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Scientific African 8 (2020) e00303
Contents lists available at ScienceDirect
Scientific African
journal homepage: www.elsevier.com/locate/sciaf
Comparative analysis of moisture-dependent physical and
mechanical properties of two varieties of African star apple
(Chrysophyllum albidum) seeds relevant in engineering design
David Nwabueze Onwe
a
, Kingsley Charles Umani b , ∗, William Adebisi Olosundea
,
Inimfon Samuel Ossom
a
a
Department of Agricultural and Food Engineering, Faculty of Engineering, University of Uyo, P.M . B. 1017, Uyo, Akwa Ibom State, Nigeria
b
Department of Agricultural Engineering, Faculty of Engineering, Akwa Ibom State University, Ikot Akpaden, Mkpat-Enin, P.M. B. 1167 ,
Uyo, Akwa Ibom State, Nigeria
a r t i c l e i n f o
Article history:
Received 12 January 2019
Revised 3 November 2019
Accepted 31 January 2020
Editor: Dr. B. Gyampoh
Keywo rds:
African star apple
Udara
Nwannu
Seeds
Physical properties
Mechanical properties
Moisture content
a b s t r a c t
This study was conducted to investigate and compare the engineering properties of
two varieties [big (Udara) and small (Nwannu)] of African star apple ( Chrysophyllum al-
bidum ) seeds in Nigeria. The objectives were to determine the effect of moisture on some
moisture-dependent physical and mechanical properties of the Udara and Nwannu seeds.
The parameters considered were geometric (axial dimensions, mean diameters, surface
area, sphericity, aspect ratio, and volume), gravimetric (bulk and true densities, 1,0 0 0 seed
mass and porosity), frictional (angle of repose and static coefficient of friction) and me-
chanical (rupture force, rupture energy and deformation at rupture point) properties of
the seeds in the moisture content range of 3.00–18.03% (w.b.). It was observed that mois-
ture content have a significant effect on all the engineering properties studied at p ≤0.05.
The result also revealed that the angle of repose of Udara seed was higher than Nwannu
seed. The coefficient of static friction of the star apple seeds was greatest on the galvanized
sheet surface, 0.526 for Nwannu seed and lowest on the aluminum surface, 0.210 for udara
seed. The coefficient of static friction was higher on all surfaces for Nwannu seed than for
Udara seed. The force required to rupture the seed, the rupture energy, and the defor-
mation at rupture point of Udara and Nwannu seeds decreased with increase in moisture
content for horizontal (x), transversal (y) and vertical (z) loading direction. The transverse
position showed the highest values of rupture force for both, Udara and Nwannu seed,
∗Correspondence: Kingsley Charles Umani, Department of Agricultural Engineering, Akwa Ibom State University, Ikot Akpaden, Mkpat Enin, Nigeria.
E-mail addresses: davidonwe@uniuyo.edu.ng (D.N. Onwe), kingsleyumani@aksu.edu.ng (K.C. Umani), williamolosunde@uniuyo.edu.ng (W.A. Olosunde),
ossominimfon@gmail.com (I.S. Ossom).
https://doi.org/10.1016/j.sciaf.2020.e00303
2468-2276/© 2020 The Author(s). Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative. This is an
open access article under the CC BY license. ( http://creativecommons.org/licenses/by/4.0/ )
2 D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303
but higher in Udara than Nwannu variety. The lowest rupture force was obtained during
horizontal loading for the two varieties but lower in Nwannu than Udara seed. Similar
results were obtained for rupture energy. The vertical loading position indicated the high-
est deformation at rupture point for both, Udara and Nwannu seed, but higher in Udara
than Nwannu, while, the lowest deformation at rupture point was recorded in transverse
loading for the two varieties, but lower in Nwannu than Udara seed. Properties of ker-
nels, grains, and seeds are essential in the development of equipment for transportation,
handling, and processing.
©2020 The Author(s). Published by Elsevier B.V. on behalf of African Institute of
Mathematical Sciences / Next Einstein Initiative.
This is an open access article under the CC BY license.
( http://creativecommons.org/licenses/by/4.0/ )
Introduction
African star apple ( Chrysophyllum albidum ) fruit belongs to the family of Sapotaceae which is of the West African origin
of exotic fruit with a tough leather-like, green to dark orange skin subject to the stage of ripeness. They are sub-spherical
in shape, slightly pointed at the tip, and ranges in sizes (diameter) and the size of the seeds are directly proportional to the
size of the fruit [43] .
The fruit contains a bunch of about 4–5 seeds stuck organized in the shape of a star. The fruit is round, oblate and
ellipsoid in shape, 5–10 cm (2–4 inch diameter), maybe yellowish or pale-green [33] . The fruit when ripe is pointed at the
apex, up to 6 cm long and 5 cm in diameter ( Figs. 1 and 2 ). The endocarp is orange or golden yellow at ripen state, while
the flesh within the endocarp is mostly orange or light yellow which contains four to five seeds. The seeds are dark brown,
ellipsoidal or oblong measuring up to 2.8 cm long and 1.2 cm wide. When the hard, bony, shiny, dark brown coat of the
seed is broken, it reveals a white-colored cotyledon [16] . Each seed is covered with an outer skin which feels like plastic
and is covered with a creamy white fibrous sweet membrane. The seeds possess a glittery brown casing which feels like
plastic and are covered with a creamy white fibrous sweet membrane.
In recent times, African star apple fruit has become a crop of marketable worth in Nigeria [36] . Chrysophyllum albidum
fruit is a famous tropical fruit tree which has gained popularity in urban and rural areas, and is widely distributed in the
low-land rain-forest zones and frequently found in villages. The fleshy tissue of the star apple fruits is mostly consumed as
a snack while its fruit has been established to contain a high amount of ascorbic acid which is greater than that of orange
and guava. It has also been reported that African star apple is a good source of iron, flavors, and vitamins to foods. The
seeds are also used for local games, livestock feed or discarded [33] .
The commercial value of the fruit is attracting interest particularly in the use of its seed oil, application in livestock feed
as well as its roots and leaves for medicinal purposes [8] . Adebayo et al. [1] described African star apple as one of the
fruits of valuable economic importance in tropical Africa owing to its many industrial, medicinal and food usages. Adesina
[3] , reported that the African star apple fruits could be used for the production of fruit jams and jellies, while Ajewole and
Adeyeye [6] , in a seeming investigation reported that the juice could be a viable element for the production of soft drinks,
wine, and alcoholic drink when fermented. This fruits when eaten fresh are good cures for tooth-ache, constipation, and
sore-throat [33] . Oil extracted from the seed is used for soap making and also the seeds are mostly used to treat intestinal
worms and hemorrhoid [4] .
