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Process Engineering Thesis

2020-03-17

ROASTING OF COFFEE BEAN USING

CIRCULATING HOT AIR IN A SINGLE

LAYER PACKED BED ARRANGEMENT

Getaneh, Ezana

http://hdl.handle.net/123456789/10453

Downloaded from DSpace Repository, DSpace Institution's institutional repository

BAHIR DAR UNIVERSITY

BAHIR DAR INSTITUTE OF TECHNOLOGY

SCHOOL OF RESEARCH AND POSTGRADUATE STUDIES

FACULTY OF CHEMICAL AND FOOD ENGINEERING

ROASTING OF COFFEE BEAN USING CIRCULATING HOT AIR IN

A SINGLE LAYER PACKED BED ARRANGEMENT

By

Ezana Getaneh

Supervised by

Solomon Workneh (Ph.D.)

September 2018

Bahir Dar, Ethiopia

ROASTING OF COFFEE BEAN USING CIRCULATING HOT AIR IN

A SINGLE LAYER PACKED BED ARRANGEMENT

Ezana Getaneh

A Thesis

Submitted to Faculty of Chemical and Food Engineering, Bahir Dar

Institute of Technology, Bahir Dar University in partial fulfilment of the

requirements for the Degree of Masters of Science in Chemical

Engineering (Process Engineering Specialization)

Bahir Dar

Ethiopia

i

DECLARATION

ii

APPROVAL SHEET

iii

ACKNOWLEDGEMENT

I would like to thank my thesis supervisor Dr. Solomon Workneh of the Faculty of

Chemical and Food Engineering at Bahir Dar University. He was constantly open at

whatever point I had an inquiry regarding my research or writing. He reliably checks

this thesis paper to be my work.

I might, likewise, want to thank the specialists at Ethiopian coffee and tea development

and marketing authority who was associated with the validation of this research project:

Mr. Berhanu Gezahegn (director of coffee quality inspection & certification center) and

MrTefera Lemma (senior coffee cupper) by providing necessary information and raw

materials.

I additionally thank the panelist to be specific Alemu Engashu, Esikindir Endalew,

Asinake Baye, Wendu Hailemichael and Tigabu Asimare without their help and input,

the research will be difficult.

At last, I might want to thank my adored family for their significant help and support.

iv

ABSTRACT

Due to its flavor, aroma and stimulant nature, coffee becomes the second most

drinkable liquid next to water all over the world. These natures develop very well during

roasting, but there is a problem of partial charring and poor temperature distribution

across the coffee bean due to the the low thermal conductivity of bean and overheating

during roasting process; hence, there is a need for uniform temperature distribution

across the roasted coffee bean. To fulfill this need, the roasting of coffee bean in a single

layer packed bed arrangement using closed system circulating hot air had been

employed. In this work, the export standard Limu coffee bean moisture content and raw

bean values were determined and it gets roasted. During the roasting process the effect

of roasting temperature (200℃, 230℃, and 260℃ ), roasting time (5-minutes, 10-

minutes, and 15-minutes), and particle size (4-6 mm, 2.36-3.35mm and 1.7-2.36mm)

on dry mass loss, bulk density, biochemical composition and sensory attribute of the

final product have been investigated. The moisture content of raw coffee bean was

found to be 8.45% in wet bases, beside this the raw value scores 37.6/40. From the

processing conditions 230℃ with particle size range 4-6mm shows the steady transition

in terms of dry mass loss and change in bulk density with time. The maximum dry mass

loss and change in bulk density were 33.14% at a processing condition of 260℃ for 15-

minutes and 2.36-3.35mm and 30.9% for 260℃ for 15-minutes for 4-6mm respectively.

The sensory attribute analysis marks that no single processing condition gives every

desired cup values, so the best one is selected from of the cumulative roasting cup

values and found to be 89.21 at processing condition of 230℃ for 10-minutes for 4-

6mm. The caffeine result shows a maximum of the caffeine content of 55.5mg/250ml

can be achieved at a processing condition of 230℃, 10-minutes, 2.36-3.35mm particle

size.

v

TABLE OF CONTENTS

DECLARATION ............................................................................................................ i

ACKNOWLEDGEMENT ............................................................................................ iii

ABSTRACT .................................................................................................................. iv

TABLE OF CONTENTS ............................................................................................... v

LIST OF ABBREVIATION AND SYMBOLS ......................................................... viii

LIST OF FIGURE......................................................................................................... ix

LIST OF TABLE .......................................................................................................... xi

1 INTRODUCTION ...................................................................................................... 1

1.1 Background ......................................................................................................... 1

1.2 Statement of the problem .................................................................................... 2

1.3 Research Objective .............................................................................................. 3

1.3.1 Specific objectives ......................................................................................... 3

1.3.2 Scope of the research ..................................................................................... 3

1.3.3 Significance of the research .......................................................................... 3

2 LITERATURE REVIEW ........................................................................................... 4

2.1 Roasting process .................................................................................................. 4

2.2 Effects of Coffee Roasting .................................................................................. 7

2.2.1 Physical Changes ........................................................................................... 7

2.2.2 Chemical changes ........................................................................................ 11

2.3 Roasting Techniques ......................................................................................... 15

2.3.1 Industrial coffee roasting ............................................................................. 16

2.3.2 High Yield Roasting .................................................................................... 18

2.4 Industrial Roasting Equipment .......................................................................... 19

2.4.1 Drum Roasters ............................................................................................. 19

2.4.2 Paddle (Tangential) Roaster ........................................................................ 20

2.4.3 Bowl Roaster ............................................................................................... 21

2.4.4 Fluidized-bed Roasters ................................................................................ 21

2.5 Thesis organization ............................................................................................ 23

3 MATERIALS AND METHODS .............................................................................. 24

3.1 Material ............................................................................................................. 24

vi

3.2 Chemicals and Equipment ................................................................................. 24

3.3 Experimental description and procedure ........................................................... 25

3.4 Experimental design of coffee roasting ............................................................. 25

3.5 Configuration of the roaster arrangement ......................................................... 27

3.6 Characterization of raw coffee bean .................................................................. 27

3.7 Physio-chemical characterization of bean ......................................................... 29

3.7.1 Determination of bulk density ..................................................................... 29

3.7.2 Moisture content determination ................................................................... 30

3.7.3 Determination of dry mass loss ................................................................... 30

3.8 Determination of caffeine content ..................................................................... 30

3.9 Sensory attribute analysis of the final product .................................................. 31

4 RESULT AND DISCUSSION ................................................................................. 33

4.1 Raw value analysis ............................................................................................ 33

4.2 Moisture content ................................................................................................ 33

4.3 Dry mass loss ..................................................................................................... 34

4.3.1 Dry mass loss for initial particle size of range 4-6 mm ............................... 34

4.3.2 Dry mass loss for initial particle size of range 2.36-3.35 mm ..................... 35

4.3.3 Dry mass loss for initial particle size of range 1.7-2.36 mm ....................... 37

4.3.4 Statistical analysis of dry mass loss ............................................................ 38

4.4 Change in bulk density ...................................................................................... 39

4.4.1 Bulk density of initial particle size of range 4-6 mm .................................. 39

4.4.2 Bulk density of initial particle size of range 2.36-3.35mm ......................... 40

4.4.3 Bulk density of initial particle size of range 1.7-2.36 mm .......................... 40

4.4.4 Statistical analysis of bulk density .............................................................. 41

4.5 Sensory quality (Cup value) analysis ................................................................ 42

4.5.1 Cup cleanness .............................................................................................. 42

4.5.2 Acidity ......................................................................................................... 45

4.5.3 Body ............................................................................................................ 48

4.5.4 Flavor ........................................................................................................... 50

4.6 Caffeine content ................................................................................................ 52

5 CONCLUSION AND RECOMMENDATION ........................................................ 55

5.1 Conclusion ......................................................................................................... 55

5.2 Recommendation ............................................................................................... 57

vii

REFERENCES ............................................................................................................ 58

Appendix A: Representative figures for secondary raw coffee defect ........................ 63

Appendix B: Average raw experimental data .............................................................. 64

Appendix C: Grade and Total Value of Roasted Coffee Bean .................................... 66

Appendix D: Checklist for Preliminary Washed Coffee Quality Assessment ........... 67

viii

LIST OF ABBREVIATION AND SYMBOLS

ABR Air to Bean Ratio

CD Cup Defect

ECTACQICC Ethiopian Coffee and Tea Authority Coffee Quality Inspection

and Certification Center

gm Gram

HTST High-Temperature Short Time

l Liter

LTLT Low-Temperature Long Time

M1 Mass of empty pycnometer before drying

M2 Mass of crucible with sample before drying

M3 Mass of crucible with sample after drying

Mp Mass of pycnometer

Ms Mass of sample

Mn Net mass

%Mc Percentage moisture content

Vp Volume of pycnometer

W Mass of coffee bean after roasting

Wi Mass of coffee bean before roasting

%Wl Percentage of dry mass loss

ix

LIST OF FIGURE

Figure 2-1 Key aspect of roasting of coffee beans (Geiger et al., 2001). ...................... 5

Figure 2-2 Coffee bean profile during roasting (Geiger et al., 2001) ............................ 5

Figure 2-3 Principal roasting techniques(Clarke et al., 2008) ..................................... 15

Figure 3-1Block diagram of coffee roasting process ................................................... 25

Figure 3-2 experimental setup of packed bed roaster ................................................. 27

Figure 4-1 Graphical representation of the percentage of dry mass loss of bean of size

range of 4-6 mm ........................................................................................................... 35

Figure 4-2 Graphical representation of the percentage of dry mass loss of bean of size

2.36-3.35 mm ............................................................................................................... 36

Figure 4-3 Graphical representation of the percentage of dry mass loss of bean of size

1.7-2.36 mm ................................................................................................................. 37

Figure 4-4 Graphical representation of the final bulk density of bean of size 4-6 mm 39

Figure 4-5 Graphical representation of the final bulk density of bean of size 2.36-3.35

mm ............................................................................................................................... 40

Figure 4-6 Graphical representation of the final bulk density of bean of size 1.7-2.36

mm ............................................................................................................................... 41

Figure 4-7 Cup cleanness value at 200℃ as a function of time and particle size ........ 43

Figure 4-8 Cup cleanness value at 230℃ as a function of time and particle size ....... 44

Figure 4-9 Cup cleanness value at 260℃ as a function of time and particle size ....... 45

Figure 4-10 Acidity value at 200℃ as a function of time and particle size ................ 46

x

Figure 4-11 Acidity value at 230℃ as a function of time and particle size ................ 46

Figure 4-12 Acidity value at 260℃ as a function of time and particle size ................ 47

Figure 4-13 Body value at 200℃ as a function of time and particle size .................... 48

Figure 4-14 Body value at 230℃ as a function of time and particle size .................... 49

Figure 4-15 Body value at 260℃ as a function of time and particle size .................... 50

Figure 4-16 Flavor value at 200℃ as a function of time and particle size .................. 51

Figure 4-17 Flavor value at 230℃ as a function of time and particle size .................. 51

Figure 4-18 Flavor value at 260℃ as a function of time and particle size .................. 52

Figure 4-19 Calibration curve for caffeine content determination of roasted coffee bean

...................................................................................................................................... 53

Figure 4-20 Bar diagram of grade1 roasted coffee value and their caffeine content ... 53

xi

LIST OF TABLE

Table 2-1 Material data of Arabica coffee beans (Eggers et al., 2001) ......................... 7

Table 2-2 Changes inside the coffee bean during roasting (Pittia et al., 2001) ............. 7

Table 2-3 The basic principles of modern roasting technology (Clarke et al., 2008). . 22

Table 3-1Chemicals and equipment used with their grade/model and purpose .......... 24

Table 3-2 Factors and levels of roasting parameters ................................................... 26

Table 3-3 Evaluation format for raw coffee bean ........................................................ 29

Table 3-4 Raw coffee bean defect checking list .......................................................... 29

Table 3-5 Cup quality value check list ........................................................................ 32

1

1 INTRODUCTION

1.1 Background

Coffee is the second most drinkable liquid next to water everywhere throughout the

world. Not only this, it is also a noteworthy item in the farming business. The interest

in a quality refreshment made of coffee with various origin and preparation method

increases with time (Malta et al., 2003).

