Moisture dependent physical properties of nigella seeds

A study on the nigella seeds (Ajmer Nigella-1) was performed to investigate the effect of seeds moisture content on their physical properties as these are very important to design post harvest equipments. The physical properties of the nigella were evaluated as a function of moisture contents in the range of 5.2 to 25.1% dry basis (d.b.). Seed geometric parameters such as average length, width, thickness, geometric mean diameter, volume, increased with the increase in seed moisture, except seed sphericity and surface area, which remain unchanged as statistically non-significant. The 1000-seed mass increased linearly with increase in moisture. Bulk density and true density of nigella decreased when seed moisture content was raised from 5.2 to 25.1% d.b. The porosity of nigella increased up to 19.9% moisture content and then decreased slightly. The angle of repose and coefficients of static friction on four different surfaces (plywood, mild steel, galvanized iron and glass) and terminal velocity increased with seed moisture.


INTRODUCTION
Nigella (Nigella sativa L.) is an annual herb belonging to the family Ranunculaceae.The plants are erect growing to a height ranging from 30 to 60 cm.Stems are branched and leaves are alternate, 2.5 to 3 cm long, blade is pinnately dissected into thin sub linear lobs usually described as feathery.Flowers are pale blue, which turned to white in later stage.The fruit is a capsule that turns into yellow brown at maturity.Seeds are black trigonous, regulose and tubercular.The seed of nigella is used to add taste and flavour in meat, vegetable dishes, pickles and other edibles.Seeds are sprinkled on naan breads before baking.The seeds contain 0.5 to 1.4% essential oil which has demand in the pharmaceutical and perfume industry.Nigella seeds are also known for its health promoting benefits and used as a medicinal grain (in traditional medicines) and as a food ingredient in several countries, including Egypt, Syria, Jordan and Iran (D'Antuono et al., 2002).The seed or its paste mixed with honey is commonly used as flavoring agent in bread and cheese (Merfort et al., 1997).The nigella oil has been reported to exhibit anti-tumor (Worthen et al., 1998), antioxidant (Burits and Bucar, 2000), anti-inflammatory (Houghton et al., 1995), and antibacterial (Morsi, 2000) activities.It has stimulatory effect on the immune system (Salem and Hossain, 2000).
For efficient processing operation, it is essential to have the knowledge of moisture dependent physical properties such as spatial dimensions, bulk density, true density, and porosity of nigella for better design of storage structures, processing equipments, and processes.The frictional properties and aerodynamic properties of food materials are important for the design of efficient oil extraction, dehulling and hull separation machines.
Limited published literature is available on the physical properties of nigella seed as a function of moisture content.The present study was, therefore, aimed to determine moisture dependent physical properties such as spatial dimensions, geometric mean diameter, sphericity, surface area, volume, 1000-seed mass, bulk density, true density, porosity, angle of repose, static coefficient of friction and terminal velocity of nigella seeds (Ajmer Nigella-1) between 5.1 and 25.2% (d.b.) moisture range, which should be helpful in designing handling, processing and packaging equipments for nigella.

MATERIALS AND METHODS
The nigella seeds of variety Ajmer Nigella-1 were arranged from National Research Centre on Seed Spices, Ajamer (Rajasthan), India.Seeds were cleaned manually to remove all impurities such as dust, chaffs, stones, insects and damaged or unhealthy seeds.Initial moisture content of the seeds was determined using standard hot air oven drying method at 105±1°C for 24 h (AOAC, 1980).Test samples of the desired moisture contents were prepared by adding measured amount of distilled water to achieve the required moisture contents followed by thorough mixing and sealing in LDPE bags.The conditioned samples were kept at 5°C in a refrigerator for 7 days to allow uniform distribution of moisture throughout the sample.Desired quantity of seeds was taken out from the bags and held at room temperature (22 to 25°C) for 2 h before conducting the test (Carman, 1996;Deshpande et al., 1993;Singh and Goswami, 1996;Cetin, 2007).The physical properties were determined at five Singh et al. 59 moisture levels at 5.1,10.2,15.0,19.9,and 25.2% (d.b.).All the experiments were replicated five times (except measurement of dimensions for which 100 seeds were taken randomly at each moisture content) and the average values used in the analysis.
To determine the average size of nigella, 100 seeds were randomly selected and length (L), width (W), and thickness (T) of the seeds were measured using a digital micrometer ((least count 0.01 mm; Mitutoyo Corporation, Japan).The arithmetic mean diameter (D a ), geometric mean diameter (D g ), sphericity (φ) and volume (V) were calculated by using the following relationships (Mohsenin, 1986;Jain and Bal, 1997): Where, B = (WT) 0.5 (5) The surface area (A s ) was determined by analogy with a sphere of same geometric mean diameter using the following relationship (Mohsenin, 1986): To determine the mass of 1000 seeds (M t ), about 250 seeds were taken randomly and weighed (M) on an electronic balance (least count 0.001 g).Then the number of seeds (n) in the sample was counted (Deshpande et al., 1993).The mass of 1000 seeds was calculated as; Bulk density ( b ) was determined following the procedure reported by Singh and Goswami (1996) by filling a 500 ml cylinder with the seeds from a height of 150 mm at a constant rate and then weighing the contents.The seeds were not compacted during the test.
True density ( t ) was determined using the toluene displacement method.Toluene was used in place of water because the seed absorbed toluene to a lesser extent than that of water and because of its low surface tension shallow dips in seeds were filled and with low dissolution power (Mohsenin, 1986).The volume of toluene displaced was found by immersing a weighed quantity of nigella in the toluene (Mohsenin, 1986;Singh and Goswami, 1996).
The bed porosity (ε) of the bulk is the ratio of spaces in the bulk to its bulk volume.The ε was calculated using the following equation (Mohsenin, 1986).

