37 and 04.87–22.69 respectively for the studied varieties.

Table 6.

Texture properties of horse gram varieties at respective moisture content

Variety	Moisture content (%)	Hardness (kg)	Springiness	Gumminess	Chewiness
GPM-6	10.08	322.50d	0.289d	79.37cd	22.69a
	15.71	286.32c	0.279cd	75.25cd	21.84a
	22.06	267.32c	0.268abcd	68.57c	18.93a
	29.91	179.51ab	0.256abcd	61.63bc	15.87a
PAYIUR-2	10.18	349.87de	0.296e	63.68bc	20.59a
	15.88	323.71d	0.289d	57.51abc	18.33a
	22.31	296.13c	0.281cd	40.26ab	14.51a
	29.79	201.23ab	0.273abcd	33.34a	13.21a
BHK	10.12	360.47e	0.246abc	50.24d	15.18a
	15.61	329.91d	0.242abc	37.65cd	11.77a
	22.38	271.13b	0.236ab	28.97cd	07.35a
	29.98	209.23a	0.232a	19.24a	04.87a

Within the rows, mean ± standard error, mean values with the same lowercase letter superscripts are not significantly different (p ≤ 0.05)

Conclusion

It is evident and necessary to establish the physical, mechanical and aerodynamical properties of major as well as minor grains. This study has shown that physical properties of horse gram do change significantly with increase in moisture content. It was also observed that the dimensions, 1000 kernel mass, angle of repose, porosity and terminal velocity of horse gram varieties got increased linearly whereas the values for the bulk and true density got decreased gradually and linearly with change in moisture content from 10 to 30%db. This information can play an important role in selecting the processing equipment, not only this they are considered as critical parameters for designing these machines. The dehulling parameters like dehulling ability, overall dehulling and yield of broken was also observed to change during increase in moisture content which dictates the importance of analyzing and bringing the grain to desired moisture content before processing as the inappropriate moisture content can affect the dehulling yield substantially. The lack of other published research work and data regarding physical, mechanical, aerodynamical and dehulling properties of horse gram varieties emphasis on need for more research in the field of agricultural produced which can help in further enhancing the knowledge in order to design and develop processing equipment that can efficiently and effectively be used in handling and processing of horse gram with minimum lose of produced.

List of symbols

GPM-6, BHK, PAYIUR-2

Horse gram varieties

D_g

Geometric mean diameter

D_a

Arithmetic mean diameter

L

Length

W

Weight

T

Thickness

V

Volume

Φ

Sphericity

ρ_t

True density

ρ_b

Bulk density

A_r

Angle of repose

H_c

Height of cone

D_c

Diameter of cone

μ

Coefficient of friction

μ_a

Dynamic viscosity of air

α

Angle of inclination

V_t

Theoretical terminal velocity of air

V_e

Experimental terminal velocity of air

Z

Shape factor

m_t

Mass of 1000 seeds in kg

g

Gravitational force

A_i

Projected area of seed in air

N_Re

Reynold’s number

Abbreviations

ANOVA

Analysis of variance

SPSS

Statistical Package for Social Science

Ply

Plywood

SS

Stainless steel

GM

Galvanized metalsheet