According to Idowu et al. [24] , the facts about the different properties of agricultural materials are very vital to the design
of suitable machines and equipment which can be used for planting, harvesting, transporting, processing and storage of the
products. A cogent approach to the design of agricultural machinery, equipment and facilities involves the knowledge of the
engineering properties of the agricultural product concerned [57] . Physical properties such as size and densities are useful in
developing equipment and processes for sorting, grading, transporting and storage. The angle of repose is vital for designing
package or storage structures, and the coefficient of friction plays an essential role in transportation, handling and storage
structures. Mechanical properties of agricultural products plays essential role in the oil extraction or expression process.
Agricultural products respond differently to tensile or compressive forces and also behave differently when subjected to
vibration [59] . Therefore, knowing the mechanical properties such as stress, strain, hardness and compressive strength is
vital to design engineers handling and processing of agricultural products [58] as well as determining the power requirement
for different operations [60] .
Oyelade et al. [44] determined some physical properties of African star apple ( Chrysophyllum albidum ) seeds at moisture
content of 8.49% d.b for unspecified variety, but a review of literature showed no information on the mechanical properties
of African star apple seeds, most especially, in relation to the design of an oil expression machine. Thus, this work was
aimed at investigating the physical and mechanical properties of two varieties of African star apple ( Chrysophyllum albidum )
seeds at different moisture content levels ranging from 3.00 −18.03 % w.b and evaluate if there are significant differences
between the properties of the two varieties.
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 3
Fig. 1. Big size variety ( Udara ) of African star apple ( Chrysophyllum albidum ) fruit.
Fig. 2. Small size variety ( Nwannu )of African star apple ( Chrysophyllum albidum ) fruit.
Fig. 3. Big size (Udara) variety of African star apple seeds.
Materials and methods
Sample preparation
Bulk quantity of the two varieties of ripe African star apple ( Chrysophyllum albidum ) fruits, namely Big and Small, was
purchased from the open market. The fruits were given out to be eaten, and the seeds were collected ( Figs. 3 and 4 ),
manually cleaned to remove impurities such as immature seed, dried and taken to the laboratory for analysis.
Moisture content determination
The initial moisture level of the two varieties of African star apple seeds used for the study was determined using the
oven drying method. Three hundred (150) seeds representing the three (3) samples A, B, C was selected from the bulk
sample for the initial moisture content determination using the method in ASABE (2006) as adopted by Olaoye [41] , Ozguven
and Vursavus [45] and Obi and Offorha [35] for castor nut, pine nuts and egusi melon seeds respectively.
4 D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303
Fig. 4. Small size (Nwanu) variety of African star apple seeds.
The three samples of 50 seeds each were weighed and then placed in an air oven at a temperature of 105 °C and allowed
to dry for sixty (60) minutes. The samples were removed from the oven, placed in a desiccator to cool and were weighed in
an electronic weighing balance. The samples were replaced in the air oven and allowed to dry for 30 min and were weighed.
The procedure was repeated until there was a constant weight. The moisture content of the samples on a wet basis (w.b)
was calculated using Eq. (1) and averaged. The moisture content, Mc was defined based on a wet basis as:
M C
wb
(
%
)
=
M
W
−M
D
M
D
×100 (1)
Where MC
wb
= moisture content, % w.b ; M
W
= initial weight of star apple seed, kg; M
D
= weight of African star apple seed
after drying, kg .
Considering the importance of moisture in the properties of agricultural materials, one kilogram each of the African star
apple seeds were conditioned to chosen levels of moisture (3.0 0, 8.0 0, 13.0 0 and 18.03 % w.b) using Eq. (2) according to
Olajide [39] ; BulentCo ¸s kun et al. [14] ; Onimisi and Ovansa [ 42 ]; Enoch et al. [17] ; Umani et al. [62] ; Obi and Offorha [35] ;
Fakayode et al. [18] .
Q = w
1
×M
2
−M
1
100 −M
2
×100 (2)
Where Q = Volume of water to be added, ml;w
1
= Weight of the material, g; M
1
= Initial moisture content of the sample,
% wet basis; M
2
= Final (desired) moisture content of sample, % wet basis .
The already conditioned samples of the two varieties of African star apple seeds were kept in tight polythene bags and
refrigerated at a temperature of about 5 °C for one week so that the required moisture can be evenly distributed. The already
conditioned samples stored in the refrigerator were then removed and placed in a desiccator to avoid moisture loss or gain
before the experiment commenced.
Determination of the physical properties of African star apple seed
One hundred samples were selected at random from the bulk sample for the four moisture levels of 3.0 0%, 8.0 0%, 13.0 0
and 18.03%. The 100 seeds selected for each moisture level was used to evaluate the following physical properties of the
two varieties of African star apple seed required for the study:
•Geometric properties
•Gravimetric properties
•Frictional properties
Geometric properties
Determination of seed size
In order to determine the physical dimensions of the African star apple seeds, 100 seeds sample each of the two varieties
was selected randomly from the bulk sample. The axial dimensions of the seeds at various moisture levels were measured
using an electronic vernier caliper with measured accuracy of 0.001 mm. The geometric, D
g
, and arithmetic mean diameter,
D
a
, of the seed were evaluated using the expression in Eqs. (3) and (4) given by Mohsenin [31] according to Ogunsina et al.
[38] , Ajav and Fakayode [57] and Tava koli et al. [55] .
D
g
=
(
LW T
)
1
/
3 (3)
D
a
=
L + W + T
3
(4)
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 5
Where D
g and D
a are geometric, and arithmetic mean diameters, mm; L, W and T = length, width, and thickness of the
African star apple seeds, mm respectively.
Determination of surface area, sphericity, aspect ratio, and volume
The surface area, S
a ( mm
2
), was determined using the expression in Eq. (5) adopted from Garnayak et al. [19] ; Tavakoli
et al. [55] ; Sharma et al. [53] ; and Obi and Offorha [35] .
S
a
= π( D
g
)
2 (5)
Where S
m
= Surface area of African star apple seed, mm
2
; D
g
= Geometric mean diameter, mm .
The degree of Sphericity, S
p expressed by Mohsenin [31] in Eq. (6) and adopted by Obi and Offorha [35] and Fakayode
[61] , was used to calculate the Sphericity of the two varieties of African star apple seeds.
