Food Science and Technology 10(3): 55-74, 2022 http://www.hrpub.org 
DOI: 10.13189/fst.2022.100301 
Physical and Mechanical Properties of Coffee Cherries 
and Beans in Africa: Review and the State of Arts 
1,* 1 1
Gaspard Bizimungu , Roger Houêchéné Ahouansou , Clarence Semassou , 
2
Jean Claude Dusabumuremyi  
1Department of Mechanical and Energy Engineering, Faculty of Science and Technology, Polytechnic School of Abomey-Calavi, 
University of Abomey-Calavi, 01 BP 2009 EPAC, Benin 
2Department of Agricultural Economics and Agribusiness, College of Basic and Applied Sciences, University of Ghana, 
P.O. Box LG 68, Legon-Accra, Greater Accra Region, Ghana 
Received April 9, 2022; Revised June 15, 2022; Accepted July 11, 2022 
Cite This Paper in the Following Citation Styles 
(a): [1] Gaspard Bizimungu, Roger Houêchéné Ahouansou, Clarence Semassou, Jean Claude Dusabumuremyi , 
"Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts," Food 
Science and Technology, Vol. 10, No. 3, pp. 55 - 74, 2022. DOI: 10.13189/fst.2022.100301. 
(b): Gaspard Bizimungu, Roger Houêchéné Ahouansou, Clarence Semassou, Jean Claude Dusabumuremyi (2022). 
Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts. Food Science 
and Technology, 10(3), 55 - 74. DOI: 10.13189/fst.2022.100301. 
Copyright©2022 by authors, all rights reserved. Authors agree that this article remains permanently open access under the 
terms of the Creative Commons Attribution License 4.0 International License 
Abstract  Coffee is an important source of income for especially Ghana. Therefore, the information on the coffee 
African smallholder farmers. The world is predominated quality attributes can help to improve the performance 
by three coffee varieties, namely Arabica, Robusta, and parameters of the coffee de-pulping machine and enhance 
Liberica originated in the African countries. However, the the price of African coffee. 
low coffee quality attributes can affect the coffee beans‟ 
price due to the improper post-harvesting processes. Keywords  Arabica Coffee, Coffee Bean, Coffee 
Therefore, this work is aimed to review the physical and Cherry, Coffee Quality, Coffee Processing, De-pulping 
mechanical properties of the African coffee cherries and Process, Robusta Coffee 
beans on the country-wise level, and review the methods 
used to measure quality parameters and compare the key 
coffee quality parameters. This work was qualitatively 
conducted using secondary data on African Arabica and 1. Introduction
Robusta coffee from journals, conference proceedings, and 
reports. Methods used for determining the coffee beans‟ Coffee varieties available in approximately 80 countries 
properties were explored country wise. The coffee [1,2] worldwide originated in East and Central African 
attributes of the two coffee varieties were compared countries. Coffee Arabica originated from Ethiopia and is 
through statistical analysis using ANOVA. The review highly cultivated in tropical countries of Africa, Asia, and 
found that the coffee quality depends on geographical Southwest America [3]. The Arabica coffee (Coffee 
characteristics, agronomic factors, and post-harvesting Arabica L.), which is a highland coffee variety originated 
processes. The coffee cherries de-pulping process is linked either from the Great Rift Valley (kaya plateau), or the 
to the coffee quality to avoid the damage of parchment Kaffa plateau of Southwestern Ethiopia, Mt. Imatong or 
coffee beans based particularly on the size and shape of Boma plateau of Sudan, and Mt. Marabit of Kenya 
coffee cherries and beans. The analysed quality parameters (Berthoud and Cherrier, 1988). It was transmitted from 
showed that the Ethiopian Arabica coffee beans were larger Ethiopia to Yemen by slaves and later spread worldwide. 
than the Ghanaian Robusta coffee beans. While the size of The Robusta coffee (Coffee anaphora L. or also namely 
Robusta coffee in Eastern African countries, particularly coffee Canephora), a lowland coffee that originated either 
Uganda, is bigger than those in Western countries, from Central Africa parts such as the Congo River basin, or 
56 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
East Africa such as Kenya or Uganda (in Bunyoro district, de-pulping process is linked with the size, moisture content, 
or Buganda Forest). It is primarily cultivated in the West sphericity, and coefficient of friction of coffee beans; and 
African (Ivory-Coast), Central [3], and Eastern (Uganda) the price is linked with tonnage, damaged beans and aroma 
regions [4]. of green coffee beans [18,19]. Consequently, the improper 
The coffee Arabica and Robusta are also producing post-harvesting of coffee cherries such as traditional 
about 98 to 99% of total African production from around de-pulping at the farmer level and small-scale de-pulping 
34 countries [4,5]. Africa contributes every year up to 10% locally across the continent can affect the quality of 
of global coffee production. The Eastern, Western, and parchment coffee beans. 
Central African countries produce 69%, 30%, and 1% Therefore, the objectives of this present review were to 
continent-wide, respectively [6]. Thus, world coffee investigate the physical and mechanical properties of both 
production increased annually due to the country‟s coffee cherries and beans from ten producing countries of 
agriculture policies. Africa, evaluate and describe the methods used in some 
The other known species are Liberica coffee (Liberian or countries, determine each coffee quality parameter 
Liberica coffee) originated either from Western Africa in obtained during wet or dry process, and compare the 
the tropical forest of the present Liberia country or properties of beans in ten African coffee-producing 
probably in Ivory-Coast as reported by Prakash et al. 2005 countries. 
and [7], and coffee Dewvrei (Excelsa) [8]. These all types 
of coffee produce less than 1% commercially in the world. 
Therefore, these less productive coffee species account for 2. Physical Parameters of Coffee
up to 103 types and are being practised by small-scale Cherries and Beans
farmers in different rural areas of the world [2,7]. 
According to Avallone (1999), from a morphological 
perspective view, the ripened coffee cherry has either 2.1. Weight, Volume, Density, and Shell Content 
ellipsoidal, ovoid, glabrous, or an oval shape of about 1 cm 
The mean weight of a coffee bean or cherry [20] can be 
x 2 cm for polar and equatorial diameters with two seeds 
determined by weighing 1000-beans of a selected sample 
curved on one side and flat on the other one. Coffee fruits 
[21,22] with replications [23,24] using a digital balance 
also have four main types of bean shapes. These include 
with 0.0001g accuracy [7,20,25,26]. The bean screen 
standard, peaberry, triage, and elephant shapes [3,4]. Their 
having specific holes of 14/64-inch [20,25], and sieve 
size mainly, length and width and thickness as described by 
number 19 [22] is generally used to determine the 
Bikila [4] can vary depending on environmental conditions, 
percentage weight of each variety [20]. To calculate the 
planting conditions [9,10], and the geographical and 
corresponding volume of green coffee, Eq. (1) was used 
climate factors [3,11–13]. Thus, temperature and altitude 
[7,20,25,27]. According to the studies [7,17,28–30], water 
are the main criteria for selecting a parcel of land for 
displacement method can be used to validate the Eq. (1). 
growing coffee as determined [14]. 
