Carbon Dioxide Evolution and Moisture Evaporation During Roasting of Coffee Beans

8/10/2019 Carbon Dioxide Evolution and Moisture Evaporation During Roasting of Coffee Beans

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JFS E: Food Engineering and Physical Properties

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ABSTRAABSTRAABSTRAABSTRAABSTRACTCTCTCTCT: E: E: E: E: Evvvvvolution of carbon dioolution of carbon dioolution of carbon dioolution of carbon dioolution of carbon dioxide and water vxide and water vxide and water vxide and water vxide and water vapor durapor durapor durapor durapor during ring ring ring ring roasting of coffee was follooasting of coffee was follooasting of coffee was follooasting of coffee was follooasting of coffee was followwwwwed in an isothered in an isothered in an isothered in an isothered in an isothermalmalmalmalmalhigh-temperhigh-temperhigh-temperhigh-temperhigh-temperaturaturaturaturature shore shore shore shore short-time and a lot-time and a lot-time and a lot-time and a lot-time and a low-temperw-temperw-temperw-temperw-temperaturaturaturaturature long-time re long-time re long-time re long-time re long-time roasting proasting proasting proasting proasting processocessocessocessocess. I. I. I. I. In addition, Cn addition, Cn addition, Cn addition, Cn addition, COOOOO

22222rrrrrelease durelease durelease durelease durelease during storing storing storing storing stor-----

age of rage of rage of rage of rage of roasted beans was follooasted beans was follooasted beans was follooasted beans was follooasted beans was followwwwwed. Ced. Ced. Ced. Ced. COOOOO22222and water vand water vand water vand water vand water vapor concentrapor concentrapor concentrapor concentrapor concentration wation wation wation wation wererererere assaye assaye assaye assaye assayed in the exhaust air bed in the exhaust air bed in the exhaust air bed in the exhaust air bed in the exhaust air by nondispersivy nondispersivy nondispersivy nondispersivy nondispersiveeeee

infrinfrinfrinfrinfrararararared gas analysised gas analysised gas analysised gas analysised gas analysis. Although C. Although C. Although C. Although C. Although COOOOO22222evevevevevolution rolution rolution rolution rolution rates differates differates differates differates differed in the 2 pred in the 2 pred in the 2 pred in the 2 pred in the 2 processesocessesocessesocessesocesses, the final total amount of C, the final total amount of C, the final total amount of C, the final total amount of C, the final total amount of COOOOO

22222rrrrreleasedeleasedeleasedeleasedeleased

after 63 d of storafter 63 d of storafter 63 d of storafter 63 d of storafter 63 d of storage rage rage rage rage remained equal. Cemained equal. Cemained equal. Cemained equal. Cemained equal. COOOOO22222evevevevevolution and differolution and differolution and differolution and differolution and differentiation betwentiation betwentiation betwentiation betwentiation between eveen eveen eveen eveen evaporaporaporaporaporation of initial water andation of initial water andation of initial water andation of initial water andation of initial water and

chemically forchemically forchemically forchemically forchemically formed water shomed water shomed water shomed water shomed water showwwwwed that chemical red that chemical red that chemical red that chemical red that chemical reactions leading to reactions leading to reactions leading to reactions leading to reactions leading to relevelevelevelevelevant amounts of Cant amounts of Cant amounts of Cant amounts of Cant amounts of COOOOO22222and water starand water starand water starand water starand water start at ap-t at ap-t at ap-t at ap-t at ap-

proximately 180 C. A mass balance established on the present measurements was able to account fairly well for theproximately 180 C. A mass balance established on the present measurements was able to account fairly well for theproximately 180 C. A mass balance established on the present measurements was able to account fairly well for theproximately 180 C. A mass balance established on the present measurements was able to account fairly well for theproximately 180 C. A mass balance established on the present measurements was able to account fairly well for thegrgrgrgrgravimetravimetravimetravimetravimetrically measurically measurically measurically measurically measured red red red red roast lossoast lossoast lossoast lossoast loss.....

