properties of pearl millet

J . agric . Engng Res . (1997) 66 , 85 – 91

Properties of Pearl Millet

R . K . Jain ; S . Bal

Post Harvest Technology Centre , Indian Institute of Technology , Kharagpur 721 302 , India

( Recei y ed 1 8 October 1 9 9 4 ; accepted in re y ised form 9 September 1 9 9 6 )

Three varieties of pearl millet seed , consisting of two hybrids GHB 30 and Bajra 28 – 15 , and one Babapuri (traditional) variety , were graded , dried to 7 ? 4% moisture content dry basis and properties of the major fraction were determined . The average three principal dimensions were 3 ? 12 , 1 ? 94 and 1 ? 70 mm for GHB 30 , 2 ? 98 , 1 ? 86 and 1 ? 82 mm for Bajra 28 – 15 and 3 ? 36 , 2 ? 24 and 2 ? 01 for Babapuri varieties . The surface area and volume of single grain were 12 ? 5 mm 2 and 3 ? 8 mm 3 for hybrid varieties and 16 ? 4 mm 2 and 5 ? 8 mm 3 for the traditional variety . Sphericity of the grain for all the varieties was 0 ? 94 and the bulk density and the grain density were 850 and 1600 kg / m 3 respectively . The shape factor of the grain was 1 ? 07 for GHB 30 , 1 ? 01 for Bajra 28 – 15 and 1 ? 06 for the Babapuri varieties . The porosity of the bulk varied between 45 and 49% . The static coef ficient of friction was approximately 0 ? 25 on galvanized steel sheet and 0 ? 26 on mild steel sheet . The angle of repose was about 23 8 to 25 8 . Hybrid seed contained about 4 ? 5% oil and the traditional Babapuri variety 7 ? 3% .

÷ 1997 Silsoe Research Institute

1 . Introduction

Pearl millet ( Pennisetum typhoides ) , a hardy cereal crop compared with wheat and rice , is grown in regions with relatively low rainfall owing to its ability to tolerate and survive under continuous or intermit- tent drought . The principal homes of this crop are Ethiopia , Malawi , Sudan , Zimbabwe , Kenya , Tanz- ania , Uganda , Zambia , Somalia , Botswana , India and Pakistan . It is a short duration crop (88 to 96 d) and suits many crop rotation programmes . In India , it is cultivated over 13 ? 64 Mha , representing 30% of the world acreage of the crop and this area produces over 7 Mt of grain per year . 1 The yield of the traditional

varieties of pearl millet in India vary from 300 – 500 kg / ha and that of the hybrid varieties 2 from 1300 – 2400 kg / ha .

Notation

D 1 largest principal dimension , mm D 2 second largest principal dimension , mm D 3 smallest principal dimension , mm D diameter of the spherical part of the grain ,

5 ( D 2 D 3 ) 1 / 2 , mm

D s surface mean diameter , mm M i mass fraction retained on i th sieve D i geometric average opening of i th and

( i 1 1)th sieve in a sieve set , mm M mass of 1000 seeds , g S 1 surface area of spherical part of the grain ,

mm 2

S 2 surface area of conical part of the grain , mm 2

D e diameter of the equivalent sphere , mm S e surface area of the equivalent sphere , mm 2

V 1 volume of the spherical part of the grain , mm 3

V 2 volume of the conical part of the grain , mm 3

f sphericity y t volume of a single grain , mm 3

S surface area of a single grain , mm 2

a volume based shape factor b surface area based shape factor r radius of the base of the cone , mm h height of the cut segment of the sphere , mm l shape factor

r b bulk density , kg / m 3

r t grain density , kg / m 3

e porosity , %

Pearl millet grain contains 11 ? 6% protein and 2 ? 3% minerals and these are higher than the corresponding contents of rice , maize and sorghum . It also contains

85 0021-8634 / 97 / 020085 1 07 $25 . 00 / 0 / ag960119 ÷ 1997 Silsoe Research Institute

R . K . J A I N ; S . B A L 86

1 ? 2% fibre , which is less than wheat , maize and sorghum . It has a greater oil content (4 – 9%) than all other cereal crops (Desikachar 3 ) . Storability of the pearl millet is poor owing to its high oil content . Whole grains of the pearl millet can safely be stored at low moisture content (7 – 9% , d . b . ) but once the grains are crushed , the grits or flour have very poor storability (Kaced et al . 4 ) .