Despite the ISO standard 6669: 115 (en) theory, the bulk 
However, de-pulping coffee cherries and de-hulling of 
density is the mass of coffee beans over their occupied 
parchment coffee beans can be considered during the 
volume of the container [17,29], and it can also be 
investigation of coffee beans‟ quality attributes that were 
calculated using the Eq. (2) [20,25]. Bulk density is 
defined according to ISO (2000). There are two types of 
measured using a graduated container of 500 mL 
cherries harvesting means [9] using either hand-picking 
[10,20,30]. The bulk density is essential in designing the 
which is time-consuming [3], or the stripping method. The 
hopper, storage, and grading equipment [20]. The true 
last one is most preferred in dry processing [15]. 
density is the ratio of the original volume of coffees to their 
In addition, the post-harvested coffee cherries and beans‟ 
real weight and it is usually greater than 1g/mL [7]. Then, 
profiles can vary in terms of size, shape, and density as 
the true density can be calculated using Eq. (3) 
well as moisture as defined by Binmahfuth [15]. Thus, 
[19,20,25,28]. Thereafter, the shell content is the silverskin 
information on seeds in terms of physical and mechanical 
material covering the parchment coffee and can be 
characteristics is also needed for the design and 
determined using Eq. (4) [28]. Hence, weight, volume and 
development of agricultural equipment such as coffee 
shell content help to separate good cherries and floaters 
pulping machines. In this concern, the ISO 9116 (standard, 
before the de-pulping process and determine the coffee 
2004a) described the quality of green coffee beans and 
pulping machine capacity.  
defined their respective measuring methods for each coffee 
3
quality parameter at each level [16]. Therefore, the high Volume (V)=π/6*L*W*T (mm )  (1) 
precision instruments that are mostly used for determining Bulk-Density (𝜌b)=(weight of washed green coffee 
the physical quality characteristics of coffee cherries and beans/bulk volume of green coffee beans) (g/mL)  (2) 
beans after harvesting the coffee cherries were described 
Apparent Density (𝜌t) (g/mL)=weight of washed green 
by Saparita et al. [17]. The coffee quality was defined 
coffee beans/geometric volume of green coffee bean (g/mL) 
based on bean size, tonnage among other physical 
(3) 
characteristics and price [10]. The coffee cherries‟ 
Food Science and Technology 10(3): 55-74, 2022 57 
Chell/Sliverskin Content (Cs)=((Wus- Ws)/ Wus)*100  c oefficient of friction can affect the separation efficiency 
(4) of parchment coffee beans with pulps during the pulping 
process. 
2.2. Linear Dimensions, Arithmetic and Geometric Filling angle of repose (μ)= tan-1(2H/D)    (9) 
Diameters and Aspect Ratio 
Dynamic angle of repose (μd)= tan-1(H/B)   (10) 
The sizes of cherries and beans are mainly parameters Coefficient of friction (Cf)=tanθ  (11) 
for defining the quality of coffee. Precision linear 
measuring instruments are used to measure the size [29] of Coefficient of Contact Surface (CSS)=(FSA-TSA)/FSA 
(12) 
the coffee seeds. High precision digital caliper with 
sensitivity or micrometer gauges (0.001 mm) where, Front surface Area (FSA)=π/4*L*W (mm2) and 
[7,10,20,25,28,29] were used to measure the size of beans Transverse Surface Area (TSA)= π/4*L*W (mm2). 
[20,23]. However, to find means, and deviations, and 
minimize errors in measurements; replication of the tests 3.2. Moisture Content, and Porosity 
has been performed [31]. Then, diameters and other 
dimensions can be calculated analytically from the premier Moisture is an essential factor for the preservation of 
dimensions using empirical formulas [7,10,20,25,28,29], coffee beans‟ quality [16]. It can also affect the de-pulping 
as shown in Eqs. (5-8). Thus, these parameters are more operation which is not performed before 24 hours after 
important for setting up the coffee pulping machine cherries are being harvested. Normally, it is recommended 
mechanism and designing the hopper of the machine.  to pulp cherries within 8 hours as practicing locally in 
Rwanda. The moisture content of dried coffee beans which 
Arithmetic Mean Diameter (AMD)= 
ranges between 10-12.5% is very important for 
= (L+W+T)/3 (mm)      (5) 
maintaining the quality and flavour of the coffee. The 
Geometric Mean Diameter (GMD)= (L+W+T)1/3 (mm) (6) moisture content can be measured using different methods. 
Aspect Ratio (AR or R )= W/L (mm)          (7) These include the direct measurement using Moisture Bulk a
2 Density Teste AMTAST JV-010S (P 30) with a range of Cross-Sectional Area (CSA)=π/12*(L+W+T) (g/mL) (8) 
0-40% (wb) [28], and mass difference of beans dried using 
an oven (UNB 400, Memmert, Germany [7,22], hot air 
oven (J.P Selecta, S. A No. Serial #0473987 Barcelona, 
3. Mechanical Parameters of Coffee Spain (Mikru Tesfa 2019) at 1050C in 24 hours. To identify 
Seeds if there are holes in the seeds or not, porosity can be 
measured using a porosity meter [22], and calculated using 
3.1. Angle of Repose, Coefficient of Static Friction, and the empirical formula using bulk and true density as shown 
Coefficient of Contact in Eq. (13) [33]. Thus, Arabica cherries harvested during 
the heavy rain season can be de-pulped well without water 
Angle of repose shows how the seeds can sit on the whereas the Robusta cherries sometimes need to be soaked 
ground or behaviours of how they can slide over to increase moisture before de-pulping operation [4]. 
themselves during the harvesting and post-harvesting 
Porosity (Є)=(1-ρb/ρt)*100 (g/mm3)  (13) 
process. It can be identified in two types, emptying and 
filling angle of repose [7]. The filling angle of repose can 
be measured as defined by [10,22,28], and can be 3.3. Shape Index, Sphericity, and Roundness 
calculated using Eq. (9). The surface area used is almost 
The shape index (SI) defines if a coffee bean has an oval 
made of stainless steel, aluminium, acrylic, and wood 
or spherical shape for SI >1.5 or SI<1.5, respectively 
sheets [28]. Therefore, the emptying angle of repose can 
[20,25]. It is an important parameter for designing 
also be measured [7]. The dynamic angle of repose can be 
pulper/huller, coffee screening, and grading machines [17]. 
measured as described [28,29], and it can be calculated 
Eq. (14 or 15) [17] and Eq. (16) [20,25], are used for 
using Eq. (10). The coefficient of static friction of coffee 
calculating the shape index. The sphericity and circularity 
beans or cherries can be measured using different materials 
of the coffee beans are the essential parameters for 
like aluminium [22], stainless steel and plywood sheet 
describing the bean shape [27]. Sphericity is obtained using 
[17,28,30], fiberglass surfaces [30], and hallowed plastic 
the Eq. (15) from measured size [22,23,28] or using the 
cylinder [22,28,29]. The coefficient of the static friction 
revealed formula [34] using Eq. (16). Therefore, the 
measuring method was described [7,32] to define how the 
roundness ratio can be determined using Eq. (17). Hence, 
seeds slide on coffee processing equipment. The formula 
de-pulping cherries regular in shape can decrease 
used to calculate the coefficient of static friction is shown 
damaging the parchment coffee beans. 
in Eq. (11) [17,29]. The coefficient of contact of washed 
coffee beans was calculated using Eq. (12) [20,25]. Thus, Shape Index (SI)= √(W+T)  (13) 
the angles of repose can affect the feed rate, whereas the or  SI=De/√(Dp*T)  (14) 
58 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
Sphericity (Sp)=(L×W×T)1/3/L  (15) coffee including bulk density, length, width, thickness, and 
2 cross-section area, sphericity and shape index [10,20,25]. Sphericity (Sp)=(Di- ̄) /(DN)2  (16) 
Thus, the physical properties of the coffee obtained in the 
Roundness ratio (Rr)=r/R     (17) two reports during the drying process cannot be only based 
on selecting/designing the equipment in wet processing in 
3.4. Average Hardness and Fracturability the country. Besides the drying process can be performed 
using different methods [3]; however, in the two-research 
The hardness is essential for evaluating cherries and conducted, the drying processes used were not specified 
beans‟ feeding amount, their reduction in size; and defining and their effect on coffee beans. Therefore, despite the 
the fracture force that helps in designing a coffee crusher advantages of the dry process [3] and wet processing 
[17]. The fracture force and deformation of cherries were methods, optimizing the coffee post-harvest processes for 
determined using the platform of Instron (Instron, 5566, both coffee Arabica and Robusta‟s physical properties at 
USA) testing machine [7,17,35]. Thus, the force is also different levels of moisture content can also be based on for 
important during pulping processing to reduce the breaking selecting, designing, and developing the coffee processing 
rate of beans. equipment. 
