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Changes of physical properties of coffee beans during roasting

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  • University of Banja Luka, Faculty of Technology, Banja Luka, Bosnia and Herzegovina

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The effects of heating time on physical changes (weight, volume, texture and colour) of coffee beans (Outspan and Guaxupe coffee) were investigated. The roasting temperature of both samples was 170°C and samples for analysis were taken at the intervals of 7 minutes during 40 minutes of roasting. Total weight loss at the end of the roasting process was 14.43 % (light roasted) and 17.15 % (medium to dark roasted) for Outspan and Guaxupe coffee beans, respectively. Significant (P < 0.05) changes in the coffee bean breaking force values were noted between the 7th and 14th minutes, and statistically not significant (P > 0.05) between the 35th and 40th minutes of the roasting. According to the L* colour parameter as a criterion for the classification of roasted coffee colour (light, medium, dark), the Outspan sample was medium and Guaxupe sample was dark roasted.
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APTEFF, 43, 1-342 (2012) UDC: 663.938.3:543.92
DOI: 10.2298/APT1243021J BIBLID: 1450-7188 (2012) 43, 21-31
Original scientific paper
21
CHANGES OF PHYSICAL PROPERTIES OF COFFEE BEANS DURING
ROASTING
Marija R. Jokanovića*, Natalija R. Džinića, Biljana R. Cvetkovićb, Slavica Grujićc and
Božana Odžakovićc
a University of Novi Sad, Faculty of Technology, Bulevar cara Lazara 1, 21000 Novi Sad, Serbia
b University of Novi Sad, Institute for food technology, Bulevar cara Lazara 1, 21000 Novi Sad, Serbia
c University of Banja Luka, Faculty of Technology, Stepe Stepanovića 73, 78000 Banja Luka,
Republic of Srpska, Bosnia and Herzegovina
The effects of heating time on physical changes (weight, volume, texture and colour)
of coffee beans (Outspan and Guaxupe coffee) were investigated. The roasting tempe-
rature of both samples was 170°C and samples for analysis were taken at the intervals of
7 minutes during 40 minutes of roasting. Total weight loss at the end of the roasting
process was 14.43 % (light roasted) and 17.15 % (medium to dark roasted) for Outspan
and Guaxupe coffee beans, respectively. Significant (P < 0.05) changes in the coffee bean
breaking force values were noted between the 7th and 14th minutes, and statistically not
significant (P > 0.05) between the 35th and 40th minutes of the roasting. According to the
L* colour parameter as a criterion for the classification of roasted coffee colour (light,
medium, dark), the Outspan sample was medium and Guaxupe sample was dark roasted.
KEY WORDS: coffee bean, roasting, texture, colour
INTRODUCTION
Coffee is one of the most widely consumed beverages in the world. The high accepta-
bility of coffee is due to many factors, one of the most contributory factors being its
flavour (1). Commercial coffee beverage is made from arabica or robusta beans or blends
of them (2). The quality of coffee used for beverages is strictly related to the chemical
composition of the roasted beans, which is affected by the composition of the green beans
and post-harvesting processing conditions (drying, storage, roasting and grinding) (3).
Green coffee is devoid of the pleasant aroma and flavour appreciated worldwide in
roasted coffee. The desired aroma and flavour of coffee beans used for beverage prepara-
tion are developed during the roasting process, where the beans undergo a series of reac-
tions leading to the desired changes in the chemical and physical composition (4). So, in
order to obtain a good quality cup of coffee with specific organoleptic properties (flavour,
* Corresponding author: Marija R. Jokanović, M.Sc., University of Novi Sad, Faculty of Technology, Bulevar
cara Lazara 1, 21000 Novi Sad, Serbia, e-mail: marijaj@tf.uns.ac.rs
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22
aroma and colour), the step of roasting is very important (5). The characteristic flavour
and aroma of coffee result from a combination of hundreds of chemical compounds
produced by the reactions that occur during the roasting (3). This implies controlling the
roasting time and temperature so that they are sufficient for the required chemical reac-
tions to occur, without burning the beans and compromising the flavour of the final beve-
rage. In general, in conventional roasting process the temperature is in the range from 200
to 230C, and the process time is ranging from 12 to 20 minutes. However, these values
can vary greatly, depending on the degree of roast required (light, medium or dark), on
the type of roaster used, and also on the variety, age, moisture content, etc. of the coffee
beans (6). The roasting process can be divided into three consecutive stages: drying,
roasting or pyrolysis and cooling (3).