100, ρ
To determine angle of repose (θ), a plywood box of 10×10×10 cm size with a removable front panel was used.The box was filled with seeds and the front panel was quickly removed allowing the seeds to flow and assume a natural slope (Joshi et al., 1993;Paksoy and Aydin, 2004).The diameter (D) and height (H) of the slope were recorded.The angle of repose (θ) was calculated by using the following equation.
The static coefficient of friction (µ), a dimensionless quantity required for calculating the friction force, was determined on four different surfaces; plywood, mild steel, galvanized iron and glass.These materials are commonly used for handling and processing of nigella and construction of storage and drying bins.For determination of µ, a wooden box of 100-mm length, 100-mm width and 40-mm height without base and lid was filled with the sample and placed on an adjustable tilting plate, faced with the test surface.The sample container was raised slightly (0.5 to 1.0 mm) so as not to touch the surface.The inclination of the test surface was increased gradually with a screw device until the box just started to slide down and the angle of tilt (α) was read from a graduated scale.The μ was taken as the tangent of this angle (Dutta et al., 1988;Joshi et al., 1993;Singh and Goswami, 1996).
Terminal velocity was measured using a cylindrical column in which the material was suspended in the air stream (Nimkar and Chattopadhyay, 2001;Vishwakarma et al., 2010).The minimum air velocity, which held the seeds under suspension, was recorded using a digital anemometer (±0.1 m/s) (Joshi et al., 1993).
The data analysis of this study was carried out by using the Statistca 6 software.The differences between the mean values of physical characteristics of nigella samples were tested for significance using t-test.The relationship between moisture content and physical properties of nigella seeds was determined using linear regression analysis.

Geometrical parameters
Dimensional characteristics, surface area and volume of nigella seeds at selected moisture contents are reported in Table 1.The L, W, T, D a , D g , and V values increased significantly (p<0.05) with moisture content.The geometric mean diameter of the seed was found more than that of its width and thickness.The relationship between L, W, T and D g and moisture content (m) can be represented by the following equations.The sphericity and surface area of the nigella remained unchanged with increase in moisture content (Table 1).

Seed mass (for 1000 seed)
Change in 1000-seed mass of nigella seeds with moisture content is shown in Figure 1.The 1000-seed mass increased linearly from 2.39 to 2.81 g (17.60% increase) with increase in moisture content from 5.1 to 25.2%.The relationship between 1000-seed mass and moisture content can be expressed by the following relationship.

Bulk density
Bulk density of nigella seeds decreased from 552.50 to 482.29 kg/m 3 (12.71%decrease) with increase in moisture content from 5.1 to 25.2% (Figure 2).Variation of bulk density with moisture content can be expressed as: Thousand seed mass, g Moisture content, % d.b.

True density
True density of nigella decreased from 1113.43 to 1054.28 kg/m 3 (5.31%decrease) with increase in moisture content (Figure 3).The variation of true density mass with moisture content can be expressed as: The true density of the nigella was higher than that of bulk density at all moisture contents.