KKKKKeyworeyworeyworeyworeywords: coffee rds: coffee rds: coffee rds: coffee rds: coffee roasting, carbon diooasting, carbon diooasting, carbon diooasting, carbon diooasting, carbon dioxide evxide evxide evxide evxide evolution, moisturolution, moisturolution, moisturolution, moisturolution, moisture eve eve eve eve evaporaporaporaporaporationationationationation

Introduction

Roasting presents 1 of the key steps in coffee technology thatleads to the desired flavor and color of the final product. Duringthe roasting process, the initial moisture content is dried off, and a

substantial part of dry matter is transformed by chemical reactions

into volatiles so that coffee beans loose between 14% and 20% of

their weight, depending on green coffee quality, roasting conditions,

and degree of roast (Clarke and Macrae 1987). Carbon dioxide and

chemical reaction water, which are generated by Strecker degrada-

tion, degradation of chlorogenic acid, degradation of sucrose, de-

carboxylation of amino acids, and Maillard reaction (Illy and Viani

1995), account for the major portion of the weight loss. In addition,

other gases evolve and silver chaffs as well as abrasive products are

removed (Clarke and Macrae 1987; Jansen and Lange 2001). In roast-

ing trials by Meister and Puhlmann (1989), a roast loss of 13% was

composed of 6% moisture loss, 6% loss of volatiles, and 1% silver

skin. As for gases, Clarke and Macrae (1987) state that 87% is formed

as carbon dioxide.

Part of the carbon dioxide evolves directly during roasting, part

is either bound to polar sites of coffee bean polymers, dissolved in

the oil fraction, or dissolved in the residual moisture of roasted

beans. Labuza and others (2001) assume that an additional amount

is entrapped in amorphous collapsed zones of roasted beans. Sivetz

and Desrosier (1979) estimated an entrapped amount of carbon di-

oxide in the cell structure of 2%. During storage of roasted beans,

the entrapped carbon dioxide is slowly released from the beans. Ac-cording to Illy and Viani (1995), carbon dioxide is released over a pe-

riod of up to 6 wk. The degassing process is highly dependent on the

process conditions, structural changes, and roasting degree (Meister

and Puhlmann 1989; Massini and others 1990; Schenker 2000).

So far, the monitoring of roast loss and moisture decrease during

the roasting process has been based on intermittent sampling and

gravimetric analysis, while carbon dioxide evolution during storage

was followed by recording headspace pressure in closed containers

(Schenker 2000), manometric measurements (Radtke 1975), or

trapping carbon dioxide on columns (Shimoni and Labuza 2000).

Not much information is available on the direct measurement of

the evolution of water vapor and gases during the roasting of cof-

fee. Yeretzian and others (2000) monitored evolving volatiles on-

line by proton-transfer-reaction mass-spectrometry (PTR-MS).

Dutra and others (2001) collected the gas by condensation in 2

traps, both submerged in an ice bath, and analyzed the conden-

sate by gas chromatography (GC). Perren and others (2002) used a

nondispersive infrared (IR) gas analyzer to monitor evolving carbon

dioxide and moisture during microscale roasting of hazelnuts with

differential scanning calorimetry (DSC). Barbera (1967) described

a method to determine the non-odorous atmosphere in the roasted

coffee beans by displacing gases from the beans in the absence of

air by means of hot water. During thermal decomposition of fir

wood, which is similar to a food-roasting process, Samolada and

Vasalos (1991) determined evolved gases by gas chromatography

flame ionization detection (GC-FID) andgas chromatography ther-

mal conductivity detection (GC-TCD).

The aim of this work was to apply the nondispersive IR gas ana-

lyzer method to follow the evolution of carbon dioxide and water

vapor in the air stream of the roasting process and to compare car-

bon dioxide and water vapor evolution with other changes over

roasting time. Because the release of carbon dioxide during storage

is directly linked to the roasting process, carbon dioxide evolution

during storage in closed containers was also analyzed.

Materials and Methods

Roasting process and process characterizationRoasting process and process characterizationRoasting process and process characterizationRoasting process and process characterizationRoasting process and process characterizationRoasting.Roasting.Roasting.Roasting.Roasting. Samples of 100 g of coffee beans per run were roasted. In

the main set of experiments, initial moisture of beans was 8.3 g/100 g

web basis (wb). In addition, coffee beans were roasted that had been

pre-dried in an air-dryer at 85C for 6 d to a residual moisture content

of 1.1 g/100 g wb. The roaster has been described in detail by Schen-

ker (2000) and Schenker and others (2000) and allowed for coffee roast-

ing under defined process conditions with accurate control of hot air

temperature and air velocity and recording of bean core temperature.