Coffee drinking is basically motivated by its wonderful flavor and aroma, the positive

sensations it produces and its physiological impacts. This desired quality of coffee is

attained after passing of different processing steps like harvesting, washing, drying,

roasting, grinding, and cupping (Pellegrini et al., 2003).

In coffee processing steps, roasting is the most essential stage by shaping sensorial,

structural, chemical and physical changes. Roasting of coffee beans is practiced to give

a particular organoleptic attribute to the brew (Clarke et al., 2008; Eggers et al., 2001).

In the roasting procedure, the coffee beans are presented to a high temperature for

different times in light of the favored last attributes of the final product (Toledo et al.,

2016).

To get well roasted coffee bean different kinds of roasters have been fabricated;

however, most coffee bean roasting machines are ordinary of rotating cylinder type,

containing inner baffle for blending and dropping beans, covered in a gas fired oven

(Eggers et al., 2001; Speer et al., 2001). According to (Kocadağlı et al., 2012; Redgwell

et al., 2002) the principal disadvantage of these conventional machines is that they

require high temperature and long time keeping in mind the end goal to roast the beans.

The after effect of these high temperatures and a more extended time of 15-18 minutes

result in scorching of a few beans, oil and char deposit on the chamber dividers. Besides,

the machines are hard to clean in the wake of handling, causing roasted beans with a

harsh, smoky taste. Consequently, these conventional coffee roasting machines are

unhygienic (Kocadağlı et al., 2012).

2

Various types of roasters have been designed and applied in the roasting of coffee bean.

In rotating coffee roaster smaller beans tends to move to the low speed center of the

bed transverse area. Hence because of poor mixing in this way and low heat transfer

rates in bed transverse area, these beans will roast to a lesser degree, and present a

lighter color (Clarke et al., 2008; S. J. Lee et al., 2017).

The other kind of roaster is a fluidized bed roaster which is utilized for substantial scale

roasting of coffee beans for making instant coffee. The merits of fluidized bed roasters

are the consistency of product and well control of process parameters. Nonetheless,

these units pose issues when holding medium and small batch (Mujumdar, 2006).

A spouted bed roaster is a variant of the fluidized bed roaster; this has one of a kind

focal point contrasted with the fluidized bed roaster when the particles to be roasted are

more prominent than around 5 mm and has a tendency to create unsteady or slugging

fluidization (Nagaraju et al., 2016).

From the previous works, it can be seen that roasting using hot air gives fairly good

roast when compared with drum roasting but still there is a problem of high operating

temperature and air to bean ratio. This shows there is a need to minimize these high

roasting temperature and heating demand of air stream. Not only have these, in the

above-mentioned roasters, none of them considered roasting of grounded coffee bean

which has the benefit of the substantial surface zone to volume proportion. In addition,

size reduction minimizes the heat transfer resistance and result in low temperature

gradient for a similar amount of heat transfer when contrasted with the unground coffee

bean. So; this research essentially focuses on consideration of the effect of size

reduction and use of closed system hot circulating air in a single layered packed bed

roaster on the final quality of the coffee bean.

1.2 Statement of the problem

In the coffee processing industries, the final roasted coffee is susceptible to partial

charring and poor temperature distribution due to the low thermal conductivity of the

coffee bean, and small surface area to volume ratio of the raw unground coffee bean.

Furthermore, the currently available roasters operate at high temperature and require

3

high hot air to bean ratio which causes organic loss to the bean. Approaching the

roasting process by using a closed system circulating hot air in a single layer packed

bed arrangement using grounded coffee bean may alleviate such problems.

1.3 Research Objective

The main objective of this thesis is to evaluate the coffee roasting potential of a single

layer packed bed roaster by applying hot circulating air.

1.3.1 Specific objectives

Physical characterization of the raw coffee bean.

Investigate the effect of roasting parameters like (temperature, time, and

particle size) on the dry mass loss.

Physio-chemical characterization of the final product.

To evaluate the effect of roasting parameters on sensory quality.

1.3.2 Scope of the research

This thesis work focuses on raw coffee bean characterization, investigation of the effect

roasting parameters such as temperature (i.e.200℃, 230℃, and 260℃) ,time (i.e.5-

minutes, 10-minutes, and 15-minutes) and particle size (i.e. 4-6mm, 2.36-3.35mm, and

1.7-3.35mm) on dry mass loss, biochemical content determination and sensory attribute

analysis.

1.3.3 Significance of the research

This research gives insight to circulating hot air based packed bed roaster potential at

laboratory scale by providing preliminary data for the change in bulk density, dry mass

loss, and caffeine content. So as it leads to an optimum operating condition which

results minimum mass loss and enhanced grade of coffee bean.

4

2 LITERATURE REVIEW

2.1 Roasting process

At essential prospect, the roasting of coffee gives off an impression of being a simple

and a basic procedure that it is only the utilization of heat to raw coffee beans

(Bonnlander et al., 2004). The fundamental thing is to create and control the right

temperatures at the correct time, at that point stop the process when the smell has

completely created and the shade of the coffee bean is the same all through the bean

(Geiger et al., 2001). However, on closer examination, questions emerge that have not

yet gotten an answer: the reliance of the dynamic temperature dispersion in the roasted

coffee bean on the cutoff points managing the process, for example, fluid flow

conditions, roast gas temperature and material properties of the coffee bean. Because

of the sudden changes, it is hard to control the entire procedure (Bonnlander et al., 2004;

Geiger et al., 2001).

From a chemical engineering perspective, the roasting procedure contains a collective

heat and mass transport superposed by endothermic and exothermic responses

(Redgwell et al., 2002).

Figure 2.1 reveals the importance of temperature distribution in a coffee bean amid the

roasting process. The bean has a convoluted shape; its internal structure is different;

during heating its volume increases or swells and the internal structure changes (Geiger

et al., 2001).

5

Figure 2-1 Key aspect of roasting of coffee beans (Geiger et al., 2001).

As mentioned by Geiger et al. (2001) besides creating temperature field heating

additionally causes inner pressure and a re-dissemination of moisture relying upon time

and area. These impacts are delineated in Figure 2.2 heat energy is passed on to the

surface of the raw bean, for the most part by outside hot gas stream, with the help of

radiation and contact heat exchange. The later one is reliant on the kind of roaster.

Figure 2-2 Coffee bean profile during roasting (Geiger et al., 2001)

transport of

water vapor

transport of

CO2 and volitiles

Inside of the bean

Temperature rise

Endothermic water vaporization

Exothermic reaction

Volume enhancement

Dry mass loss

Temperature dependent transport

Change of material properties

Inner heat transport

Conduction superposed by

swelling with counter current gas

flow (H2O,CO2,,,)

Outer heat transport

By convection/radiation

and contact

6

Because of the temperature gradient, the surface temperature achieves the vaporization

temperature of the bean moisture and evaporation starts at the surface and move towards

the center of the bean. This vaporization makes the wall firm and can't permeate passage

of vapor subsequently pressure develops and affecting the bean volume to expand (M.

J. Lee et al., 2013).

Because of an endothermic process, for example, drying and swelling, the heat

conductivity between the vapor front and external surface of the bean reduce radically

besides the temperature gradient is more extreme in the dried area of the bean in favor

of creating protection from the heat exchange (M. J. Lee et al., 2013). The development

of thermal stress towards the middle encourages the development of mechanical stress

and makes the bean break or even burst if the superposed pressure conquers the

elasticity of the bean (Mujumdar, 2006).

Bonnlander et al. (2004) indicate roasting response begins at hoisted internal pressure

(P>10bar) and high temperature (T>160℃) which prompts browning with the

arrangement of flavor mixes in which the procedure goes from the surface to the

internal dry pre-extended structure of the bean. This procedure is exothermic and

prompts the development of vaporous item basically carbon dioxide, yet the

entanglement of the gases within the cell structure, increment the internal pressure until

the point when it penetrates through the wall that is split by high temperature (Geiger

et al., 2001).

The coffee roasting process can be seen as a counter-current process in which volatile

components transport outside while heat transport inside. The transport of heat relies

upon the moving zone of vaporization, roast temperature, and the mass transport is

associated with the origin of coffee to be roasted (Mussatto et al., 2011). The roasting

process is required to be homogeneous; along these lines, with a specific end goal. To

do this the process must be all around controlled going for little temperature distinction

all through the bean. As opposed to this, fast roasting prompts an inhomogeneous

profile by overlapping of the evaporation and roasting steps (S Schenker et al., 2000).

After accomplishing the desired degree of roasting, the beans should be cooled quickly

by chilly air aiming to stop additionally changes in color, flavor, and volume. The

7

organic loss as shown in table subsequent to roasting is estimated by the dry mass or

total mass loss.

Table 2-1 Material data of Arabica coffee beans (Eggers et al., 2001)

Coffee Green Medium to dark

Mass(g) 0.15 0.13

Moisture (Wt. %) 10-12 2-3

Roasting loss (Wt. %) 0 15-18

Organic loss (Wt. %) 0 5-8

Density (g/ml) 1.2-1.4 0.7-0.8

Volume (ml) 0.11-0.13 0.16-0.19

Equivalent radius(mm) 3 3.5

porosity <0.1 0.5

2.2 Effects of Coffee Roasting

2.2.1 Physical Changes

The principle physical change occurring in the bean with increasing temperature is a

change in color from brown to dark, lose its original strength, volume grows up to 100%

for a dark roast and relatively reduce in the density around 40-50% as shown in table

2.2 (Pittia et al., 2001)

Table 2-2 Changes inside the coffee bean during roasting (Pittia et al., 2001)

Temperature change

with in bean (℃)

Effect

20–130 Liquid-vapor transition of water (bean drying).

Color fades

130–140 First endothermic maximum.

Yellow coloring and swelling of bean with the

beginning of non-enzymatic browning.

Roast gases are formed and begin to evaporate

140–160 Complex series of endothermic and exothermic

peaks.

Color changes to light brown. Large increase in bean

volume and micro pores. Rests of silver skin are

removed.

8

Temperature change

with in bean (℃)

Effect

140-160 Bean is very brittle. Some little fissures at the surface

occur.

Aroma formation starts

160–190 Roasting reactions move towards the inner dry

structure of the bean

190–220 Micro fissures inside the bean. Smoke escapes.

Large volumes of carbon dioxide escape and leave

the bean very porous.

Typical flavor of roasted coffee appears

2.2.1.1 Product Temperature

Contrasted with other roasting processes in food application of coffee requires the most

elevated product temperature for developing the desired product characteristics. In

general, the coffee bean temperature is required to surpass 190℃ for a certain minimum

duration to trigger the typical chemical reactions of roasting (Hernández et al., 2007).

The increment of product temperature during traditional coffee roasting is characterized

by a consistent increment up to the final maximum at which the process is then ceased

by abrupt optional water quenching based precooling and cooling. A typical final

product temperature may be in the range of 200-250℃. The roasting time might be from

3 to 20 minutes (S Schenker et al., 2000).

2.2.1.2 Color Development

The color change during roasting is the most evident and visible sign of increasing

degree of roast (Ottinger et al., 2001). Coffee beans change color from greenish-gray-

blue which is the color of the raw bean to yellow, orange, brown, dark brown, lastly to

relatively black. The color improvement is especially interlinked with flavor

development. In this way, the bean color is the best indicator of the level of degree of

roast and a most imperative quality criterion (Perrone et al., 2012). Generally, the best

quality and the best specific final color is determined by the desire of the consumer.

