Variation of the Physical Properties of Sheanut (Vitellaria Paradoxa Gaertn.) Kernels during...

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, [email protected]

†Department of Process Engineering, National School of Agro-Industrial Sciences, Universityof Ngaoundere, Cameroon, [email protected]

‡Department of Process Engineering, National School of Agro-Industrial Sciences, Universityof Ngaoundere, Cameroon, d [email protected]

∗∗Department of Electrical Engineering, Energetics and Automation, National School of Agro-Industrial Sciences, University of Ngaoundere, Cameroon, kuitche [email protected]

††Department of Chemistry, Advanced Teachers Training College ENS, University of Yaounde,Cameroon, [email protected]

‡‡Department of Food Science and Nutrition, National School of Agro-Industrial Sciences, Uni-versity of Ngaoundere, Cameroon, [email protected]

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.

1

Bup et al.: Physical Properties of Sheanut Kernels during Drying

Published by The Berkeley Electronic Press, 2008

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.

2

International Journal of Food Engineering, Vol. 4 [2008], Iss. 7, Art. 7

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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

3



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

4



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.

5



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

6



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

7



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

8



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

9



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

10



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.

11



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

12



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

13



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)

14



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



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

16



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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