Roasting and cooling were performed in separate sections.

MS 20040485 Submitted 7/20/04, Revised 8/23/04, Accepted 9/24/04. Au-thors Geiger, Perren, and Escher are with Inst. of Food Science and Nutri-tion, Swiss Federal Inst. of Technology (ETH) Zurich, CH-8092 Zurich, Swit-zerland. Author Kuenzli is with DMP Ltd., Fehraltdorf, Switzerland. Directinquiries to author Escher (E-mail:[email protected]).
mailto:[email protected]:[email protected]


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Gas evolution during coffee roasting . . .

A

CAC

T

Tvv = (1)

where vC = exhaust air velocity at the sample drawing position

(m/s); vA= air velocity of the suction air, measured by anemometer

(m/s); TC=temperature of the exhaust air at sample drawing position

(K); TA=temperature of the suction air (ambient temperature) (K).

The data in Table 1 show that measured and calculated valueswere sufficiently near to claim that the system did not have appre-

ciable amounts of leakage.

Exhaust air flow rates vC(m3/s) were then calculated as the prod-

uct of the exhaust air velocity vCand the cross-section of the exhaust

tube (0.0079 m2).

Roasting trials were carried out with vA= 2.037 m/s for LTLT and

vA= 3.007 m/s for HTST roasting. These values were recorded after

fluidization of the coffee bean bed started so that pressure drop due

to product was taken into account.

Monitoring of carbon dioxide andMonitoring of carbon dioxide andMonitoring of carbon dioxide andMonitoring of carbon dioxide andMonitoring of carbon dioxide andwater vapor during roastingwater vapor during roastingwater vapor during roastingwater vapor during roastingwater vapor during roasting

The concentration of carbon dioxide and water vapor was mea-sured in the exhaust air by infrared absorption with a LI 6400 Por-

table Photosynthesis System (LI-COR Inc., Lincoln, Nebr., U.S.A.).

An aliquot of the air (approximately 0.2%) was led from the exhaust

air tube through a sampling tube (r= 0.0025 m) to the detector. The

sampling tube had a length of approx. 1.5 m so that the air could

cool down before entering the detector system. To compensate for

potential variations of air velocity over the exhaust tube cross-sec-

tion (r= 0.05 m), the sampling tube was positioned in the exhaust

tube. Therefore, each roasting trial was repeated 5 times, and the

position of the sampling tube was moved from the peripherical

position at sampling tube entering (p1) to the tube center (c) and

to the opposite peripherical position (p2).

For the infrared absorption, the major absorption band for CO2

was centered on 4.26m, whereas for water vapor, band pass filterswere centered on the 2.59-m band (Long and others 1996). The in-

strument reading wasmol CO2/mol air and mmol H2O/mol air. Cal-

ibration of the instrument was carried out as recommended by the

LI-COR(LI-COR Inc., Lincoln, Nebr., U.S.A.) company. Before each

roasting run, the carbon dioxide and water vapor concentration of

the exhaust air were recorded for at least 2 min. Over the experiment

period, the average carbon dioxide concentration of the hot air was

around 540mol/mol air (0.054%), whereas the water vapor concen-

tration of the hot roasting air was between 6 and 8 mmol/mol air.

These baseline values were subtracted from the carbon dioxide and

water vapor readings during the roasting process itself.

Quantification of carbon dioxide andQuantification of carbon dioxide andQuantification of carbon dioxide andQuantification of carbon dioxide andQuantification of carbon dioxide andwater vapor content in the exhaust airwater vapor content in the exhaust airwater vapor content in the exhaust airwater vapor content in the exhaust airwater vapor content in the exhaust airQuantities of carbon dioxide and moisture evolved were calculat-

ed from the molar concentration value according to the following

procedure:

airOH,COOH,CO ncn 2222&& = (2)

C

Cair

TR

Vpn

=

&

& (3)

where nair= molar air flow rate (mol/s); p = ambient atmospheric

pressure (98000 1000 Pa); v

c= volumetric air flow rate (m3

/s); R =

gas constant (8.314 J/mol K); TC= air temperature at sample drawing

position (K); nCO2,H2O= moisture and carbon dioxide evolution rate

(mol/s); cCO2,H2O= carbon dioxide and moisture concentration, read-

ings corrected for baseline values [mol/mol (CO2), mmol/mol

(H2O)].