Pearl millet , though nutritive , is considered as inferior stable food and consumed only by economi- cally weaker sections of the community . The crop is processed by the age old traditional methods (Jain and Bal . 5 ) . For its significant contribution to cereal production in the drought-prone region of the world and for its inherent disadvantages in post production , conservation and consumption , this crop requires special attention . This study was undertaken to deter- mine and analyse some of the important properties of single grain and grains in bulk . Oil content of the pearl millet was determined . These characteristics are often required for designing cleaning , grading , pearl- ing (debraning) and handling machinery for pearl millet .

2 . Materials and methods

Pearl millet , variety GHB 30 was obtained from Gujarat Agricultural University , Anand Campus , An- and , Gujarat state ; variety Bajra 28 – 15 from Panjab- rao Krishi Vidhyapeeth , Akola , Maharastra and Babapuri variety from farmers’ fields in Kharagpur , West Bengal state during the rainy seasons of 1990 – 91 and 1991 – 92 . Experiments reported in this study were conducted during two successive years except for the traditional variety , which was carried out during 1990 – 91 only . The grains were sun dried to a safe storage moisture content of 7 . 4 Ú 0 ? 2% (d . b . ) . Samples were cleaned with a laboratory cleaner to remove foreign matter and manually to remove ergot af fected seeds . Grains were stored in a metal bin for the study period of 4 months . During the storage period temperature varied between 15 and 29 8 C and relative humidity varied between 35 and 70% .

The grains were classified into dif ferent grades on the basis of their physical dimensions by sieving for 10 min in a Rotap sieve shaker using Taylor sieves (2 ? 4 , 2 ? 0 , 1 ? 7 , 1 ? 4 , 1 ? 2 mm and pan) having successive openings 2

1 – 4 times less than the previous sieve openings . Mass retained over dif ferent sieves was collected and weighed with a single pan electronic balance (least count 0 ? 001 g) . The per cent mass retained on each sieve was calculated using four replications . The mean size of the grain was calculated , using the

definition of surface mean diameter ( D s ) given by Coulson and Richardson 6

D s 5 o 5

i 5 1 M i

o 5 i 5 1 ( M i / D i )

(1)

where M i is the mass fraction retained on the i th sieve and D i is the geometric average opening of i th and ( i 1 1)th sieve . D s is also known as the Sauter mean diameter and is the diameter of the particle with the same specific surface as powder .

One thousand grains of the main fraction were counted and weighed . The mass is termed the seed mass of the main fraction .

2 . 1 . Grain shape and dimensions

The shape of the pearl millet grain was assumed to be cono-spherical (like a liquid drop) . The grain is assumed to consist of a base sphere (with one flat surface) and a round base cone on the cut portion of the sphere as shown in Fig . 1 .

The three principal dimensions of 100 randomly selected grains (Dutta et al . 7 ) of the principal grade , were measured with the help of a dial gauge (least count 0 ? 01 mm) . The size of the grain is defined as the geometric mean of the second largest and the smallest principal dimensions of the grain ( D 5 D 2 D 3 )

1 / 2 . These data are useful in designing the grader for pearl millet .

The principal dimensions were used to calculate the volume ( V t ) and surface area ( S ) of a single grain (Jain Ω ) with the help of the following equations . Their derivation is given in Appendix 1 and dimensions are shown in Fig . 1 .

V t 5 π D 2 D 2

1

6(2 D 1 2 D ) (2)

S 5 π DD 2

1

(2 D 1 2 D ) (3)

The volume of the pre-counted graded seeds was also found by means of an air comparison pycnometer (Beckman Model-930) . Six replications were taken and compared with the volume calculated using Eqn 2 for verifying the developed equations .