S
p
=
(LW T )
1 / 3
L
(6)
Where S
p
= Sphericity of African star apple seed; L, W , and T = length, width, and thickness of African star apple seeds,
mm .
The aspect ratio ( R
a
) was calculated using Eq. (7) according to Tabatabaeefa [54] and Obi and Offorha [35] .
R
a
=
W
L
×100 (7)
Where R
a
= Aspect ratio of African star apple seed; L and W = length and width of African star apple seed, mm .
Volume: The average volume of the seed was determined using Archimedes’s principle of water displacement as de-
scribed by Nelkon [34] . The one hundred (100) seed samples were weighed and immersed in a measuring cylinder con-
taining a known volume of water. Water was used because the texture of the Star apple is like plastic and does not absorb
water. The difference in water volume between the new level of water in the measuring cylinder and the initial volume of
water was recorded as the volume of the star apple seed, V.
Gravimetric properties
Determination of 10 0 0 seed mass
The masses of a 10 0 0 sample seeds were determined using electronic weighing balance reading to an accuracy of 0.001 g .
Two samples containing 100 seeds each from the two varieties of African star apple seeds were multiplied by 10 to give the
mass of 10 0 0 seeds [11] which were used the determination of the 10 0 0 seed mass.
Determination of bulk and right density, and porosity
The bulk density was determined by weighing and filling an empty graduated cylinder with the seed and then reweighed
with the seeds [32] . The weight of the seeds was obtained by subtracting the weight of the cylinder from the weight of both
the cylinder and seed. The volume occupied was recorded. The process was repeated five times, and the bulk density ( ρb
)
for each replicate were evaluated using the expression in Eq. (8) as adopted by Sharma et al. [53] .
ρb
=
W
s
V
b
(8)
Where ρb
= Bulk density of African star apple seeds, kg / m
3
; W
s
= Weight of the seed, kg; V
b
= Bulk volume occupied by
the sample, m
3
.
The true density was determined as the ratio between the mass of Star apple seeds and the true volume of the seeds
using the expression in Eq. (9) as:
ρt
=
W
s
V
t
(9)
Where ρt
= True density of African star apple seeds, kg / m
3
; W
s
= Weight of the seed, kg; V
b
= True volume of the sample,
m
3
.
The porosity was calculated from the values of bulk and true densities using the relationship in Eq. (10) by Mohsenin
[32] as:
ε =
1 −ρb
ρt
×100 (10)
Where ɛ = porosity, dimensionless; ρb
= Bulk density of African star apple seeds, kg / m
3
ρt
= True density of African star apple
seeds, kg / m
3
.
6 D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303
Fig. 5. Schematic and pictorial view of the screw device used for determination of the coefficient of friction.
Frictional properties
Angle of repose
The angle of repose of African star apple was determined by the use of a cylinder made of cardboard paper, open at
both ends. The seeds were poured into the cylinder placed on a flat surface to form a pile. The cylinder was then gradually
lifted until it was completely removed, allowing the samples to spread and form a pile. The experiment was replicated five
times, the radius and the height of piled samples were determined and recorded, and the relationship in Eq. (11) according
to Bhatia et al. [13] , Fakayode [61] and Razavi et al. [49] was used to determine the angle of repose, ( θ).
θ= ta n
−1
(
h/r
) (11)
Where θ= Angle of repose of African star apple seeds, °C; h = height of piled sample, mm; r = radius of spread, mm .
Determination of static coefficient of friction
The static coefficient of friction, μ, of the Star apple seeds was determined for four different surfaces; wood, aluminum,
glass, and galvanized steel. Sliding motion occurs only when an applied force has overcome static friction. The surfaces were
gently inclined using a screw device ( Fig. 5 ), and the angle of inclination at which the sample started sliding was recorded
as θ. The procedure was repeated five times for all the surfaces. The static coefficient of friction was determined using the
relation in Eq. (12) as:
μ= tan θ=
h
b
(12)
Where μ= Coefficient of static friction, dimensionless; θ= Angle of repose of African star apple seed, °C; h = height raised,
mm; b = base distance, mm .
Determination of mechanical properties of African star apple seed
The mechanical properties, namely; rupture force, rupture energy and deformation were determined following standard
procedures. A Universal Testing Machine (Model Cussons M500-25AT), which incorporates 30 kg compression load and in-
tegrator, was utilized for the determination of the mechanical properties. The values were measured to 0.001 N accuracy
for the force and 0.001 mm accuracy for the deformation [32] . Seeds of the two varieties of African star apple at four dif-
ferent moisture content levels were loaded individually in the plates of the machine ( Fig. 6 ) and compressed until rupture
occurred as is indicated by a bio-yield point in the force-deformation curve. Once the bio-yield was detected, the loading
was stopped. rupture force, deformation at rupture point, and hardness were measured [40 , 59] .
Experimental procedure and data analysis
The standard method of determining moisture content in the laboratory as approved by International Seed Testing Asso-
ciation, ISTA, was adopted for the determination of the initial moisture level of the African star apple seed, and the rewetting
techniques were used to attain the desired moisture content of the seed. Regression was used to fit the coefficient of the
linear model of the response (physical and mechanical properties) in order to correlate the response variable to the inde-
pendent variable (moisture content). ANOVA was conducted at 5% probability level to determine the significance and the
effect of the factor (moisture content) on the responses. Turkey, pairwise comparison test, was also used to check the differ-
ence in means of the responses for the two varieties of African star apple seeds at 95% confidence level between the levels
of moisture content used for this study using Minitab 17.0 software.
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 7
Fig. 6. Universal Testing Machine.
Tabl e 1
Equations representing the relation between the axial dimensions ( L, W and T ) and moisture content for big and
small varieties of African star apple seed.