Release of carbon dioxide during storageRelease of carbon dioxide during storageRelease of carbon dioxide during storageRelease of carbon dioxide during storageRelease of carbon dioxide during storageThe whole roasted batch of initially 100 g green coffee was placed

in a 500-mL septum flask immediately after roasting. The flask was

closed tight with a special rubber septum of 12-mm thickness. The

existing pressure in the flask after the sample preparation was mea-

sured by placing the flasks in a headspace sampling device

equipped with a pressure transducer as described in detail by Gn-

tensperger and Escher (1994). The flasks were stored at room tem-

perature (23 1C, 73.4F) while bean gas release took place. The

headspace pressure was determined periodically. After each mea-

surement, the rubber septum was opened and the flask was vent-

ed. After closing the flask again, the existing headspace pressure

was remeasured and the gas release could restart for the next time

period. A release rate was calculated. The gas release of 3 batches

was measured.

Results and Discussion

Process characterization and reproducibilityProcess characterization and reproducibilityProcess characterization and reproducibilityProcess characterization and reproducibilityProcess characterization and reproducibilityof on-line measurements of evolved gasof on-line measurements of evolved gasof on-line measurements of evolved gasof on-line measurements of evolved gasof on-line measurements of evolved gas

Figure 2 shows the development of coffee bean core tempera-

0 50 100 150 200

0

4

8

12

16

20HTST

Roastloss(g/10

0gwb)

Watercontent(g/100gwb)

Roasting time (s)

0 200 400 600 800

0

4

8

12

16

20

Hot air temp.

Hot air temp.

water content roast loss

water content roast loss

Roasting time (s)

Roastloss(g/100gwb)

Watercontent(g/100gwb)

LTLT

0

50

100

150

200

250

Temperature(C)

0

50

100

150

200

250

Temperature

(C)

Figure 2Bean core temperature, water content changesand roast loss changes depending on roasting time for high-temperature-short-time (HTST) and low-temperature-long-

time (LTLT) roasting
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ture, roast loss, and moisture content of beans during the isother-

mal HTST and LTLT roasting process to equal roast loss (15.38% and

15.86%, respectively). In both processes, the bean core temperature

did not reach the pre-set air temperatures of 260C and 228C, re-

spectively, because of excessive heat transfer by radiation. This is

a typical phenomenon in small-scale roasting equipment with a

large air-to-bean ratio. The increase of roast loss and the decrease

of moisture content were almost linear at HTST conditions. Only

toward the end, the curve for moisture content started to level off.

In contrast, exponential curves over the roasting time were observed

at LTLT conditions for both roast loss and moisture content. These

results confirm earlier results (Schenker 2000; Schenker and others

2000, 2002) that showed roasting temperature is the most decisive

parameter in controlling overall changes in coffee beans.

eroasting. In both figures, the concentration data measured at 3

locations of the cross-section of the exhaust are plotted. In gener-

al, there was good agreement of all concentration curves. Part of the

differences within 1 set probably originated from the differences in

air velocity. Nevertheless, one can see that the distribution of car-

bon dioxide and water vapor concentration over the cross-section

of the exhaust tube was fairly homogeneous.

When comparing the HTST (Figure 3) and LTLT processes (Figure

4), clear differences were observed in the development of evolvedgas concentration over roasting time. The carbon dioxide concen-

tration increased sharply in the end phase of HTST roasting,

whereas the concentration in LTLT roasting stayed much lower and

leveled off in the end phase. One could imagine that HTST roast-

ing moved toward pyrolytic conditions at the end of the process. As

far as the formation of water vapor is concerned, a maximum con-

centration and a subsequent decline were observed. The peak con-

centration in the HTST roasting was higher than in the LTLT roasting.

It is concluded that the on-line measuring system can be ap-

plied to monitor gas evolution over the roasting process.

