Post on 09-May-2023
International Journal of FoodEngineering
Volume 4, Issue 7 2008 Article 7
Variation of the Physical Properties of Sheanut(Vitellaria Paradoxa Gaertn.) Kernels during
Convective Drying
Divine Nde Bup∗ Cesar Kapseu† Dzudie Tenin‡
Alexis Kuitche∗∗ Charles Fon Abi†† Clerge Tchiegang‡‡
∗Department of Process Engineering, National School of Agro-Industrial Sciences, Universityof Ngaoundere, Cameroon, bupdiv@yahoo.fr
†Department of Process Engineering, National School of Agro-Industrial Sciences, Universityof Ngaoundere, Cameroon, kapseu@yahoo.fr
‡Department of Process Engineering, National School of Agro-Industrial Sciences, Universityof Ngaoundere, Cameroon, d tenin@yahoo.fr
∗∗Department of Electrical Engineering, Energetics and Automation, National School of Agro-Industrial Sciences, University of Ngaoundere, Cameroon, kuitche a@yahoo.fr
††Department of Chemistry, Advanced Teachers Training College ENS, University of Yaounde,Cameroon, cfonabi@justice.com
‡‡Department of Food Science and Nutrition, National School of Agro-Industrial Sciences, Uni-versity of Ngaoundere, Cameroon, clerge.tchiegang@caramail.com
Copyright c©2008 The Berkeley Electronic Press. All rights reserved.
Variation of the Physical Properties of Sheanut(Vitellaria Paradoxa Gaertn.) Kernels during
Convective Drying∗
Divine Nde Bup, Cesar Kapseu, Dzudie Tenin, Alexis Kuitche, Charles Fon Abi,and Clerge Tchiegang
Abstract
The effect of moisture content and drying temperature of Vitellaria paradoxa Gaertn ker-nels on some of its physical properties was investigated. The kernels which were harvested at amoisture content of about 60% (wet basis) and hence prone to high post harvest losses from twoecological zones of Cameroon (Bangoua in West province and Tchabal in Adamawa province)were dried in a forced convection dryer at 40oC, for 6, 20, 48, 72 and 96 hours to give moisturecontents ranging from 10 to 60% wet basis. Ten trees from each of the zones were carefully se-lected to serve as sources for the ripe kernel bearing shea fruits that were used as samples for thisstudy. For each parameter studied, a sample population of 30 kernels selected at random per treewas used. The results obtained revealed that there was a significant difference in the physical prop-erties of the kernels from different trees irrespective of the locality. The bulk density, true density,sphericity and porosity varied non-linearly with the moisture content. Kernels with larger massesshowed a different variation pattern of bulk density and porosity with moisture content comparedto the lighter kernels. The variation of the bulk density, sphericity, porosity of sheanut kernels withmoisture content and temperature was satisfactorily modelled with empirical equations. The sam-ples underwent considerable shrinkage (up to 35%) during the drying process. Three empiricalmodels were used to describe the shrinkage behaviour of the kernels and it is proposed that thesemodels could be incorporated in drying models.
KEYWORDS: sheanut kernel, drying, temperature, physical properties, moisture content, shrink-age models
∗This research was supported by the International Foundation for Science, Stockholm, Swedenand the Conseil Ouest et Centre Africain pour la recherche et le Development Agricoles – WestAfrican Council for Agricultural Research and Development (CORAF/WECARD), Dakar, Sene-gal, through a grant to Bup Nde Divine.
1.0 INTRODUCTION
Vitellaria paradoxa Gaertn, produces kernels which have an oil content of about
35-60% (Tano-Debrah and Ohta, 1994). These kernels are harvested at a moisture
content of about 60% wet basis and hence pruned to high post harvest losses.