Table 1.  Ethiopian Arabica Coffee Beans Properties [10, 20] 
3.5. Colour and Coefficient of Variation 
Parameters Range Mean 
Referring to [7], the colour was measured by using a 
colour meter (CR-10, Konica Minolta, Japan). Then, the 0.11 to 0.19 [20] 0.15 Weight of bean (g) 
Coefficient of Variation (CV) was also used to define the 0.127 to 0.2033 [10] 0.16 
quality of the measured parameters regarding their mean of Bulk density (g/mL) 0.66 to 0.73 0.68* 
coffee beans, which are naturally irregular in shape and 
Apparent density (g/mL) 0.98 to 1.68 1.35 
size [20,25]. During selective harvesting, the farmers can 
only pick either the ripped, overriped or immature cherries Moisture content (%)  9.77 to 13.0 11.17 
based on colour. Therefore, the cherries‟ colour is the most Weight over screen 14 (%) 71.94 to 99.09 92.82 
important parameter [10,20] because pulping immature 8.28 to 10.4 [20] 9.34* 
and avorriped damages the beans and reduces parchment Length (mm) 
9.01 to 11.34 [10] 11.95 
coffee beans‟ quality. Hence, these affect the efficiency 
and capacity of coffee pulping machines indirectly. 5.59 to 6.82 [20] 5.935* 
Width (mm) 
6.48 to 7.55 [10] 8.45 
4. Country-Wise Coffee Beans Quality 3.52 to 4.48 [20] 4* Thickness (mm) 
Parameters and Research Gaps 2.82 to 4.66 [10] 4.90 
Volume (mm3) 97.17 to 158.30 128.8 
4.1. Ethiopia 85.55 to 142.10 [20] 103.71 
Cross-section area (mm2) 
The properties of green coffee beans as shown in Table 1 122.12 [20] 122.12* 
were investigated on coffee seeds obtained using the 63.11 to 72.10 66.45 
Sphericity (%) 
drying method [10,20]. The coffee beans investigated were 65 to 72 70 
from 25 regions of Ethiopia to select the equipment for 
1.63 to 2.01 1.87 
sorting, separating, and cleaning. Shape-index (%) 
The experiments started by weighing the 100-beans 1.19 to 1.94 1.5 
weight selected randomly from 16 regions using a digital Coefficient of contact (mm2) 50.49 to 60.78 56.38 
balance with an accuracy of 0.0001 g, ER-120a, Japan; Geometric mean diameter 
5.09 to 7.26 6.175* 
with triplications measurements to find the individual (mm) 
mean weight of a bean. Reviewed data showed that weight Arithmetic mean diameter 
is almost the same with a mean of 0.15 g [20], and 0.16 g 5.31 to 7.81 6.17* (mm) 
[10]. However, Tesfa [19] found that, the coffee bean 
Equivalent mean diameter 
weight depends on moisture content, geographical 5.16 to 7.38 6.27* (mm) 
conditions, and coffee varieties. The linear dimensions 
Front surface area (mm2) 32.86 to 65.91 49.385* 
were taken for each bean using the electronic digital caliper 
with 0.01 mm accuracy [10,20]. The other physical and Aspheric ratio 0.66 to 0.81 0.735* 
mechanical properties were calculated based on these 
parameters in both reports. They were analysed using 4.2. Ivory-Cost 
One-Way ANOVA. The properties of the coffee beans 
were calculated from the work conducted on Ethiopian In Ivory-Coast, the chemical quality and sensorial 
Food Science and Technology 10(3): 55-74, 2022 59 
characteristics of the coffee beans were investigated [36]. coffee. The difference in density happened because the 
Those properties were investigated on the Robusta coffee density of coffee beans depends on the mass variation, not 
harvested from three regions of the Ivory-Coast. These on volume. This means that the de-pulping operation is 
regions contribute about 41% of coffee production in the linked with the coffee beans‟ size and weight. However, in 
country. The properties investigated varied depending on general, the volume and variations in the size were 
different moisture content (Abengourou: 10.7%, Aboisso: influenced by climate and agronomic factors rather than 
11.01%, and Divo: 11.2%) with a mean of 10.9%. The both processing methods and the season. Conventionally, 
moisture content can affect physical parameters and the there are sun drying and mechanical drying methods [3]. 
quality of coffee [3]. The variation of moisture content Therefore, the study was conducted on one coffee variety 
directly affects the chemical properties, sensorial from one region and parameters can show a difference 
properties, weight, and density of coffee [25,37]. However, from one region to another, or between different coffee 
the work was focused only on the chemical properties of varieties. Thus, besides that, the ratio in mass or weight of 
coffee Robusta, and it had not been extended to any cherry mass to parchment coffee beans mass can be 
physical properties of coffee beans. Therefore, moisture evaluated to assess the effect of the coffee pulping machine 
content level can also cause the crushing of beans during on parchment coffee mass. 
the pulping process. 
4.4. Ghana 
4.3. Kenya The effect of moisture content of Robusta coffee beans 
Table 2.  Kenyan Coffee Arabica Coffee Beans Properties [9,34] on physical and some aerodynamic or hydrodynamic 
properties from Ghana has been analysed in Uganda by 
Parameters Range Mean Niwagaba et al. [37]. The linear dimensions, diameters, 
Moisture content 10.17 to 10.84 10.46 mass, densities, sphericity, surface area and porosity, 
Mass (g) for 100 beans 14.67 to 17.33 16 coefficient of static friction, emptying and filling angle of 
repose were investigated as shown in Table 3. All these 
Volume (cm3) 14.11 to 17.55 15.83 
parameters increased as moisture content increased from 8 
density (kg/cm3) 895.81 to 1227.0 1061.405* to 12, 24, and 30% except sphericity which had minimal 
Weight (g) 0.149 to 0.163 0.154667 change; and porosity which was decreasing. This was the 
Weight-100 dry fruits (g) 17.11 to 20.7 18.448 same for the coffee cherries during the drying process, 
where moisture content in cherries significantly affects 
10.97 to 11.6 11.34333 
Length (mm) their physical properties by decreasing in superficial area, 
10.46 to 12.18 10.812 volume, and diameter as reported by Ghosh et al. [3]. 
Width (mm) 8.26 to 8.78 8.58333 However, the [33], reported that the parchment coffee 
beans size don‟t increase with the increase in moisture 
Seed width (mm) 4.56 to 5.02 4.75 
content, even if mass can be increased. Moreover, most of 
Depth (mm) 4.90 to 5.07 4.96 
the measured properties of coffee beans and the calculated 
Seed thickness (mm) 2.49 to 2.90 2.791 ones from the measured linear dimensions were having 
0.006536 to P-value≤0.05. The investigation was not done on directly 
Sphericity 0.69 
0.006899 harvested and processed coffee beans; this can slightly 
Volume (mm3) for 100 affect the results. However, using the same approach and 
241 to 272 257.2 
beans the correlation equation as initiated, it can be better to 
analyse the effect of moisture content on physical and 
The comparison of effects of pulping methods (disc, engineering parameters of coffee beans at a controlled 
drum, and eco-pulper) on the physico-chemical and moisture content. This is because there is a contradiction in 
sensorial properties of coffee Arabica in Kenya was the science as reported by Ghosh et al. [31], that some of 
explored by Koskei et al. [38]. The heavier parchment the properties of coffee, such as length, width, and 
coffee beans were dried up to 10.46% (wb) to each sample thickness were not affected by moisture content as 
for every pulping method. The analysed physical mentioned earlier. Then after, different drying methods can 
parameters for the case of Kenya were mass, volume, and also be used to determine the moisture content of coffee 
density as shown in Table 2. The review showed that the beans and cherries to validate the correlation equations 
mean moisture content was higher for coffee from disc established. The effect of moisture content on coffee 
pulper of 10.75% wb than that of other pulping methods attributes and de-pulping machine performance in wet 
due to immediate drying after pulping operation and, processing can also be investigated using freshly harvested 
coffee beans mass was lower of 14.5g disc pulper. Density cherries. Hence, the work can be extended to investigate 
obtained from season 1 using eco-pulping of 1222.62kg/m3 the breakage force of cherries and beans in de-pulping and 
was lower than that of season 2 obtained of 1226.2kg/m3 hulling operations, respectively, considering the level of 
using also eco-pulping process due to the higher mass of moisture content [4]. 
60 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
4.5. Nigeria machines for other types of seeds can be used for 
processing coffee after evaluating that they are fitting with 
Olukunle et al. [29], conducted a study to explore the the properties of coffee to provide optimum yield with 
coffee seeds and beans‟ properties in Nigeria. Those good quality of coffee beans. The expression of the relation 
properties are size, sphericity, geometric diameter, ratio between axial dimensions has been initiated. 
emptying angle of repose, co-efficiency of friction, However, the research did not cover the filling angle of 
densities (Bulk & true), and porosity as shown in Table 4. repose, volume surface area, cracking force, and specifying 
The coffee beans at 10.7% moisture content had higher the drying method used as well as the type of coffee 
sizes compared to the other types of seeds that were investigated. It can also be better to compare coffee 
investigated for validation. The research revealed parchment beans size from wet to dry process method of 
differences in comparisons but the bulk density and true cherries from different regions to provide the information 
density were lower and higher for coffee seeds and the needed for improving post-harvest activities at the local 
other seeds, respectively. Therefore, the designed level. 
