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Physical properties of kernels, grains, and seeds are necessary for the design of equipment to handle, transport, process and store the crop. The physical properties of popcorn kernels have been evaluated as a function of kernel moisture content, varying from 8.95% to 17.12% (db). In the moisture range, kernel length, width, thickness, arithmetic mean diameter, and geometric mean diameter increased linearly from 8.18 to 9.14 mm, 5.71 to 6.32 mm, 3.65 to 4.90 mm, 5.85 to 6.79 mm and 5.54 to 6.55 mm respectively with increase in moisture content from 8.95% to 17.12%. The sphericity, kernel volume, kernel surface area, and thousand seed weight increased linearly from 0.677 to 0.717, 73.24 to 125.14 mm3, 96.26 to 134.92 mm2, and 136 to 157 g, respectively. The true density and bulk density decreased linearly from 1.304 to 1.224 g/cm3 and 0.771 to 0.703 g/cm3 respectively while porosity increased from 40.87% to 42.56%. The highest static coefficient of friction was found on the plywood surface. The static coefficient of friction increased from 0.55 to 0.74, 0.47 to 0.62, and 0.46 to 0.61 for plywood, galvanized iron, and aluminium surfaces respectively. The angle of repose increased linearly from 25.3° to 30.8° with the increase of moisture content.
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Physical properties of popcorn kernels
Ers
ß
an Karababa
*
Department of Food Engineering, Faculty of Engineering, University of Mersin, 33343 C¸ iftlikko
¨
y, Mersin, Turkey
Received 11 May 2004; accepted 11 November 2004
Available online 21 January 2005
Abstract
Physical properties of kernels, grains, and seeds are necessary for the design of equipment to handle, transport, process and store
the crop. The physical properties of popcorn kernels have been evaluated as a function of kernel moisture content, varying from
8.95% to 17.12% (db). In the moisture range, kernel length, width, thickness, arithmetic mean diameter, and geometric mean diam-
eter increased linearly from 8.18 to 9.14 mm, 5.71 to 6.32 mm, 3.65 to 4.90 mm, 5.85 to 6.79 mm and 5.54 to 6.55 mm respectively
with increase in moisture content from 8.95% to 17.12%. The sphericity, kernel volume, kernel surface area, and thousand seed
weight increased linearly from 0.677 to 0.717, 73.24 to 125.14 mm
3
, 96.26 to 134.92 mm
2
, and 136 to 157 g, respectively. The true
density and bulk density decreased linearly from 1.304 to 1.224 g/cm
3
and 0.771 to 0.703 g/cm
3
respectively while porosity increased
from 40.87% to 42.56%. The highest static coefficient of friction was found on the plywood surface. The static coefficient of friction
increased from 0.55 to 0.74, 0.47 to 0.62, and 0.46 to 0.61 for plywood, galvanized iron, and aluminium surfaces respectively. The
angle of repose increased linearly from 25.3 to 30.8 with the increase of moisture content.
2004 Elsevier Ltd. All rights reserved.
Keywords: Popcorn; Physical properties; Moisture content; Density; Coefficient of friction
1. Introduction
Popcorn is one of the most popular snack food for
consumer in large part of the world. Popcorn is a form
of flint corn and differs from dent and other soft corns in
two ways. The first is that it contains almost entirely
hard starch. The second is that it has a very hard peri-
carp and outer layers of endosperm, which permit the
internal pressure and temperature to rise high enough
to pop (Cretors, 2001).
There are two distinct shapes of popcorn kernels,
described as rice and pearl type. Rice type have long
kernels with a sharp point at the top and are historically
associated commercially with white popcorns. Pearl
types are round with no sharp point at the top and are
historically associated commercially with yellow pop-
corns. Recently the pearl type hybrids have gained
importance in industry (Ziegler & Ashman, 1994).
The kernel popcorns are classified by kernel sizes.
They are classified as small, medium and large. The
small yellow types are preferred by home consumers be-
cause usually produce more tender flakes with few hulls.
The larger kernel types are preferred by vendors as large
kernels produce larger flakes that have good eye appeal
and are tougher, to reduce breakage from handling even
though they may have more hulls. The medium kernel
size hybrids can be used by both end users (Ziegler,
Ashman, White, & Wysong, 1984).