9

2.2.1.3 Volume Increase and Structural Changes

The structure of the coffee bean is by all accounts basic for the creation of a typical

roast flavor of coffee. The intact bean acts as an essential mini-reactor for the chemical

reactions. It controls the reaction condition in a way that the correct precursors can react

with each other in the correct sequence (Frisullo et al., 2012). Temperature, water

activity, pressure, and also mass transport phenomena are especially identified with

structure and oversee the energy of chemical reactions that create flavor (S Schenker et

al., 2000).

Coffee beans swell during roasting and may build the volume up to factor 2. The

microstructure changes from a dense to an exceptionally permeable or very porous

structure. Unlike other crop beans, coffee beans swell constantly in a steady process.

The expanding gas pressure inside the bean is the principle main impetus for expansion,

though the thick plant cell walls hold against it (Redgwell et al., 2002).

Volume increment, dehydration, and chemical reactions during roasting result in

significant changes in the microstructure of the bean tissue. The green bean is described

by an extremely smaller and thick structure and sophisticated intracellular organization

of native biological cells (Geiger et al., 2001).

The cell wall of coffee beans is remarkably thick when contrasted with plant material

of different species (Eira et al., 2006). They are outfitted with reinforced rings that give

them the run of the mill nodular appearance in the cross-sectional view. Roasting

destroys this local structure and step by step prompts development of uncovered cells

(Geiger et al., 2001; Pittia et al., 2001).

In spite of the fact that the structure of cell wall stays unbroken, the reduced cytoplasm

is pushed toward the wall offering a path to an extensive gas-filled void possessing the

middle (Frisullo et al., 2012). A portion of the staying denatured cytoplasm extends

along the cell walls. This layer winds up more slender on the continuation of roasting

since more cell mass is changed over into gases and water vapor and the cell sizes

increment. In parallel to volume increment, estimated porosity increments likewise

progressively increase during roasting (S Schenker et al., 2000).

10

2.2.1.4 Dehydration

Green coffee beans enter the roasting procedure with a standard moisture content of

around 10-12% g/g, wet base (Pittia et al., 2001). During roasting, dehydration depends

on the roasting conditions, the roasted beans may leave the process with the last

moisture of around 2.5%. Obviously, the last moisture content of roasted beans may

likewise be impacted by water cooling conditions on the grounds that the beans may

somewhat retain water that is sprayed onto the bean surface during the precooling step

(S Schenker et al., 2002).

According to Perrone et al. (2010), isothermal roasting happens in a steady and

continuous manner. In non-isothermal conditions the dehydration kinetic energy is

multistep process conditions, rely upon the roasting profile. In addition to the water

present in the green bean, there is likewise a lot of water that is produced because of

chemical reactions.

2.2.1.5 Roast Loss

During roasting, water is vaporized and dry mass is somewhat changed into volatiles.

At the end of roasting coffee beans may lose 12-20% weight during roasting, depending

upon green bean quality, roasting parameters, and desired final degree of roast (Ciampa

et al., 2010).

Perrone et al. (2010) say that roast loss consists of several parts, such as water

evaporation, the transformation of organic matter into gas and volatiles, physical loss

of silver skins, dust, and bean fragments or other light material. The roast loss is always

product specific. It increases in a steady and continuous manner during roasting. The

highest rate of roast loss is usually found in the early process stages and is mainly

caused by dehydration, whereas loss of organic matter is initiated later during the more

advanced stages (Frisullo et al., 2012).

2.2.1.6 Oil Migration to the Bean Surface

Coffee beans may contain up to 18% lipid which is coffee oil (Gaffney et al., 2015).

Lipids are surrounded by the cytoplasm of the native plant cell inside separate

membrane-protected oil bodies situated along the cell walls. Structural changes in the

11

coffee bean tissue during roasting destroy the native biological cell organization, break

up the oil bodies, and mobilize the coffee oil. Roasted coffee beans display once in a

while a more or less oil sweating (Baggenstoss et al., 2008). The gas pressure inside the

bean pushes the coffee oil through a tiny microchannel in the cell wall to the bean

surface. During the underlying phases of the oil migration, various little oil droplets

show up on the bean surface. Oil droplets may coalesce and turn out to be more, in the

long run covering the whole bean with a shiny oil film (Baggenstoss et al., 2008).

2.2.2 Chemical changes

2.2.2.1 Endothermic and Exothermic Roasting Phase

The increasing bean temperature during roasting prompts complex chemical reactions

that finally in an extremely altered composition of the roasted bean. The most essential

chemical reactions influencing carbohydrate to incorporate Maillard reaction, Strecker

degradation, pyrolysis, and caramelization (Buffo et al., 2004).

A typical mailard reaction, strecker degradation, pyrolysis and caramelization is shown

below

Maillard reaction: when carbonyl group of a sugar react with amino group of amino

acid

12

strecker degradation : converts an α-amino acid into an aldehyde containing the side

chain, by way of an imine intermediate.

Pyrolysis: chemical decomposition of organic materials through the application of heat

C7H16 C2H4 +C5H12

Caramelization reaction: - the browning of sugar when heated beyond the melting point

and give caramelized sugar.

Roasting additionally leads to protein denaturation and degradation. Numerous acids

present in the green bean are additionally degraded. During the underlying phases of

roasting a considerable energy, input is required to drive the evaporation of water and

to induce chemical reactions which are an endothermic stage (Hernández et al., 2007).

At a certain point during roasting, the energy balance of chemical reactions ends up

autocatalytic (exothermic). The beans, in the end, begin to create heat without anyone

else (Tsai et al., 2017). Henceforth, the last phases of the roasting process are described

by increasing rate of process progression and step by step approach conditions of a

combustion process. Process control ends up being crucial at this stage (Baggenstoss et

al., 2008). A few seconds can have the effect between a correctly roasted product with

Heat

13

the desired degree of roast and an over-roasted product. Roasting should be ceased

suddenly at the desired degree of roast with efficient precooling or cooling step. In the

event that roasting proceeds in an uncontrolled way the beans may burst into flames

and create risky conditions in a roaster (Baggenstoss et al., 2007).

2.2.2.2 Gas Formation

Roasting creates a lot of gas because of pyrolysis and Maillard reaction. The gas

formation rate during isothermal roasting is low toward the start of the process,

however, accelerates forcefully in the second half of the process. However, it is much

subjected to the roasting conditions (Tsai et al., 2017). The prevalent gas formed after

roasting is carbon dioxide, CO2. Other critical components incorporate CO and N2.

Partially the gas is discharged to the environment during roasting, another major part

remains captured inside the beans and is just discharged later during a moderate

desorption process and consequent handling steps like grinding and brewing (Redgwell

et al., 2002).

Gas measurements and model calculations arrive at the end that the gas pressure inside

the bean upon roasting may surpass values higher than 10 bars (S Schenker et al., 2000).

The thick cell walls of coffee are naturally built up to stand this pressure without

breaking, however, get continuously stretched and span an expanding pore volume.

However, some minor structural cracks and breaks happen during the last roasting

stages, releasing a little amount of gas in a sudden microburst and show in splitting and

popping sounds. The gas together with the water vapor constitutes the main impetus for

bean expansion during roasting (Perrone et al., 2010).

2.2.2.3 Formation of Aroma Compounds

The volatile fraction of roasted coffee is extremely complex and comprises of in excess

of 1000 mixes (Mayer et al., 2001). The formation kinetics of aroma compounds during

roasting is controlled by the particular conditions for chemical reactions like

temperature, water activity and pressure as controlled by the process parameters

(Poisson et al., 2014).

14

Subsequently, different time-temperature conditions during roasting lead to particular

flavor profiles acquired from a similar raw coffee bean. Quantitative development of

key aroma impact compounds in function of process conditions have been studied using

various methodologies (S Schenker et al., 2002). Wieland et al. (2012) analyzed the

volatile part of coffee tests taken at various phases of the roasting process, using six

different roasting profiles. The first roasting stage does not create substantial aroma

amounts, but rather might be vital for the development of aroma precursors.

A greater part of aroma compounds demonstrated the most remarkable development

rate at medium stages of the roasting process and medium phases of bean dehydration

with the water content extending from 2-7% wet basis (Fadai et al., 2017). The majority

of important aroma compounds e.g., most pyrazines begin to reduce at high temperature

during advanced stages of the process. The concentration of these volatiles decrease

with increasing degree of roast. By contrast, a small number of aroma compounds keep

on being made at high temperature (M. J. Lee et al., 2013).

2.2.2.4 Evolution of the Acidity/Bitterness ratio with increasing degree of roast

Good cupped coffee is described by an adjusted acidity/bitterness proportion. As a

general guideline, increasing degree of roast prompts decreasing acidity and increasing

bitterness. Accordingly, choosing the ideal level of roast is critical for an adjusted taste

profile (Perrone et al., 2010). Chlorogenic acids are strongly degraded during roasting.

However, their contribution to overall sensory perception is extremely restricted. By

contrast, citric and malic acids are highly relevant to sensory perception (Balzer, 2001).

These acids are already available in the green bean and are then likewise gradually

reduced during roasting. Acetic acid and formic acid are likewise strong contributors to

total sensory perceived acidity. Their concentration in green coffee is low. These acids

are produced during the initial stages of roasting a carbohydrate precursor but then

degraded at higher temperatures during the final stages of roasting (Perrone et al.,

2010). The concentrations of quinic acid and some volatile acids are increasing to some

extent during roasting. Generally speaking, the sensory perceivable total acidity is

obviously reduced over the course of roasting. Lightly roasted beans unfold more

acidity in the cup than dark roasted coffee (Willems et al., 2016).

15

M. J. Lee et al. (2013) said that roasting creates a bitter taste in coffee. The identification

and development pathways of bitterness components in roasted coffee have been

illustrated in recent years and are still subject to ongoing scientific research. In spite of

the fact that caffeine which is available in the green bean has a solid bitter taste, it

contributes just somewhere in the range of 10-20% to the tangible high intensity in

coffee (Blank et al., 2002). The primary supporters of bitterness are shaped by roasting.

The class of chlorogenic acid lactones-a decomposition result of chlorogenic acids has

been recognized as one of the principal supporters of bitterness in coffee (T Hofmann

et al., 2008).

2.3 Roasting Techniques

From an engineering viewpoint, the principles of roasting can be described by roaster

type, dominant heat transfer mechanism and operational perspective as shown in figure

2.3 (Clarke et al., 2008).

Figure 2-3 Principal roasting techniques(Clarke et al., 2008)

Speer et al. (2001) explain that traditional or customary roasters have a high tendency

of giving inconstancy in roasted coffee product prompted a reduction of interest for the

vast persistent roaster in the roast and ground market; in opposite batch, roasters gives

extremely steady product where the heating input can fluctuate after some time.

Type of roaster

Rotating drum

Horizontal

Vertical fixed chamber

With rotating paddle

Swirling bed

Spouted bed

Fluidized bed

Heat transfer

Contact

Radiation

Convective radiation

Operation

Batch-wise

Continuous

Conventional

Pressurized

Technique

16

The majority of modern roasters utilize hot air in such away a constrained convective

hot air flows through a moving bed of coffee bean this movement of bean is made by

the rotation or by the flow of roasting gas (Nagaraju et al., 2016).

At present the vitality and ecological issues turn out to be all the more tight,

accordingly, current roasting advances consider distribution of the fumes gas; the gas

is either return to the burner or will be subjected to ignition so as to produce vitality and

reduce residual particle and the heat generated is utilized to preheat the approaching

outside air (Ciampa et al., 2010).