Variety MC (%w.b) Equation R
2
Big (Udara) 3.00–18.03 L = 27 . 18 + 0 . 1866 M C −0 . 01630 M C
2
+ 0 . 0 0 0484 M C
3 0.9539
W = 15 . 43 −0 . 17 27 MC + 0 . 02712 M C
2
−0 . 0 0 0905 M C
3 0.9273
T = 8 . 096 + 0 . 1898 MC −0 . 01846 M C
2 + 0 . 0 0 0569 M C
3 0.9983
Small
(Nwannu)
3.00–18.03 L = 17 . 26 + 1 . 726 MC −0 . 1646 M C
2
+ 0 . 004808 M C
3 0.7003
W = 13 . 39 −0 . 2343 MC + 0 . 01808 M C
2 0.9208
T = 7 . 481 + 0 . 1311 MC −0 . 01256 M C
2
+ 0 . 0 0 0497 M C
3 0.9340
Notes: MC is the moisture content (%, w.b.); L is the length (mm); W is the width (mm); T is the thickness (mm);
Results and discussion
Geometric properties
The results obtained for the geometric properties of the two varieties of African star apple (udara and nwannu) that is,
the length, L (mm), width, W (mm), thickness, T (mm), geometric mean diameter, D
g (mm), arithmetic mean diameter, D
a
(mm), surface area, S
a (mm
2
), sphericity, S
p (%), aspect ratio, R
a (%), and the volume V (mm
3
) are presented in Supplemen-
tary Table 1. The moisture level investigated for the two varieties of the African star apple seed ranged between 3.00–18.03%
(w.b). It was generally observed that the average values of geometric properties of the African star apple seed showed rapid
increase relative to increase in the moisture level from 3.00 to 18.0 3% (w.b) and afterward the increment was gradual.
Effect of moisture content on the axial dimensions
Considering the geometric properties studied, it was observed that the average results for the two varieties of African
star apple (Udara and Nwanu) increased as the moisture increases. The length, width and thickness of the big (Udara) vari-
ety of the African star apple seed varied from 27.61–28.09 mm, 15.14–15.84 mm and 8.52–8.86 mm, respectively, when the
moisture level increased from 3.00 to 18.0 3% (d.b) ( Fig. 1 ), whereas the length, width, and thickness of the small (nwannu)
variety are in the range of 21.09–23.06 mm, 12.97–15.01 mm and 7.78–8.68 mm, respectively, as the moisture content in-
creased from 3.00 to 18.03% (d.b) ( Fig. 7 ). These properties are of importance in the determination of the volume of seeds
at various moisture contents. A similar trend was stated by Tavakoli et al. [55] for soybean grains, Bamgboye and Ade-
jumo [9] for Roselle seeds and Bamgboye and Adebayo [11] for jatropha seeds. The results from the Analysis of Variance
(ANOVA) for the effect of moisture content on the average values of length, width, and thickness of the big (udara), and
small (nwannu) varieties of African star apple seed indicate that the effects are statistically significant at 5% level of signifi-
cance.
The results of the regression analysis of the values obtained for the axial dimensions of the two varieties of African star
apple seeds at the various moisture level studied revealed that there is a strong positive correlation, R
2 relating the axial
dimensions and moisture content ( Table 1 ). The average values obtained were fitted in the developed regression model
presented in Table 1 . The coefficient of determination R
2
, obtained for the big (udara) and small (nwannu) varieties of the
African star apple seed ranged between 0.9539 to 0.9983, and 0.7003 to 0.9340 respectively, for the developed regression
equations. The high value of R
2 indicates that the seeds of the two varieties of African star apple enlarge according to
its dimensions when the seed absorbs water [55] . Pairwise comparison between the average values of dimensions of the
big (udara) and small (nwannu) varieties of African star apple seed (Supplementary Tables 5a, 5b, and 5c), revealed that
8 D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303
Fig. 7. Variation of geometric dimensions of big and small varieties of African star Apple with moisture content. Error bars represent the standard deviation
of the mean ( n = 3 ).
their differences are statistically significant at α= 0 . 05 . The variation in the axial dimensions of the African star apple seed
varieties could be a function of the soil type, water, species, as well as the cultivation method used during planting.
Effect of moisture content on the mean diameters, surface area, sphericity, aspect ratio, and volume
The average values obtained for mean diameters (geometric and arithmetic mean diameters), surface area, sphericity,
aspect ratio and volume of the big and small varieties of African star apple seed between the moisture content of 3.00–
18. 03% (w.b.) are presented in Supplementary Table 1. It was observed that these properties increased with increasing mois-
ture content level ( Figs. 8 , 9 , 10 and 11 ) as was reported for jatropha by Bamgboye and Adebayo [11] . The geometric and
arithmetic mean diameters of the udara variety of African star apple seed ranged from 15.26 to 15.77 mm and 17.09 to
17.6 0 mm, respectively, with increase in the moisture from 3.00 to 18.03% (w.b.), while that of nwannu variety was in the
range of 12.56–14.11 mm and 13.95–15.58 mm, respectively, as the moisture increased, as shown in Figu. 8. The surface area,
sphericity, aspect ratio and volume of the big (udara) and small (nwannu) varieties of African star apple seed increased
from 731.67 to 781.39 mm
2
, and 495.66–625.55 mm
2
; 55.17–56.33%, and 46.50–47.82%; 54.84–56.39%, and 61.50–65.09%;
and 1533 to 2222 mm
3
, and 1445–1618 mm
3
, respectively. A similar trend in the result was reported by Paksoy and Aydin
[47] , Sachin et al. [50] , Sacilik et al. [51] , Bamgboye and Adebayo [11] , and Baryeh and Mangope [12] for squash, soybean,
jatropha, hemp, and millet seeds, respectively, but Hsu et al. [23] reported a decrease in surface area as the moisture level
increase for pistachios seed. Tavakol i et al. [55] , reported that the small increment of the sphericity of seeds is due to an
insignificant change in their dimensions as the seeds absorb moisture.
The result from the Analysis of Variance (ANOVA) for the effect of moisture content on the average values of the mean
diameters, surface area, sphericity, aspect ratio and volume of the big (udara), and small (nwannu) varieties of African
star apple seed indicate that the effects are statistically significant at 5% level of significance. The results of the regression
analysis of the values obtained for the mean diameters, surface area, sphericity, aspect ratio and volume of the two varieties
of African star apple seeds at the various moisture level studied revealed that, there is a good correlation, R
2 relating the
studied properties and moisture content ( Table 2 ). The average values obtained were fitted in the developed regression
model presented in Table 2 . The coefficient of determination R
2
, obtained for the big (udara) and small (nwannu) varieties of
the African star apple seed range between 0.8221 to 0.9990, and 0.4 4 4 4 to 0.9969 respectively, for the developed regression
equations. The R
2 of 0.4 4 4 4 for the aspect ratio of nwannu variety is low, this signifies that the correlation between the
different levels of the moisture is weak, though the model can still be used to navigate the response within this moisture
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 9
Fig. 8. Variation of geometric mean diameters of big and small varieties of African star Apple with moisture content. Error bars represent the standard
deviation of the mean ( n = 3 ).