The ratio of mass of the sample to the actual grain volume is termed the grain density of the sample . It was determined from the ratio of seed mass of a sample to the actual volume measured for 1000 grains of the same grade size sample by the air comparison pycnometer .

P R O P E R T I E S O F P E A R L M I L L E T 87

y

r

O

D1

D1 – D

h

B

E

Ay

P

x x D

D3

D2

C

Fig . 1 . Cono - spherical shape of the pearl millet grain

The sphericity of the grain is an index of its roundness . For non-spherical particles , this is calculated as the ratio of the surface area of the equivalent sphere to the surface area of the grain (McCabe and Smith 9 ) .

f 5 F D (2 D 1 2 D ) D 2

1 G 1/3

(4)

The volume and surface area obtained from Eqns 2 and 3 were compared with the regular shape (sphere) considering the second largest dimension of the grain as characteristic diameter . Shape factor based on volume and surface area of the grain and overall shape factor were determined (McCabe and Smith 9 ) from

a 5 V t / D 3 2

b 5 S / 6 D 2 2

l 5 b / a (5)

Bulk density of the grain was measured by means of a hectometer at storage moisture content of 7 ? 4% d . b . The vessel was filled with clean grains and gently tapped five times to cause the grain to settle . After initial settling the hectometer vessel was further filled with grains and again tapped twice . A sharp edge flat

was used to remove excess grain to level the surface at the top of the vessel .

Porosity is the percentage of volume of voids in the test sample at a given moisture content . It was calculated as ratio of the dif ference in the grain and bulk densities to grain density and expressed as a percentage .

Porosity , » , % 5 ( r t 2 r b )100

r t (6)

The static coef ficient of friction for the pearl millet grains was determined on galvanized sheet and mild steel sheet . A topless and bottomless circular cylinder 100 mm in diameter , 30 mm high made of galvanized sheet (22 gauge) was placed on an adjustable tilting plate with the sample . The surface with the cylinder resting on it was inclined gradually with a screw device until the cylinder began to slide . The angle of tilt was read from the graduated scale and the tangent of this angle is the static angle of friction on that surface . 7 , 1 0 , 1 1

The dynamic angle of repose was found using a plywood box 300 3 300 3 300 mm with a removable front panel . The box was filled with the sample and the front panel was quickly removed , allowing the

R . K . J A I N ; S . B A L 88

grains to flow and make a natural heap . 7 , 1 0 , 1 2 , 1 3 The angle of repose was calculated from the measurements of the depths of free surfaces of the grains at two known horizontal distances from one side of the box .

Pearl millet seeds were ground in a Willey mill to pass through a 0 ? 85 mm screen opening . Oil from the ground samples (30 g each) was extracted using Soxhlet extraction with n -hexane as solvent (boiling temperature 58 8 C) and refluxed for 6 h . The oil was collected after removal of hexane by distillation and the per cent oil content was found 1 4 on the basis of seed mass .

3 . Results and discussion

3 . 1 . Grading

Cumulative mass retained on each sieve of three varieties of pearl millet is shown in Fig . 2 . It shows that more than 70% of both hybrid varieties of pearl millet were retained on a 1 ? 7 mm opening screen , whereas , 65% of Babapuri variety was retained on a 2 ? 00 mm screen .

Surface mean diameters ( D s , Eqn 1) for the three varieties were found to be 1 ? 72 , 1 ? 74 and 2 ? 08 mm respectively for GHB 30 , Bajra 28 – 15 and Babapuri variety .