When shea fruits are harvested, they undergo the following stages to produce shea
butter: depulping and dehusking; the resulting nuts are boiled and then cracked to
give the kernels. These kernels are sun dried to a safe storage moisture content of
about 10-15% before oil extraction. Drying, an old age technique for the
preservation of food stuffs which is equally employed in the processing of sheanut
kernels has been identified as one of the key processing steps critical to shea
kernel quality (Womeni, 2004; Lovett, 2004; Zangue Adjia, 2005). However the
physical properties of food materials may change enormously during the drying
process. A thorough understanding of the factors responsible for this change in
physical properties during the dehydration process is thus of major relevance. The
physical properties of sheanut kernel like those of other agricultural materials are
important to design the equipment for processing, transportation, sorting,
separation and storing. Designing such equipment without taking these into
consideration may yield poor results. Therefore the determination and
consideration of these properties in future works has an important role. 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, 1986). The major moisture-dependent
physical properties of biological materials are shape, size, mass, bulk density, true
density and porosity (Mohsenin, 1986). Other researchers have studied these
properties for various grains and seeds such as pop corn (Karababa, 2006),
pistachio nuts and kernels (Kashaninejad et al., 2006) safflower seeds (Baümler et
al., 2006), apple, guava and potato (Hawlader et al., 2006), mango (Jha et al.,
2006), bambara groundnuts (Atiku et al., 2004). A closer look at the literature
indicates that very few works have been done on the physical properties of shea
kernels in relation to thermal processing. For example, Olajide et al., (2000)
described the physical properties (mass, length, diameter, bulk and true densities,
porosity) of Nigerian sheanut at one moisture content and in relation to
mechanical processing. These properties were later used by Oluwole et al. (2004)
to develop a sheanut cracker. The effect of moisture content on the physical
properties of sheanut was investigated by Aviara et al. (2005). These two studies
were carried out on shea kernels and nuts from Gwoza local government area of
Borno state in Nigeria in the moisture range 6-29% dry basis.
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Bup et al.: Physical Properties of Sheanut Kernels during Drying
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Bup (2003), Womeni (2004) and Zangue Adjia (2005) reported some physical
properties of shea kernels (mass, length and diameters) at their initial moisture
contents (55-60% wet basis) from some localities of Cameroon. When shea
kernels are harvested at a moisture content of 60% wet basis (150% dry basis),
they have to be dried to a recommended moisture content of 10-15% wet basis for
oil extraction. It is therefore neccessary to understand the behaviour of the
physical properties of these kernels during drying in the entire moisture content
range 10-60% wet basis (11-150% dry basis). Many foods suffer reduction of its
external dimensions during drying (Mayor and Sereno, 2004). This decrease in
dimensions is generally referred to as dimensional shrinkage. Shrinkage of food
materials causes in most cases a negative impression in the consumer (Schultz et
al., 2007). Sheanut kernels like other food materials may undergo considerable
shrinkage during drying. To the best of our knowledge, no studies exist at the
moment on the shrinkage of shea kernels.
Womeni (2004) claimed that, the differences in the physical
properties reported for sheanut kernels from different localities (Bangoua and
Ngaoundere-Cameroon) could be due to tree to tree variation in the properties of
the kernels within the same locality. A study on the tree to tree variation of the
physical properties of sheanut kernels will be of great value to researchers and
potential investors in the field.
From the foregoing, it is observed that no attempt has been made at
understanding the tree to tree variation of the physical properties of sheanut
kernels. A study of the properties of the kernels undergoing a drying process as
well as the influence of temperature on these properties have equally not been
investigated. The objective of the study was therefore to evaluate the physical
properties of shea kernels from different trees undergoing a drying process in the
range 10-60% as well as the influence of drying temperature on the sphericity
and dimensional shrinkage of the kernels.
2. MATERIALS AND METHODS
Shea fruits were collected tree by tree from Bangoua (West Province) and
Tchabal (Adamawa Province) villages in Cameroon. These regions belong to the
afromontane and soudano-guinean ecological zones respectively. In each locality
10 trees containing an appreciable quantity of mature kernels were sampled.
Mature fruits that had fallen down were picked for the work. The fruits from the
same tree were then packaged in a tissue bag and labelled as TTi or BTi
representing samples from tree i (i = 1, 2…) in the Tchabal or Bangoua locality
respectively. On reaching the laboratory, the fruits were washed, depulped and the
nuts were stored at –18°C until analysis.
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The nuts were removed from the freezer and allowed on a laboratory bench
overnight for the equilibration of water contained in the product. They were then
cracked to give the kernels. The physical properties of the fresh kernels as well as
the influence of moisture content and temperature on some of these physical
properties were measured as described in the following sections.
2.1. Determination of the Physical Properties of Fresh Shea Kernels
2.1. 1. Mass, Size and Shape
In order to determine the size and shape of the kernels, 30 kernels were randomly
selected from kernels of each tree. The three linear dimensions namely major
diameter x ¸ in mm, intermediate diameter, y, in mm, and the minor diameter, z in
mm were measured with a digital vernier calliper (Model SV-03-150,
SCHLENKER enterprises LTD, USA) having a precision of 0.01 and the mass of
each kernel was obtained with the help of a precision electronic balance (model
Scout Pro SPU402, OHAUS, USA). The sphericity ω of the kernels was given by
Mohsenin (1986).