Tabl e 2
Equations representing the relation between the mean diameters, surface area, sphericity, aspect ratio and volume
of big and small varieties of African star apple seed.
Variety MC (% w.b) Equation R
2
Big (Udara) 3.00–18.03 D
g
= 14 . 90 + 0 . 1469 MC −0 . 01078 M C
2
+ 0 . 0 0 0293 M C
2 0.9733
D
a
= 16 . 94 + 0 . 05167 MC −0 . 0 0 0861 M C
2 0.9990
S
a
= 697 . 90 + 14 . 11 MC −1 . 032 M C
2
+ 0 . 02811 M C
3 0.9746
S
p
= 55 . 32 −0 . 1507 MC + 0 . 03778 M C
2
−0 . 001460 M C
3 0.8319
R
a
= 56 . 76 −0 . 9921 MC + 0 . 1298 M C
2
−0 . 004211 M C
3 0.8221
V = 819 . 1 + 319 . 6 MC −30 . 04 M C
2
+ 0 . 9221 M C
3 0.9157
Small
(Nwannu)
3.00–18.03 D
g
= 11 . 20 + 0 . 5885 MC −0 . 050 0 0 M C
2
+ 0 . 0 01458 M C
3 0.9574
D
a
= 12 . 56 + 0 . 6319 MC −0 . 06235 M C
2
+ 0 . 002029 M C
3 0.9016
S
a
= 386 . 40 + 47 . 52 MC −4 . 054 M C
2
+ 0 . 119 5 MC
3 0.9616
S
p
= 46 . 30 + 0 . 05861 MC + 0 . 001405 M C
2 0.9969
R
a
= 71 . 34 −4 . 365 MC + 0 . 3889 M C
2
−0 . 009208 M C
3 0.4444
V = 115 4 + 125 . 2 MC −10 . 26 M C
2
+ 0 . 2630 MC
3 0.7152
Notes: MC is the moisture content (%, w.b.); D
g is the geometric mean diameter (mm); D
a
is the arithmetic mean
diameter (mm); S
p is the sphericity (%); S
a
is the surface area (mm
2
); R
a is the aspect ratio; and V is the volume
(mm
3
). Standard deviations are in parentheses.
range. A pairwise comparison between the average values of mean diameters, surface area, sphericity, aspect ratio and
volume for big (udara) and small (nwannu) varieties of African star apple seed (Supplementary Tables 6a, 6b, 6c, 6d, 6e and
6f), indicate that the difference in their means is statistically significant at α= 0 . 05 .
Physical properties of seeds such as size, shape, specific gravity, surface roughness, colour etc. are essential for developing
various types of cleaning, grading and separation equipment For designing an air screen grain cleaner, the shape and size
of the seeds determine the shape and size of screen openings, angle of inclination and vibration amplitude and frequency
of screens. The shape of seed is an important parameter which affects conveying characteristics of solid materials by air
or water. The shape is also considered in calculation of various cooling and heating loads of food materials. The surface
characteristics, colour and appearance obtained for the African star apple are important parameters for selecting separation
and storage medium.
10 D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303
Fig. 9. Variation of the surface area of big and small varieties of African star apple with moisture content.
Effect of moisture content on the gravimetric properties
The average values obtained for the gravimetric properties (bulk and true densities, 1,0 0 0 seed weight and porosity)
of the big (udara) and the small (nwannu) varieties of the African star apple seeds between the moisture levels of 3.00–
18. 03% (w.b.) are presented in Supplementary Table 2. From Figs. 12 , 13 and 14 , it is shown that the values of the properties
increased when the seed moisture content was increased.
It was generally observed that the average values of geometric properties of the African star apple seed showed rapid
increase relative to increase in the moisture level from 3.00 to 18.0 3% (w.b) and afterward the increment was gradual.
From the result, it was evident that, the average values obtained for the gravimetric properties for udara seed increased
rapidly with an increase in the seed moisture from 3.00–18.03% (w.b.), but the average values obtained for the gravimetric
properties for nwannu seed was less rapid when the moisture level was increased within the studied range ( Figs. 12–14 ). The
bulk and true densities of the African star apple seeds ranged from 700 to 762 kg/m
3
and 980 to 1071 kg/m
3
(Supplementary
Table 2), respectively for the big (udara) variety; and 457 to 461 kg/m
3 and 858 to 881 kg/m
3 (Supplementary Table 2),
respectively for the small (nwannu) variety, when the moisture level was increased between 3.00 to 18.03 % (w.b.). The bulk
and true densities increased due to increase in the seed mass as a result of water addition, which makes the seed mass
higher than its volumetric expansion of the seeds, since density is the quotient of seed mass and its volume [48 , 55] . The
results are in agreement with those reported by Oyelade et al. [44] for African Star Apple ( Chrysophyllum albidum ). This
increasing trend was reported by Bamgboye and Adebayo [11] for Jatropha seeds, Kingsly et al. [27] for dried pomegranate
seeds and Adejumo and Abayomi [2] for Moringa Oleifera Seed. However, Sachin et al. [50] reported that the true density
of soybean decreased when the moisture level of the seed was increased.
The 1,0 0 0 seed mass increased from 1504.43 to 2382.78 g for the udara variety, and 1252.64 to 1428.98 g for the nwannu
variety of the African star apple as the moisture content of the seed increased between 3.00 to 18.03 % (w.b.). The average
values of porosity for udara seed is linear ( Fig. 14 ) when the seed moisture was increased, but that of nwannu seed increased
between 46.50 to 47.82% with a corresponding increase in moisture between 3.00 to 18.03 % (w.b.). The porosity of the seed
is an essential parameter which affects the resistance of airflow within bulk seeds [35] . Bamgboye and Adebayo [11] ; and
Seifi and Alimardani [52] reported an increase in porosity at increased moisture for jatropha and corn. On the contrary,
Koocheki et al. [29] , Adejumo, and Abayomi [2] , and Hosain [22] reported a decrease in porosity at increased moisture level
for watermelon, Moringa Oleifera, and sesame seeds, respectively.