The degree of roast can be monitored by the colour of the beans, the loss of mass, the
developed flavour and aroma or by the chemical changes of certain components (5, 6).
Control of temperatures and duration of roasting, in industry, are only effective if the
quality of the raw material does not vary (5).
Therefore, the objective of this study was to evaluate the changes of different physical
characteristics such as total weight loss, breaking force, and colour of Outspan and
Guaxupe coffee beans during roasting.
EKSPERIMENTAL
Sample preparation
Two green (crude) coffee samples of Rio Minas, Outspan and Guaxupe, used for the
production of commercial blends, were provided by a local industrial coffee roaster.
Beans of each variety (20 kg/batch) were roasted separately using an oven with direct
heating. The roasting conditions were the same; the highest roasting temperature was
170C, and roasting time was 40 min. For each sampling step (0, 7, 14, 21, 28, 35, 40
min) the coffee beans (100 g) were taken from the oven, and the following determina-
tions were carried out:
Determination of moisture, protein, carbohydrate and ash content
Moisture content was measured based on the sample weight-loss after oven-drying at
105C for 16 h (3). The nitrogen (N2) content of the coffee sample was determined on a
Kjeldahl Digestion System. Protein content was calculated as nitrogen x 6.25 (3). Ash
content was calculated from the weight of the sample after burning at 580C for 17 h (3).
Carbohydrate content was estimated by volumetric method of Luff-Schoorl, which is
based on the reduction of alkaline Cu2+-complex (7). Results are expressed as the mean
value of three measurements.
Total weight loss (WT)
Total weight loss is expressed as g/100 g, and is calculated by weighing coffee
samples before (WI) and after roasting (W), as follows: WT=100 (WI-W)/WI (8). The
results are expressed as the mean value of three measurements.
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Mass, volume and bulk density
Mass was measured for 100 beans. Volume was measured for 100 beans in a 50 mL
graduated recipient.
Bulk density was evaluated as the ratio between the weight and volume of the 100
beans sample in a 50 mL graduated recipient (8). The results are expressed as the mean
value of three measurements.
Mechanical testing
An Instron Universal Testing Machine, equipped with a 25 kN load cell was used.
The uniaxial compression was carried out at a rate of 50 mm/min. For the measurements,
20 beans of each sample were taken at random. Each bean was positioned on its longest
side and with the flat side up between two parallel metal plates. A compression force was
applied until failure occurred; the working temperature was 23C. The breaking force (N)
corresponded to the force at the major failure event. It was considered as an empirical
measure of the strength (8). The results are expressed as the mean value.
Colour analysis
Colour was analysed by using a tristimulus colorimeter MINOLTA CP410. Standard
CIE conditions with illuminate were used. The configuration included the illuminant D65
and an angle of 10. The readings were made using the CIELAB system (L*, a*, b*), and
presented as L* value (colour brightness).
Colour was evaluated for ground coffee beans placed in a suitable tank. The results
are expressed as the mean value of five measurements.
Statistical analyses
Analysis of variance (one-way ANOVA) was used to test the hypothesis about the
differences among the mean values. The software package STATISTICA 8.0 (9) was
used for the analysis.
RESULTS AND DISCUSSION
Table 1 presents the composition of raw Outspan and Guaxupe coffee beans. The data
show that the raw Outspan and Guaxupe coffee beans, according to their chemical
composition, are of the same quality category.
Table 1. Chemical composition of raw Outspan and Guaxupe coffee beans
Parameters Outspan Guaxupe
Moisture (%) 10.43±0.02 9.59±0.06
Protein (%) 13.66±0.07 13.41±0.06
Mono and disaccharide (%) 7.45±0.09 5.16±0.11
Total ash (%) 4.06±0.12 3.92±0.07
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The contents of moisture and total ash of both samples are in agreement with Serbian
national legislation (10). The protein contents of the analysed coffee samples (13.66 %
for Outspan and 13.41 % for Guaxupe) are in the range for green coffee reported in the
literature: 11.0–16.5 g/100 g (11). Franca et al. (11) reported that the higher quality
coffee samples have higher protein levels, but there is no evidence suggesting that the
protein contents in the varieties of different quality or even of different species (arabica
vs. robusta) should be noticeably different.
The changes in the weight, volume, bulk density and total weight loss (measured for
100 beans) of Outspan and Guaxupe coffee beans during roasting are presented in Tables
2 and 3, respectively.