Table 3.  Ghanaian Robusta Coffee Beans Properties [37] 
Moisture Content (wb) Mean 
Parameters 
6% 12% 18% 24% 30% 
Mass (g) 0.1045 0.12 0.134 0.1492 0.1678 0.1351 
Bulk density (g/cm3) 0.7889 0.7512 0.7619 0.7275 0.692 0.7444 
Particle density (g/cm3) 1.187 - - - 0.960 1.0735* 
Length (mm) 8.49 8.85 9.86 9.46 10.22 9.376 
Width (mm) 5.831 6.048 6.582 6.924 7.163 6.509 
Thickness (mm) 3.732 4.144 4.33 4.457 4.715 4.2538 
Porosity (%) 33.61 - - - 21.92 27.765* 
Static co-efficient of friction 0.245 0.276 0.293 0.343 0.404 0.3122 
Geometric mean diameter (mm) 5.626 6.042 6.547 6.627 7.015 6.3706 
Arithmetic mean diameter (mm) 5.982 6.347 6.924 6.949 7.367 6.7138 
Angle of repose (degree) 29.80 - - - 45.44 37.24* 
Sphericity 0.9239 0.9329 0.9249 0.9398 0.9340 0.9311 
Surface area (mm2) 84.049 96.931 113.334 116.814 130.484 108.324 
Moisture content (%) 11.6 14.0 13.04 
Table 4.  Nigerian Properties of Seeds and Beans [29] 
Parameters Range Mean 
Bulk density (g/cm3) 0.400 to 0.4167 0.4077 
True density (g/cm3) 0.60 to 0.86 0.71 
8.10 to 10.5 9.78 
Length (mm) 
7.30 to 9.40 8.19 
6.10 to 8.30 7.24 
Width (mm) 
5.40 to 7.30 6.11 
4.50 to 6.00 5.23 
Thickness (mm) 
3.60 to 5.30 4.60 
Sphericity - 0.75 
Coefficient of friction 0.383 to 0.424 0.40 
Geometric mean diameter(mm) - 7.18 
22.7 to 29.6 25.5 
Angle of repose 
23.8 to 26.7 24.8 
Bulk density (g/cm3) 0.5882 to 0.6098 0.4974 
True density (g/cm3) 0.9091 to 1.25 1.063 
Coefficient of friction 0.2942 to 0.3635 0.3335 
Food Science and Technology 10(3): 55-74, 2022 61 
4.6. Cameroon quality of coffee and based on them during designing the 
coffee processing equipment. Therefore, it is better to 
The mass and caracolis properties of Robusta coffee in research the two different coffee varieties harvested in the 
Cameroon were evaluated from the cherries harvested to same area to confirm the comparison of their quality 
the different clones [39] as shown in Table 5. These clones parameters.  
that were cultivated in Cameroon were coming from 
Central Africa, Ivory Coast, Java, and Madagascar. The Table 6.  Ugandan Robusta Coffee Beans Properties [31] 
average mass of 100-beans evaluated was 19.14 g at 12% 
of moisture content obtained after 3 weeks of drying and Parameters Range Mean 
the significant difference was P-value≤0.05. The Length (in) 0.4050 to 0.4011 0.417 
significant difference between the parameters was 
Width (in) 0.2982 to 0.3164 0.311 
calculated using the Newman-Keuls method. The results 
revealed that mass did not depend on the clone types apart Depth (in) 0.1931 to 0.2004 0.213 
from locations of Robusta coffee. The research conducted 
Area of flat surface (sq, in) 0.089 to 0.095 0.093 
in Cameroon focused on mass only and could also be 
extended to the other parameters as well as for Arabica Area of Transverse Surface (sq, 0.047 to 0.053 0.054 
coffee to provide enough information on the de-pulping in) 
process at the local level. This is necessary because setting 
up the de-pulping machine depends on the cherries‟ size 4.8. Tanzania 
that differ from varieties and growing area. 
The description of Arabica coffee grain size and shape 
Table 5.  Cameroonian Robusta Coffee Seeds [39] 
from Tanzania and Kenya as the key important quality 
Parameters Range Mean parameters of the coffee was reported by Libor [34]. 
Weight (g) -100 Samples were ordered using a commercial distribution 
10.99 to 19.14 14.193 near to C6 
seeds network from 20 different regions in 13 countries, 
including Kenya and Tanzania. Apart from the size and 
Moisture content 12 
mass of 100-beans measured using precision instruments; 
the other parameters were calculated using empirical 
4.7. Uganda formula and their corresponding deviations were also 
calculated as shown in Table 7. The equations that define 
The study of the shape and size of two main coffee 
the shape contour of coffee beans and sphericity were 
species Arabica and Robusta coffee from Rukera Estate 
(Kenya) and Magigye Estate of Uganda respectively were derived. These equations are useful for coffee processing 
reported in 1970 [31] at Makerere University. The 52 and handling operations. The comparative analysis of 
Arabica coffee samples and 10 Robusta coffee samples shape variety using elliptical Fourier descriptor showed a 
with 100-parchment beans in each sample were relevant difference for all varieties. The description was 
investigated. The size was measured directly on randomly limited to numeral simulation of coffee grain mechanical 
selected samples, and the surface areas were measured behaviour that can be supported by the derived formulas 
using graph paper with squares. The research revealed four that are also describing the grain contour and sphericity. 
main types of shapes of coffee; triangle, peaberry, elephant, According to [40], the investigation was conducted using 
and standard with double cuts. The parameters of two the same data, but with a sample of 20 beans selected 
varieties of coffee were compared in terms of size (length, randomly (while in previous was 100 beans). The analyses 
width, depth, and surface areas), and shapes as shown in of Arabica coffee roasted experimentally had described the 
Table 6. The properties of these shapes were similar to fracture behaviour of the grains and given geometry. 
those reported by Ghosh et al. [31]. However, after However, [40] was confused about the number of beans 
investigation of size, surface areas, standard deviation, used to calculate the mean value where 1000-beans were 
significant difference, the correlation between size reported in previous work [34]. But the two researchers got 
parameters, and the empirical formulas were determined the same results on the physical characteristics of coffee 
and validated with the experimental method. It was selected in 20 regions. Therefore, in a mechanical context, 
revealed that Arabica coffee beans had size and areas the strength of Arabica coffee beans under static and 
greater than that of Robusta coffee beans. The other quality dynamic loading, and the crushing of coffee beans under 
parameters of the coffee beans were investigated like quasi-static and dynamic compression were found that are 
parchment skin, angle of repose, bulk and true density, more important for coffee grinding. The relation between 
airflow characteristics, and drying rate. However, the size crushing stress and grain shape was not significantly 
and shape, as well as the weight are used to define the different. 
62 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
Table 7.  Tanzanian Arabica Coffee Cherries and Beans [34] size of beans during the drying process, and separation of 
Parameters Range Mean pulps to the parchment coffee beans [3]. 
Weight (g) 0.149±0.021 0.149 
5.2. Linear Dimensions of Arabica and Robusta Coffee 
Length (mm) 11.28±0.68 11.28 Beans 
Width (mm) 8.60±049 8.60 
The coffee cherries‟ de-pulping plays a big role to the 
Depth (mm) 4.78±0.30 4.78 quality parameters of the coffee beans [18]. Then, the 
reviewed data as compiled in Tables 1-7, in the appendix 
Sphericity 0.007269 
were used to analyse coffee attributes based on 
countrywide in the African countries per each parameter. 