[1]
2.1.2. Determination of True Density
The true density was determined by the water displacement method (Olajide et
al., 2000). A kernel of known mass (m) was dropped into a can and the volume of
water (v) displaced into a measuring cylinder noted. The kernel density ρp in
g/cm3 was given by
v
mp =ρρρρ [2]
( )x
xyz 31
ω=
x
y
z z
Figure 1: Photo of shea kernel showing the dimensions measured
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Bup et al.: Physical Properties of Sheanut Kernels during Drying
Published by The Berkeley Electronic Press, 2008
where v is the kernel volume. At least ten nuts were used for each experiment.
2.1.3. Determination of Bulk Density
The bulk density was determined using the AOAC (1980) method. This involved
the filling of a 500 cm3 cylinder with nuts from a height of 15 cm and weighing
the contents. The bulk density (ρb) in g/cm3 was given by
V
mb =ρ [3]
where V is the bulk volume. Each experiment was replicated thrice.
2.1.4. Determination of Porosity
The Porosity was calculated from the kernel and bulk densities using the
relationship given by (Pabis et al., 1988). The porosity, ε, was given by the
equation
[4]
2.1.5. Determination of Moisture Content
To determine the moisture content, the kernels were cut into smaller chips to ease
moisture removal. About 5 g of the ground sample (m1) were weighed into a
previously weighed crucible, (m0) and dried in an oven at 105°C to constant mass.
The moisture content (X) was then calculated from
[5]
The dimensionless ratio of the moisture content was reported as X/X0 where X0 is
the original moisture content.
−=
pρρρρρρρρ
εεεε b1100
100 w.b.)(% X0
01 ×
−=
m
mm
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2.2. Influence of Moisture Content on the Physical Properties of Sheanut
Kernels
In order to evaluate the influence of moisture content on the physical properties of
sheanut kernels, they were dried in a forced convection dryer (Kapseu et al.,
2007) at 40°C for 6, 20, 48, 72 and 96 hrs. The different drying times
corresponded to different moisture contents. The physical properties were then
evaluated at these moisture contents as described in sections 2.1.1. Fifteen to
twenty kernels were used at each moisture content.
2.2. 1. Determination of dimensional shrinkage
The dimensional shrinkage (hitherto referred to simply as shrinkage) of sheanut
kernels from different trees was calculated by measuring all the three dimensions
of the kernel after 96 hours of drying using a precision vernier calliper. The
shrinkage was expressed as the percentage change in dimension (equation 6). At
least 10 kernels were measured at each moisture content.
Sx = 100*(xo-x)/xo)
Sy = 100*(yo-y)/yo) [6]
Sz = 100*(zo-z)/zo)
The average shrinkage was then calculated from
S = (Sx + Sy + Sz)/3 [7]
xo, y0 and z0, and x, y, and z were the major, intermediate and minor diameters
before drying and at the end of drying (after 96 hours) respectively. Sx, Sy and Sz
were the percentage dimensional shrinkages along the major, intermediate and
minor diameters respectively. Mcminn and Magee, (1998) described the
shrinkage of food products by measuring their reduction in length as a function of
moisture content.
2.3. Influence of Temperature on the Sphericity and Dimensional Shrinkage
of Sheanut Kernels
To evaluate the influence of temperature on the sphericity and dimensional
shrinkage of sheanut kernels, kernels having initial major, intermediate and
minor diameter values of 42.94 ± 2.71, 29.58 ± 1.76 and 25.58 ± 2.09mm were
dried as described in section 2.2 at 40, 50 and 60°C. These kernels were selected
from one tree to limit the degree of variation of the properties.
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Bup et al.: Physical Properties of Sheanut Kernels during Drying
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The sphericity and dimensional shrinkage were then evaluated as described in
section 2.1.4 at each moisture content and temperature. Three polynomial
equations were also tested to model the shrinkage phenomenon at the three
different temperatures:
S = y0 + aX [8]
S = y0 + aX + bX2
[9]
S = y0 + aX + bX2
+ cX3 [10]
The constants y0, a, b and c were obtained through multiple regression analysis of
the experimental data for each model on STAGRAPHICS PLUS 5.0 software
(Statistical graphic corp.).