Bed porosity
Bed porosity of nigella increased from 50.37 to 54.78% at 19.9% moisture content and then decreased slightly to 54.25% at 25.2% moisture content (d.b.) as shown in Figure 4.The variations in ε with moisture content was significant (p<0.05).The variation of porosity with moisture content can be expressed as:

Angle of repose
Angle of repose of nigella seeds increased linearly from 24.4 to 33.23º with moisture content (Figure 5).Variation of angle of repose with moisture content can be expressed as:

Coefficient of static friction
Variation of static coefficient of friction for nigella seeds on four surfaces (plywood, mild steel, galvanized iron and glass) with moisture content are presented in Figure 6.The static coefficient of friction increased significantly (p<0.05) with moisture content for all the surfaces.This was due to the increased adhesion between the seeds and the material surfaces at higher moisture values.The static coefficient of friction ranged from 0.50 to 0.65, 0.59 to 0.78, 0.27 to 0.70 and 0.26 to 0.77, respectively for plywood, mild steel, galvanized iron and glass surfaces, respectively in the experimental moisture content range.Variation of μ with moisture content of nigella can be expressed mathematically as follows: μ ms = 0.538 + 0.009 m (R 2 = 0.99) (21) μ gi = 0.148 + 0.022 m (R 2 = 0.96) ( 22) Where μ pb , μ ms , μ gi , and μ g are static coefficient of friction of nigella seeds against plywood, mild steel, galvanized iron and glass surfaces, respectively.The coefficient of friction at all moisture contents was highest on mild steel.The μ increased drastically with increase in moisture content beyond 15%, except for plywood.It showed that the material would tend to stick in hoppers at higher moisture contents and tendency of sticking to the surface might be observed.
The μ of the nigella seeds increased with increase in moisture content for all the surfaces under study.The seeds may become rougher and sliding characteristics are diminished at higher moisture contents so that the static coefficient of friction is increased.

DISCUSSION
The L, W, T, D a , D g , and V values increased significantly (p<0.05) with moisture content.The linear increase in spatial dimension was probably due to expansion resulted from moisture uptake by grain in the intercellular space in the seeds.This indicated that drying of nigella at higher moistures should result in shrinkage due to decrease in seed dimensions.Increase in seeds dimensions for black cumin, ajwain, soybeans and pigeon pea have been reported by Gharib-Zahedi et al. (2010), Zewdu (2011), Deshpande et al. (1993) and Baryeh and Mangope (2002), respectively.The geometric mean diameter of the seed was found more than that of its width and thickness.The sphericity and surface area of the nigella remained unchanged with increase in moisture content (Table 1).Zewdu (2011), Deshpande et al. (1993) and Sobukola and Onwuka (2010) reported increase in sphericity of ajwain, soybean and locust bean seeds, respectively.The 1000-seed mass increased linearly from 2.39 to 2.81 g (17.60% increase) with increase in moisture content from 5.1 to 25.2%.Similar results have been reported for barbunia beans (Cetin, 2007), black cumin (Gharib-Zahedi et al., 2010), locust bean seed (Sobukola and Onwuka, 2010), ajwain (Zewdu, 2011) and guar seeds (Vishwakarma et al., 2010).
Bulk density of nigella seeds decreased from 552.50 to 482.29 kg/m 3 (12.71%decrease) with increase in moisture content from 5.1 to 25.2%.This decrease was due to the higher rate of increase in volume relative to the increase in weight.Similar relationships have been reported for chickpea (Konak et al., 2002), locust bean seed (Sobukola and Onwuka, 2010) and black cumin (Gharib-Zahedi et al., 2010).However, increase in bulk density with moisture content was reported for cashew nut (Balasubramanian, 2001).Zewdu (2011) reported non-significant decrease in bulk density of ajwain seeds.
True density of nigella decreased from 1113.43 to 1054.28 kg/m 3 (5.31%decrease) with increase in moisture content.The decrease in true density with increase in moisture content was mainly due to the significant increase in volume, which was higher than the corresponding increase in the mass of the material.The behaviour of true density with moisture content is contradictory as reported in the literature.Increase in true density with moisture content has been reported by Singh and Goswami (1996), Altuntas and Yildiz (2007) and (Gharib-Zahedi et al., 2010) for cumin, faba bean, and black cumin, respectively.These seeds have lower volume change in comparison to change in weight with moisture content.However, Tunde-Akintunde and Akintunde (2007), Cetin (2007) and Zewdu (2011) have reported that the true density of beni seeds, barbunia seeds and ajwain decreased with increased moisture content.The true density of the nigella was higher than that of bulk density at all moisture contents.
Bed porosity of nigella increased from 50.37 to 54.78% at 19.9% moisture content and then decreased slightly to 54.25% at 25.2% moisture content (d.b.).The variations in ε with moisture content was significant (p<0.05).Increase in porosity with moisture content was reported by Singh and Goswami (1996), Altuntas and Yildiz (2007) and (Gharib-Zahedi et al., 2010) for cumin, faba bean and black cumin seeds, respectively.However, Zewdu (2011), Tunde-Akintunde and Akintunde (2007), Joshi et al. (1993), and Shepherd and Bhardwaj (1986) have reported decrease in porosity of ajwain, beniseeds, pumpkin and pigeon pea seeds, respectively with increased moisture content.Higher porosity values provide better aeration and water vapor diffusion during deep bed drying and the data may be utilized for design of aeration system.
Angle of repose of nigella seeds increased linearly from 24.4 to 33.23º with moisture content.Similar behavior has been observed for cumin, black cumin, and guar seeds (Singh and Goswami, 1996;Gharib-Zahedi et al., 2010;Vishwakarma et al., 2010).At higher moisture content, seeds tend to stick together, causing less flowability and angle of repose is increased.The data may be useful for design of hoppers, and storage bins for the nigella.
The static coefficient of friction increased significantly (p<0.05) with moisture content for all the surfaces.This was due to the increased adhesion between the seeds and the material surfaces at higher moisture values.The static coefficient of friction ranged from 0.50 to 0.65, 0.59 to 0.78, 0.27 to 0.70 and 0.26 to 0.77, respectively for plywood, mild steel, galvanized iron and glass surfaces, respectively in the experimental moisture content range.
The coefficient of friction at all moisture contents was highest on mild steel.The μ increased drastically with increase in moisture content beyond 15%, except for plywood.It showed that the material would tend to stick in hoppers at higher moisture contents and tendency of sticking to the surface might be observed.The order of decrease in coefficient of friction reported for cumin seeds (Singh and Goswami, 1996), karingda seeds (Suthar and Das, 1996) and locust bean seed (Sobukola and Onwuka, 2010) was plywood followed by mild steel and galvanized iron.However, Amin et al. (2004) have reported that no variation existed between plywood and galvanized iron for lentil seeds.The μ of the nigella seeds increased with increase in moisture content for all the surfaces under study.The seeds may become rougher and sliding characteristics are diminished at higher moisture contents so that the static coefficient of friction is increased.
Terminal velocity (V t ) of nigella seeds exhibited significant increase (p<0.05) from 2.49 to 3.33 m/s as the moisture content increased from about 5.2 to 25.1% (d.b.).Singh and Goswami (1996), Baryeh and Mangope (2002), Isik andUnal (2007), andGharib-Zahedi et al. (2010) have reported a linear increase in terminal velocity with moisture content for cumin, pigeon pea, white speckled kidney beans and black cumin, respectively.The increase in terminal velocity with increase in moisture content within the study range can be attributed to the increase in mass of the individual seed per unit frontal area presented to the airflow.