4.9. Rwanda and Benin The three main dimensions (length, width and thickness) of 
the Arabica coffee beans from Ethiopia, Kenya and 
The Rwandan target is to earn $95 million from coffee Tanzania; and Robusta coffee beans from Ghana, Nigeria 
export by 2024 according to NAEB, (2021) report, and the and Uganda were analysed through statistical analysis. The 
harvesting target is 700 kg/ha (Introducing National Coffee length of Arabica coffee from the three countries was 
Investment Agendas for Africa Challenge: Case of Rwanda, recorded and the smaller average length of 9.34 mm was 
2016). Rwanda produces 99% of Arabica coffee and the from Ethiopian coffee beans as shown in Figure 1. 
remained percentage is for Robusta coffee. However, there Therefore, the mean average length of Arabica coffee on 
is no official document reported on the physical properties the African continent was 10.65 mm as shown in Table 8. 
of cherries and beans from Rwanda, but some literature The Robusta coffee showed a higher value of length, width 
described the coffee attributes of Arabica coffee beans [4] and thickness of 10.59 mm, 7.90 mm and 5.41 mm, 
in Ethiopia that are almost similar to the ones available in respectively from Uganda as shown in Figure 1 compared 
Rwanda. Besides that, the Rwandan coffee cherries have to Nigerian and Ghanian grains. Then, the mean average 
19.6 mm long and 10 mm in width on average. length of Robusta coffee revealed was 9.91 mm. Thereafter, 
Like Rwanda, Benin produces only the Robusta coffee the length of Arabica coffee beans looked bigger than that 
and its annual production is around 42 tons as reported by of Robusta coffee beans. But, the smallest length in the two 
world Atlas in 2019 [41,42]. Therefore, Benin Robusta varieties was found for Arabica coffee too. The width of 
coffee and Rwandan Arabica coffee‟s quality parameters Arabica coffee was found bigger while its thickness was 
will be investigated in the next work. This will provide the smaller than Robusta coffee as shown in button distribution 
information needed to develop a coffee pulping machine in Figures 1 & 2. According to [2] it can be confirmed that 
that can help local women farmers and enhance the quality the length and width of the coffee Arabica were higher than 
of parchment coffee beans. that of the Robusta coffee as shown in Figure 1. This 
consideration was found different from other many studies 
[2,7]. 
5. Statistical Analysis and Comparison Thus, considering the mean length of 10.65 mm and 9.91 
of the Recorded Parameters of mm and width of 7.705 mm and 7.21mm and thickness of 
Coffee Arabica and Robusta 4.58 and 4.9 mm for Arabica and Robusta, respectively, 
there small deviation of 1 mm to the length and less than 1 
mm to width and thickness as shown in Figure 1 and Table 
5.1. Analysis of Moisture Content of Arabica and 
8. Therefore, it can be suggested to use one coffee pulping
Robusta Coffee Beans 
machine for de-pulping African Arabica and Robusta 
The mean moisture content calculated from the recorded coffee cherries by adjusting the de-pulping mechanism at 
moisture content countrywide during the analysis were each instant. However, to enhance machine efficiency 
11.17 % wb, and 10.46% wb for Ethiopian [10,20] and despite the importance of separating of good cherries to the 
Kenyan [38] Arabica coffee respectively, and 13.04% wb floaters based on size. The well-designed pulping and 
for Ghanian Robusta coffee [37] as shown in Tables 1, 2, & dehulling machines were proposed to be used for two 
3. It was observed that all moistures are closer, so recorded coffee varieties [10,20] based on the average size of beans. 
moisture cannot affect the analysis and comparison of the  Pearson correlation between varieties was also 
quality parameters recorded from different African performed to verify the significant difference between 
countries. The maximum moisture found is 13.04% wb for linear dimensions of Arabica and Robusta coffee beans. 
Ghanian Robusta coffee, and the average moisture content The highest correlation ratio of 0.999 was revealed 
of 10.84% was obtained in the African countries. between the length of Arabica and the width of Robusta 
It is useful for storage and to preserve the flavour of the beans as shown in Table 8. The lowest correlation ratio of 
coffee at the buyer but due to this parameter not being 0.628 was revealed between Robusta length and Arabica 
controlled by a human being, the comparison was not due thickness. But, the thickness of the beans is smaller 
to variety or regions. However, moisture content affects the compared to their length and width; due to this, thickness 
Food Science and Technology 10(3): 55-74, 2022 63 
cannot affect the performance of the depulping machine as machine parameters according to the coffee varieties and 
it is like a third of the length, and a half of the width of harvesting period curve. Hence, the Principal Component 
cherry. This confirms that the dimensions of the two Analysis (PCA) also showed a strong positive correlation 
varieties are not much different, therefore pulping of between linear dimensions of the two coffee varieties, as 
operation can be performed by adjusting or setting-up indicated in Figure 9. 
Figure 1.  Distribution of Mean linear dimensions of coffee Arabica and Robusta beans for African coffee-producing countries 
15
10
5
0
, La a,
 L , W , 
T , Ta a a,
 T
ic st cab u i us
t c
bi s
t
ra ob a
b
ob ra ob
u
A R rA R A R
Figure 2.  Comparison in length, width and thickness of Arabica and Robusta coffee beans 
Dimensions, mm
64 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
5.3. Analysis of Weight of Arabica and Robusta Coffee between coffee varieties as shown in Figure 4b. Therefore, 
Beans the bulk-density can vary geographically from one country 
to another, not depending mainly on coffee variety. In the 
The analysis revealed that the weight of African Arabica case of depulping process, the bulk density is an important 
coffee 0.155g/bean is greater than that of Robusta coffee parameter for defining the quality of cherries in the sorting 
0.138g/bean as shown in Figure 3a. The means weights process and can directly affect the performance of the 
from three countries (Ethiopia, Kenya, Tanzania and pulping machine. The parchment beans with less bulk 
Ghana) showed non-significant differences in countrywide density cannot slide easily by the gravitational force from 
comparison, but with a significant difference between the hopper to the pulping chamber or from the pulping 
varieties (P-value<0.05), as shown in Figure 3b. Therefore, chamber to the machine outlet et even if it depends mainly 
the Arabica coffee beans from the Eastern regions of Africa on the coefficient of friction but density can also decelerate 
had a higher weight than that from the Western region, the friction resistance. 
maybe this happened due to the climate and geographical The apparent density is one of the major parameters for 
conditions in the Eastern African countries. However, identifying the quality of the coffee beans which is relative 
according to [7], the average mass of coffee Liberica to the volume and weight as well as the size of beans. 
(0.26g) is higher than coffee Robusta and Arabica weight, Therefore, apparent density from Ethiopia and from Ghana 
but Robusta coffee weight was also bigger than of Arabica and Nigeria for Arabica and Robusta, respectively were 
[7,43]. This property is the most important to fix price of recorded countrywide. Unless the apparent density from 
the parchment beans [10] and to determine coffee pulping Ethiopia Arabica coffee of 1.35g/mL was assumed to 
capacity, beans‟ breakage rate, mass ratio of pulps, represent Arabica coffee at inter country-wise level. It can 
mucilage, and wet parchments to the pulped cherries. be assumed that there is a significant difference between 
Arabica and Robusta coffee beans density as shown in 
5.4. Analysis of Density of Arabica and Robusta Coffee Figure 5a and 5b. Then, it showed that Arabica coffee has a 
Beans greater density (1.3g/mL) than coffee Robusta (0.9g/mL) 
between varieties as shown in Figure 5a and between 
Based on Figure 4b, mean bulk density from Ethiopia countries as it is also shown in Figure 5b. The apparent 
and Kenya for Arabica coffee and Robusta coffee from density doesn‟t be affected by pulping process because it is 
Ghana and Nigeria were analysed as recorded in Tables 1, a parameter independent to the performance of the machine. 
2 & 3, respectively. The analysis of mean bulk-density The apparent density is important for defining the capacity 
revealed that Kenya had a better-quality bulk-density of of the hopper in relation to the feed rate of the pulping 
1.1g/mL for Arabica coffee beans followed by the machine. However, the parchment with apparent not 
bulk-density of 0.8g/mL for Robusta coffee from Ghana as proportional to the size due to inappropriate growth or 
shown Figure 4b. The Arabica beans showed bulk density undergrowth beans can results more beans crushing during 
of 0.8g/mL higher than of Robusta beans of 0.6g/mL as depulping process. 
shown in Figure 4a. This can be confirmed by a report According to Ismail et al. [7], the Ethiopian Arabica 
conducted by Ismail et al. [7] that Arabica coffee bean had apparent density weighed more than Robusta (Ghana and 
higher bulk density than that of Robusta coffee. The plotted Nigeria) and Liberica (Malasia). This means that African 
means bulk density showed a significant difference coffee will cost more at the market level, if all the 
(P-value<0.05) in countrywide comparison as well as post-harvesting are performed perfectly. 