2.4. Validation of Models
The criteria for evaluating the reliability of the simulations were the regression
coefficients and/or the standard relative error of deviation observed between the
experimental and theoretical results. The standard relative error (SRE) of
deviation was evaluated with the help of the following equation
[11]
Where Yexp and Ymod are the values obtained from experiments and from the
model respectively. p is the number of points at which measurements were carried
out.
Statistical analysis (ANOVA) of the physical properties was carried
out on STATGRAPHICS PLUS 5.0 (Statistical graphic corp.) and the
DUNCAN’s multiple range test was used to detect the differences between
means.
3. RESULTS AND DISCUSSION
3.1. Physical Properties of the Fresh Kernels
3.1.1. Mass, Size and Shape
The results of the mass and dimensions of the shea kernels from different trees
harvested from Bangoua (West Province) and Tchabal (Adamawa Province)
villages in Cameroon are presented on table 1 (only results of some trees are
shown).
( ) ∑=
−=
p
1i expY
modY
expY
p
100%SRE
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Analysis of variance indicated that there was a significant difference (p< 0.05) in
the mass and dimensions of the kernels from different trees. The mass of the fresh
kernels, major, intermediate and minor diameters ranged from 10.2 ± 2.1 to 28 ±
6.1g, 33.9 ± 3.2 to 45 ± 3.6mm, 22.64 ± 1.9 to 33.9 ± 4.9mm and 18.8 ± 2.4 to
30.5 ± 3.1mm respectively. The Duncan multiple range tests grouped the physical
properties of the kernels from different trees into at least five homogenous groups
irrespective of the origin of the kernels. A similar observation was made with the
geometric mean and sphericity of the kernels from the different trees studied
which ranged from 24.7 ± 1.9 to 36.1 ± 3.5mm and from 0.70 ± 0.05 to 0.841 ±
0.05 respectively. The three diameters and the geometric mean were all positively
correlated (r2 > 0.9) to the mass of the kernels at the 95% confidence limit.
Table 1: Mass, size and shape of fresh sheanut kernels from different trees
Tree/
parameter
Mass x (mm)
y (mm)
z (mm)
ω ρb
(g/cm3
)
ρp
(g/cm3
)
ε (%)
28.01f 45.54e 33.87f 30.53e 0.79cd 0.47 bc 1.01c 52.94 b
TT1 ± 6.08 ± 3.58 ± 4.94 ± 3.09 ± 0.04 ± 0.03 ± 0.03 ± 3.5
17.34d 38.73c 28.29d 25.33c 0.78bcd 0.47 bc 0.92a 48.86 a
TT2 ± 4.18 ± 2.97 ± 2.62 ± 3.45 ± 0.04 ± 0.04 ± 0.08 ± 2.6
15.31cd 36.45bc 25.42bc 22.92b 0.76bc 0.47bc 1.02c 53.79 b
TT3 ± 3.06 ± 2.66 ± 2.67 ± 3.26 ± 0.05 ± 0.04 ± 0.05 ± 1.3
10.15a 35.44ab 22.64a 18.79a 0.70a 0.49 c 1.13d 56.73 c
TT4 ± 2.12 ± 2.74 ± 1.93 ± 2.42 ± 0.05 ± 0.04 ± 0.03 ± 0.89
22.56e 41.94 d 31.35e 27.74d 0.79cd 0.45b 0.95b 53.04 b
BT1 ± 4.72 ± 3.91 ± 2.62 ± 2.66 ± 0.04 ± 0.04 ± 0.04 ± 1.3
21.61e 41.8d 31.26 e 27.34cd 0.79cd 0.43a 0.90a 52.18b
BT2 ± 6.24 ± 5.29 ± 3.38 ± 4.71 ± 0.07 ± 0.03 ± 0.06 ± 4.1
21.74e 42.05 d 29.51 de 27.62d 0.78bcd 0.47 0.91a 48.33a
BT3 ± 3.06 ± 2.66 ± 2.67 ± 3.26 ± 0.05 ± 0.08 ± 0.04 ± 3.5
14.21bcd 34.71ab 27.35cd 26.02cd 0.84e 0.42a 0.92a 54.5 bc
BT4 ± 4.49 ± 4.50 ± 3.49 ± 2.80 ± 0.05 ± 0.04 ± 0.10 ± 1.2
*Values with different superscripts in the same column are significantly different (P < 0.05)
x = Major diameter, y = Intermediate diameter, z = minor diameter, De =
Geometric mean diameter, ω = sphericity, ρb = bulk density, ρp = kernel density, ε
= porosity
3.2. Influence of Moisture Content on some Physical Properties of Sheanut
Kernels
3.2. 1. Sphericity
The influence of moisture content on the sphericity of sheanut kernels harvested
from different trees from two different localities and undergoing convection
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Bup et al.: Physical Properties of Sheanut Kernels during Drying
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drying is presented on figure 1. Generally, the sphericity decreased non-linearly
as the moisture content decreased irrespective of the kernel size and mass (figure
2). The sphericities, however, varied significantly (p<0.05) from one tree to the
other.