Expansion volume is primary measures of popping
characteristics of popcorn kernels (Song, Eckhoff, Paul-
sen, & Litchfield, 1991). Many studies have indicated
that popping properties were mostly affected by physical
properties of the popcorn kernels. These physical prop-
erties include kernel size (Allred-Coyle, Toma, Reiboldt,
& Thakur, 2000; Ceylan & Karababa, 2002; Eldredge &
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2004.11.028
*
Tel.: +90 324 361 00 01; fax: +90 324 361 00 32.
E-mail addresses: ekarababa@yahoo.com, ekarababa@mersin.
edu.tr (E. Karababa).
www.elsevier.com/locate/jfoodeng
Journal of Food Engineering 72 (2006) 100–107
Lyerly, 1943; Haugh, Lien, Hanes, & Ashman, 1976; Lin
& Anantheswaran, 1988; Lyerly, 1942; Pordesimo,
Anantheswaran, Fleischmann, Lin, & Hanna, 1990;
Song et al., 1991; Tian, Buriak, & Eckhoff, 2001; Willier
& Brunson, 1927), kernel sphericity (Eldredge & Lyerly,
1943; Haugh et al., 1976; Hoseney, Zeleznak, &
Abdelrahman, 1983; Lyerly, 1942; Pordesimo et al.,
1990; Tian et al., 2001; Willier & Brunson, 1927) kernel
density (Chang, 1988; Haugh et al., 1976; Lyerly, 1942;
Tian et al., 2001) and test weight (Eldredge & Thomas,
1959; Eldredge & Lyerly, 1943; Haugh et al., 1976).
The physical properties of popcorn kernels are also
essential for the design of equipment for handling, har-
vesting, processing, storing and, packaging the grain
(Baryeh, 2002). They affect the conveying properties of
solid materials by air or water and heating and cooling
loads of food materials (Sahay & Singh, 1994). There-
fore it is necessary to determine physical properties of
popcorn kernels. Henderson and Perry (1981) specified
sorting, cleaning grading or classification of agricultural
products as being based on their physical properties.
The physical properties are also needed to define and
quantify heat transfer problems during heat processing
of the seeds (Mohesenin, 1980).
Lin and Anantheswaran (1988) reported that large
kernels had higher expansion volumes than small ker-
nels in microwave popping. Pordesimo et al. (1990)
found that kernels retained on 5 < D < 6 mm sieves
had higher expansion volumes than those of retained
on 6 < D < 7 mm sieves. Song et al. (1991) showed the
middle-sized (5 mm) kernels had the highest expansion
volume by oil popping. Allred-Coyle et al. (2000) re-
ported that medium-sized kernels (retained on the No.
4 sieve) produced the greatest expansion volume. In con-
trary, Tian et al. (2001) found that kernel size had no
effect. Ceylan and Karababa (2002) also concluded the
smallest sized (5 < D < 6 mm) fraction gave the highest
expansion volume and lowest number of unpopped
kernels.
Haugh et al. (1976) and Pordesimo et al. (1990) re-
ported that kernel sphericity was related to expansion
volume. Smaller, shorter and broader kernels produced
higher expansion volumes. Tian et al. (2001) found that
kernel sphericity had minor effect on expansion volume.
Lyerly (1942) suggested that kernel density had a low
positive correlation with expansion volume. Haugh et al.
(1976) and Pordesimo et al. (1990) showed that hybrids
with higher specific gravities had higher expansion vol-
umes. Tian et al. (2001) reported that the variety with
the highest density showed a tendency higher expansion
volume.
Previous researchers mostly studied on kernel size,
density and sphericity as physical properties. In this
study, some physical properties of popcorn kernels were
studied and also geometric mean diameter, kernel sur-
face area, kernel volume, thousand seed weight, true
density, bulk density, porosity, angle of repose, and
static coefficient of friction.
The objective of this study was to investigate the
moisture dependent physical properties of popcorn ker-
nels namely, linear dimensions, geometric mean diame-
ter, thousand seed weight, sphericity, volume, surface
area, true density, bulk density, porosity, angle of
repose, and static coefficient of friction against three
structural surfaces at different levels of moisture content.