2.3.1 Industrial coffee roasting

Although alternative technologies, for example, infrared, microwave, superheated

steam, and others have been created and evaluated, hot air roasting innovation is yet the

main widespread innovation connected in modern activities. Hot air roasting machines

might be grouped with respect to different criteria, for example, such as product flow

i.e. batch or continuous, mechanical principle, heat transfer, the air-to-bean ratio

(ABR), air flow (open system and air recirculation system), and automation principles

(Mussatto et al., 2011).

2.3.1.1 Product Flow

Roasting machines utilize the idea of either continuous product flow or batch roasting.

Although constant roasting frameworks used to be prevalent a few decades back, they

are almost out of operation today. The benefits of batch principles led to the absolute

predominance of industrial batch roasters. Batch principles give more process

flexibility and are simpler to control (Bonnlander et al., 2004).

2.3.1.2 Mechanical Principle

The beans must be kept constantly in movement inside the roasting chamber to

guarantee homogeneous heat transfer from the hot air to the coffee. From a rotating

drum or bowls to stirring devices, different mechanical principles have been introduced

to satisfy this assignment (Clarke et al., 2008). By contrast, fluidized-bed roasters use

sufficiently high air speeds as opposed to moving parts to agitate the beans. However,

17

any means of bean movement exposes the beans to some mechanical stress. An

optimized design avoids and minimizes bean breakage (Nagaraju et al., 2016).

2.3.1.3 Heat Transfer

In any hot air roasting system heat is always transported by convection, conduction,

and radiation at the same time. Convection transfers heat from the hot air right to the

bean surface (Fadai et al., 2017). Conduction happens when heat is transferred from the

hot walls of the roasting chamber to the beans. The extents of contribution may vary

from one system to another. The contribution of radiation is usually very limited and

negligible (Stefan Schenker, 2000). Concerning conduction and convection, the amount

of process air used for roasting plays a key role. In a fluidized-bed roaster, convection

is the main way of heat transfer, whereas in a drum roaster a considerable amount of

heat may be transferred via conduction. An exact calculation or measurement of the

conduction/convection ratio is difficult to accomplish (Hernández et al., 2007).

2.3.1.4 Air-to-Bean Ratio

A similar amount of heat can be transferred to the beans using either a small quantity

of air at a higher temperature or using a larger amount of air at a lower temperature.

The amount of hot air used in a roasting process in relation to the batch size of coffee

beans is defined as ABR (Nagaraju et al., 2016; Stefan Schenker, 2000).

2.3.1.5 Air Flow

Smaller roasting systems usually suck in the process air from one side and emanate the

off-gas at another end. Since the discharged off-gas is still at high temperature, open

system is not energy effective. This is why most large-scale operations make use of air

recirculation for substantially improved energy efficiency (Tsai et al., 2017). In

recirculation systems, a major part of the off-gas stream approximately around 80% is

driven back to the heating unit then reinjected into the roasting chamber. However, the

remaining part of the off-gas stream must leave the system to avoid accumulation of

problematic gas concentrations with the potential for explosion. This off-gas stream

passes a pretty much modern cleaning step for off-gas pollution control and compliance

with air pollution regulation (Oliveros et al., 2017; Vargas-Elias et al., 2016).

18

2.3.1.6 Water quenching device

Most media to large scale roasting machines is furnished with a water quenching

device. When the beans achieve their last temperature the roasting process may

alternatively be ended through a sudden precooling step by splashing a predefined

amount of cold water onto the beans called water quenching. Water evaporates on the

bean surface and cools the beans. In spite of the fact that this precooling step is optional,

it accomplishes a consistent degree of roast, batch by batch (Baggenstoss et al., 2007).

2.3.1.7 Process Automation Principles

Although small-scale roasting machines are often operated manually, larger systems

usually use more sophisticated process control systems. The conventional method for

process mechanization is to set and control a proper hot air temperature, either in a

single & isothermal or in multistage process & profile roasting (Eggers et al., 2001). In

a conventional machine, the control system regulates the burner power to reach and

maintain the pre-set hot air temperature. However, the disadvantage is that the actual

product temperature progression may not be consistent from one batch to the next and

remains subject to many factors that affect roasting (Mondello et al., 2005).

As described by Wieland et al. (2012) more advanced process control systems are

guided by the actual development of product temperature rather than hot air

temperature. The desired product time-temperature master curve is registered in the

recipe and is then precisely reproduced in the roaster batch by batch by continuous and

careful fine tuning of the energy input. This type of process control results in superior

quality consistency because the beans experience always the same temperature

development. It requires a sophisticated hardware and software design for continuous,

rapid, and accurate modulation of energy input into the roasting chamber (Speer et al.,

2001).

2.3.2 High Yield Roasting

Low temperature roast preparing has the upside of homogeneity in the bean under ideal

condition yet it requires a long investment and there is an option for upgraded heat

19

exchange by expanding the gas temperature and the hot gas to coffee ratio (Baggenstoss

et al., 2008).

In fast roasting, the required thermal energy is given so it minimizes the processing

time, to 1.5-minutes or even less. This is accomplished by the advancement of forced

convection roasting at a temperature around 300-400℃ (Redgwell et al., 2002). The

poor thermal conductivity of the bean causes roasting gradient with in the bean, bean

volume additionally expanded with a trademark puffing by 10-15% concerning

ordinary roasting, it likewise lessens the bulk density beneath 300g/l and improves

extraction during preparing by 20% (Perez-Alegria et al., 2001).

Heat transfer and technology connected specifically influence both bean temperature

and roasting time. The aftereffects of the low temperature long time (LTLT) and high

temperature short time (HTST) forms have been accounted (Bottazzi et al., 2012).

Pittia et al. (2001) examine conventional roaster with a fluidized bed roaster and found

that fluidized bed roaster creates a fundamentally quicker increment in bean

temperature. Even so, high return roasting isn't viewed as ideal in light of owing high

remaining substance of chlorogenic acids, which conveys into the glass an astringent

sour note (Sacchetti et al., 2009).

2.4 Types of Industrial Roasting Equipment

Industrial roaster design has been demonstrated and explained by various authors in

terms of advancement, pollution control, and energy efficiency.

2.4.1 Drum Roasters

The most widespread batch roaster design is the drum roaster. In this traditional design,

the batch of beans is kept in a flat pivoting drum. Hot air enters at the drum back-end

through a screen, pass through the drum and leaves at the front-end by means of an

expansion chamber. The drum rotation as well as baffles installed in the interior of the

drum keep the beans in motion and assure a through the mixing of beans with the hot

air for uniform heat transfer (Chiang et al., 2017). After completion of roasting and

precooling the batch is transferred trough an opening gate or gap at the front-end of the

20

drum and falls into the cooling section. The rotation of the drum and the baffles help

for rapid drum discharge. Most often the cooling area includes a round-bed cooler with

a pivoting delicate blending device. Depending on the air handling, the cooling air may

flow through the coffee bed either in a bottom-up or top-down direction (Eggers et al.,

2001).

Stefan Schenker (2000) indicates drum roasters usually work at a relatively low ABR.

The maximum applicable amount of air is limited by the maximum exit air velocity at

which beans may be carried away with the air. Typical roasting time is in the range of

8-20 minutes. The convection conduction ratio is largely influenced by the selection of

direct or indirect drum heating. In direct drum heating, the furnace is located directly

underneath the roasting drum. The resulting wall temperature is relatively high and

conductive heat transfer of beans in contact with the hot wall becomes substantial. By

contrast, in indirect drum heating, the drum is insulated and the hot air directed to the

back of the drum for more convective heat transfer inside the drum (Chiang et al., 2017).

2.4.2 Paddle (Tangential) Roaster

In this design, the roasting chamber is stationary and contains a rotating mixing device

with paddles. Hot air enters in the lower portion of the roasting chamber, very often

tangentially to the half cylindrical-shaped contour of the roasting chamber (Bonnlander

et al., 2004). It passes then in a bottom-up direction across the batch of beans into a

broad expansion chamber at the upper part of the roasting chamber before it exits. In

the expansion chamber, the air velocity is reduced considerably so that no beans are

carried away with the exit air, even at a high air-to-bean ratio (Clarke et al., 2008).

Alternatively, toward the finish of the roasting process water quenching can be applied

as a precooling step. The beans are then released at the base of the roasting chamber

through an opening gate. They fall by gravity into the cooling area. The cooling area

may comprise of a round bed cooler with a gently rotating agitator or a rectangular

cooling sieve without any mechanical agitation devices (Baggenstoss et al., 2007). The

cooling air typically streams in bottom-up direction over the coffee bed. Since the beans

are kept in movement in the roasting chamber by the turning paddles relatively

independent from the air flow, the roaster design allows operating within a wide range

21

of ABR. Consequently, the conduction-convection ratio is also variable. Depending

upon the requirements, the roasting time may shift in a range from 2 to 20 minutes

(Speer et al., 2001).

2.4.3 Bowl Roaster

Kelly et al. (2016) describe, a rotating bowl keeps the batch of beans in motion.

Centrifugal forces cause the bean movement to the bowl periphery where the beans

encounter with stationary guiding baffles that bring them back to the center of the bowl

in a spiral-shaped circuit. The hot air is guided top-down in a vertical shaft along the

rotation axis and enters the roasting chamber at the bottom of the bowl where it converts

into bottom-up direction. After having passed the coffee beans it exits the bowl on top.

When the beans have reached their final temperature an optional precooling step may

be applied by water quenching. The bowl then moves to a lower position, opening a

gap at the bowl edge for bean discharging into the cooling section. The design allows

operating within a certain range of ABR. Typical roasting time may be in the range of

3-12 minutes (Eggers et al., 2001).

2.4.4 Fluidized-bed Roasters

There are no moving parts inside the roasting chamber of a fluidized-bed roaster. The

beans are kept in motion solely by the current of the hot air. A relatively high air

velocity is required to generate sufficient buoyancy for fluidization of coffee beans. The

air enters at the bottom of the roasting chamber through a perforated plate (Clarke et

al., 2008; Eggers et al., 2001). Alternatively, a particular geometry of the roasting

chamber may be used to create a rotation whirl in the air stream. At last the hot air exits

over the roasting chamber. Convection represents the main heat transfer. However, the

roasting chamber geometry may also incorporate a zone with slanted walls on which

beans slide down and experience a phase of a higher share of conduction before they

get back to the zone of high air velocity. At the predefined final product temperature,

the beans are transferred by gravity into the cooling unit (Bonnlander et al., 2004).

A summarized roaster type and their characteristics are well explained in table 2.3

22

Table 2-3 The basic principles of modern roasting technology (Clarke et al., 2008).

Type Characteristics

Rotating cylinder Horizontal/vertical

With/without perforated walls

Direct heating by convective flow of hot gases

Indirect heating by hot drum walls

Batch-operated

Continuously operated by an inner conveyer

Gas temperatures: 400–550℃

Roasting times: 8.5–20 minutes

Bowl Direct heating by convective flow of hot gases

Continuously operated across the gas stream;

rotating

Gas temperatures: 480–550℃

Roasting times: 3–6 minutes

Fixed drum Direct heating by convective flow of hot gases

Batch operated

Gas temperatures: 400–450℃

Roasting times: 3–6 minutes

Fluidized bed Direct heating by fluidizing gas

Batch operated

Gas temperatures: 240–270℃

Roasting times: 5-minutes

Spouted bed Direct heating by fluidizing gas

Batch operated

Fast roasting:

o Gas temperatures: 310–360℃

23

Type characteristics

o Roasting times: 1.5–6 minutes

Slow roasting:

o Gas temperatures: 230–275℃

o Roasting times: 10–20 minutes

Swirling bed Tangential gas inlet

Spiral upward motion of the beans

Direct heat transfer of a moved packed bed

Gas temperatures: 280℃

Roasting times: 1.5–3 minutes

2.5 Thesis organization

This thesis compiles five chapter. chapter1: Introduction which describes the state of

art and research objective with its specific task and scope; chapter2: literature review

revises key facts and new findings related to coffee roasting, beside it also describes

what has been done in the roasting technologies and processes; chapter3: shows how

the roasting process, characterization and sensory attribute was done in this research;

chapter4: presents the findings of this research work in comparison with previously

published articles; Chapter5: decrees conclusion based on the findings in chapter4 and

recommend further studies to enhance current research .