For all of the kernels studied (except TT4) the sphericity remained in the range
0.70-0.84 indicating that the kernels could be considered as spheres through out
the drying process. This information is important in modelling drying processes.
3.2.2 Kernel density
The variation of kernel density with moisture content is illustrated in figure 3. The
kernel density increased from 0.92 to 0.96 g/cm
3 and then decreased to values of
about 0.88 g/cm3
and then increased to values of about 0.92 g/cm3. These values
were used to determine the porosities of the samples during the drying process
3.2.3 Bulk density
The influence of moisture content on bulk density in the studied moisture range
10-60% wet basis is presented on figure 4. The variation of the bulk density was
clearly dependent on the kernel mass. For kernels with masses ranging from 10 to
Figure 2: Influence of moisture content on the sphericity of sheanut kernels
Moisture ratio (Xrr)
0.4 0.6 0.8 1.0
Sph
eric
ity
0.65
0.70
0.75
0.80
0.85
TT1
TT2
TT3
TT4
BT1
BT2
BT3
BT4
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17g the bulk density was seen to decrease non-linearly from 0.487 to 0.251 g/cm3
in the entire moisture content range 10-60%. In the mass range 21-28 g the bulk
densities decreased from 0.471 to about 0.373 g/cm3 at a moisture content of
about 26 % and then increased non-linearly to values of about 0.425 g/cm3
with a
decrease in the moisture content down to 10%. This observed phenomenon could
be due to the fact that the pattern of variation of the masses and volumes of
sheanut kernels with different initial masses are not the same during convective
drying probably due to the different rates of heat transfer through kernels with
different masses. Like the sphericities, the bulk densities of the kernels, varied
significantly from one tree to the other. The bulk densities of the kernels were
successfully modelled by empirical third order polynomial equations (equation
13) generated on SigmaPlot 2004 for windows version 9.01 (Systat software Inc.)
with regression coefficients ranging from (0.990-0.993) of the form
[13]
Moisture ratio (Xr)
0.4 0.6 0.8 1.0
Ker
nel
den
sity
(g
/cm
3)
0.88
0.90
0.92
0.94
0.96
BT3
BTI
BT4
TT2
Figure 3: Influence of moisture content on of the true density of sheanut kernels
32
rrrb DXCXBXA +++=ρ
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Bup et al.: Physical Properties of Sheanut Kernels during Drying
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Moisture ratio (Xr)
0.4 0.6 0.8 1.0
Bu
lk D
ensi
ty (
g/c
m3)
0.30
0.32
0.34
0.36
0.38
0.40
0.42
0.44
0.46
0.48
BT2
BT3 TT1 BT1
Model
Moisture (Xr)
0.4 0.6 0.8 1.0
Bu
lk d
ensi
ty (
g/c
m3)
0.25
0.30
0.35
0.40
0.45
0.50
BT4
TT4
TT3
TT2
Model
b) Mass range 21-28 g
a) Mass range 10-17 g
Figure 4: Influence of moisture content on the bulk density of sheanut kernels
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Table 2: Model constants for bulk density and porosity.