2. Materials and methods
Popcorn variety (Ant-Cin 98) was obtained from
Mediterranean Research Institute in Antalya. The ker-
nels were cleaned manually to remove all foreign matter
and broken kernels.
The popcorn kernels were conditioning by adding a
calculated quantity of water, mixing thoroughly and
then sealing in separate polyethylene bags. The samples
were kept at 5 C in a refrigerator for 15 days for the
moisture to distribute uniformly throughout the sam-
ples. Before each test, the required quantity of samples
was taken out of refrigerator and allowed to warm up
to room temperature. All the physical properties were
determined at the moisture contents of 8.95%, 11.24%,
13.15%, 15.08%, and 17.12% (db). Moisture content of
the popcorns was determined by oven drying at
Nomenclature
L length, mm
W width, mm
T thickness, mm
D
g
geometric mean diameter, mm
U sphericity, %
V kernel volume, mm
3
S kernel surface area, mm
2
B diameter of the spherical part of the kernel,
mm
e porosity, %
q
b
bulk density, g/cm
3
q
k
kernel density, g/cm
3
W thousand seed weight, g
l static coefficient of friction
M
c
moisture content, % (d.b.)
R
2
determination coefficient
h angle of repose,
E. Karababa / Journal of Food Engineering 72 (2006) 100–107 101
103 C. The samples were allowed to cool in a desiccator
before final weighing. Three samples were used and the
average moisture content was found to be 12.2% (db).
The kernel moisture content range investigated 8.95–
17.12% (db) since transportation, storage, handling and
popping operations of the kernels are performed in this
moisture range.
A digital caliper was used to measure three dimen-
sions of the popcorn kernels. The length (L) was defined
as the distance from the tip cap to the kernel crown.
Width (W) was defined as the widest point to point mea-
surement taken parallel to the face of the kernel. Thick-
ness (T) was defined as the measured distance between
the two kernel faces as described by Pordesimo et al.
(1990). Hundred kernels of each fraction samples were
measured. To determine the average size of the seed, a
sample of hundred seeds was randomly selected. Mea-
surements of the three major perpendicular dimensions
of the seed were carried out with a digital caliper to an
accuracy of 0.01 mm. The geometric mean diameter D
g
of the seed was calculated by using the following rela-
tionship (Mohesenin, 1980):
D
g
¼ðLWT Þ
1=3
ð1Þ
where L is the length, W is the width and T is the thick-
ness in mm.
The sphericity U of chick pea seeds is calculated using
the following formula (Mohesenin, 1980):
U ¼
ðLWT Þ
1=3
L
ð2Þ
Thousand seed weight was determined by counting
100 kernels and weighing them in an electronic balance
and then multiplied by 10 to give the mass of 1000
kernels.
Jain and Bal (1997) have stated kernel volume, V and
kernel surface area, S may be given by:
V ¼
pB
2
L
2
6ð2L BÞ
ð3Þ
S ¼
pBL
2
2L B
ð4Þ
where
B ¼ðWT Þ
0:5
ð5Þ
The bulk density is the ratio of the mass sample of the
kernels to its total volume. It was determined by filling
a 1000 ml container with kernels from a height of about
15 cm, striking the top level and then weighing the con-
tents (Deshpande, Bal, & Ojha, 1993; Gupta & Das,
1997; Konak, C¸ arman, & Aydin, 2002; Paksoy & Aydin,
2004).
The kernel density defined as the ratio of mass of the
sample to its kernel volume, was determined using the
water displacement method. Five hundred milliliter of
water was placed in a 1000 ml graduated measuring cyl-
inder and 25 g seeds were immersed in that water. Ow-
ing to the short duration of the experiment and the
nature of the skin of the kernel which did not allow
water to be absorbed easily, the kernels were not coated
to prevent moisture adsorption. The amount of dis-
placed water was recorded from the graduated scale of
the cylinder. The ratio of weight of seeds to the volume
of displaced water gave the kernel density (Amin,
Hossain, & Roy, 2004; Olajide, Ade-Omowaye, &
Otunola, 2000).