1·0

0·8

0·6

0·4

0·2

01·2 1·4 1·6 1·8 2·0 2·2 2·4

Screen opening, mm

Cum

ulat

ive

mas

s fr

actio

n re

tain

ed

Fig . 2 . Sie y e analysis of pearl millet . 3 GHB 3 0 ; s Bajra 2 8 – 1 5 ; n Babapuri

3 . 2 . Seed dimensions

The three principal dimensions of specific grades of pearl millet are given in Table 1 . It shows that the second largest and smallest principal dimensions of the grain are close to each other , whereas the grain is elongated towards the largest principal dimension . The grain formed a cono-spherical shape . In that shape , the second largest and the smallest principal dimensions form a spherical base .

3 . 3 . Volume , surface area and sphericity

The volume of a single grain calculated by Eqn 2 was 3 ? 795 , 3 ? 832 and 5 ? 794 mm 3 for GHB 30 , Bajra 28 – 15 and Babapuri varieties respectively . The volume of 1000 grains was also found by the air comparison pycnometer and the values are compared in Table 1 . The dif ference in both these data were not significant at 5% probability level ( t calculated 5 1 ? 498) using the standard t-test . The good comparison of the calculated and measured volumes confirms the as- sumption of the cono-spherical shape of the grain .

The surface area of a single grain was calculated by Eqn 3 to be 12 ? 55 , 12 ? 49 and 16 ? 38 mm 2 for GHB 30 , Bajra 28 – 15 and Babapuri varieties respectively . The sphericity of all three varieties of seeds was found to be the same at 0 ? 94 . The shape factor of the grain was calculated by Eqn 5 to be 1 ? 067 , 1 ? 011 and 1 ? 056 for GHB 30 , Bajra 28 – 15 and Babapuri varieties respectively .

3 . 4 . Densities and porosity

The bulk density of the grain varied between 830 and 866 kg / m 3 . The lower value of the bulk density applied to the Babapuri variety , which has a bigger grain size . The grain density of all the three varieties of the grain was found to be about 1600 kg / m 3 . This is higher than other grains such as wheat (1390 kg / m 3 ) , maize (1390 kg / m 3 ) , pigeon pea (1330 kg / m 3 ) , rice (1240 kg / m 3 ) , sorghum (1240 kg / m 3 ) and ergot af fected pearl millet seeds (1325 kg / m 3 ) (Alam 1 5 ) . Porosity of the sample varied between 45 and 49% .

3 . 5 . Frictional properties

The static coef ficient of friction was determined on galvanized sheet and mild steel sheet . The static coef ficient of friction did not vary significantly for the three varieties at 5% probability level . The value was


Table 1 Properties of pearl millet seed

Property GHB 3 0 Bajra 2 8 – 1 5 Babapuri

(1) Size , mm Largest dimension ( D 1 )

Second dimension ( D 2 )

Smallest dimension ( D 3 )

Geometric mean diameter ( D ) Surface mean diameter ( D s )

3 ? 118 ( Ú 0 ? 059)

1 ? 936 ( Ú 0 ? 092)

1 ? 702 ( Ú 0 ? 078)

1 ? 815 1 ? 720

2 ? 984 ( Ú 0 ? 062)

1 ? 862 ( Ú 0 ? 088)

1 ? 824 ( Ú 0 ? 047)

1 ? 843 1 ? 740

3 ? 361 ( Ú 0 ? 086)

2 ? 240 ( Ú 0 ? 113)

2 ? 007 ( Ú 0 ? 098)

2 ? 120 2 ? 080

(2) Volume of a single grain , mm 3

Calculated from 3 ? 795 3 ? 832 5 ? 794 dimensions (Eqn 2)

Determined by pycnometer

3 ? 793 ( Ú 0 ? 022)

3 ? 796 ( Ú 0 ? 075)

5 ? 673 ( Ú 0 ? 046)

(3) Surface area of single grain , S , mm 2 , (Eqn 3) 12 ? 547 12 ? 486 16 ? 379

(4) Sphericity , f (Eqn 4) 0 ? 9374 0 ? 9425 0 ? 9392

(5) Shape factor , l (Eqn 5) 1 ? 067 1 ? 011 1 ? 056

(6) Mass of 1000 seed of main fraction , g

5 ? 985 ( Ú 0 ? 019)