The results from the Analysis of Variance (ANOVA) for the effect of moisture variation on the average values of the
mean gravimetric properties of the big (udara) and small (nwannu) varieties of African star apple seed show that the effects
are statistically significant at 5% level of significance. The results of the regression analysis of the values obtained for the
gravimetric properties of the two varieties of African star apple seeds at the various moisture level studied also revealed
that there is a good correlation, R
2 relating the studied properties and moisture content ( Table 3 ). The values were fitted in
the developed regression model presented in Table 3 . The coefficient of determination R
2
, obtained for the big (udara) and
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 11
Fig. 10. Variation of sphericity and aspect ratio of big and small varieties of African star Apple with moisture content. Error bars represent the standard
deviation of the mean ( n = 3 ).
Fig. 11. Variation of the volume of big and small varieties of African star apple with moisture content.
small (nwannu) varieties of the African star apple seed range between 0.9289 to 0.9998, and 0.8097 to 0.9969 respectively,
for the developed regression equations. The gravimetric properties studied indicate that there is a good correlation between
the various moisture levels of the two varieties of African star apple seed. Pairwise comparison between the average values
of gravimetric properties for big (udara) and small (nwannu) varieties of African star apple seed (Supplementary Tables 7a,
7b, 7c, and 7d), indicate that the difference in their means is statistically significant at α= 0 . 05 .
12 D.N. Onwe, K.C. Umani and W.A. Olosunde et al. / Scientific African 8 (2020) e00303
Fig. 12. Variation of bulk and true densities of big and small varieties of African star Apple with moisture content. Error bars represent the standard
deviation of the mean ( n = 3 ).
Fig. 13. Variation of 10 0 0 seed mass of big and small varieties of African star apple with moisture content.
The gravimetric properties such as bulk and true densities, 1,0 0 0 seed weight and porosity are important in designing
particular equipment or determining the behavior of the African star apple seeds for its handling. The density of the seeds
obtained will decide the size of screening surface, while the surface area and related diameters and density are essential
for the determination of terminal velocity of the African star apple seeds. The terminal velocity is necessary to decide about
the winnowing velocity of air blast for separation of lighter materials in air screen grain cleaners. The density and specific
gravity are needed for calculating the thermal diffusivity in heart transfer operations, in determining Reynold’s number, in
pneumatic and hydraulic handling of the seeds.
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 13
Fig. 14. Variation of porosity of big and small varieties of African star apple with moisture content.
Tabl e 3
Equations representing the relation between the gravimetric properties and moisture content of big and small vari-
eties of African star apple seed.
Variety MC (% w.b) Equation R
2
Big (Udara) 3.00–18.03 ρb
= 694 . 93 + 3 . 774 MC 0.9580
ρt
= 900 . 60 + 34 . 37 MC −2 . 956 M C
2
+ 0 . 08717 M C
3 0.9998
S
1 , 0 0 0
= 662 . 40 + 376 . 0 MC −35 . 03 M C
2
+ 1 . 080 M C
3 0.9289
p = 28 . 20 + 0 . 1580 MC −0 . 01337 M C
2
+ 0 . 0 0 0387 M C
3 0.9525
Small
(Nwannu)
3.00–18.03 ρb
= 455 . 90 + 0 . 2365 MC 0.9409
ρt
= 838 . 70 + 8 . 234 MC −0 . 7273 M C
2
+ 0 . 02213 M C
3 0.9966
S
1 , 0 0 0
= 1006 + 106 . 0 MC −8 . 611 M C
2
+ 0 . 2236 M C
3 0.8097
p = 46 . 30 + 0 . 05861 MC + 0 . 001405 M C
2 0.9969
Notes: MC is the moisture content (%, w.b.); ρb
is the bulk density (kg/m
3
);
ρt
is the true density (kg/m
3
); S
1,0 0 0
is
the 1,0 0 0 seed mass (g); p is the porosity (%).
Effect of moisture content on the frictional properties
The average values of the frictional properties of the big (udara) and the small (small) varieties of the African star apple
seed at various moisture levels are shown in Supplementary Table 3. The frictional properties of the udara and nwannu
seed generally increased as the moisture level increased from 3.00 to 18. 03% (w.b.) with an exception in the angle of repose
which decreased as the moisture was increased ( Fig. 15 ). The variation of the dynamic angle of repose and static coefficient
of friction of udara and nwannu seed with moisture content is shown in Figs. 15 and 16 respectively.
The angle of repose for udara seed ranged from 15.156 to 18.7 20 °, while the coefficient of static friction for udara seed
on the galvanized sheet, plywood, glass, and aluminum surfaces ranged between 0.297 to 0.438, 0.278 to 0.421, 0.257 to
0.383 and 0.210 to 0.336, respectively (Supplementary Table 3). For the nwannu seed, the angle of repose ranged between
13.321 to 14.845 °. The coefficient of static friction of nwannu seed on a galvanized sheet, plywood, glass, and aluminum
surfaces varied between 0.379 to 0.526, 0.362 to 0.517, 0.361 to 0.506 and 0.352 to 0.490, respectively (Supplementary
Table 3). The angle of repose is higher in the big (udara) variety than the small (nwannu) variety. It also decreased with
increase in moisture content ( Fig. 15 ). This result agrees with those reported by Oyelade et al. [44] for African Star Apple
( Chrysophyllum albidum ) seed which was in the range of 13.01–14.69
o
. A similar trend of decrease in the angle of repose with
seed moisture content was reported by Bamgboye and Adebayo [11] for jatropha seed. The static coefficient of friction on
the four surfaces (galvanized sheet, plywood, glass, and aluminum) increased with an increase in moisture levels of the two
varieties ( Fig. 16 ). A similar trend of an increased coefficient of friction with moisture content was reported by Zareiforoush
et al. [56] on paddy grains and Sachin et al. [50] for soybean. It was also observed that the smoother the structural surface,
14 D.N. Onwe, K.C. Umani and W.A. Olosunde et al. / Scientific African 8 (2020) e00303
Fig. 15. Variation of the angle of repose of big and small varieties of African star apple with moisture content.
Fig. 16. Variation of Static Coefficient of friction on different surfaces for big and Small varieties of African star apple with moisture content. Error bars
represent the standard deviation of the mean ( n = 3 ).
the lower the coefficient of friction of the nwannu seeds on the surface, while that of udara seeds was higher with smoother
structural surface. The knowledge of the coefficient of friction will be useful during the calculations of the various forces
required to translate and compress the African star apple seeds during the seed oil expression process. Similar observation
was reported by Ajav and Fakayode [57] for Moringa seeds. The variation in the frictional behavior of the udara and nwannu
seeds could be as a result of the morphological characteristics of the two varieties of African star apple.