The porosity is the fraction of the space in the bulk
grain which is not occupied by the grain (Thompson &
Isaacs, 1967). The porosity of bulk seed was computed
from the values of kernel density and bulk density using
the relationship given by Mohesenin (1980) as follows:
e ¼
q
k
q
b
q
k
100 ð6Þ
where q
b
is the bulk density and q
k
is the kernel density.
The coefficient of static friction was determined with
respect to three surfaces: plywood, galvanised iron and
aluminium. These are common materials used for han-
dling and processing of grains and construction of stor-
age and drying bins. A hollow metal cylinder 50 mm
diameter and 50 mm high and open at both ends was
filled with the seeds at the desired moisture content
and placed on an adjustable tilting table such that the
metal cylinder does not touch the table surface. The
tilting surface was raised gradually by means of a screw
device until the cylinder just starts to slide down. The
angle of the surface was read from a scale and the static
coefficient of friction was taken as the tangent of this an-
gle. Other researchers have used this method for other
grains and seeds (Dutta, Nema, & Bhardwaj, 1988;
Joshi, Das, & Mukherjee, 1993; Singh & Goswami,
1996; Suthar & Das, 1996).
To determine the dynamic angle of repose, h a
plywood box measuring 300 mm · 300 mm · 300 mm,
having a removable front panel was used. The box
was filled with the seeds at the desired moisture content,
and the front panel was quickly removed, allowing the
seeds to flow to their natural slope. The angle of repose
was calculated from measurements of seed free surface
depths at the end of the box and midway along the
sloped surface and horizontal distance from the end of
the box to this midpoint. This method has been used
by other researchers (Dutta et al., 1988; Jain & Bal,
1997; Shepherd & Bhardwaj, 1986; Singh & Goswami,
1996). The angle of repose may also be determined from
the diameter and height of a heap of seeds on a circular
plate (Visvanathan, Palanisamy, & Sreenarayanan,
1996).
All the experiment were replicated five times for
each popcorn samples and the average values were
reported.
102 E. Karababa / Journal of Food Engineering 72 (2006) 100–107
3. Results and discussion
3.1. Kernel dimensions
The results of popcorn kernel size at different mois-
ture content were displayed in Table 1. All the dimen-
sions increased with moisture content in the moisture
range of 8.95–17.12% (db). The relationships between
the axial dimensions (L,W, T, and D
g
) and kernel mois-
ture content (M
c
) can be expressed using the regression
equations as:
L ¼ 7:118 þ 0:117M
c
R
2
¼ 0:997
W ¼ 5:018 þ 0:77M
c
R
2
¼ 0: 995
T ¼ 2:258 þ 0:154M
c
R
2
¼ 0: 997
D
g
¼ 4:402 þ 0:126M
c
R
2
¼ 0: 998
All the dimensions were significantly and positively
correlated to kernel moisture content. This result indi-
cates that the kernels expand in length, width, thickness
and geometric diameter within the moisture range 8.95–
17.12%. Similar results have been reported by some
researchers (Aviara, Gwandzang, & Haque, 1999; Bar-
yeh, 2001, 2002; Deshpande et al., 1993).
The average expansion from 8.95% to 17.12% kernel
moisture content was largest along the kernel thickness
(34%) and least along its width (10.68%). Deshpande
et al. (1993) have found similar results with soybean
seeds. The L/W, L/T and L/D
g
ratios are shown in
Table 2. The L/T and L/D
g
ratios are statistically signif-
icant. However L/W ratio did not vary significantly as
the kernel moisture content increases. L/T exhibited the
highest ratios, followed by L/D
g
and L/W in descending
order. Baryeh (2001) reported similar results. The L/D
g
and L/W ratios were very similar. This indicates the
thickness and geometric mean diameter of the kernels
are closely related to its length while width shows less
association with the length of popcorn kernels.