Table 2. Changes in weight, volume, density and total weight loss of Outspan coffee
beans during roasting (100 beans)
Heating time
(min)
Weight
(g)
Volume
(mL)
Bulk density
(g/mL)
Weight
loss (%)
0 14.86±0.04 23.03±0.06
7 14.43±0.07 27.10±0.36 0.53±0.009 2.88
14 13.81±0.07 28.13±0.15 0.49±0.000 7.09
21 13.64±0.04 28.10±0.17 0.49±0.002 8.23
28 13.63±0.04 29.10±0.10 0.47±0.000 8.29
35 12.98±0.04 31.07±0.12 0.42±0.000 12.64
40 12.71±0.04 34.10±0.10 0.37±0.002 14.43
Table 3. Changes in weight, volume, density and total weight loss of Guaxupe coffee
beans during roasting (100 beans)
Heating time
(min)
Weight
(g)
Volume
(mL)
Bulk density
(g/mL)
Weight
loss (%)
0 12.62±0.04 19.13±0.12 0.66±0.006
7 12.10±0.09 20.47±0.06 0.59±0.003 4.11
14 11.69±0.03 21.07±0.12 0.56±0.004 7.32
21 11.46±0.04 22.10±0.10 0.52±0.001 9.16
28 11.35±0.03 24.13±0.15 0.47±0.002 9.90
35 11.37±0.03 25.10±0.10 0.45±0.003 10.03
40 10.45±0.03 28.57±0.12 0.37±0.001 17.15
Protein contents of analysed coffee samples (13.66 % for Outspan and 13.41 % for
Guaxupe) are in the range for green coffee reported in the literature: 11-16.5 g/100 g
(11). Franca et al. (11) reported that the higher quality coffee samples present higher pro-
tein levels, but there is no evidence suggesting that the protein contents in varieties of dif-
ferent qualities or even of different species (arabica vs. robusta) should be noticeably dif-
ferent.
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Changes in weight, volume, bulk density and total weight loss (measured for 100 be-
ans) of Outspan and Guaxupe coffee beans during roasting are presented in Tables 2 and
3, respectively.
At the beginning of the heating process, the total mass of the 100 beans was 14.86 g
for Outspan sample, and 12.62 g for Guaxupe coffee. During the heating, the processed
coffee beans lose mass due to the water loss and loss of volatile materials (11). The total
weight loss at the end of the roasting process was 14.43 % and 17.15 % for Outspan and
Guaxupe coffee beans, respectively. The total weight loss of green coffee beans after
roasting can be one of the criteria for determining the degree of roasting. According to
Oosterveld et al. (12), the weight losses of 11 %, 15 % and 22 % represent light, medium,
and dark roasted coffee beans, respectively.
The increase in the beans volume at the end of the heating process was 48 % for
Outspan coffee beans, and 50 % for Guaxupe beans. The results of Franca and co-wor-
kers (11) for the volume increase of samples of different quality were 40 – 65 %, where
the volume increase of non-defective beans was higher than for black beans. Also, these
authors reported that the beans which swell less should be roasted more slowly.
Further, the changes in the density are noticeable during the roasting. These changes
are caused by the simultaneous increase of the volume and internal gas formation, pro-
ducts of the heat-induced reactions (mainly water vapour and carbon dioxide, and pyro-
lysis reaction products), and the decrease in the mass (due to the loss of volatiles) (8).
Bulk density changes are implied in bean expansion and in the formation of a charac-
teristic porous structure of the roasted coffee bean (13). The variations in the bean density
and volume probably reflect the bean porosity and compressibility of ground coffee, thus
being a consequence of commercial percolation (11). If the changes of density are detec-
ted, they can determine eligible roasting degree. An adequate roasting degree is needed
for coffee beans to be fragile and breakable, and as such proper for grinding and making
coffee beverages with pleasant sensory properties (8).
The results of breaking force for Outspan and Guaxupe coffee beans after different
heating time are presented in Table 4.