(a)   (b) 
Figure 3.  Weight of Arabica and Robusta coffee beans: (a) Comparison between varieties and (b) Distribution of countrywide weight 
Food Science and Technology 10(3): 55-74, 2022 65 
1.5
1.0
0.5
0.0
Arabica Robusta
(a)                                              (b) 
Figure 4.  Bulk-density of Arabica and Robusta coffee beans: (a) Comparison between varieties and (b) Distribution of countrywide Bulk-density 
1.4
1.2
1.0
0.8
Arabica Robusta
(a)  (b) 
Figure 5.  Apparent-Density of Arabica and Robusta coffee beans: (a) Comparison between varieties and (b) Distribution of countrywide apparent 
density 
5.5. Analysis of Coefficient of friction and Angle of Therefore, the coefficient of friction and angle of repose 
repose for Robusta Coffee Beans are independent variables at each other for coffee beans of 
the same variety. Hence, sliding of parchment beans on 
The coefficient of friction and angle of repose data were drum can be a factor of defining the de-pulping and 
recorded for only Robusta coffee beans for two countries separation rate. The Nigerian Robusta coffee can resist 
Ghana and Nigeria from the Western region of Africa. more in sliding on coffee pulping drum surface than 
However, from experiments conducted in different regions, Ghanian Robusta coffee according to the coefficient of 
it had revealed that the coefficient of friction is 0.33 of friction as shown in Figures 6 & 7. However, filling the 
Ghanian Robusta coffee is lower than of Nigerian Robusta de-pulping machine hopper for the Ghanian Robusta coffee 
coffee of 0.34 as indicated in Figure 6. Hence, angle of beans is easier than that of Nigerian Robusta coffee beans. 
repose defines of how cherries and beans are sitting each During the pulping process, if the coefficient is not good it 
other in the hopper which can increase the feeding rate can increase the cloggage of beans in pulping chambers 
directly and pulping performance indirectly. This is that can also increase the breakage of beans. Then if the 
contrary to an angle of repose of 37.24°C and 25.5°C for flowing of beans is not at the standard the beans will not 
Ghana and Nigerian respectively, as indicated in Figure 7. flow from the hopper to the pulping chamber. 
Density, g/mL
Density, g/mL
66 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
0.4 coffee of 0.693 as shown in Figure 8a. 
Therefore, keep in mind that the sphericity depends on 
length, width, and thickness. This parameter does not affect 
much geometric properties and weight. Thereafter, 
0.3 geometry properties define the strength of the beans until 
the first crack [44] in order to perform pulping without 
crushing the parchment. The grater sleeve of drum pulper, 
0.2 disc pulper contains bulbs that separate pulps and beans 
and help the beans to slide in pulping clearance toward the 
outlet of machine. Therefore, according to [40] the analysis 
showed that the crushing strength of a single bean under 
0.1 static force applied perpendicularly to the flat surface of 
Arabica coffee beans from Ethiopia, Kenya and Tanzania 
varies in between 30N and 60N and the correlated dynamic 
0.0 force can rise up 75N. The corresponding displacement is 
about 0.3mm. Therefore, keeping the correct clearance in 
Ghana Nigeria
the pulping mechanism in relation to the bean‟s 
Figure 6.  Variation of coefficient of coefficient of Friction of Robusta compression strength can avoid the breakage of the 
coffee beans parchment. However, the crushing strength of cherries is 
lower than that of the beans [7], and the crushing strength 
40 of Liberica coffee cherries (128.52N) is greater than of the 
Arabica and Robusta with 30N and 50N, respectively. This 
means that pulps separated with beans before attaining the 
fracture force of the parchment; hence, crushing of 
35 parchment beans depends on setting up of the machine. 
The summary of the means coffee parameter of the two 
main African coffee varieties from the above analysis is 
30 represented in Table 8. The pairwise Pearson correction 
coefficient and P-value between parameters showed that 
they are no significant differences for the means of one 
variety and showed a significant difference between coffee 
25 varieties (P-value<0.05). The beans‟ size and shape differ 
from one region to another due to different factors [10], but 
adjustment of the pulping machine should take into 
20 consideration based on the correlation between the sizes of 
the beans among the varieties to reduce the damage of 
Ghana Nigeria parchments during the pulping process which leads to the 
low quality of parchment coffee beans. Thus, as much as 
Figure 7.  Variation Angle of Repose of Robusta coffee beans 
the beans are more spheric, the pulping operation becomes 
easier due to proportion in the size and it reduces the 
5.6. Analysis of Sphericity of Arabica and Robusta 
breakage rate because the strength increases as the 
Coffee Varieties 
thickness that most defines sphericity increases as also the 
The analysis of sphericity was conducted for the ratio of geometric diameter to length increases. 
sphericity that were determined using Eq. (16) of 0.695, Pearson multivariant correlation analysis between the 
0.6531 and 0.7269 recorded for one bean for Ethiopian, bulk-density to the Apparent density (g/mL), weight of 
Kenyan and Tanzanian coffee, respectively as shown in bean (g), length (mm), width (mm), thickness (mm), 
Figure 8b [34]. The sphericity of coffee beans that is coefficient of friction and sphericity were 0.562, -0.655, 
assumed to be ellipsoidal or sphere from four different -0.339, -0.651, 0.228, -0.658, and -0.909, respectively. The 
African countries shows that Ghana is the one with a high apparent density was the one showed a positive correlation 
sphericity value of coffee. While, the Ethiopian Arabica with bulk density while the length and width showed 
coffee shows minimum sphericity. negative correlation. The highest negative correlation was 
The sphericity of Arabica and Robusta coffee with sphericity of -0.909 which was significant in the 
comparison analysis showed a significant difference correlation analysis. The density relative was analysed due 
(P-value<0.05) between the two verities. Unless the results to its importance on defining the ratio of size and volume 
showed that there is no significant difference between as well as the mass of coffee seeds that can fill the pulping 
sphericity countrywide. The Robusta coffee beans have machine hopper, design dimensions of hopper, and 
bigger average sphericity of 0.84 rather than Arabica evaluate the capacity of depulping machines [20]. 
Angle, °C Coefficient of friction
Food Science and Technology 10(3): 55-74, 2022 67 
(a)  (b) 
Figure 8.  Variation of sphericity of Arabica and Robusta coffee beans: (a) between varieties and (b) distribution in countries 
Table 8.  Summary of some mean parameters of Arabica and Robusta to the size of cherries of the two varieties couldn‟t affect 
coffee beans recorded from 8 African coffee producing countries 
much the quality of coffee beans. The de-pulping 
Parameters Arabica Robusta mechanism can be adjusted based on the quality of the 
available cherries. 