Evolution of carbon dioxideEvolution of carbon dioxideEvolution of carbon dioxideEvolution of carbon dioxideEvolution of carbon dioxideduring roasting and storageduring roasting and storageduring roasting and storageduring roasting and storageduring roasting and storage

The data illustrated in Figure 3 and 4 for carbon dioxide were

averaged and converted into evolution rate and cumulative

evolved carbon dioxide as shown in Figure 5 for the HTST and LTLT

process. The evolution rate again follows the pattern already seen

in Figure 3 and 4. In contrast, the cumulative value show that more

carbon dioxide evolves in the LTLT process than in the HTST pro-

cess due to the much longer roasting time.

One has to bear in mind that cumulative values in Figure 5 do not

Table 2Mass balance for the roasting and storage of cof-fee beans and comparison with overall roast loss

Weight (g)

Step HTST LTLT

Initial beansTotal solids 91.7 91.7Moisture 8.3 0.2 8.3 0.2Sum 100.0 100.0

Roasting loss (on-line)

Carbon dioxide 0.4 0.0 0.50 0.3Total water 10.2 1.2 11.4 1.4Silver chaff 1.0 1.0Sum 11.6 1.2 12.9 1.7

Cooling loss (calculated)Carbon dioxide 0.1 0.0 0.0 0.0Water 1.6 0.2 0.1 0.0Sum 1.7 0.2 0.1 0.0

Total weight loss 13.3 1.4 13.0 1.7(on-line/calculated)

Roast loss (gravimetric) 15.38 0.05 15.86 0.02Storage loss

Carbon dioxide 0.99 0.02 0.83 0.01

Figure 4Development of carbon dioxide and water va-por concentration in the exhaust air during low-tempera-ture-long-time (LTLT) roasting. Positions p1, p2, and c as

explained in the Materials and Methods section.

500

550

600

650

700

750

run 1 (c)run 2 (p2)run 3 (c)run 4 (p1)run 5 (p2)

CO2conc.(mol/molair)

0 200 400 600 800

6

8

10

12

14

Roasting time (s)

H2Oconc.(mmol/mola

ir)

500

600

700

800

900

run 1 (c)run 2 (p2)

run 3 (c)run 4 (p1)run 5 (p2)

CO2conc.(mol/molair)

0 40 80 120 160 200

6

8

10

12

14

Roasting time (s)

H2Oconc.(mmol/mol

air)

Figure 3Development of carbon dioxide and water va-por concentration in the exhaust air during high-tempera-ture-short-time (HTST) roasting. Positions p1, p2, and c as

explained in the Materials and Methods section.


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reflect the quantity of carbon dioxide formed. A substantial part of

this quantity is trapped in the coffee beans and is released only

during storage. Therefore, cumulative evolved carbon dioxide for

the roasting process and the storage period were combined in Fig-

ure 6. Roast loss and storage time, respectively, were chosen as in-

dependent variables for the 2 steps. The lower gas evolution dur-

ing HTST roasting is more than compensated for during storage by

a much higher cumulative gas release. Assuming that both the

HTST and LTLT curves level off after a storage period of 63 d, HTST

and LTLT roasting result in an almost equal total formation and

evolution of carbon dioxide. Total carbon dioxide formation and

evolution seem to be dependent on only the degree of roast and

not on roasting temperature.

Evaporation of water vaporEvaporation of water vaporEvaporation of water vaporEvaporation of water vaporEvaporation of water vaporfrom nonpre-dried beansfrom nonpre-dried beansfrom nonpre-dried beansfrom nonpre-dried beansfrom nonpre-dried beans

Figure 7 presents the data for water evaporation averaged from

the data represented in the curves in Figure 3 and 4 and again con-

verted into evaporation rates and cumulative evaporated water,

respectively. As stated earlier, both roasting processes lead to a

peak evaporation rate and a subsequent decrease. The HTST and

LTLT processes differ primarily in the extent of the peak rate. Mois-

ture evaporation rate depends on the roasting temperature.