The angle of repose is essential in the design of machines for mass flow and transportation of the product. The coefficient
of static friction of seed increased as the moisture content of the seed is increased, this is because the presence of water
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 15
Tabl e 4
Equations representing the relation between the frictional properties and moisture content of big and small varieties
of African star apple seed.
Variety MC (% w.b) Equation R
2
Big (Udara) 3.00–18.03 F
ar
= 21 . 80 −1 . 390 MC + 0 . 133 9 M C
2
−0 . 004282 M C
3 0.9191
S
gs
= 0 . 08824 + 0 . 09292 MC −0 . 008582 M C
2
+ 0 . 0 0 0250 M C
3 0.8378
S
pw
= 0 . 09311 + 0 . 08136 MC −0 . 0 07257 M C
2
+ 0 . 0 0 0208 M C
3 0.8828
S
gl
= 0 . 1336 + 0 . 05466 MC −0 . 0 05017 M C
2
+ 0 . 0 0 0153 M C
3 0.9330
S
al
= 0 . 09194 + 0 . 05149 MC −0 . 004499 M C
2
+ 0 . 0 0 0133 M C
3 0.9483
Small
(Nwannu)
3.00–18.03 F
ar
= 16 . 34 −0 . 6724 MC + 0 . 06349 M C
2
−0 . 001968 M C
3 0.9195
S
gs
= 0 . 2925 0 . 03795 M C −0 . 003441 M C
2
+ 0 . 0 0 0114 M C
3 0.9594
S
pw
= 0 . 2611 + 0 . 04480 MC −0 . 0 0420 0 M C
2
+ 0 . 0 0 0139 M C
3 0.9458
S
gl
= 0 . 2622 + 0 . 04604 MC −0 . 004 94 8 M C
2
+ 0 . 0 0 0174 M C
3 0.8406
S
al
= 0 . 2768 + 0 . 03458 MC −0 . 003625 M C
2
+ 0 . 0 0 0131 M C
3 0.8829
Notes: MC is the moisture content (%, w.b.); F
ar
is the angle of repose ( °); S
gs
is the static coefficient of friction on
galvanized sheet surface; S
pw
is the static coefficient of friction on plywood surface; S
gl
is the static coefficient of
friction on glass surface; S
al
is the static coefficient of friction on aluminum surface.
Fig. 17. Variation of Rupture force of big and small varieties of African star apple with Moisture content. Error bars represent the standard deviation of the
mean ( n = 3 ). Note: TV = Transverse position; VT = Vertical position; HT = Horizontal position .
increases the cohesion between the surface of the material and the seed which causes the surface to be sticky and in turn
limits the ease at which the seed should slide [29 , 55] . For all the frictional properties studied, the lowest value of the angle
of repose was recorded for the small (nwannu) seed, while the highest value was obtained for the big (udara) seed. For all
the levels of the moisture content of the udara and nwannu varieties of African star apple seed, galvanized sheet surface
offered the highest coefficient of static friction. The coefficient of static friction is significant in the development of conveyors
as friction can necessitate the hauling of the seeds to the transporting surface without slippage [55] . Co ¸s kuner and Karababa
[15] , Konak [28] , Baryeh and Mangope [12] and Kaleemullah and Gunasekar [25] established that the coefficient of static
friction of for coriander, chickpea, QP-38 pigeon pea, and areca nut seeds, respectively have a non-linear connection with
moisture content. The frictional properties of the seeds such as coefficient of friction and angle of repose are important in
the design of storage bins, hoppers, chutes, pneumatic conveying system, screw conveyors, forage harvesters, and threshers.
16 D.N. Onwe, K.C. Umani and W.A. Olosunde et al. / Scientific African 8 (2020) e00303
Fig. 18. Variation of Rupture energy of big and small varieties of African star apple with Moisture content. Error bars represent the standard deviation of
the mean ( n = 3 ). Note: TV = Transverse position; VT = Vertical position; HT = Horizontal position.
The result for the Analysis of Variance (ANOVA) for the effect of moisture content on the average values of the frictional
properties of the big (udara), and small (nwannu) varieties of African star apple seed indicate that the effects are statistically
significant at 5% level of significance. The results of the regression analysis of the values obtained for the angle of repose
and coefficient of static friction of the two varieties of African star apple seeds at the various moisture level studied indicate
that there is an excellent correlation, R
2
relating the frictional properties and moisture content ( Table 4 ). The average values
obtained were fitted in the developed regression equation presented in Table 4 . The coefficient of determination R
2
, obtained
for the big (udara) and small (nwannu) varieties of the African star apple seed ranged between 0.8378 to 0.9483, and 0.8406
to 0.9594 respectively, for the developed regression equations. Pairwise comparison between the average values of frictional
properties of the big (udara) and small (nwannu) varieties of African star apple seed (Supplementary Tables 8a, 8b, 8c, 8d,
and 8e), show that their differences are statistically significant at α= 0 . 05 .
Effect of moisture content on mechanical properties
The summary of the values of the mechanical properties of the two varieties of African star apple seeds in transverse,
vertical and horizontal loading positions concerning moisture content is presented in Supplementary Table 4. The result for
the Analysis of Variance (ANOVA) for the effect of moisture content on the average values of the mechanical properties of
the big (udara), and small (nwannu) varieties of African star apple seed revealed that the effects are statistically significant
at 5% level of significance.
Effect of moisture content on rupture force
The force required to break the udara and nwannu seeds generally decreased as moisture content increased from 3.00 to
18. 03% w.b at the transverse, vertical and horizontal loading positions with exception in the transverse loading position of
the udara seed which increased when the moisture content was increased, but suddenly decreased with further increase in
the moisture content of the seed ( Fig. 17 ). The highest values of breaking or rupture force of 138.03 N and 123.10 N, 99.62 N
and 81.65 N and 95.55 N and 80.92 N were obtained at transverse, vertical and horizontal loading positions, respectively
for the big (udara) and small (nwannu) varieties respectively. This trend indicates that seeds with lower moisture content
require high compression force to crack. The rupture force is least in the horizontal loading position and this could be
because the horizontal axis exposes a greater length of the area where the two halves of the seed are seamed. The transverse
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 17
Fig. 19. Variation of Deformation of big and small varieties of African star apple with Moisture content. Error bars represent the standard deviation of the
mean ( n = 3). Note: TV = Transverse position; VT = Vertical position; HT = Horizontal position .
axis gave the highest force, and cracking the seed on that axis destroys the kernel inside. The trend of decrease in braking
force with an increase in moisture content was reported by Seifi and Alimardani [52] for corn, Ahmadi et al. [5] for fennel
seed, Aktas et al. [7] for safflower seed, Oduma et al. [ 37 ] for african oil bean seed and Tavako li et al. [55] for soybean
grains.