3.2. Sphericity
The relationship between sphericity and moisture
content of popcorn kernels are shown in Fig. 1. The val-
ues of the sphericity for different moisture levels varied
from 0.677 to 0.717. This relationship can be repre-
sented by the following equation:
U ¼ 0:632 þ 0:005M
c
R
2
¼ 0:99
The sphericity of popcorn kernels was much higher than
those reported for sunflower (Gupta & Das, 1997), and
was found close to cowpea seed (Olapade, Okafur,
Ozumba, & Olatunji, 2002). However it was lower than
these reported for millet (Baryeh, 2002), pearl millet
(Jain & Bal, 1997), pigeon pea seed (Baryeh & Mangope,
2003), bambara groundnuts (Baryeh, 2001).
3.3. Thousand seed weight
The thousand seed weight increased linearly from 136
to 157 g increasing kernel moisture content (Fig. 2). The
variation can be expressed by the following equation:
W ¼ 114:12 þ 2:58M
c
R
2
¼ 0:98
Table 1
Axial dimensions of popcorn kernels (standard deviation in parentheses)
Moisture
content % d.b.
Length
(L)mm
Width
(W)mm
Thickness
(T)mm
Arithmetic mean
diameter (L + W + T/3) mm
Geometric mean
diameter (L · W · T)
1/3
mm
L/WL/TL/D
g
8.95 8.18 (0.25) 5.71 (0.42) 3.65 (0.38) 5.85 5.54 1.43 2.24 1.48
11.24 8.42 (0.28) 5.86 (0.27) 3.95 (0.22) 6.08 5.79 1.44 2.13 1.45
13.15 8.64 (0.29) 6.05 (0.37) 4.27 (0.24) 6.32 6.06 1.43 2.02 1.43
15.08 8.85 (0.31) 6.19 (0.34) 4.53 (0.24) 6.52 6.27 1.43 1.95 1.41
17.12 9.14 (0.36) 6.32 (0.30) 4.90 (0.32) 6.79 6.55 1.45 1.87 1.39
L/W, kernel length/kernel width; L/T, kernel length/kernel thickness; L/D
g
, kernel length/geometric mean diameter.
Table 2
The relationship between moisture content and static coefficients of
friction for various surfaces
a
Surface Equations R
2
Plywood l
p
= 0.358 + 0.021M
c
0.99
Galvanized iron sheet l
g
= 0.303 + 0.019M
c
0.99
Aluminium sheet l
a
= 0.289 + 0.018M
c
0.99
a
R
2
, coefficient of determination.
Fig. 1. Effect of moisture content on sphericity of popcorn.
E. Karababa / Journal of Food Engineering 72 (2006) 100–107 103
Similar results have been reported by Deshpande et al.
(1993), Aviara et al. (1999), Singh and Goswami
(1996), Visvanathan et al. (1996), O
¨
g
˘
u
¨
t (1998), Chandr-
asekar and Viswanathan (1999), Baryeh (2001, 2002),
and Baryeh and Mangope (2003) for soybean, cumin
seeds, neem nut, white lupin, coffee, guna seeds bambara
nuts, millet, and pigeon pea respectively.
3.4. Kernel volume
The volume variations with kernel moisture content
are shown in Fig. 3. The volume increases linearly with
moisture content. When the moisture content changed
from 8.95% to 17.12%, the volume increased from
73.24 mm
3
to 125.14 mm
3
. This relationship can be writ-
ten as:
V ¼ 14:56 þ 6:35M
c
R
2
¼ 0: 99
Similar results have been reported by Bal (1978) for
paddy, Dutta et al. (1988) for gram, Deshpande et al.
(1993) for soybean, O
¨
g
˘
u
¨
t (1998) for white lupin, Aviara
et al. (1999) for guna seeds, and Baryeh (2002) for
millet.
3.5. Surface area
The popcorn kernel surface area, S, is shown in Fig.
4. The surface area increased with increasing moisture
content. There is a 40.16% increase in surface area from
a moisture content of 8.95–17.12%. Shepherd and
Bhardwaj (1986), Baryeh (2001, 2002) found a similar
result with soybeans, bambara nuts, and millet seeds
respectively. However, Hsu, Mannapperuma, and Singh
(1991) obtained the surface area of pistachios to de-
crease with increasing grain moisture content. The vari-
ation in surface area with moisture content of popcorn
kernel can be represented by the following equation:
S ¼ 52:96 þ 4:73M
c
R
2
¼ 0: 99
3.6. Bulk density
The bulk density of the kernel was observed to de-
crease linearly from 0.771 to 0.703 as the moisture con-
tent increased from 8.95% to 17.12% (Fig. 5). The
relationship can be expressed by the following equation:
q
b
¼ 0:849 0:0086M
c
R
2
¼ 0:99
Fig. 2. Effect of moisture content on 1000 kernel mass of popcorn.