Evaporation of water vapor from pre-dried beansEvaporation of water vapor from pre-dried beansEvaporation of water vapor from pre-dried beansEvaporation of water vapor from pre-dried beansEvaporation of water vapor from pre-dried beansThe cumulative evaporated quantity of water is composed of

Figure 5Carbon dioxide evolution rate and cumulativeevolved carbon dioxide per 100 g of green beans for high-temperature-short-time (HTST) and low-temperature-long-

time (LTLT) roasting

0 40 80 120 160

0

2

4

6

8

10

12

CO2evolution rate

CO2evolution rate

CO2evolutio

nrate(mg/s)

Roasting time (s)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Cumulative evolved CO2

Cumulative

evolvedCO2

(g/100ggreenbeans)

Cumulative evolved CO2

0 200 400 600 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Roasting time (s)

CO2evolutionrate

(mg/s)

0.0

0.1

0.2

0.3

0.4

0.5

0.6LTLT

HTST

CumulativeevolvedCO2

(g/100ggreenbeans)

Figure 6Cumulative evolved carbon dioxide during roast-ing and storage for high-temperature-short-time (HTST) andlow-temperature-long-time (LTLT) roasting. Storage time was63 d at room temperature (23 1 C).

0 6 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

HTST

LTLT

Cumulativeevo

lvedCO2

(g/100ginitialgre

enbeans)

Roast loss (g/100g wb)

10 20 30 40 50 60 70

Storage time (d)

Figure 7Water evaporation rate and cumulativeevaporated water per 100 g of green beans for high-temperature-short-time (HTST) and low-temperature-

long-time (LTLT) roasting

0 40 80 120 160

0.00

0.02

0.04

0.06

0.08

0.10

0.12

H2Oevaporationrate(g/s)

Roasting time (s)

0

2

4

6

8

10

12

14

Cumulativee

vaporatedH2O

(g/100ggreenbeans)

0 200 400 600 800

0.00

0.02

0.04

0.06

0.08

0.10

0.12H

2O evaporation rate

H2O evaporation rate

Roasting time (s)


0

2

4

6

8

10

12

14

Cumulative evaporated water

Cumulative evaporated water

LTLT

HTST

Cumulativeevapora

tedH2O

(g/100ggreenbe

ans)


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water that evaporates because of dehydration of initial moisture of

coffee beans and of water that is generated by chemical reactions

mentioned in the Introduction. To evaluate the formation of mois-

ture in chemical reactions, HTST and LTLT roasting trials with pre-

dried beans (1.1 g/100 g wb) were carried out. It was assumed that

in this case, the detected moisture in the exhaust air was exclusive-

ly formed by chemical reactions because of a negligible initial water

content of the green coffee. In Figure 8, the chemical reaction wa-

ter and the initial water evaporation rates are shown. The evapora-

tion rate of initial water was calculated from the difference between

the evaporation rate of total moisture during roasting of nonpre-

dried coffee (water content: 8.30 g/100 g wb) and the evaporation

rate of total moisture during roasting of pre-dried coffee.

As already mentioned, water evaporation is temperature-de-

pendent. The peak rate for HTST roasting was higher than the

peak rate under LTLT roasting conditions, whereas more moisture

evaporates during LTLT roasting because of the much longer roast-

ing time. In an early roasting phase of both roasting processes, only

initial water has been released from the beans. With increasing

roasting time, in particular in the case of LTLT roasting, total mois-

ture evaporation mainly consists of evaporating chemical reaction

water. Initial water evaporation became almost negligible a fter

approximately 300 s, and the formation and evaporation of chem-ical reaction water were equaled. The maximum in the total mois-

ture evaporation curve is caused by the overlapping effect of initial

and chemical reaction water. The decrease of chemical reaction

water evaporation could be the consequence of a depletion of the

Figure 8Evaporation rate of total moisture, initial water,and chemical reaction water during high-temperature-short-

time (HTST) and low-temperature-long-time (LTLT) roasting

0 40 80 120 160 200

0.00

0.02

0.04

0.06

0.08

0.10


Total moisture evaporation rate (online determined in exhaust air)

from beans (moisture content 8.3 g/100g)

Total moisture evaporation rate (online determined in exhaust air)

from pre-dried beans (moisture content 1.1 g/100g)Initial water evaporation rate (calculated by subtracting total

moisture evaporation rate of pre-dried beans of the total

moisture evaporation rate of non-pre-dried beans)Rate of initial moisture loss, gravimetrically determined

Rate of chemical reaction water evaporation (calculated by subtrac-

ting the rate of initial moisture loss (gravim etrically deter-

mined) from total moisture evaporation rate of non-pre-dried beans)

H2Oevaporationrate

(g/s)