6 ? 027 ( Ú 0 ? 024)

9 ? 190 ( Ú 0 ? 015)

(7) Bulk density , r b , kg / m 3 866 ? 1 ( Ú 8 ? 8)

853 ? 6 ( Ú 7 ? 5)

830 ? 3 ( Ú 9 ? 0)

(8) Grain density , r t kg / m 3 1578 ( Ú 20 )

1591 ( Ú 30 )

1623 ( Ú 37 )

(9) Porosity , e % (Eqn 6) 45 ? 1 46 ? 3 48 ? 8

(10) Coef ficient of friction on (a) galvanized sheet (b) mild steel sheet

0 ? 247 0 ? 263

0 ? 251 0 ? 260

0 ? 249 0 ? 261

(11) Angle of repose , degrees 25 8 23 8 23 8

(12) Oil content , % by mass of seed

4 ? 44 ( Ú 0 ? 04)

4 ? 53 ( Ú 0 ? 07)

7 ? 31 ( Ú 0 . 10)

Values in parenthesis are standard deviations .

0 ? 25 on galvanized sheet and 0 ? 26 on mild steel sheet . The angle of respose of the pearl millet varied between 23 and 25 8 . In comparison with other grains (Gupta and Singh 1 6 ) such as rice , wheat , soybean and pigeon pea , its static coef ficient of friction and angle of repose are lower because of its smoother outer surface and higher value of sphericity .

3 . 6 . Oil content

Both the hybrid varieties have 4 ? 5% oil , whereas the local variety had 7 ? 3% oil (Table 1) . It was the experience of the authors during the study , that insect attack on the Babapuri variety was frequent on the whole seed and its flour became rancid earlier than

hybrid varieties . It may be owing to the higher oil content of the Babapuri variety .

4 . Conclusions

(1) The average principal dimensions were 3 ? 0 , 1 ? 9 and 1 ? 75 mm for hybrid and 3 ? 4 , 2 ? 2 and 2 ? 0 mm for Babapuri variety of the pearl millet grain .

(2) Mean size based on surface area of the grain was 1 ? 73 mm for hybrid varieties and 2 ? 08 mm for Babapuri variety .

(3) The shape of the grain was confirmed to be cono-spherical . The sphericity was 0 ? 94 and the

R . K . J A I N ; S . B A L 90

shape factor of the grain was 1 ? 07 for GHB 30 , 1 ? 01 for Bajra 28 – 15 and 1 ? 06 for the Babapuri varieties respectively .

(4) The surface area and the volume of a single pearl millet grain were 12 ? 5 mm 2 and 3 ? 8 mm 3

for hybrid varieties and 16 ? 4 mm 2 and 5 ? 8 mm 3

for Babapuri variety respectively . (5) The bulk and grain densities of the grain were

850 and 1600 kg / m 3 respectively , which are significantly higher than other cereal grains .

(6) Pearl millet exhibited low values of coef ficient of friction of 0 ? 25 and angle of repose of 23 to 25 8 .

(7) Porosity of the grain varied between 45 and 49% .

(8) Oil content of hybrid varieties of pearl millet was 4 ? 5% and of Babapuri variety 7 ? 3% .

Appendix 1

1 . Volume and surface area of a single grain

The shape of the pearl millet grain was assumed to be cono-spherical . To calculate the volume and surface area of a grain , it has been divided into two main parts , (1) a spherical part and (2) a conical part as shown in Fig . 1 .