24

3 MATERIALS AND METHODS

3.1 Material

Raw coffee bean (C. Arabica) used for this study was export standard limu coffee. It is

obtained from Ethiopian coffee and tea authority, coffee quality inspection and

certification center (ECTACQICC). This variety was selected due to its availability

during the study time. In this study, the raw coffee bean used was free from impurities

such as husk, irregular beans like a piece of parchment and beans with an irregular

visual appearance such as bean having black color or attacked by insects.

3.2 Chemicals and Equipment

In order to complete the experiments the following major chemical and equipment were

used as shown in table 3.1

Table 3-1Chemicals and equipment used with their grade/model and purpose

Name of Chemical or

equipment

Grade/model Used for

Caffeine powder Reagent plus, SIGMA-ALDRICH

Standard for generation of a

calibration curve

water Distilled brewing of roasted coffee

Chloroform Reagent grade

99.8%

Extraction of caffeine from

grounded coffee bean

Digital balance Pw124(120,0.001g) Mass measurement

pycnometer 290/I reference

class

Determination of bulk density

Glass jar Known volume Handling and brewing of coffee

Cup Food grade use and

throw

Providing brewed coffee for

panelist

Sieve 0-3.35 mm To screen out the size of a

grounded coffee bean

Perforated plate Stainless steel Packing the bean to be roasted

Crucible Ceramic Determination of moisture

content

PID oven M40-VF

bernareggio, Italy)

Roasting of coffee

UV Perkin Elmer

lamda35

Determination of absorbance

25

3.3 Experimental description and procedure

The coffee bean brought from ECTACQICC was screened using a sieve to remove

fragmented coffee bean less than 4mm size and the under sized coffee beans had been

placed distinctly. Then the desired one was sorted based on their color, shape, and

appearance using ECTACQICC standard (ECX, 2011).

As shown in Figure 3.1 before roasting the bean was dried in an oven at 105℃ for 3-

hours and then crushed using Nima coffee grinder. In order to get the desired size of

ground bean sieving is done by using a sieve of 1.7-3.35 mm size range. Finally, it gets

roasted at designed temperature and time, and then it gets cooled in the open air.

Figure 3-1Block diagram of coffee roasting process

3.4 Experimental design of coffee roasting

During roasting the experiment is designed using factorial design i.e. (factor) level with

three factors that were operating temperature, time and particle size and three levels as

shown in table 3.1 and the experiment was performed.

Temperature range is selected based on the characteristics of coffee bean. When the

bean temperature reaches in the range of 190-220℃ smoke escapes and typical flavor

Screening Sorting Drying (105℃,

3hrs)

Crushing Roasting (200-260℃, 5-

15-minutes)

Screening (1.7-3.35

mm)

Cooling

26

of roasted coffee appears. As a result, in order to have a heat transfer, the hot air should

be greater than 190℃ and selected to be 200℃. For the upper limit, 260℃ is selected

by considering the effect of the poor thermal conductivity of the coffee bean and air on

the roasting process.

The time range is selected by comparing previously done experiments in which; for

fluidized spouted bed it takes 1.5-6 minutes in a temperature range of 310-360℃; for

rotating cylinder it takes 8.5-20 minutes in a temperature range of 400-550℃ (Nagaraju

et al., 2016). For this study, a minimum of 5-minutes and maximum of 15-minutes were

set in order to give compensation for the reduction in temperature. Particle size ranges

were determined by the available sieve in the laboratories. The experiment response

(R) is randomized as shown in table 3.2

Table 3-2 Factors and levels of roasting parameters

Temperature (℃)

200 230 260

Time

(minutes)

particle

size(mm)

5 10 15 5 10 15 5 10 15

1.7-2.36mm R2 R11 R22 R1 R17 R6 R5 R10 R14

2.36-3.35mm R15 R7 R18 R9 R4 R3 R25 R21 R16

4-6mm R8 R20 R26 R19 R12 R24 R13 R27 R23

Number of experimental run for roasting = 33 =27

Number of replication = 3

Total no roasting experiment =3*27=81

27

3.5 Configuration of the roaster arrangement

In order to have good circulation of hot air, beans were placed in the stainless perforated

plate and placed near to the fan which operates by forced ventilation and natural air

convection oven which has opening and closing for air (model: M40-VF bernareggio,

Italy) as indicated in Figure3.2. During roasting, once the samples were inserted inward

and outward flow of air was blocked and roasting proceeds.

Figure 3-2 experimental setup of packed bed roaster

3.6 Characterization of raw coffee bean

Forty percent of the quality of coffee roasting is dependent on the raw coffee bean

quality before roasting. In this work, five panelists had been invited and trained. Then

350 gram raw coffee bean is placed in a cup and each individual inspects the quality

using ECTACQICC standard evaluation format and raw value checklist tables 3.3 and

table 3.4 respectively (ECX, 2011). This experiment is replicated five times.

For secondary raw value observation both Ethiopian and specialty coffee association of

America (SCAA) standard used side by side. For further pictorial information

concerning the terms used in standard see appendix A

28

Definition of the terms used for primary (count) defect

Full black: completely black colored bean

Full sour: biting and pungent sourness, ferment, rotten fruit flavored bean

Fungus: Unroasted coffee beans with a light green or white fur-like texture

characteristic of mold

Foreign matter: impurities such as sand and grit

Insect defects: beans with wholes created by assimilation of insects.

Definition of the terms used for secondary (weight) defect for Ethiopian Checklist

Foxy: Unroasted coffee beans with a brown or rust color

Under dried: bean with high moisture content stick to each other

Over dried: Ragged shaped, pale, and light weight unroasted coffee beans

Mixed dried: presence of under dried, well dried and over dried bean.

Stinker: A coffee bean that produce an unpleasant or fowl taste

Faded: Unroasted coffee beans that have lost much of their original color, a

characteristic of old crop and beans that were dried too rapidly

Coated: beans enclosed by shell

Light: bean have lighter mass compared with other

Starved: bean that grow in mineral-starved soil

29

Table 3-3 Evaluation format for raw coffee bean

Defects (20%) Odor (10%) Make up

& shape

(5%)

Color (5%)

Primary

(count)

10%

points Secondary

(weight)

10%

points Quality points Very

good

5 Bluish 5

1 10 <5 10 Clean 10 Good 4 grayish 4

2-5 8 <8 8 Fairly

clean

8 Fairly

good

3 Greenish 3

6-10 6 <10 6 Trace 6 average 2 coated 2

11-15 4 <12 4 Light 4 small 1 faded 1

16-20 2 <14 2 Moderate 2

>20 1 >14 1 Strong 0

Table 3-4 Raw coffee bean defect checking list

Raw values

Primary defect Secondary defects observation

type bean grade SCAA 0 1 2 3 Ethiopia 0 1 2 3

Full black Partial

black

foxy

Full sour Partial sour Under

dried

Fungus Floater Over dried

Foreign

matter

Immature Mixed

dried

Insect

defects

Withered Stinker

Total shell Faded

S. insect Coated

Total Light

Starved

3.7 Physio-chemical characterization of bean

3.7.1 Determination of bulk density

Bulk density was determined by measuring the mass of three known volume i.e 10ml,

25ml and 50ml of empty pycnometer (Mp) followed by measuring of pycnometer with

the sample (Ms). Then the mass of each empty pycnometer was subtracted from each

sample holding measurement and by considering the net mass (Mn) of sample and

30

volume of pycnometer the bulk density was calculated as mass of sample divided by

volume of pycnometer as shown in equation 1 and 2. Finally, the bulk density is the

average of the three determination (Liu et al., 2008).

n p s pM M M M 1

ns

p

M

V

2

3.7.2 Moisture content determination

Moisture content was determined first by measuring the mass of empty crucible before

drying (M1) then 50 gram of sample was placed in the crucible (M2) and placed in an

oven at 105℃ to remove the moisture. The sample gets placed in the oven until it

reaches constant mass (M3), after that, it gets cooled inside of a desiccator to prevent

losing or gaining moisture so as to get their tare weights. The moisture content was

calculated using equation 3 in wet basis (Burmester et al., 2010).

2 3

2 1

% *100%c

M MM

M M

3

3.7.3 Determination of dry mass loss

The total dry mass loss was determined by measuring the mass of the dried raw coffee

bean before and after roasting. The net mass change before and after roasting divided

by the mass of dried raw coffee bean before roasting as shown in equation 4. Here

desiccators were used to prevent losing or gaining of moisture (Clarke et al., 2008).

% *100%l

i

Wi WW

W

4

3.8 Determination of caffeine content

The caffeine content determination method was adopted from (Ahmad et al., 2016) and

determined as follow.

First 100ppm stock solution was prepared by dissolving 20mg caffeine in 200ml

chloroform solvent in 250ml holding capacity volumetric flask. Then the standard

31

preparation follows by pipetting (10, 20, 30, 40, 50) ml aliquots of stock standards

solution into a separate volumetric flask of 100ml and dilute it with chloroform and

forms (10, 20, 30, 40, 50) mg/L standard solution. The absorbance of each solution was

measured at absorption maximum of 205nm using 10mm quartz cuvettes.

The caffeine in the roasted coffee was extracted by pouring of 5ml brewed coffee in

separating funnel followed by addition of 1ml of sodium carbonate solution, which is

prepared by dissolving 20gm sodium carbonate into distilled water in a 25ml volumetric

flask. 20 ml of chloroform was also added in the separating funnel to dilute the caffeine.

The caffeine was extracted by inverting separating funnel at least three times venting

the separating funnel after each inversion. The non-aqueous chloroform layer was

removed. This procedure was repeated for all brewed samples and the absorbance of

the coffee extract was measured on UV/Visible Spectrophotometer at 205 nm using

quartz cuvette

3.9 Sensory attribute analysis of the final product

Twelve gram of roasted coffee bean was milled by using Nima grinder and brewed in

250 ml of boiled water based on ECTACQICC standard. After that five panelists

evaluate the sensory attribute i.e. cup cleanness, acidity, body, and flavor for seven

times based on ECTACQICC standard and record their result using table 3.5. After

each taste all the panelist rinse their mouth with tap water in order to avoid the residual

effect. All the sensory were conducted at 10:00-10:30 in the morning and 3:00-3:30 in

the afternoon to reduce the hunger effect on sensory data.

Cup cleanness: stands for no presence of ‘’ non-coffee’’ smell, proper settlement after

brewing, to develop foam well and sustain the foam for a long time without breaking

and overall visual appearance. If the brewed coffee fulfill all the five criteria it scores

15 and if not, it will be given the corresponding value.

Acidity: refers the concentration of extracts in the cup for a given sensory analysis. It

was done by aspirating the liquor into the mouth in such a way as to cover as much area

as possible, especially the tongue and upper palate. For strong feeling it is called pointed

and score 15. In contrast, for too diluted it is called not detected and score 1.

32

Body: represents the combined effect of viscosity and mouth feel property of the

brewed coffee. Whenever the mouth feel and viscosity of the liquor is satisfactory it

scores 15 and labeled as full. In contrast, if the mouth feel and viscosity is not too far

from tap water it is labeled as not detected and score 1.

Flavor: Flavor of brewed coffee represents the originality of taste of the coffee in which

the brewed coffee represents the true flavor of limu brand coffee. If it is purely of limu

brand it is good and scores 15 and if the bean is not representative of limu brand coffee

it is called not detected and score 1.