0 200 400 600 800

0.00

0.02

0.04

0.06

0.08

0.10

Roasting time (s)

LTLT

HTST

Figure 9Influence of bean core temperature on the for-mation and evaporation of chemical reaction water and

carbon dioxide per degree Celsius and 100 g of green beans

0 50 100 150 200 250

0.000

0.005

0.010

0.015

0.020

0.0

0.1

0.2

0.3

0.4

0.5

CO

2evolution(g/C)

Bean core temperature (C)

HTST

LTLT

HTST

LTLT

Chemicalreactionwater(g/C)

substrate, or the generated chemical reaction water is used up in

the cells for other chemical reactions and therefore does not appear

in the exhaust air. It seems in Figure 8 that chemical reaction water

was already produced from the beginning of the process. In fact,

most of the residual moisture of 1.1 g/100 g wb in the pre-dried

green beans had to be removed before the effective chemical reac-

tion water could be detected.

As illustrated in Figure 9, the required product temperature to

start the chemical reactions was 180C to 200C. Below these tem-

peratures, no significant increase of chemical reaction water evap-

oration or carbon dioxide evolution was observed. The calculation

of the evolved gases per degree Celsius considers the dynamic pro-

cess conditions, which were different for HTST and LTLT roasting.

Figure 10 shows the cumulative evaporated water for nonpre-

dried HTST and LTLT roasting (initial water content: 8.3 g/100 g wb)

and for the cumulative moisture evaporation because of chemical

reactions as calculated from the difference between total moisture

evaporated and loss of initial moisture content. The chemical reac-

tion water amounted to 41% for HTST roasting and 36% for LTLT

roasting. The area between the curves represents the evaporation

of initial moisture.

Mass balanceMass balanceMass balanceMass balanceMass balanceIn Table 2, a mass balance over the roasting and storage is pre-

sented on the basis of evolved carbon dioxide, evaporated water,

and losses of solids in the form of silver chaffs. An estimation of the

loss of gases during the cooling step was done by linear extension

of the cumulative carbon dioxide and moisture evaporation values

for another 20 s after the end of roasting. A standard deviation of the

evolved carbon dioxide and moisture was calculated from the de-

viation of air velocity and gas concentration measurements.

Taking the variations of the data in Table 2 into account, approx-


7/7


E

F

d E

i

i

&

Ph

i

lP

ti


Figure 10Cumulative values of evaporated total moistureand chemical reaction water during high-temperature-short-time (HTST) and low-temperature-long-time (LTLT) roasting

imately 93% (LTLT) and 96% (HTST) of the gravimetrically deter-

mined roast loss could be explained by measuring gas evolution and

determination of silver chaff. It must be pointed out that the balance

values in Table 2 still do not account for all of the roast loss. Material

from abrasion, for example, tippings, and evolving gases other than

CO2and water vapor also contribute to the total roast loss. The re-

maining difference for both roasting processes could be explained

by inaccuracies of air velocity and gas concentration measure-

ments, as well as by inaccuracies of the raw material.

Conclusions

The results of the present investigation show that the methodology that has been developed for measuring gas evolution duringroasting of coffee provides a valuable tool for analyzing complex

roasting processes. It is clear that to receive a more conclusive pic-

ture of gas evolution, these measurements have to be expanded

over other coffee varieties and coffees of different origin and han-

dling as well as to other roasting conditions. With the necessary

technical adaptations, the measuring system may also be applied

to larger roasting equipments.

Then, as a next step in the investigations on coffee roasting pro-

cesses, it will be interesting to relate the evolution of carbon diox-

ide and water vapor during roasting to the development of coffee

been structure and to the aroma retention and release during roast-

ing and storage.

AcknowledgmentsThe authors gratefully acknowledge the funding of this work by G.

W. Barth Ltd., Freiberg/Neckar, Germany, as well as technical sup-

port by Keme Food Engineering Ltd., Haco Swiss Ltd., and Migros

Betriebe Birsfelden Ltd, Birsfelden, Switzerland. Special thank are

due to Dr. Jrg Leipner, Inst. of Plant Science at ETH Zurich, for

support with the instrumentation for IR gas analysis.

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Carbon Dioxide Evolution and Moisture Evaporation During Roasting of Coffee Beans