Triangle OAC

( AC ) 2 5 ( OA ) 2 2 ( OC ) 2

P 2 5 h D 1 2 ( D / 2) j 2 2 ( D 2 / 4)

P 2 5 D 2 1 2 DD 1 (A1)

Triangle OBC

( OC ) 2 5 ( OB ) 2 1 ( BC ) 2

( D 2 / 4) 5 h ( D / 2) 2 h j 2 1 r 2

r 2 1 h 2 5 Dh (A2)

Triangle ABC

( AC ) 2 5 ( AB ) 2 1 ( BC ) 2

P 2 5 ( D 1 2 D 1 h ) 2 1 r 2 (A3)

Therefore , from Eqns A1 , A2 and A3

h 5 D ( D 1 2 D ) (2 D 1 2 D )

(A4)

r 2 5 D 1 D 2 ( D 1 2 D )

(2 D 1 2 D ) 2 (A5)

1 . 1 . Volume of a single grain

1 . 1 . 1 . Volume of spherical part of a grain ( V 1 ) The volume of one base sphere ( V 1 ) was determined

by rotating the major segment of the circle ( Fig . 1 ) along the y-axis and integrating with the limits from [( 2 D / 2) to h ( D / 2) 2 h j ]

E y 1

0 d V 5 E h ( D /2) 2 h j

( 2 D /2) π x 2 d y

V 1 5 E h ( D /2) 2 h j

( 2 D /2) π S D 2

4 2 y 2 D d y

V 1 5 π 12

[2 D 3 1 4 h 3 2 6 Dh 2 ]

Substituting the value of h from Eqn A4

V 1 5 π D 3 D 2

1 (4 D 1 2 3 D ) 6(2 D 1 2 D ) 3 (A6)

1 . 1 . 2 . Volume of conical part of a grain ( V 2 )

V 2 5 π 3

( BC ) 2 h AE 1 BE j

V 2 5 π 3

r 2 ( D 1 2 D 1 h )

Substituting the value of h and r 2 from Eqns A4 and A5

V 2 5 2 π D 2 D 2

1 ( D 1 2 D ) 2

3(2 D 1 2 D ) 3 (A7)

Volume of a single grain ( V t )

(A8)

V t 5 V 1 1 V 2

V t 5 π D 2 D 2

1

6(2 D 1 2 D )

1 . 2 . Surface area of a single grain

1 . 2 . 1 . Surface area of the spherical part ( S 1 ) The surface area of one base sphere was deter-

mined by integrating the area of small segment on the periphery with the limits from [( 2 D / 2) to h ( D / 2) 2 h j ] .

S 1 5 E h ( D /2) 2 h j

( 2 D /2) 2 π x d p

x 5 ( D / 2) sin ( a )

y 5 ( D / 2) cos ( a )

d y 5 2 ( D / 2) sin ( a ) d a

d p 5 d y / sin ( a ) 5 2 ( D / 2) d a


limits changed to

y 5 2 ( D / 2) , a 5 π y 5 h ( D / 2) 2 h ) j , a 5 cos 2 1 [ h ( D / 2) 2 h j / ( D / 2)]

S 1 5 E [cos 2 1 h ( D /2) 2 h j /( D /2)]

π 2 π h ( D / 2) sin ( a ) jh ( 2 D / 2) d a j

S 1 5 π D ( D 2 h )

Substituting the value of h from Eqn A4

S 1 5 π D 2 D 1

(2 D 1 2 D ) (A9)

1 . 2 . 2 . Surface area of the conical part of a grain ( S 2 )

S 2 5 (1 / 2) h 2 π ( BC )( AC ) j S 2 5 π rp

Substituting the value of p and r from the Eqns A1 and A5

S 2 5 π DD 1 ( D 1 2 D )

(2 D 1 2 D ) (A10)

Total surface area of a grain from Eqns A9 and A10

(A11)

S 5 S 1 1 S 2

S 5 π DD 2

1

(2 D 1 2 D )

2 . Sphericity

Sphericity ( f ) was calculated as the ratio of surface area of the equivalent sphere to the surface area of the grain .

Diameter of equivalent sphere

D e 5 F 6 V t

π G 1/3

Substituting the value of V t from Eqn A8

D e 5 F D 2 D 2 1

(2 D 1 2 D ) G 1/3

Surface area of equivalent sphere

(A12) S e 5 π D 2

e

f 5 S e / S

Substituting the values from Eqns A11 and A12

f 5 F D (2 D 1 2 D ) D 2

1 G 1/3

(A13)

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