Conclusions
The physical properties of nigella seeds are function of moisture content.The following conclusions are drawn from this investigation on physical properties of nigella seeds for the moisture content range of 5.2 to 25.1% (d.b.): (1) The length, width, thickness, geometric mean diameter and volume of seed increased with moisture content whereas sphericity and surface area remained unchanged.
(2) The thousand seed mass increased from 2.39 to 2.81 g with the increase in moisture content from 5.1 to 18.75% w.b.
(3) The bed porosity increased from 50.37 to 54.78 at 19.9% moisture content and then decreased slightly to 54.25 at 25.2% moisture content (d.b.).(4) The bulk density decreased linearly from 552.50 to 482.29 kg/m 3 whereas the true density decreased from 1113.43 to 1054.28 kg/m 3 with increase in moisture content.
(5) The terminal velocity increased from 2.49 to 3.33 m/s and angle of repose increased from 24.4 to 33.23° in the moisture range from 5.2 to 25.1% (d.b.).( 6) The static coefficient of friction increased on four structural surfaces namely, galvanized iron sheet (0.27 to 0.70), mild steel (0.59 to 0.78), glass surface (0.26 to 0.77) and plywood (0.50 to 0.65) in the moisture range

Figure 3 .Figure 4 .
Figure 3.Effect of moisture content on true density of nigella seeds (R 2 =0.98, bars show standard deviation from mean).

Figure 5 .Figure 6 .
Figure 5.Effect of moisture content on angle of repose of nigella seeds (R 2 =0.99, bars show standard deviation from mean).

Figure 7 .
Figure 7. Effect of moisture content on terminal velocity of nigella seeds (bars show standard deviation from mean).
d *:Figures in a row followed with different superscripts are significant (p<0.05).