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Abstract

CO2 is one of the major gases formed during coffee roasting, which has important implications on coffee’s quality and packaging requirements. In this study, the residual CO2 content and CO2 degassing behavior of an Arabica coffee processed using a fluidized bed roaster, as affected by the roasting temperature-time conditions, were investigated. The results showed that positive correlations existed between the degree of roast (expressed as lightness value) and residual CO2, implying that lightness could be used as an indicator of initial CO2 content in roasted coffee. At the same degree of roast, coffee roasted with high-temperature-short-time process had significantly higher CO2 degassing rate than those with low-temperature-long-time process. Moreover, the CO2 releasing rate increased with the degree of roast. The degassing rate of CO2 in ground coffee was highly dependent on the grind size and roasting temperature, but less dependent on the degree of roast. The different degassing behaviors observed between roasted coffee samples were explained on the basis of chemical composition and microstructural differences.
Effect of Roasting Conditions on Carbon Dioxide Degassing
Behavior in Coffee
XIUJU WANG, LOONG-TAK LIM*
Department of Food Science, University of Guelph, Guelph, On, N1G 2W1, Canada
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*Corresponding author. Tel: +1 (519) 824-4120 x 56586. Fax: +1 (519) 824-6631. E-mail:
llim@uoguelph.ca
Abstract
CO2 is one of the major gases formed during coffee roasting, which has important implications on
coffee’s quality and packaging requirements. In this study, the residual CO2 content and CO2
degassing behavior of an Arabica coffee processed using a fluidized bed roaster, as affected by the
roasting temperature-time condition, were investigated. The results showed that positive correlations
existed between the degree of roast (expressed as lightness value) and residual CO2, implying that
lightness could be used as an indicator of initial CO2 content in roasted coffee. At the same degree of
roast, CO2 degassing rate was significantly faster (p<0.05) in samples processed with high-
temperature-short-time than those processed with low-temperature-long-time. Moreover, the CO2
releasing rate increased with the degree of roast. The degassing rate of CO2 in ground coffee was
highly dependent on the grind size and roasting temperature, but less dependent on the degree of
roast. The different degassing behaviors observed between roasted coffee samples were explained on
the basis of chemical composition and microstructural differences.
Keywords
Coffee roasting; FTIR; CO2 degassing; Diffusion; Porous structure
Highlights
• An accurate and easy method based on FTIR is developed to study the CO2 degassing in roasted
coffee.
• Systematic investigation on the residual CO2 and its degassing behavior in coffee is presented.
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• Weibull distribution model shows accurate prediction of the CO2 degassing data.
• CO2 degassing rate in roasted coffee is affected by both physical and chemical properties developed
during the roasting process.
1. Introduction
Green coffee beans are normally roasted above 200oC to develop the characteristic flavors,
colors, and aromas of roasted coffee beans (Schenker et al. 2002; Yeretzian et al. 2002; Baggenstoss
et al. 2008; Moon and Shibamoto 2009). As a result of the roasting process, CO2 is produced that
accounts for more than 80% of the gases formed (Clarke and Macrae 1987; Geiger et al. 2005). By
and large, the formation of CO2 in roasted coffees is attributed to Maillard, Strecker, and pyrolysis
reactions (Shimoni and Labuza 2000; Anderson et al. 2003; Geiger et al. 2005).
During the roasting process, the formations of CO2 and other volatile compounds result in an
increased internal pressure, causing the beans to expand and eventually crack (Schenker et al. 2000;
Clarke and Vitztbum 2001). A typical roasting process can cause the beans to swell 40~60% at about
20% roast loss (Illy and Viani 1995). The porous structure developed, which is dependent on the
roasting temperature-time conditions applied, determines the residual CO2 content after roasting, as
well as the subsequent mass transport phenomena occurred during storage (Illy and Viani 1995;
Schenker 2000; Clarke and Vitztbum 2001; Geiger et al. 2005). The expanded porous matrices of
roasted coffee beans are made up of evacuated cells with a framework of cell walls. The evacuated
cells, with a diameter of 20-40 µm, can be regarded as macropores based on the pore size
classification of International Union of Pure and Applied Chemistry (Schenker et al. 2000; Anderson
et al. 2003). These macropores are the main contributors to the porosity of roasted coffee beans.
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Porosity values of roasted coffee beans varies from 0.474-0.738 depending on the measurement
technique used and roasting conditions (Schenker et al. 2000; Shimoni and Labuza 2000). Part of
residual CO2 is believed trapped within these evacuated cells as occluded gas, which is in
equilibrium with absorbed/adsorbed CO2 in oil, polysaccharides, and moisture (Illy and Viani 1995;
Schenker 2000; Clarke and Vitztbum 2001). Besides, the aforementioned cell walls are porous as
well with typical pores diameter ranging from 20 to 50 nm, which can be regarded as mesopores
(Schenker et al. 2000; Anderson et al. 2003). High-temperature roasted coffees were found have
larger mesopores in the cell wall as compared to low-temperature roasted beans (Schenker et al.