Table 4. Breaking force of Outspan and Guaxupe coffee beans during roasting (n=20)
Heating time
(min) Outspanns Guaxupens
Breaking force (N) Breaking force (N)
7 96.76±18.86 a94.75±15.87 a
14 69.58±15.49
b
69.30±18.00
b
21 64.13±18.75
b
68.43±17.79
b
28 63.03±16.99
b
66.10±15.66
b
35 52.12±15.14
b
,c 52.78±15.61
b
40 37.82± 5.67 c 35.39±9.50
b
a,b,c Means within a column with different superscripts differ (P < 0.05)
ns Means within a row no significant difference (P > 0.05)
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The textural characteristics of roasted beans could be related to the effects of some
chemical changes induced on the raw bean components by the severe thermal process
(13). The braking force values after 7 minutes of heating were similar (P > 0.05) for both
samples, 96.76 N for Outspan and 94.75 N for Guaxupe. According to Pittia et al. (13), a
higher breaking force value for raw coffee beans could be attributed, partly, to the
presence of a certain amount of some structural polysaccharides. Also, the force needed
to break the bean depends on the content of water in the bean. When the content of water
is low, the bean has more fragile and breakable structure, and when the content of water
is high, the bean is no longer crunchy, and becomes viscose and plastic (8). In both cases,
for Outspan and Guaxupe coffee, as the heating time went by, the force at failure tended
to decrease, reaching, again, similar values (37.82 N and 35.39 N) at the end of the
process. The reduction of the breaking force indicates a progressive reduction in the
strength of the bean.
(a)
(b)
Figure 1. Changes in moisture (a), bulk density (b) and breaking force value of Outspan
coffee beans during roasting
0
20
40
60
80
100
120
0
1
2
3
4
5
6
7
8
0 1020304050
Breaking force (N)
Moisture (%)
Heating time (min)
Moisture (%)
Breaking force (N)
0
20
40
60
80
100
120
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 1020304050
Breaking force (N)
Density (g/mL)
Heating time (min)
Density (g/mL)
Breaking force (N)
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The breaking force can also show the degree of roasting, but to a lesser extent than the
water content can (13). The characteristic brittleness and fragility induced by roasting is
the primary attribute of roasted coffee beans. The reaching of a certain degree of brit-
tleness is very important in the grinding process, which is carried out on roasted coffee
beans before the extraction of coffee brew (8).
(a)
(b)
Figure 2. Changes in moisture (a), bulk density (b) and breaking force value of Guaxupe
coffee beans during roasting
The influence of moisture content and bulk density on the breaking force values of
Outspan and Guaxupe coffee beans are shown in Fig. 1 and 2. The coffee beans with the
lowest moisture content and the lowest bulk density had the lowest breaking force values.
The water content has a great influence on the texture of roasted beans, and therefore
influence the work applied during the grinding process (13). According to the results of
Pittia et al. (13) the lowest breaking force values are reached by the samples of coffee
0
20
40
60
80
100
0
1
2
3
4
5
6
7
8
0 1020304050
Breaking force (N)
Moisture (%)
Heating time (min)
Moisture (%)
Breaking force (N)
0
20
40
60
80
100
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 1020304050
Breaking force (N)
Density (g/mL)
Heating time (min)
Density (g/mL)
Breaking force (N)
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beans having very low moisture content. Water, acting as plasticizer, is expected to
decrease the bean’s stiffness. Therefore, decreasing the moisture content should increase
the stiffness of the material up to the glassy state. When the moisture of coffee bean is
very low (1.5 - 2 g/100g), it permits a glassy-like structure to form, which is very easy to
break (13). As can be seen from Figs. 1 and 2, there is an initial (between 7th and 14th mi-
nutes of roasting) significant (P < 0.05) decrease in the breaking force that could be attri-
buted to the higher water loss and higher density decrease caused at high temperatures.
This can be explained by the finding of Massini et al (14), who analysed coffee beans
under industrial roasting conditions (200 - 210C), and found formation of cavities and
cracks after 4 minutes of roasting in the internal and external bean surface due to the re-
levant increase in the internal pressure and volume of coffee bean. Pittia et al. (13) repor-
ted the relation of texture changes of coffee beans roasted at 170 and 200C to the chan-
ges of density.
Figure 3. Changes in L* value of Outspan and Guaxupe coffee beans during roasting
In coffee, the characteristic colour, aroma and flavour are developed during roasting,
and thus it is necessary to adapt the roasting process to the type of coffee being roasted.