Pairwise comparison between the average values of force required to break the big (udara) and small (nwannu) varieties
of African star apple seed (Supplementary Table 9), revealed that their differences are statistically significant at 5% level of
significance.
Effect of moisture content on rupture energy
The relationship between the rupture energy and moisture content is shown in Fig. 18 . The rupture energy for the udara
and nwannu seed generally does not show a definite increase or decrease with increase in moisture content from 3.00 to
18. 03% w.b at the transverse, vertical and horizontal loading positions ( Fig. 18 ). The breaking or rupture energy of 0.160 J and
0.106 J were the highest in the transverse loading, 0.091 J and 0.087 J in the vertical loading, and 0.085 J and 0.086 J in the
horizontal loading for the udara and nwannu varieties respectively. The rupture energy for the udara variety of the African
star apple was the highest across the three loading positions compared to the nwannu variety. The trend in the results is
similar to those reported by Seifi and Alimardani [52] for corn, Ozumba and Obiakor [46] for palm-kernel seed, Manuwa
and Muhammad [30] for shea kernel, Bamgboye and Adejumo [10] for roselle seed and Tavakoli et al. [55] for soybean.
A pairwise comparison between the average values of the rupture energy of the big (udara) and small (nwannu) varieties
of African star apple seed (Supplementary Table 10) , show that their differences is statistically significant at α= 0 . 05 , except
the udara and nwannu levels comparisons of 13%w.b - HLP and 13%w.b –TLP, 18.03%w.b - VLP and 13%w.b –TLP, 3%w.b -
HLP and 13%w.b –TLP, 3%w.b - VLP and 13%w.b –TLP, 5%w.b - VLP and 13%w.b –TLP, 13%w.b - HLP and 3%w.b –HLP, 13%w.b
- HLP and 3%w.b –VLP, 18.03%w.b - VLP and 3%w.b –HLP, 18.03%w.b - VLP and 3%w.b –VLP, 3%w.b - HLP and 3%w.b –HLP,
5%w.b - VLP and 3%w.b –HLP, 3%w.b - HLP and 3%w.b –VLP, 3%w.b - VLP and 3%w.b –VLP, and 5%w.b - VLP and 3%w.b –
VLP respectively, were statistically insignificant (Supplementary Table 10).
Effect of moisture content on deformation
The deformation at rupture point for the udara and nwannu seeds generally decreased as moisture content of the seeds
increased from 3.00 to 18.0 3% w.b when the seeds were placed at transverse, vertical and horizontal loading positions
with the exception in the transverse loading position of the udara seed which increased with increase in moisture content
18 D.N. Onwe, K.C. Umani and W.A. Olosunde et al. / Scientific African 8 (2020) e00303
( Fig. 19 ). The deformation at rupture point of 1.690 mm and 1.137 mm, 2.670 mm and 1.651 mm, and 2.640 mm and
1.596 mm were the highest values in the transverse, vertical and horizontal loading positions respectively, for the big (udara)
and small (nwannu) varieties respectively. The highest deformation was obtained in the transverse position and closely
followed by horizontal loading position of the udara variety compared to the nwannu variety, this could be because the
rupture force was maximum at the transverse loading position, and cracking the seed on that axis destroys the seed. In
comparison with other oil-bearing crops, maximum values of 0.96, 1.11, and 0.92 mm was reported for rupture deformation
in horizontal (x), transverse (y) and vertical (z) loading position, respectively for jatropha seeds by Karaj and Müller [26] .
The trend of decrease in deformation with increased moisture content was reported by Gupta and Das [20] for sunflower
kernel, Seifi and Alimardani [52] for corn, Ahmadi et al. [5] for fennel seed, Aktas et al. [7] for safflower seed, Tavakoli et al.
[55] for soybean grains and Heidarbeigi et al. [21] for khinjuk seed.
Pairwise comparison between the average values of deformation at rupture point for the big (udara) and small (nwannu)
varieties of African star apple seed (Supplementary Table 11), revealed that their differences are statistically significant at
5% level of significance.
The mechanical properties such as hardness, compressive strength, impact and shear resistance affect the various oper-
ations of agricultural processing. The impact and shear resistance are important for size reduction. This information on the
mechanical properties of African star apple is useful in determination of the appropriate methods of crushing, breaking or
cracking the seeds. These properties also play important roles towards seed resistance to cracking during processing.
Conclusion
Some moisture-dependent properties of the two varieties of African star apple ( Chrysophyllum albidum ) seed in the mois-
ture content range of 3.00 - 18.0 3% (w.b.) was studied. The results were concluded as follows:
(a) The effect of moisture content on the geometric, gravimetric and frictional properties of big (udara) and small
(nwannu) seeds was significant at 5% significant level ( p < 0.05).
(b) It was shown that the increase in the moisture content of the two varieties of African star apple seed results to
increase in the average values of the geometric, gravimetric and frictional properties, except for the angle of repose.
(c) The force required to rupture the seed decreased as moisture content increased at the three loading positions. It was
highest in the transverse direction and least in the horizontal direction.
(d) The rupture energy does not show a definite increase or decrease with increased moisture content.
(e) The deformation at rupture point decreased with increased moisture content.
(f) The correlation between the physical properties studied and the moisture content was mostly quadratic and polyno-
mial except for the bulk density of the two varieties of African star apple seed which were linear.
(g) The coefficient of determination R2, for the developed regression models, was high for the two varieties of African
star apple seed, except for the aspect ratio of the small (nwannu) seed (0.4 4 4 4).
(h) Considering the respective moisture levels, the highest coefficient of static friction was obtained when a galvanized
sheet was used for nwannu seed.
Notation
TLP = Transverse Loading Position; VLP = Vertical Loading Position ; HLP = Horizontal Loading Position
Acknowledgment
The authors acknowledge Ukpong, Unyime Udo (07/EG/AE/134), and all the technologist of the Laboratory of Agricultural
and Food Engineering of University of Uyo, Uyo, Akwa Ibom State, Nigeria.
Declaration of Competing Interest
The authors wish to declare that they have no conflict of interests.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.sciaf.2020.
e00303 .
D.N. Onwe, K.C. Umani and W.A . Olosunde et al. / Scientific African 8 (2020) e00303 19
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