Constants/
Tree
A B C D r2
Bulk densities
TT1 8.29 -1.98 2.61 -9.65 1.000
TT2 -0.55 4.24 -6.02 2.80 0.991
TT3 0.67 -2.25 4.20 -2,17 0.991
TT4 0.70 -2.03 2.30 -1.10 0.990
BT1 -0.03 2.51 -5.00 2.99 0.960
BT2 0.31 0.71 -1.57 0.98 0.766
BT3 0.34 0.78 -2.19 1.53 0.981
BT4 0.37 -1.00 2.46 -1.13 0.993
Porosities
TT1 53.87 -29.73 137.62 -108.64 0.983
TT2 129.94 -302.52 393.69 -167.25 0.996
TT3 73.53 -48.55 64.29 -40.42 0.956
TT4 7.36 365.90 -648.4 331.70 0.841
BT1 1,99 191.94 -262.64 103.93 0.741
BT2 -2.41 297.13 -404.46 162.13 0.959
BT3 91.94 -21.48 454.23 -283.05 0.999
BT4 50.78 151.42 -327.23 179.53 0.968
3.2.4. Porosities
The variation of the porosity with moisture content of the samples from different
trees like the bulk density showed two-pattern behaviour according to their
masses (figure 5). The porosity of the samples with masses in the range 10-17 g
increased non-linearly (49.6-72.8%) in the course of the drying process as the
moisture content decreased from 60-10%. For kernels in the mass range 21-28 g
the porosities increased (53-64%) as the moisture content decreased to a value of
about 26% and then increased to about 62% with a further decrease in the
moisture content down to about 10%. The reasons earlier adduced for the
variation of bulk density with moisture content during the drying process could be
used to explain the variation of porosity of the kernels undergoing drying since
porosity is a direct function of bulk density as shown in equation 5. The porosities
of the kernels equally variedly significantly from one tree to the other irrespective
of the sampling site. The Model constants for bulk density and porosity are
presented on table 2.
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Moisture ratio (Xr)
0.2 0.4 0.6 0.8 1.0
Po
rosi
ty (
%)
45
50
55
60
65
70
75
TT4 TT3 TT2BT4
Moisture ratio (Xr)
0.2 0.4 0.6 0.8 1.0
Poro
sity (
%)
50
55
60
65
TT1 BT3BT2 BT1
b) Mass range 21-28 g
Figure 5: Influence of moisture content on porosity of sheanut kernels
a) Mass range 10-17 g
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3.2.5 Shrinkage
Results indicated that sheanut kernels from different trees underwent considerable
reduction in size (shrinkage) in the three different dimensions measured. There
was a significant difference (P<0.05) observed in the degree of shrinkage on
kernels from different trees. The shrinkage variation was more pronounced in the
direction of the minor diameter, getting up to 35% in some cases. This high
degree of shrinkage observed in the direction of the minor diameter could be
explained by the fact that the minor diameter constituted the natural rest position
of the kernels which was perpendicular to the direction of flow of the drying air
(figure 1). Hence the removal of water was more pronounced in this direction thus
explaining the enhanced structural collapse in the direction of the minor diameter
(figure 6) compared to the other two diameters for kernels from more than 90% of
the trees studied and at all temperatures.
Figure 6: Percentage shrinkage of sheanut kernels from different trees
along the three diameters measured after 96 hours of drying.
Different trees
NT1 NT2 NT3 NT4 BT1 BT2 BT3 BT4
Sh
rin
kag
e (%
)
0
10
20
30
40
Major diameter
Intermediate diameter
Minor diameter
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3.3. Influence of Temperature on Sphericity and Shrinkage of the Kernels
3.3.1. Sphericity
The effect of air drying temperature (40-60°C) on the sphericity (figure 7) was
evaluated on BT3 (chosen randomly) kernels. At 50°C, the sphericity decreased
more rapidly with moisture content compared to 40°C. This could be due to the
higher drying rate of the kernels at 50°C since drying rate increases with
temperature. Water evaporates more rapidly than the rate at which air comes in to
replace it leading to a collapse of the structure. However, contrary to our
expectation, the rate of decrease of the sphericity with moisture content at 60°C
was lower than that at 40 and 50°C. This was probably due to the development of
a surface coat at such relatively high temperatures which slowed down mass
transfer and the kernel diameters decreased relatively slowly with moisture
content compared to the decrease at 40 and 50°C. Kapseu et al. (2007) had earlier
reported lower values for the drying constants of shea kernels at 60°C than at
45°C and suggested that this phenomenon was perhaps due to the development of
a surface coat on the kernels shortly after drying commenced.