Table 3-5 Cup quality value check list

Cup value (60%)

Cup cleanness

(15%)

Acidity (15%) Body (15%) Flavor (15%)

Quality points Intensity points quality points quality points

Clean 15 Pointed 15 Full 15 Good 15

Fairly

clean

12 Medium

pointed

12 Medium

full

12 Fairly good 12

1 CD 9 Medium 9 Medium 9 Average 9

2CD 6 Light 6 Light 6 Fair 6

3CD 3 Lacking 3 Thin 3 Commonish 3

>3CD 1 Not

detected

1 Not

detected

1 Not

detected

1

33

4 RESULT AND DISCUSSION

4.1 Raw value analysis

The raw value result comprises primary and secondary defect, the odor of bean, make

up and shape, and color of the coffee bean as shown in table 4.1

Table 4-1 Average value of the raw coffee bean value analysis

Defects (20%) Odor (10%) Make up and

shape (5%)

Color (5%)

Primary (10%) Secondary (10%)

10 10 10 5 5

8 8 10 4 4

10 10 10 5 5

10 8 10 5 4

10 8 10 4 5

Average 9.6 Average 8.8 Average 10 Average 4.6 Average 4.6

The average defect value is 9.6 and 8.8 for primary and secondary defects respectively.

Primary defect includes full black, full sour, fungus attacked and insect damaged beans

as well as the presence of foreign matter and as the result indicates that probability of

counting bad bean according to ECTACQICC standard result is 0.4 (ECX, 2011).

Secondary defect indicates that whether the beans have shell, immaturity or mixed dried

this is due non-uniform growth of coffee bean and do not have equal handling after

harvesting. No problem has been seen on the odor of the raw bean and scores 10 but

for makeup and shape some of the beans range from very good to good. The irregularity

in some beans comes from storage and washing breakage. When it comes to the color

of the beans some of the beans are faded due to a secondary defect and creates color

variation. The overall raw coffee bean quality is 37.6/40.

4.2 Moisture content

The moisture content of the three sample was found to be 7.89%, 9.08%, and 8.38%.

The average value is 8.45% as shown in table 4.2

34

Table 4-2 One-sample and T-test statistical analysis of moisture content

One-Sample Statistics

N Mean Std. Deviation Std. Error Mean

moisture 3 8.45 0.59 0.34

One-Sample Test

Test Value = 0

t df Sig. (2-

tailed)

Mean

Difference

95% Confidence

Interval of the

Difference

Lower Upper

moisture 24.499 2 .002 8.45 6.97 9.9

From one sample T test, it can be seen that the moisture content is significantly affected

by a different run at 𝛼=95%. The mean moisture content 8.45% agrees with the

international coffee organization criteria; in which dried coffee bean should have a

moisture content of 8-12.5% (Pittia et al., 2001).

4.3 Dry mass loss

The dry mass loss analysis was conducted for three different particle size ranges is

discussed in the following sub sections. The average raw data can be seen in Appendix

B.

4.3.1 Dry mass loss for initial particle size of range 4-6 mm

The raw coffee bean particle size range from 4-6mm dry mass loss figurative

representation is shown by figure 4.1. The diagram is plotted as a function of roasting

temperature and roasting time

35

Figure 4-1 Graphical representation of the percentage of dry mass loss of bean of size range

of 4-6 mm

From the above figure, the dry mass loss for all roasting time increases as the

temperature increase due to escaping of organic matter in the form of smokes (Redgwell

et al., 2002). Roasting time of 15-minutes shows nonlinear loss of dry mass and 5-

minutes roasting process results in insignificant loss of dry matter when the temperature

increases from 230℃ to 260℃. This is due to insufficient time for the bean to interact

with the hot air. The 10-minutes process shows a linear smooth increment in the dry

mass loss which is a good sign for a quality roast and it agrees with previous studies on

coffee roasting (Eggers et al., 2001). When the result obtained by Perrone et al.

(2010)compared with the above results, it shows increased dry mass loss of 30% due to

the convoluted shape of the bean which makes uniform temperature distribution

difficult so as the bean has to wait more minutes (15-20 minutes) to get well roasted

but the outer layer get roasted and result high dry mass loss)

4.3.2 Dry mass loss for initial particle size of range 2.36-3.35 mm

The raw coffee bean particle size range from 2.36-3.35mm dry mass loss figurative

representation is shown by figure 4.2 the diagram is plotted as a function of roasting

temperature and roasting time

0

5

10

15

20

25

30

195 215 235 255 275

% o

f d

ry m

ass

loss

Roasting temperature in ℃

5-minutes

10-minutes

15-minutes

36

Figure 4-2 Graphical representation of the percentage of dry mass loss of bean of size 2.36-

3.35 mm

Similar to particle size range of 4-6 mm roasting process, the dry mass loss for the

particle size of 2.36-3.35 mm increases as the temperature increase for all roasting time

due to escaping of smokes (Redgwell et al., 2002). Here all roasting times result in

linear increment on the dry mass loss with 15-minutes roasting process become more

stepper than 5 and 10-minutes. As shown in the figure 4.2 the dry mass loss achieved

by 15-minutes and 260℃ process is more than 15% when compared with the other

operating times at similar temperature, this is due to the interaction of size reduction

and extended time for the roasting process. The low dry mass loss is observed at 5 and

10-minutes roasting process. The results at 10-minutes and 260℃ agree with the

recommended value of 15-18% dry mass loss for medium to dark roast (Eggers et al.,

2001). When it comes to 15-minutes roasting process it is more than the recommended

value and may cause degradation of essential flavors and organic matter when it reaches

260℃ roasting condition. When the result obtained by Perrone et al. (2010)compared

with the above results, it shows low value of dry mass loss of 30% for high roasting

temperature and time due to the larger size of the ungrounded bean offer large mass

transfer resistance than the reduced bean size.

0

5

10

15

20

25

30

35

195 215 235 255 275

% o

f d

ry m

ass

loss

Roasting temperature in ℃

5-minutes

10-minutes

15-minutes

37

4.3.3 Dry mass loss for initial particle size of range 1.7-2.36 mm

The raw coffee bean particle size range from 1.7-2.36mm dry mass loss figurative

representation is shown by figure 4.3. The diagram is plotted as a function of roasting

temperature and roasting time.

Figure 4-3 Graphical representation of the percentage of dry mass loss of bean of size 1.7-

2.36 mm

For a particle size of 1.7-2.36mm, the temperature has a significant effect on the dry

mass loss because as temperature increases the percentage of dry mass loss increases.

Beside this, long time of roasting cause a high dry mass loss when compared to the

lowest roasting time. Unlike the other two particle size ranges, there is no sign of

linearity in the dry mass loss for the particle size of 1.7-2.36mm, this is due to

decrement of porosity with reduction of size. It is more pronounced at lower roast

temperature, 200℃ the air molecule movement is lower than that of 230℃ and 260℃.

The 1.7-2.36mm sized particle size bean shows larger dry mass loss than the coffee

bean subjected to the same roasting process condition as reported by Eggers et al.

(2001). The reason is that as the particle gets smaller and smaller it facilitate heat and

mass transfer, which results in bigger organic loss than the ungrounded coffee bean.

0

5

10

15

20

25

30

195 215 235 255 275

% o

f d

ry m

ass

loss

Roasting temperature in ℃

5-minutes

10-minutes

15-minutes

38

4.3.4 Statistical analysis of dry mass loss

The statistics of dry mass loss generated using SPSS as indicated in table 4.2. The

statistic basis univariate generalized linear model, which analyzes statistics such as the

significance of each factor, the interaction effect between factors and also the overall

interaction effect of factors on the target.

As shown in table 4.2 all the roasting parameters and their interaction affect the target

dry mass loss significantly.

Table 4-3 Univariate statistical analysis of dry mass loss

Tests of Between-Subjects Effects

Dependent Variable: Dry_mass_loss

Source Type III Sum

of Squares

df Mean Square F Sig.

Corrected Model 1592.124a 26 61.236 . .

Intercept 4587.212 1 4587.212 . .

temperature 906.364 2 453.182 . .

time 485.135 2 242.567 . .

particle_size 13.186 2 6.593 . .

temperature * time 140.435 4 35.109 . .

temperature *

particle_size 1.276 4 .319 . .

time * particle_size 25.393 4 6.348 . .

temperature * time *

particle_size 20.336 8 2.542 . .

Error .000 0 .

Total 6179.336 27

Corrected Total 1592.124 26

a. R Squared = 1.000 (Adjusted R Squared = .)

39

4.4 Change in bulk density

The change in bulk density analysis was conducted for three different particle size

ranges is discussed in the following sub sections. The average raw data can be seen in

appendix B

4.4.1 Bulk density of initial particle size of range 4-6 mm

The bulk density of the raw coffee bean of the particle size range of 4-6 mm was 550g/l

on average and figure 4.4 shows how much the bulk density of the bean has changed.

At 200℃ the roasted coffee bean bulk density was in the range 470-490 g/l for all

roasting time which is 14.5-10.9 % of the initial bulk density. Then increased to 24.5-

16.3% at 230℃ with a density value of 415-460 g/l and finally it reaches 30.9-20% at

260℃ with a density value of 380-440g/l. For a shortest roasting time as the temperature

increases, the bulk density decreases almost linearly due to a proportional change in the

bean volume and mass that escape in the form of gas(S Schenker et al., 2000). Whereas,

for 10 and 15-minutes the density slope decrease after 230℃ which implies that at 10-

minutes as temperature increase the mass leaves more and the bean volume expansion

reduces. Relatively for 15-minutes as temperature increase the volume almost reaches

to its final capability but the amount of mass leaving reduce progressively.

Figure 4-4 Graphical representation of the final bulk density of bean of size 4-6 mm

350

370

390

410

430

450

470

490

510

195 205 215 225 235 245 255 265

bulk

den

sity

g/l

Roasting temperature in ℃

5-minute

10-minute

15-minute

40

4.4.2 Bulk density of initial particle size of range 2.36-3.35mm

The bulk density of the raw coffee bean of particle size range of 2.36-3.35mm was

563g/l on average and the change in the final bulk density of the bean is shown in figure

4.5 below.

Figure 4-5 Graphical representation of the final bulk density of bean of size 2.36-3.35 mm

The roasting process at 5 and 15-minutes show a reduction of slope for bulk density

change as the temperature increases. This is due to short contact time for 5-minutes

process and removal of the volatile component due to prolonged time for 15-minutes.

Whereas, for 10-minutes the slope shows increment which implies temperature

increment for this particular roasting result much reduction in bulk density which is

associated with high organic loss (Perrone et al., 2010). Moreover, this particle size

range increases the exposed surface area per unit volume for the roasting process and

increases irregularity of shape for roasting which causes such variations.

4.4.3 Bulk density of initial particle size of range 1.7-2.36 mm

For the smallest particle size range 1.7-2.36 mm the result in bulk density are close to

one another as indicated in figure 4.6. The reason is that, instead of the advantage of

the large surface area to volume ratio, the package porosity reduction influence the

process and it inhibits the circulation of hot air. As a result, it causes larger bulk density

when compared with the other two particle size ranges.

350

370

390

410

430

450

470

490

510

530

195 205 215 225 235 245 255 265

bulk

den

sity

g/l

Roasting temperature in ℃

5-minute

10-minute

15-minute

41

Figure 4-6 Graphical representation of the final bulk density of bean of size 1.7-2.36 mm

4.4.4 Statistical analysis of bulk density

The significance of each operating parameter (factor) i.e. roasting temperature , roasting

time and particle size, the interaction between two operating parameter and the overall

interaction effect of factors on the bulk density is generated using SPSS as indicated in

table 4.3. The statistic basis univariate generalized linear model,

As shown in table 4.3 all the roasting parameters with in a group (i.e. individual levels

in each roasting time, roasting temperature and particle sizes), between a group (i.e.

between roasting time roasting, temperature and particle size) and overall affects the

target bulk density significantly. That means a change in one of the factor results on the

change of the amount of dry mass loss.