2000). Due to the complicated porous structure, the mass transfer of CO2 in roasted coffee matrices
is complex, involving Knudsen diffusion, transition-region diffusion, pressure driven viscous flow,
surface diffusion, and desorption from various constituents (Anderson et al. 2003).
Although much of the CO2 produced is lost during roasting, a significant amount remains
trapped in the roasted beans. Thus, in packaged products, roasted coffee is tempered to remove the
CO2 before packaging to prevent package swelling or failure. Alternatively, coffees are partially
tempered to minimize aroma loss and packaged in active packaging systems that are equipped with a
one-way vent valve to allow the releasing of CO2 during storage. In order to strike a balance between
adequate CO2 degassing and minimized aroma loss during the tampering process, systematic
understanding of CO2 degassing kinetics as affected by roasting conditions is important. This
information will be also useful during the design optimization of pressure relief valve for active
packages of coffee. The first objective of this study is to investigate the CO2 formation and degassing
behaviors of an Arabica coffee, processed using a fluidized bed hot air roaster under high-
temperature-short time (HTHT) and low-temperature-long-time (LTLT) processing conditions. The
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second objective is to understand the different degassing phenomena observed by investigating
various physicochemical properties of the roasted coffees.
2. Materials and methods
Brazilian Arabica green coffee beans (Strictly Soft grade) were supplied by Mother Parkers Tea
& Coffee (Mother Parkers Tea & Coffee Inc., Mississauga, ON, Canada). Sodium carbonate, hexane,
sodium hydroxide, tri-sodium citrate, concentrated sulfuric acid, Drierite desiccant, and Ascarite II
column (sodium hydroxide coated silica gel) were all purchased form Fisher Scientific International
Inc. (Ottawa, ON, Canada).
2.1 Roasting procedure and sample preparation
Green coffee beans were roasted by using a commercial fluidized bed roaster (Fresh Roast SR 500
roaster; Fresh Beans Inc., UT, USA), which was modified to allow accurate hot air temperature
control by means of a microprocessor temperature process controller (Model CN 7200, Omega
Engineering, Inc., Stamford, CT, USA) (Wang and Lim 2013). Green coffee beans were roasted
using either LTLT process at 230°C or HTST process at 250°C to achieve light (L), medium (M), dark
(D) and very dark (VD) roast degrees. The roast degree was determined by comparing the color of
the roasted coffee fine grinds with SCAA (Specialty Coffee Association of America) standard disks,
and expressed as CIE L* value determined using a spectrophotometer reflectance system (Model
CM-3500d, Konica Minolta Sensing, Inc., Osaka, Japan) of the same coffee grinds.
Medium and dark roasted coffee beans (LTLT-M, LTLT-D, HTST-M and HTST-D) were grinded
to coarse, medium, and fine grinds, using a commercial blur grinder (BODUM® Inc., NY, USA). The
particle size distribution of these three grinds was measured by a series of sieves and presented in
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Figure 1. Table 1 summarizes the roasting conditions used to obtain various coffee samples.
2.2 Determination of residual CO2 content
Residual CO2 content was determined by a gravimetric method (Shimoni and Labuza 2000;
Anderson et al. 2003; Wang and Lim 2013). Briefly, 17 grams coffee sample were mixed with 50 mL
of an alkali tri-sodium citrate solution (50 g/L tri-sodium citrate in 0.3 M NaOH) to trap CO 2. The
mixture was transferred into a three-entries distillation flask, and sulfuric acid solution (5M) was
then added to release the CO2, which was purged with nitrogen gas to a Drierite desiccant column to
remove the moisture. The dried gas was then passed through a Ascarite II column in which the CO 2
was trapped. The weight gain of the Ascarite II column was recorded and converted to mg of CO2 per
gram of the coffee sample.
2.3 CO2 degassing behavior test
The degassing of CO2 from roasted coffee at 25°C was monitored by a Fourier transform infrared
(FTIR) spectrometer (IR Prestige-21; Shimadzu Corp., Tokyo, Japan). Two and a half grams of
coffee was placed into a modified 250 mL clear French squares glass bottle (Fisher Scientific
International Inc., Ottawa, ON, Canada) equipped with a PTFE lined closure. Two 15 mm diameter
holes were drilled on the opposite side walls of the bottle, which were sealed with CaF2 windows
(Pike Technologies, Madison, WI, USA) using epoxy adhesive. The CaF2 windows allowed the
infrared beam of the spectrometer to transmit across the headspace of the bottle, allowing the
detection of CO2 without disrupting the headspace air (Figure 2a). At predetermined time intervals,
the headspace air was analyzed. A typical FTIR spectrum of the headspace air is shown in Figure 2b,
showing a strong absorbance at 2341 cm-1 wavenumber that can be attributed to CO2 gas. To
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determine the CO2 concentration, a calibration curve was established by mixing various known
amounts of sodium carbonate with excess amount of 1N sulfuric acid solution to generate headspace
air with different levels of CO2. High linear correlation was observed between the absorbance value
and CO2 concentration, with a coefficient of determination (R2) of 0.998.