This implies controlling the roasting time and temperature so that they are sufficient for
the required chemical reactions to occur, without burning the beans and compromising
the flavour of the beverage (6). During roasting, due to the non-enzymatic browning and
pyrolysis reactions, changes in the coffee bean colour take place. So, beside the loss of
mass and the chemical changes of certain components which could serve as tools in the
control of the process, the effects of heating and the degree of roast can also be monitored
by the colour of the beans (5). Yellow-green colour of the raw bean changes to a brown-
black roasted colour (13). Browning is, in turn, described by a decrease of L* as well as
of a* and b* parameters. These colour changes for samples of Outspan and Guaxupe
coffee beans are shown by the decrease of the L* value in Fig. 3. The initial L* value of
48.72 and 49.32 for raw coffee beans decreased during roasting to the final values of
26.77 and 24.45 for Outspan and Guaxupe, respectively. In their paper Pittia et al. (8)
stated the criteria for the classification of differently roasted coffee samples on the basis
0
10
20
30
40
50
60
0 7 14 21 28 35 40
L*
Heating time (min)
Outspan
Guaxupe
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of the L* colour parameter: the samples are classified as light, medium or dark roasted
when L* value is 31.1, 26.0 and 24.3 respectively. According to these criteria, it can be
concluded that Outspan sample was medium and Guaxupe was dark roasted.
CONCLUSION
On the basis of obtained the results, it is possible to summarise that the moisture
content, and density changes, mainly affect the mechanical properties of coffee beans
during roasting. Bulk density values at the end of the heating process were the same for
both coffee samples, 0.37g/mL, and the mean values of bean's breaking force were also
very similar (P > 0.05), 37.82 N for Outspan and 35.39 N for Guaxupe. According to the
total weight loss of green coffee beans after roasting the Outspan coffee was medium
roasted, and Guaxupe coffee was between medium and dark roasted. According to the
values of L* colour parameter at the end of roasting process it can be concluded that the
Outspan sample was medium and Guaxupe dark roasted.
It is possible to control and standardize the quality of ground roasted coffee as the
final product by way of applying an adequate roasting procedure. In conventional
roasting, the temperature range and the time can vary greatly, depending on the degree of
roast required (light, medium or dark), on the type of used roaster, and also depend on the
variety, age, moisture content, and other quality characteristics of the coffee.
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ПРОМЕНЕ ФИЗИЧКИХ СВОЈСТАВА ЗРНА КАФЕ ТОКОМ ПЕЧЕЊА
Марија Р. Јокановића*, Наталија Р. Џинића, Биљана Р. Цветковићб, Славица
Грујићц и Божана Оџаковићц
а Универзитет у Новом Саду, Технолошки факултет, Булевар цара Лазара 1, 21000 Нови Сад, Србија
б Универзитет у Новом Саду, Институт за прехрамбене технологије, Булевар цара Лазара 1, 21000 Нови
Сад, Србија
ц Универзитет у Бањој Луци, Технолошки факултет, Степе Степановића 73, 78000 Бања Лука, Република
Српска, Босна и Херцеговина
У овом раду испитане су промене физичких својстава (маса, запремина, тексту-
ра и боја) зрна кафе сорти Outspan и Guaxupe, које се користе за производњу ко-
мерцијалних мешавина, у различитим временским интервалима топлотне обраде.
Топлотна обраде обе соре била је индентична, примењена је максимална темпера-
тура од 170C у току 40 минута. Узорци за анализе узиману су током просеца топ-
лотне обраде у временским интервалима од 7 минута.
Укупан губитак масе зрна на крају процеса печења био је 14,43% и 17,15% за
Outspan и Guaxupe, редом и према том критеријуму узорак Outspan кафе био је
средње печен, док је узорак Guaxupe кафе био средње до тамно печен. Током топ-
лотне обраде смањивала се и просечна вредност силе лома зрна кафе. Највеће, ста-
тистички значајне (P < 0,05) разлике вредности силе лома за оба узорка уочене су
APTEFF, 43, 1-342 (2012) UDC: 663.938.3:543.92
DOI: 10.2298/APT1243021J BIBLID: 1450-7188 (2012) 43, 21-31
Original scientific paper
31
имеђу 7. и 14., као и нумеричке, али не и статистички значајне (P > 0,05) разлике
између 35. и 40. минута топлотне обраде.
L* вредност, као један од параметара за дефинисање степена печености зрна
кафе, током процеса печења се смањује, односно зрно постаје тамније. Према кри-
теријумима за дефинисање степена печења на основу L* вредности може се закљу-
чити да је узорак Outspan (26,77) кафе средње, а узорак Guaxupe (24,45) кафе тамно
печен.
Кључне речи: зрно кафе, печење, текстура, боја
Received: 03 September 2012
Accepted: 12 October 2012
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