X/X0
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1
Sp
her
icit
y
0,70
0,71
0,72
0,73
0,74
0,75
0,76
0,77
0,78
60 °C
40 °C
50 °C
Theoritical
Figure 7: Influence of temperature on the sphericity of sheanut kernels
Moisture ratio (Xr)
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3.3.2. Shrinkage
The influence of three different drying temperatures (40, 50 and 60°C) on the
shrinkage of sheanut kernels is illustrated in figure 8. As expected, the degree of
shrinkage increased with a reduction in moisture content in the moisture range
studied 10-60%. Shrinkage was enhanced when the air drying temperature
increased from 40 to 50°C. However, the shrinkage decreased at 60°C contrary to
our expectations probably due to the phenomenon of surface hardening as earlier
advanced for sphericity. In order to better model the phenomenon (equations 7-
11), a dimensionless ratio of the shrinkage of the kernels was defined along the
three different dimensions measured as follows:
Srx = x/x0, Sry = y/y0, Srz = z/z0 [13]
where Srx, Sry, and Srz are the shrinkage ratios along the major, intermediate and
minor diameters respectively. The average shrinkage (Sr) ratio was then
calculated from
Sr = (Srx+Sry + Srz )/3 [14]
The constants for the models together with the regression coefficients and
the standard relative error between the predicted and experimental values are
presented on table 3.
Table 3: Model constants and r2 and SRE values for Shrinkage.
Constants/
Temperature (oC) y0 A B c r2 SRE (%)
Linear
40 0.8345 0.1342 0.808 1.66
50 0.4342 0.5715 0.994 1.84
60 0.8958 0.0818 0.79 1.06
Quadratic
40 0.8969 -0.139 0.2316 0.932 0.91
50 0.4301 0.6462 -0.0849 0.998 1.46
60 0.9119 0.0023 0.0681 0.82 1.07
Cubic
40 0.8258 0.3978 -0.8386 0.6157 0.969 0.67
50 0.4058 1.0707 -1.1012 0.6243 0.999 0.22
60 0.8401 0.6118 -1.3366 0.8813 0.954 0.58
Judging from the values of the regression coefficients (greater than 0.941)
and the SRE (less than 10%), the models are suitable for use in the indicated
temperature and moisture content range. These shrinkage models can therefore be
incorporated in drying models to better describe the drying behaviour of sheanut
kernels.
15
Bup et al.: Physical Properties of Sheanut Kernels during Drying
Published by The Berkeley Electronic Press, 2008
4.0 CONCLUSION
There is a tree to tree variation of the physical properties of the kernels. The
variation of the bulk density and the porosities of the kernels with moisture
content was found to be dependent on its size or mass. The sphericity and
shrinkage of the kernels is affected by temperature. The physical properties of
sheanut kernels vary significantly during drying. At 60°C, the kernels develop a
surface coat which reduces the sphericity and the shrinkage rate of the kernels.
Shrinkage was more pronounced in the direction of the minor diameter of the
kernels in all cases. The variation of some of the physical properties of sheanut
kernels with moisture content and temperature was satisfactorily modelled with
empirical equations. Four models were used to describe the shrinkage behaviour
of the kernels and it is proposed that these models can be incorporated in drying
models. The results of the study can be useful in the design of processing
equipments for sheanut kernels.
Figure 8: Influence of temperature on the dimensional shrinkage of sheanut kernels
Moisture ratio (Xr)
0.0 0.2 0.4 0.6 0.8 1.0
Shri
nkag
e ra
tio (
d/d
0)
0.4
0.5
0.6
0.7
0.8
0.9
1.0
400C
500C
600C
Linear model
Quadratic model
Cubic model
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International Journal of Food Engineering, Vol. 4 [2008], Iss. 7, Art. 7
http://www.bepress.com/ijfe/vol4/iss7/art7
Notations
m Mass x Major diameter (mm)
S Average shrinkage (%) X Moisture content at time t (% wet base)
Sr Average Dimensionless shrinkage X0 Initial moisture content (% wet base)
Srx Dimensionless shrinkage along the major diameter Xr Dimensionless moisture ratio (X/X0)
Sry Dimensionless shrinkage along the intermediate
diameter
y Intermediate diameter (mm)
Srz Dimensionless shrinkage along the minor diameter y0, a, b, c, A, B, C, and D Empirical
constants
Sx Shrinkage along the major diameter (%) z Minor diameter (mm)
Sy Shrinkage along the intermediate diameter (%) ε Porosity (%)
Sz Shrinkage along the minor diameter (%) ω Sphericity
T Temperature °C ρb Bulk density (g/cm3)
v Kernel volume (cm3) ρp Kernel density (g/cm3)
V Bulk volume (cm3)
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