Fig. 3. Effect of moisture content on volume of popcorn.
Fig. 4. Effect of moisture content on surface area of popcorn.
Fig. 5. Effect of moisture content on density of popcorn. (n) True
density, (h) bulk density.
104 E. Karababa / Journal of Food Engineering 72 (2006) 100–107
This was due to the higher rate of increase in volume
than weight. The negative linear relationship was found
by Sreenarayana, Visvanathan, and Subramanijam
(1988) and Deshpande et al. (1993) for soy bean,
Visvanathan et al. (1996), Baryeh and Mangope
(2003), C¸ arman (1996), Gupta and Das (1998) and O
¨
g
˘
u
¨
t
(1998) also observed bulk density of neem nut, lentil
seeds, sunflower seeds and white lupin, respectively, to
decrease linearly with increase in grain moisture content.
3.7. True density
The true density decreases linearly from 1.309 to
1.224 g/cm
3
as the kernel moisture content increases
(Fig. 5). The true density of kernel can be represented
by following equation:
q
k
¼ 1:395 0:01M
c
R
2
¼ 0:997
This may be due to higher rate of increase in seed vol-
ume than weight. The negative linear relationship was
also observed by Shepherd and Bhardwaj (1986) for
pigeon pea, Deshpande et al. (1993) for soy bean,
Suthar and Das (1996) for karingha seeds, Sahoo and
Srivastava (2002) for okra seed, Konak et al. (2002)
for chickpea seeds, and Kaleemullah and Gunasekar
(2002) for arecanut kernels.
3.8. Porosity
The porosity was calculated from the bulk density
and true density of the kernel. The porosity was found
to decrease from 42.56% to 40.87% with increase in
moisture content from 8.95% to 17.12% (Fig. 6). The
values obtained were lower than those reported for
hazelnut (Aydin, 2002), raw cashew nut (Balasubrama-
nian, 2001), chickpea (Konak et al., 2002), okra seeds
(Sahoo & Srivastava, 2002), millet (Baryeh, 2002), sun-
flower kernels (Gupta & Das, 1997), karingha seeds
(Suthar & Das, 1996), and higher than those reported
for pigeon pea (Baryeh & Mangope, 2003), gram
(Chowdhury, Sarker, Bala, & Hossain, 2001), arecanut
kernels (Kaleemullah & Gunasekar, 2002), and lentils
(C¸ arman, 1996).
The relationship between the porosity and moisture
content for popcorn kernels can be represented by the
following equation:
e ¼ 46:52 0:452M
c
R
2
¼ 0:989
Similar trends were reported for gram (Dutta et al.,
1988), sunflower kernels and seeds (Gupta & Das,
1997), white lupin (O
¨
g
˘
u
¨
t, 1998), guna seeds (Aviara
et al., 1999), hazel nuts (Aydin, 2002), gram (Chowdhury
et al., 2001), chickpea seeds (Konak et al., 2002), areca-
nut kernels (Kaleemullah & Gunasekar, 2002), raw cash-
ew nut (Balasubramanian, 2001), green gram (Nimkar &
Chattopadhyay, 2001), and quinoa seeds (Vilche, Gely,
& Santalla, 2003), but are different from the behaviour
reported for soya bean Deshpande et al. (1993), pumpkin
seeds (Joshi et al., 1993), karingha seeds (Suthar & Das,
1996), coffee (Chandrasekar & Viswanathan, 1999),
bambara groundnuts (Baryeh, 2001), okra seeds (Sahoo
& Srivastava, 2002), millet (Baryeh, 2002), Turkish
mahlep (Aydin, O
¨
g
˘
u
¨
t, & Konak, 2002), and pigeon pea
(Baryeh & Mangope, 2003).