450

470

490

510

530

550

570

590

195 205 215 225 235 245 255 265

bulk

den

sity

g/l

Roasting temperature in ℃

5-minute

10-minute

15-minute

42

Table 4-4 Univariate statistical analysis of final bulk density

Tests of Between-Subjects Effects

Dependent Variable: bulk_density

Source Type III Sum

of Squares

df Mean Square F Sig.

Corrected Model 60646.768a 26 2332.568 . .

Intercept 6191487.412 1 6191487.412 . .

temperature 9443.707 2 4721.854 . .

time 8995.716 2 4497.858 . .

particle_size 38301.433 2 19150.716 . .

temperature * time 135.247 4 33.812 . .

temperature *

particle_size 620.527 4 155.132 . .

time * particle_size 2317.252 4 579.313 . .

temperature * time *

particle_size 832.884 8 104.111 . .

Error .000 0 .

Total 6252134.180 27

Corrected Total 60646.768 26

a. R Squared = 1.000 (Adjusted R Squared = .)

4.5 Sensory quality (Cup value) analysis

After the coffee was brewed its sensory quality for cup cleanness, acidity, body, and

flavor had been evaluated and each of the sensory attribute results is discussed as

follows.

4.5.1 Cup cleanness

Cup cleanness stands for the characteristics of brewed coffee to settle properly, to

develop foam well and sustain the foam for a long time without breaking and overall

visual appearance. It accounts for 15% of the total quality of the coffee roasting process

(ECX, 2011).

4.5.1.1 Cup cleanness value at 200℃

The value for cup cleanness for roasting temperature of 200 ℃ is shown in figure 4.7.

For all particle size, the cup cleanness value increases with roasting time. When

43

comparing the effect of particle size, the 1.7-2.36mm bean particle size range has a

large cup cleanness value. This is due to ease of heat transfer in smaller particle size

than the larger particle and releases more foam forming component than the larger

particle size (Gmoser et al., 2017).

Figure 4-7 Cup cleanness value at 200℃ as a function of time and particle size

4.5.1.2 Cup cleanness value at 230℃

The value for cup cleanness for roasting temperature of 230 ℃ is shown in figure 4.8.

The cup cleanness value increases with roasting time and forms plateau at 10 minutes

then approaches to same value, when the time reaches 15-minutes. At a roasting

temperature of 230℃, the maximum cup cleanness value is observed at 15-minutes and

particle size of 1.7-2.36mm. When comparing the effect of particle size the smaller

bean particle size range has high cup cleanness value, the reason is that during roasting

the interaction between the hot air and the coffee bean was boosted. For all particle

size range as the time increase from 10 to 15-minutes the value for cup, cleanness

approaches nearly to the same value due to saturation of foaming components in the

coffee bean (Arii et al., 2017).

6

7

8

9

10

11

12

13

14

0 1 2 3 4

cup

cle

anes

s val

ue

in n

um

ber

out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

44

Figure 4-8 Cup cleanness value at 230℃ as a function of time and particle size

4.5.1.3 Cup cleanness value at 260℃

The value for cup cleanness for roasting temperature of 260 ℃ is shown in figure 4.9.

For the particle size ranges, 4-6mm and 2.36-3.35mm the cup cleanness value show

increment as the time increase from 5-minutes to 10-minute at which peak value is

observed and then and it decreases when the time reaches 15-minutes. The reason is

that short time develops the foam forming chemical well but extending the time

denature their characteristics (Nunes et al., 1997).

For particle size range of 1.7-2.36mm, the cup cleanness value drops as the roasting

time increases. As particle size reduces, the cup cleanness value reduces due to

degradation of foam forming components at 260℃ and the particle becomes less dense,

so it won`t settle so easily.

8

9

10

11

12

13

14

0 1 2 3 4

cup

cle

anes

s val

ue

in n

um

ber

out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

45

Figure 4-9 Cup cleanness value at 260℃ as a function of time and particle size

4.5.2 Acidity

Acidity value refers to the concentration of extracts in the cup for a given sensory

analysis and here below the acidity value is discussed for 200℃ 230℃ and 260℃

processes independently. It accounts for 15% of the total quality of the coffee roasting

process (ECX, 2011).

4.5.2.1 Acidity value at 200℃

Figure 4.10 shows for each particle size range, the sensory value of acidity increases as

roasting time increases because at 200℃ roasting the mono-saccharide and

polysaccharides inside the bean become easily available for extraction.

4-6 mm range particle sized beans show a relatively lower value of cup acidity whereas

the smallest particle in size i.e 1.7-2.36 mm brings high value for cup acidity because

as the particle size reduces surface to volume ratio increases and facilitate extraction of

saccharide components (Redgwell et al., 2002). The intermediate particle size range

shows linear progress in the acidity value which indicates a reduction of overlapping of

internal reactions.

7

8

9

10

11

12

13

0 1 2 3 4

cup

cle

anes

s val

ue

in n

um

ber

out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

46

Figure 4-10 Acidity value at 200℃ as a function of time and particle size

4.5.2.2 Acidity value at 230℃

Figure 4.11 shows for each particle size range the sensory value of acidity increases as

roasting time increases until 10-minutes and then it starts bending down for smallest

particle size and keep plateau for the larger particle sizes. At 230℃ the mono saccharide

and polysaccharides inside the bean become easily degrades, for example, the

polysaccharide xylose completely degrade at 220℃ (Redgwell et al., 2002).

Figure 4-11 Acidity value at 230℃ as a function of time and particle size

6

7

8

9

10

11

12

0 1 2 3 4

acid

ity v

alue

in n

um

ber

out

of

15

Axis Title

4-6mm

2.36-3.35mm

1.7-2.36mm

8

9

10

11

12

13

14

0 1 2 3 4

acid

ity v

alue

in n

um

ber

out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

47

When compared with 200℃ the 230℃ roasting process, 230℃ gives a smaller value of

cup acidity for particles size range of 1.7-1.36mm. For the particle size range 2.36-

3.35mm at 230℃ the acidity value gets its peak value at 10-minutes, but for a roasting

temperature of 200℃ no peak value is observed; simply it shows increment with time

which means it needs additional time to reach to its peak value at 200℃.

4.5.2.3 Acidity value at 260℃

At roasting temperature of 260℃, as the particle size reduces, acidity value reduces, but

for 4-6 mm sized particles it slightly increase until 10-minutes then reduce like the other

particle size ranges. This is due to the effect of roasting temperature increment which

degrades the essential polysaccharides (Redgwell et al., 2002).

For 5-minutes roasting the particle size 2.36-3.35mm gives a higher acidity value than

the other two particle size and at 10 and 15-minutes the particle size range 4-6mm

results in the highest score than other two particle size ranges. This implies size

reduction result high acidity value at the lower roasting time, but not for too fine particle

size.

Figure 4-12 Acidity value at 260℃ as a function of time and particle size

6

7

8

9

10

11

12

13

14

0 1 2 3 4

acid

ity v

alue

in n

um

ber

out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

48

4.5.3 Body

The body of brewed coffee represents the combined effect of viscosity and mouth feel

property. It accounts for 15% of the total quality of the coffee roasting process (ECX,

2011).

4.5.3.1 Body value at 200℃

Body value of brewed coffee bean at 200℃ is displayed in the figure 4.13. As the graph

indicates particle size 1.7-2.36mm gives a higher score for body value to the brewed

coffee, because of extraction of lipid and trigonelline increases as the size of bean

particle reduces (Gaffney et al., 2015).

Figure 4-13 Body value at 200℃ as a function of time and particle size

As shown in the figure 4.13 above, the 2.36-3.35mm particle size follows the 1.7-

2.36mm particle size body value then it approaches to the same value at 10-minutes

roasting. After that, both shows increment with no peak value up to 15-minutes. The

reason is size reduction of the bean did not facilitate the migration of lipids; the lipids

appear on the surface due to their initial high concentration on the bean (Bertrand et al.,

2006). Similarly 4-6mm particle size increases in its body value, but not as much as

that of the 2.36-3.35 and 1.7-2.36mm particle size range. This is due to poor heat

transfer inside the bean caused by its larger size and low thermal conductivity (Perez-

Alegria et al., 2001).

6

7

8

9

10

11

12

13

14

0 1 2 3 4

bo

dy v

alue

in n

um

ber

out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

49

4.5.3.2 Body value at 230℃

Body value of brewed coffee bean at 230℃ is displayed in the figure 4.14. The size

range 2.36-3.35 mm shows a linear increment in body value whereas the 1.7-2.36 mm

and 4-6mm particle size the increment is nonlinear; this is due to the overlapping of

internal processes such as evaporation and roast gas formation (Vargas-Elias et al.,

2016). For all particle size as the roasting time increases the body value also increases.

The reason is associated with lipid and trigonelline viscosity reduction due to

temperature increment, and high mass transfer resistance on the larger particle size

range than on the smaller one (Bertrand et al., 2006).

Figure 4-14 Body value at 230℃ as a function of time and particle size

4.5.3.3 Body value at 260℃

For 260℃ roasting process body value of the brewed coffee is shown in the figure 4.15.

For smaller particle size range (1.7-2.36 mm) body value is high among all others in

the first 5-minutes. Then after, due to the high roasting temperature (260℃) degradation

of lipid and trigonelline components occurs (Stadler et al., 2002). This loss is magnified

on particle size range of 1.7-2.36 mm because there is low resistance for mass and heat

flow due to their size.

9

9.5

10

10.5

11

11.5

12

12.5

13

13.5

14

0 1 2 3 4

bo

dy v

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in n

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out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

50

Figure 4-15 Body value at 260℃ as a function of time and particle size

4.5.4 Flavor

Flavor of brewed coffee represents the originality of taste of the coffee. It accounts for

15% of the total quality of the coffee roasting process (ECX, 2011).

4.5.4.1 Flavor value at 200℃

For 200℃ roasting process flavor value of the brewed coffee is shown in the figure

4.16. The value of flavor increases as roasting time increases, but for smaller particle

size (1.7-2.36mm) the value of flavor is much lower than that of the larger particle size

(4-6mm). This comes from the development of bitter taste and escaping of responsible

flavor forming organic chemicals due to small mass transfer resistance of smaller bean

(Buffo et al., 2004).

0

2

4

6

8

10

12

14

16

0 1 2 3 4

bo

dy v

alue

in n

um

ber

out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

51

Figure 4-16 Flavor value at 200℃ as a function of time and particle size

4.5.4.2 Flavor value at 230℃

For 230℃ roasting temperature, flavor value of the brewed coffee is shown in the figure

4.17. For all particle size ranges at 230℃ the flavor value reaches a peak value at the

roasting time of 10-minutes and subsequently, it decreases as a result of the bitter taste

of caffeine and reduction of chlorogenic acid due to degradation (Blank et al., 2002).

Figure 4-17 Flavor value at 230℃ as a function of time and particle size

0

2

4

6

8

10

12

14

0 1 2 3 4

flav

or

val

ue

in n

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out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

0

2

4

6

8

10

12

14

16

0 1 2 3 4

flav

or

val

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in n

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out

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15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

52

4.5.4.3 Flavor value at 260℃

For 260℃ roasting temperature, the flavor value of brewed coffee is shown in the

figure4.18. For particle size4 -6 mm the flavor value decreases linearly with time, but

for the others the flavor value decreases nonlinearly showing that more of flavor

containing chemicals leave the bean in gas form or undergo chemical reaction and result

in minimum flavor value (Thomas Hofmann et al., 2001; M. J. Lee et al., 2013).

Figure 4-18 Flavor value at 260℃ as a function of time and particle size

4.6 Caffeine content

After collecting the absorbance data of pure caffeine solution at different concentration

(10, 20, 30, 40, and 50mg/l), absorbance verses concentration is plotted as shown in

figure 4-19 and then the curve is fitted to linear regression in order to read the

concentration of caffeine in the brewed coffee bean.