Weight of bean (g) 0.1546±0.00 0.13855±0.00 
Table 9.  Mean Size of Arabica and Robusta coffee beans harvested in 8 
Bulk density 
0.871±0.27 0.576±0.24 African Countries 
(g/mL) 
L_A W_A T_A L_R W_R T_R 
Apparent density 
1.350 0.980±0.13 
(g/mL) 
8.28 5.09 3.52 8.49 5.83 3.73 
Porosity - 27.765 
10.4 6.82 4.28 10.22 7.16 4.71 
Length (mm) 10.653 9.915 
9.01 6.48 2.82 8.1 6.1 4.5 
Width (mm) 7.705±1.53 7.216±0.70 
11.34 7.55 4.66 10.5 8.3 6 
Thickness (mm) 4.58±0.51 4.965±0.62 
10.97 8.26 4.9 7.5 5.4 3.6 
Volume (mm3) 0.0736 - 
11.6 8.78 5.07 9.4 7.3 5.3 
Coefficient of 
- 0.339475 
friction 10.46 8.58 4.75 10.29 7.57 4.9 
Angle of Repose - 31.195 12.8 8.58 4.75 10.19 8.03 5.09 
Sphericity 0.693±0.031 0.84±0.128 11.28 8.6 4.78 10.23 7.8 4.99 
Moisture Content 11.085 13.04 
L_A: Length_ Arabica, W_A: Width_ Arabica, T_A: Thickness_ Arabica, 
L_R: Length_ Robusta, W_R: Width_ Robusta, and T_R: Thickness_ 
Robusta  
5.7. Distribution in Dimensions of African Arabica and 
Robusta Coffee Beans The analysis of variance showed that there is a 
significant difference among the means of three main 
The knowledge of the mean size of African Arabica and dimensions of African Arabica and Robusta coffee beans at 
Robusta coffee cherries and beans as shown Table 9 helps P-value< 0.05 and R-squared of 0.834. Based on the 
to develop the optimal coffee pulping machine with respect dimensions of two varieties as shown in Table 9, the design 
to the homogeneous dimensions of the two coffee varieties of experiment produced a factorial model with 16 runs of 
at a small-scale level. Therefore, taking into considering coffee pulping machine efficiency from the range 
the African coffee beans, the means size of the Arabica and (maximum and minimum) of length, width and thickness 
Robusta coffee beans, were summarized in Table 8. The of Arabica and Robusta. The model of efficiency of the 
variation in dimensions between the two varieties from depulper is represented in Table 10. The three main 
African coffee producers is illustrated in Figure 2. It shows dimensions of coffee beans are the results from the premier 
that the development of coffee pulping machines according size of cherries that are based on for developing a coffee 
68 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
pulping machine. The model equation produced by 5.8. Principal Composition Analysis (PCA) of 
Response Surface Methodology (RSM) from the Dimensions of African Arabica and Robusta 
dimensions can be based on for predicting the size of Coffee Varieties 
coffee in Africa. The mathematical relationship between 
the dimensions of Arabica and Robusta coffee to the coffee The information about differences and correlation 
pulping machine efficiency (see Eq. 18) was a reast-square between physical properties of beans is important in 
(R²) of 0.98 and the adjusted R² of 0.92 obtained using fit developing coffee treatment operation including pulping 
statics method in One-Way ANOVA. The R² showed the operation or others [43]. The Arabica and Robusta coffee 
variation of size accounts 92-98% to the machine beans obtained by the coffee pulping machine or by hands 
efficiency. In this case, A, AD, AF, and BD are significant de-pulping in African showed a perfect positive correlation 
model terms at (P-value<0.05) in the regression equation. between the three main dimensions as indicated in Figure 
This means that one dimension of coffee bean or cherry 9. 
cannot affect the efficiency of the machine, but the The grouped dimensions showed a correlation range 
combination of three main dimensions of coffee cherries or (r=0.78, 0.84 and 0.94). This confirms the insignificance 
beans. Therefore, this model equation can be used to difference correlation of the terms in the model equation, 
navigate the design of de-pulping machines on the African as shown in Eq. 18. Therefore, the relation in internal 
continent based on coffee beans size with an efficiency of variations of the dimension help in decreasing the damage 
at least 97.17%. of beans and increase the de-pulping efficiency for a 
machine [4] adjusted based on the main dimensions of the 
Efficiency= +97.19 +0.67A +0.09B -0.62C +0.39E -0.1 cherries and their corresponding parchment coffee beans. 
6F +0.81AD -0.65AF -0.96BD +0.40ABF   (18) 
Table 10.  Analysis of variance for the relationship between dimensions of African Arabica and Robusta coffee for Pulping Machine Efficiency Model 
Response  
Source Sum of Squares Df Mean Square F-value p-value 
Model 63.62 13 4.89 14.46 0.0665 
A-Length_A 7.33 1 7.33 21.66 0.0432 
B-Width_A 0.1425 1 0.1425 0.4211 0.5829 
C-Thickness_A 6.26 1 6.26 18.51 0.0500 
D-Length_R 0.0008 1 0.0008 0.0022 0.9666 
E-Width_R 2.49 1 2.49 7.35 0.1133 
F-Thickness_R 0.4389 1 0.4389 1.30 0.3728 
AC 4.44 1 4.44 13.12 0.0685 
AD 10.48 1 10.48 30.97 0.0308 
AE 5.28 1 5.28 15.60 0.0585 
AF 6.77 1 6.77 20.01 0.0465 
BD 14.76 1 14.76 43.63 0.0222 
BF 2.60 1 2.60 7.68 0.1092 
ABF 2.62 1 2.62 7.73 0.1087 
Residual 0.6768 2 0.3384 
Cor Total 64.30 15 
R²=0.98, adjusted R²=0.92, df=degree of freedom. 
Food Science and Technology 10(3): 55-74, 2022 69 
Loadings
PC1 PC1
1.0
0.78
0.84
0.91
0.5
0.0
-0.5
-1.0
-1.0 -0.5 0.0 0.5 1.0
PC1
Figure 9.  Correlation between Arabica and Robusta coffee dimensions (Length, Width and Thickness) based on Principal Component Analysis 
6. The Inter-Zone Coffee Beans the Western Africa (Robusta) due to coffee verities which 
Parameters Comparison are favourite to the climate in the two regions while the 
bulk density of Arabica was greater than of Robusta 
The physical and mechanical parameters investigated in variety. 
different previous review works were not similar in every Arabica coffee is one of the coffee varieties with higher 
country. Therefore, inter-zone comparisons in terms of dimensions but they are less than of the coffee Liberica [7]. 
dimensions, moisture content, sphericity maturity of the Although there are varying from one country to another. 
parchment coffee and maturity of coffee cherries were The results show small differences that account for less 
compared in this section. The Arabica and the Robusta than one millimetre for length, width, and thickness as 
coffee were investigated in ten African countries where the shown in Figure 1, and Table 8. However, the length of the 
database information of physical properties of coffee beans is in the range of more than 11 mm and 9 mm for the 
cherries and beans were developed in Tables 1-7. The Arabica and Robusta coffee, respectively. The highest and 
dimensions measured directly and calculated parameters smallest lengths are 11.95 mm and 11.28 mm from 
play a key role in designing the coffee processing machines Ethiopia and Tanzania, respectively. From the above view, 
such as pulping and dehulling machines. Besides that, the highest and smallest width of 8.60 mm and 8.58 mm are 
these dimensions are used to identify the quality of coffee coming from Tanzania and Kenya for Arabica coffee, 
in proportional to the bulk density. The dimensions of respectively. The higher and smaller thickness of Arabica 
beans from Easter Africa (Arabica) were bigger than that of coffee of 4.96 mm and 4.9 mm were found from Kenya and 
PC1
70 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
Ethiopia, respectively. For the Robusta coffee among beans depends on geographical conditions and processing 
Nigeria, Ghana, and Uganda countries; the higher and method and has an influence on density, crushing strength, 
smaller average width obtained from the conducted price and impact on the optimal operational performance of 
research is 7.89 mm and 6.51 mm for Uganda and Ghana the pulping machine[4]. 