3.9. Static coefficient of friction
The plots of static coefficient of friction obtained
experimentally on three structural surfaces against mois-
ture content in the range of 8.95–17.12% (d.b) are dis-
played in Fig. 7. The static coefficient of friction
varied significantly at 5% probability level. The coeffi-
cient of friction of popcorn kernel increased linearly
with moisture content and varied according to the sur-
face. Plywood had the highest coefficient of friction
followed by galvanized iron, and aluminium. This order
has been reported by other researchers (Baryeh, 2001;
Baryeh & Mangope, 2003; Gupta & Das, 1998; Joshi
et al., 1993; Singh & Goswami, 1996; Suthar & Das,
1996; Visvanathan et al., 1996). This may be due to
the smoother surface of galvanized iron compared to
Fig. 6. Effect of moisture content on porosity of popcorn.
Fig. 7. Effect of moisture content on static coefficient of friction of
popcorn. (n) Plywood, (h) galvanized iron, (s) aluminium.
E. Karababa / Journal of Food Engineering 72 (2006) 100–107 105
plywood, and the smoother surface of aluminium com-
pared to galvanized iron.
The popcorn kernel also may become rougher and
sliding characteristics are diminished at higher moisture
contents, so that the static coefficient of friction in-
creased. The relationship between moisture content
and static coefficients of friction for three surfaces is pre-
sented in Table 2.
3.10. Angle of repose
The variation of the angle of repose, h with seed
moisture content is plotted in Fig. 8. The angle of repose
increases linearly with seed moisture content from 25.3%
at 8.95% seed moisture content to 30.8% at 17.12% seed
moisture content. The relationship can be expressed in
equation form as follows:
h ¼ 18:95 þ 0:703M
c
R
2
¼ 0: 991
A linear increase in angle of repose as the seed moisture
content increases has also been noted by Chandrasekar
and Viswanathan (1999) for coffee, Suthar and Das
(1996) for karingda seeds, Dutta et al. (1988) for gram,
Shepherd and Bhardwaj (1986) for pigeon peas, Oje and
Ugbor (1991) for oilbean seeds, Joshi et al. (1993) for
pumpkin seeds, Fraser, Verma, and Muir (1978) for
fababeans, Singh and Goswami (1996) for cumin seeds
and Gupta and Das (1998) for sunflower seeds. The val-
ues are lower than those reported for karingda seeds
(Suthar & Das, 1996), gram (Dutta et al., 1988), type
17 pigeon-peas (Shepherd & Bhardwaj, 1986), oilbean
seed (Oje & Ugbor, 1991), pumpkin seed (Joshi et al.,
1993), fababeans (Fraser et al., 1978), pearl millet (Jain
& Bal, 1997) and cumin seeds (Singh & Goswami, 1996).
The differences could be due to differences in surface
roughness of seeds. This could also be responsible for
the increasing trend of the angle of repose at higher
moisture contents. Visvanathan et al. (1996) also found
this to be true for neem nuts, while Aviara et al. (1999)
found the increase with moisture content to be nonlinear
for guna seeds.
4. Conclusion
(1) The length, width, thickness, arithmetic mean diam-
eter, and geometric mean diameter of popcorn ker-
nels increased linearly with increase of moisture
content.
(2) Sphericity, kernel volume, kernel surface, and thou-
sand seed weight increased linearly with increase of
moisture content.
(3) True density was higher than bulk density at all ker-
nel moisture contents studied.
(4) True and bulk density slightly decreased linearly
with increase of moisture content but porosity
increased with increase in moisture content.
(5) The static coefficients of friction was highest for ply-
wood, followed by galvanized iron and aliminium
among the material tested.
(6) Angle of repose increased from 25.3 to 30.8, from
kernel moisture content of 8.95–17.12%.
(7) The physical properties of popcorn kernels were
expressed in the linear regression equations as a
function of moisture content. High correlation coef-
ficients were found with a significance level of 95%.
Acknowledgments
The author thanks Dr. Nermin Koc¸ (Mediterranean
Research Institute, Antalya, Turkey) for supplying of
popcorn samples. Also author acknowledges Dr. Ali
Nazmi Ozan (Field Crops Central Research Institute,
Ankara, Turkey) for providing necessary facilities to
carry out the above work and for performing some
analysis.
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