0

2

4

6

8

10

12

14

16

0 1 2 3 4

flav

or

val

ue

in n

um

ber

out

of

15

Roasting time

4-6mm

2.36-3.35mm

1.7-2.36mm

53

Figure 4-19 Calibration curve for caffeine content determination of roasted coffee bean

From the sensory quality of brewed coffee five processing conditions scores grade 1

(i.e. point≥ 85, see appendix C and D) and selected for caffeine content determination.

The bar graph in figure 4-20 displays the total point of roasted coffee bean and

corresponding caffeine content.

Figure 4-20 Bar diagram of grade1 roasted coffee value and their caffeine content

y = 0.0205x - 0.0154R² = 0.9902

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60

Ab

sorb

ance

Concentration mg/l

absorbance

Linear (absorbance )

89.21 89.11 87.96 85.96 85.44

43.147.7

35.2

55.6

42.5

0102030405060708090

100

val

ue

processing condition

total value

caffeine mg/l

54

The caffeine content result indicates that the maximum scored caffeine content using

closed system circulated hot air roasting for limu coffee is 55.6mg/250ml at the roasting

condition of 230℃, 10-minutes, and 2.36-3.35mm particle size. For roasting condition

which scores highest total value, the caffeine content did not give as highest value as

their corresponding total value, because of the bitter taste of caffeine. As the Caffeine

content increases the flavor value decreases and it lowers its sensory quality which

agrees with Streit et al. (2007).

In support to this study different value of caffeine contents in coffee beans have been

reported by the previous researchers. For example it was reported that the percentage

of caffeine in coffee beans was 49.5mg/l (Bertrand et al., 2006). Moreover, the caffeine

content in Ethiopian Arabica coffee of origin Bench Maji, Gediyo Yirgachefe, Tepi, and

Godere has been determined by UV-Vis spectroscopy to be 56.3mg/l ,55.8mg/l

56.2mg/l and 56.15mg/l, respectively (Belay et al., 2008). Therefore, these values are

in reasonable degree of agreement with the findings of the present work. In summary,

the caffeine level of limu coffee beans roasted is comparatively enhanced to the other

Arabica coffee brands.

55

5 CONCLUSION AND RECOMMENDATION

5.1 Conclusion

The export standard limu brand raw coffee bean utilized for this research has 8.45%

moisture content and it satisfies the international coffee organization criteria for the

dried coffee bean. The raw coffee bean value analysis for the primary and secondary

defect, for odor, make up and shape, and color shows a score of 37.6/40 which is a good

sign for a raw coffee bean quality.

The dry mass loss for all particle size ranges increases as roasting time and roasting

temperature increases. As the particle size increases the linearity of dry mass loss with

temperature increases but for too much fine particle size it doesn’t work because of

porosity reduction between the beans. From the statistical data analysis changing the

roasting variable will cause a significant change in the amount of dry mass loss.

The final roasted coffee bean has low bulk density than the raw bean due to mass loss

and volume expansion during roasting. The change in bulk density decreases as the

particle size reduces, this is due to the immediate removal of the produced gas to the

environment without creating volume expansion and internal pore. In contrary, larger

initial particle size brings a larger reduction in bulk density as a result of volume

expansion. Increasing roasting time and temperature also leads to a reduction of bulk

density. As the statistics indicate roasting parameters play a significant role in bulk

density change.

For cup value, the cup cleanness for all particle sizes at 200℃ increases with time but

form plateau at 230℃ after 10-minutes, and give better foam stability and well

settlement. At 260℃ and particle size of 1.7-2.36mm the cup cleanness value decreases

with time; for the 4-6mm and 2.36-3.35mm size ranges it reaches a peak value at 10-

minutes and reduces because of reduction in density and degradation of foam forming

components.

56

For each particle size range the acidity value increase with time at 200℃ roasting

temperature, because the monosaccharide and polysaccharides become easily available

at this temperature. Whereas, at 230℃ roasting temperature, the acidity value reduces

after 10-minutes because of degradation of xylose. For 260℃ only the 4-6mm particle

size shows a linear profile in the first 10-minutes, the rests decline in their value due to

degradation of essential saccharides by high temperature.

At 200℃ and 230℃, the migration of lipid and trigonelline to the bean surface

facilitated and increase body value of the brewed coffee as a function of time. This

further increases as the particle size reduce due to low mass transfer resistance

associated with the size of the particle. In contrary, at 260℃ all particles reduce in their

body value as the time increases except for 4-6mm particle size which resists the

degradation of lipid and trigonelline but reduce with time.

For flavor value, size reduction facilitates volatilization of essential flavor and aroma

component and minimizes the value. For 4-6mm particle size time temperature

superposition is possible in which low temperature long time and high temperature

short time gives nearly the same result.

In general, the cup value results in general shows there is no specific roasting time,

roasting temperature and particle size to which maximum value in each attribute. All

are variable due to different chemical formation and degradation at each increment in

time and temperature. Among the coffee total values the processing condition of 230℃,

10-minutes and 4-6mm particle size gives the maximum total value of 89.21/100.

The caffeine result shows a maximum of 55.6mg/250ml caffeine content obtained at a

processing condition of 230℃, 10-minutes, and particle size range of 2.36-3.35mm.

The maximum caffeine value doesn`t much with the maximum coffee total value

because the bitter taste affects the sensory.

57

5.2 Recommendation

Based on the observation of this thesis the following further research works are

recommended.

This thesis focused on a single variety of coffee bean; considering other types of variety

may produce a different result. So, comparative analysis of species type on the effect

of final quality of roasted coffee bean can be a good research topic.

The sensory attribute analysis is age and sex sensitive, considering this factor may give

a broad insight into sensory value. Not only has this, including trained senior cuppers

can give a better result.

During size reduction, under sized coffee beans were observed; therefore, considering

green coffee preparation may reduce the size reduction loss.

Brewing methods also have an effect on the amount of extract from grounded coffee

.as a result studying the effect of brewing methods on the quality of cup value can be a

good research topic.

This research focused on single layer packed bed roasting, but for larger scale, it is

better to consider multiple layers packed bed roasting with ease mechanism of air flow

system.

58

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63

Appendix A: Representative figures for secondary raw coffee defect

64

Appendix B: Average raw experimental data

temperature time

particle

size

cup

cleanness acidity body flavor

bulk

density

dry

mass

loss

in %

200 ℃

5-

minutes 4-6mm 8.08 7.1 9.1 8.25 483.38 2.73

200 ℃

5-

minutes

2.36-

3.35mm 8.48 7.82 10.23 7.43 517.46 3.67

200 ℃

5-

minutes

1.7-

2.36mm 9.11 8.92 10.54 6.08 571.06 4.69

200 ℃

10-

minutes 4-6mm 8.28 8.32 11.32 10.14 471.28 5.09

200 ℃

10-

minutes

2.36-

3.35mm 9.38 9.01 12.4 8.44 473.79 6.23

200 ℃

10-

minutes

1.7-

2.36mm 10.25 10.51 12.52 8.21 558.63 5.35

200 ℃

15-

minutes 4-6mm 9.42 9.14 12.1 12.3 467.35 8.04

200 ℃

15-

minutes

2.36-

3.35mm 11.4 10.51 13.24 9.23 437.21 9.24

200 ℃

15-

minutes

1.7-

2.36mm 12.83 11.33 13.57 8.92 544.79 9.55

230 ℃

5-

minutes 4-6mm 10.85 10.4 10.42 10.47 466.93 8.83

230 ℃

5-

minutes

2.36-

3.35mm 11.02 10.62 11.07 8.33 491.22 8.69

230 ℃

5-

minutes

1.7-

2.36mm 11.98 11.53 11.6 8.3 535.03 10.26

230 ℃

10-

minutes 4-6mm 12.5 13.52 11.92 13.67 453.31 11.87

230 ℃

10-

minutes

2.36-

3.35mm 12.71 13.2 12.15 10.3 463.51 10.86

230 ℃

10-

minutes

1.7-

2.36mm 13.05 11.69 12.27 9.91 525.62 13.94

230 ℃

15-

minutes 4-6mm 12.94 13.55 12.67 12.35 413.25 13.85

230 ℃

15-

minutes

2.36-

3.35mm 12.97 12.35 13.01 6.28 419.53 19.82

230 ℃

15-

minutes

1.7-

2.36mm 13.08 11.02 13.35 4.45 522.06 16.96

260 ℃

5-

minutes 4-6mm 11.5 12.5 12.49 13.87 446.25 9.53

260 ℃

5-

minutes

2.36-

3.35mm 11.72 13.46 13.1 9.56 483.65 12.61

260 ℃

5-

minutes

1.7-

2.36mm 11.74 11.2 13.52 7.05 514.97 13.07

260 ℃

10-

minutes 4-6mm 12.4 12.69 10.86 11.35 419.21 20.36

65

260 ℃

10-

minutes

2.36-

3.35mm 12.46 12.51 8.25 8.74 437.05 16.91

260 ℃

10-

minutes

1.7-

2.36mm 10.87 10.4 8.28 6.32 509.51 20.36

260 ℃

15-

minutes 4-6mm 9.5 9.52 8.54 9.4 384.54 28.14

260 ℃

15-

minutes

2.36-

3.35mm 8.9 8.18 6.08 5.05 417.76 33.14

260 ℃

15-

minutes

1.7-

2.36mm 8.22 7.91 4.11 4.21 501.08 28.14

66

Appendix C: Grade and Total Value of Roasted Coffee Bean

roasting temperature

time particle size raw value

cup value

total value2

cup

230℃ 10-min 4-6mm 37.6 51.61 89.21 grade1

230℃ 15-min 4-6mm 37.6 51.51 89.11 grade1

260℃ 5-min 4-6mm 37.6 50.36 87.96 grade1

230℃ 10-min 2.36-3.35mm 37.6 48.36 85.96 grade1

260℃ 5-min 2.36-3.35mm 37.6 47.84 85.44 grade1

260℃ 10-min 4-6mm 37.6 47.3 84.9 grade2

230℃ 10-min 1.7-2.36mm 37.6 46.92 84.52 grade2

200℃ 15-min 1.7-2.36mm 37.6 46.65 84.25 grade2

230℃ 15-min 2.36-3.35mm 37.6 44.61 82.21 grade2

200℃ 15-min 2.36-3.35mm 37.6 44.38 81.98 grade2

260℃ 5-min 1.7-2.36mm 37.6 43.51 81.11 grade2

230℃ 5-min 1.7-2.36mm 37.6 43.41 81.01 grade2

200℃ 15-min 4-6mm 37.6 42.96 80.56 grade2

230℃ 5-min 4-6mm 37.6 42.14 79.74 grade2

260℃ 10-min 2.36-3.35mm 37.6 41.96 79.56 grade2

230℃ 15-min 1.7-2.36mm 37.6 41.9 79.5 grade2

200℃ 10-min 1.7-2.36mm 37.6 41.49 79.09 grade2

230℃ 5-min 2.36-3.35mm 37.6 41.04 78.64 grade2

200℃ 10-min 2.36-3.35mm 37.6 39.23 76.83 grade2

200℃ 10-min 4-6mm 37.6 38.06 75.66 grade2

260℃ 15-min 4-6mm 37.6 36.96 74.56 grade3

260℃ 10-min 1.7-2.36mm 37.6 35.87 73.47 grade3

200℃ 5-min 1.7-2.36mm 37.6 34.65 72.25 grade3

200℃ 5-min 2.36-3.35mm 37.6 33.96 71.56 grade3

200℃ 5-min 4-6mm 37.6 32.53 70.13 grade3

260℃ 15-min 2.36-3.35mm 37.6 28.21 65.81 grade3

260℃ 15-min 1.7-2.36mm 37.6 24.45 62.05 grade3

67

Appendix D: Checklist for Preliminary Washed Coffee Quality

Assessment