respectively. The thickness of the Robusta coffee showed a Therefore, it has been proved in Ghana [37] that 
bigger size than those of Arabica coffee as shown in moisture content affects the size and the weight of beans 
Figures 1 & 2. The Ugandan Robusta coffee thickens is increasing linearly as increases moisture. The grading of 
greater than that of Ghana with around 1mm. This means coffee can also be referred to as moisture content as 
that the length and width of coffee beans can vary from one investigated by Niwagaba et al. [37] in Uganda. However, 
country to another according to different factors including the moisture content varies from one region to another due 
environmental conditions, and soil fertility; where the to the degree of drying the coffee or post-harvesting 
environmental conditions have a higher effect on the process in general and coffee varieties. Moisture content 
quality of coffee [17] also contributes to the separation of the pulp and coffee 
For standardization of the coffee lot based on size, the beans, the results indicated that as decreasing moisture 
coffee beans graded into A, AA, AB, and C retained into content, the pulping force increases or vice versa. The 
the screen size of >18, 18>17, 16>15, and 15>14, standard moisture content is in the range of 10 to 12.5% wb, 
respectively and E-Elephant categories (8.3mm) based on while that of beyond this range can affect the quality of 
Kenyan grading system using weighing screen [45]. coffee flavor. The moisture content investigated in 
However, in Rwanda traditional grading of Arabica based Ethiopia, Kenya and Ghana was in this range, the average 
on percentage started using the screen 18 for grade A1+, moisture content of 10.84% wb was investigated to 
A1- A2+, A2-, A3+, A4+ for non-retained percentage of represent African coffee in this review. This value was 
(7%), 6%), (3%), (1.5%), (2%), (21%) respectively, and almost closer to the one determined on Indonesian Robusta 
Ivory-Cost is 0, 1, 2, 3, and 4 for retained screen of >18, coffee which is 12.2% wb on a wet basis [22]. The highest 
18/16, 16/14, 14/12, and 12/10, respectively. The Arabica moisture content from the coffee used in this work was 
coffee beans in a range of 6.75 mm to 7.14 mm and 5.95 13.04% wb for Robusta from West Africa and the lowest 
mm to 6.35 mm are graded in AA and AB, respectively as was 10.46% wb Arabica from East Africa in Ghana and 
by the Kenyan grading while the Robusta coffee beans in a Ethiopia, respectively. The moisture content level is an 
range of in the range of 5.95 mm to 7.10 mm 4.76 mm to important parameter to define aroma and flavor for the 
5.55 mm are in grade 1 and 2, respectively as by buyer, weight and density, and is a very important 
Ivory-coast grading [46]. The African coffee showed, the parameter in defining the depulping performance of the 
average mean size of beans as shown in Table 8, with a machine and the quality of both cherries and beans. The 
length of 10.46 mm and 9.91 mm, width of 7.705 mm and cherries take a growing period of 9-11 months to ripen; 
7.21mm and thickness and 4.58 to 4.9 mm for Arabica and then after, the harvesting period of Arabica coffee takes 2 
Robusta, respectively. The Arabica coffee is mostly to 3.5 months while the Robusta harvesting period is 
available in East Africa and the Robusta coffee is available irregular. The Arabica cherries in Ethiopia (East Africa) 
in West and East Africa. Therefore, based on size and are harvested when they are about 75% ripened [48]while 
Kenyan and Ivory-coast grading, the African Arabica the Robusta cherries are 60% ripened. The good cherries 
coffee is generally ranked in grade AA and African that provide the good performance of the machine are 
Robusta coffee is ranked in grade 1. Then, the Robusta always in the range of 59% to 65% wb, if not soaking of 
coffee from Uganda in East Africa with bigger size with cherries is needed. Thus, in this range, the efficiency of the 
the length of 10.56, with of 7.899 and thickness of 5.410 machine varies depending on the speed of the machine [49]. 
mm can be in grade 1; while the Robusta from Ghana with The Arabica cherries from highland (East Africa) are 
smaller size with length of 9.37 mm, width of 6.50 mm and higher in size, weight and moisture due to heavy rain 
thickness of 4.25 mm is also in grade 1. Hence, based on compared to the lowland Arabica that is mostly from West 
size and shape, Arabica and Robusta are commercially Africa, except in Uganda [4]in East Africa where there is 
homogenous until the sieve 19 [47].  little rain seasons and high temperature. Besides that, the 
The bean weight is positively correlated with AA bean Robusta yields more than the production of Arabica coffee 
size [45], then the A grade cost at New York mark 4.2$/kg for 2300-4000kg/ha and 1500-3000kg/ha of green bean in 
of Arabica and 2$/kg of Robusta in 2022 based on statista average respectively [50]. The depulping parchment coffee 
2022. The price of the beans depends also on the grade [10], beans are dried to standard moisture at a market level of 
AA is 5$/kg and AB is 4$/kg. Therefore, based on grade 10-12.5% for both Arabica and Robusta. Therefore, the 
and weight as shown in Figures 1 & 3 the Arabica coffee based-on Speciality Coffee Association of America 
price will be higher than that of Robusta and Easter Africa, (SCAA) green coffee classification, the green coffee in 
coffee price will be better to the western coffee price. grades 1 and 2 should attain a drying level of 9-13% per 
Therefore, grading is confusing from one country to 300g of coffee for both Arabica and Robusta. This 
another because there is no universal accepted standard of moisture content is similar to Banana fruits to maintain 
grading. Hence, the size distribution (grade) of coffee sensorial attributes (15.9-13.3g/300g [51]. Hence, size and 
Food Science and Technology 10(3): 55-74, 2022 71 
moisture, and pulping ripened good cherries can be defined reports used during the investigation. Besides, Arabica 
as the main determinator factors for market sale and coffee was found to contribute up to 69% of coffee 
processing of coffee as described by Nesterenko et al. [52], production in Africa and its size is slightly smaller. 
and flavor to enhance the quality and price of coffee However, it is needed to conduct, identify the properties 
needed by the coffee consumer. of the coffee cherries and beans in different countries to 
Density of the coffee bean depends on the size, and design their post-harvesting processing machines. The 
moisture content. Despite that, there are seeds with bulk pulping machines can help African coffee farmers to 
and apparent density which are less and greater than 1, increase their revenue. Comparing to intra-country quality 
respectively. The minimum and maximum bulk density parameters of the Arabica coffee grains are larger than 
revealed is 0.4077 g/mL and 0.7444 g/mL in Nigeria and Robusta coffee for the case of Ethiopia and Ghana. While 
Ghana, respectively. Similar results on density were the size of Robusta coffee in Eastern countries is bigger 
reported for Indonesia Arabica coffee [22,28] and in India than those of western countries which imply that the 
[33]. high-quality coffee is from Easter Africa producing 
The minimum and maximum true or apparent density countries. This confirms that Eastern Africa is producing 
revealed in Nigeria and Ethiopia are 1.063 and 1.35 g/mL, high-quality coffee worldwide. Further investigation is 
respectively. The bulk density of Arabica coffee beans in needed to verify any contrast that the African Arabica 
all countries is less than that of Arabica coffee in other coffee geometric size is greater than Robusta coffee. The 
countries as conducted in India for both dry and wet mean dimensions of Arabica and Robusta coffee have to 
processes [53]. Besides, Ghana and Nigeria produce be explored in each country from where data are not 
Robusta coffee and this implies that coffee density can vary available. Then, the quality parameters for Arabica and 
based on agro-ecological conditions apart from the season, Robusta coffee which are the most available in Rwanda 
processing, and varietal differences. Hence, according to and Benin, respectively and variation in size of their 
the size, moisture content, and maturity of the cherries and cherries during harvesting period will be investigated in 
parchment coffee; the coffee quality is linked to the the next work. 
pulping machine performance parameters such as pulping 
mechanism setting-up, de-pulping rate of more than 95 
percent, and damaging beans rate of less than 3.5 per cent Acknowledgements 
according to different studies [54–56]and standard (PAES 
252:2011). The pulps take up to 1m thick of the cherries. We would like to thank the MASET project under 
The pulp and parchment take 40% [57] and, 20% of the European sponsorship for their support during this study. 
weight of the Robusta and Arabica cherry [50]. Then, if the We also thank UAC-EPAC and RP-IPRC Karongi for their 
small-scale coffee producers can follow the process support. 
investigated in their respective countries, the quality of 
African coffee can also be increased. Considering these 
parameters can increase the coffee price at the market level Nomenclature 
based on ISO (2004a) or ISO 9116 the standard. 
 ̄ average diameter 
  Number of measurements 
7. Conclusions AMD Arithmetic Mean Diameter 
AR or Ra Aspect Ratio 
Nowadays coffee has been involved in the economy of Cf Coefficient of friction 
African countries. The main properties of coffee grains Cs Chell/Sliverskin Content 
depend on coffee beans variety, location, soil fertility, CSA Cross-Sectional Area 
season, climate, and growing care. The quality of coffee at CSS Coefficient of Contact Surface 
the market level is affected by inappropriate harvesting D Diameter of the cone cylinder 
and post-harvesting processes such as pulping process. Di Measured diameter 
Therefore, the information about the size, shape and FSA Front surface Area 
engineering properties of coffee grain can play a key role GMD Geometric Mean Diameter 
in improving the coffee quality up to the consumer level. H Height of the head 
The information about coffee beans in different countries L Length 
could contribute to the modern cultivation of coffee in r small radius 
Africa. On top of these, the significant role of identifying R larger radius 
the characteristics of coffee beans is about elaborating the Rr Roundness ratio 
design of the post-harvesting equipment for different SI Shape Index 
regions and fit the requirements of smallholders at the Sp Sphericity 
village level. The coffee beans of Arabica were revealed T Thickness 
to have a bigger length and width from three countries‟ TSA Transverse Surface Area 
72 Physical and Mechanical Properties of Coffee Cherries and Beans in Africa: Review and the State of Arts 
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