2.4 CO2 degassing modeling
The CO2 degassing plots of the coffee beans and ground coffee were fitted with Weibull
distribution model, which is an empirical model applied to describe various kinetics in food, such as
drying and hydration processes (Menges and Ertekin 2006; Cunningham et al. 2007; Bakalis et al.
2009; Meisami-sal et al. 2010):
C = C * [1-exp [-(t/α) β]]
where t is degassing time (h); C (mg/L) is CO2 concentration in the headspace at time t; C (mg/L) is
headspace CO2 concentration at infinity time; α is scale parameter (h) and β is shape parameter
(dimensionless). The α parameter defines the rate and is related to the reciprocal of the process’s rate
constant, representing the time needed to accomplish approximately 63% of the process. The
calculated diffusion coefficient (Dcalc) can be obtained from α parameter by taking the sample
geometries into account (Marabi et al. 2003; Marabi and Saguy 2004; Corzo et al. 2008):
Dcalc =
where L is the sample radius for spherical samples. The L of roasted coffee beans is calculated by the
equation (Dutra et al. 2001):
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L= a + b+ c
where a, b, c represent the measurements of major, minor, and intermediate diameters of individual
bean. For the ground coffee, the average radius for each grinding level was calculated based on
particle size distribution in Figure 1. Coarse grinding resulted in an average particle size of 654 µm
in radius. For medium and fine grounds, the values were 467 and 299 µm, respectively. The shape
parameter β is related to velocity of the mass transfer at the beginning; the lower the value, the faster
the degassing rate at the beginning. When β=1, the Weibull distribution model reduces to 1st order
kinetic equation, while larger β value predicts a lag phase in the mass transport process (Marabi et al.
2003; Marabi and Saguy 2004; Saguy, Marabi, and Wallach 2005; Corzo et al. 2008).
2.5 Attenuated Total Reflectance (ATR)-FTIR analysis of roasted coffee power
Roasted coffees were grinded using a laboratory ball mill (PM 200; Retsch Inc., Newtown, PA,
USA) at 450 rpm for 30 min to obtain a fine power. The power samples were then analyzed with
Shimadzu FTIR spectrometer (IR Prestige-21; Shimadzu Corp., Tokyo, Japan) equipped with an ATR
accessory (Pike Technologies, Madison, WI, USA). During the analysis, the coffee power was spread
onto the crystal and then compressed using a clamp. Five samples from each treatment were scanned
from 600 to 4000 cm-1 wavenumber in absorbance mode at a resolution of 4 cm-1. Each spectrum
represented an average of 40 scans. For chemometric analysis, the FTIR spectra obtained were
exported as ASCII format, organized in Excel spreadsheets and then analyzed using Pirouette v.4.0.
chemometric software package (Infometrix, Inc., Bothell, WA, USA). Before the principle
component analysis (PCA), second derivative and mean-center treatments were applied to reduce the
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noise from baseline variation and to enhance spectral features.
2.6 Moisture, oil content and density
Samples of roasted beans were finely grinded and then analyzed using a moisture analyzer (IR-50;
Denver Instruments, Bohemia, NY, USA). Approximately 1 g of ground coffee was weighted
accurately onto the loading tray and the temperature was increased from 60°C to 105°C in 2 min and
then kept the temperature at 105°C for 10 min. Soxlet method was used to measure the oil content of
roasted coffee by using hexane as the extraction solvent. Around 5 grams of coffee sample was used
for the test and the extraction cycle was allowed to repeat for 6 hours.
Bean density was determined using a displacement method as reported by Schenker (2000).
Briefly, a known quantity of roasted bean samples was added to a 100 mL graduated cylinder filled
with 70 mL of water. A customer-made plunger was used to submerge the floating beans into the
water, followed by sliding the plunger up and down to remove the entrapped air. The final volume
was then noted. Similarly, the plunger was submerged into the graduate cylinder without the beans.
From the volume difference, the bean volume and thus the bean density were calculated. Three
measurements were taken for each sample during moisture content, oil content, and density analyses.
2.7 Cell wall porosity test by mercury intrusion porosimetry
A mercury-porosimeter (PoreMaster 60; Quantachrome Instruments, FL, USA) was used to
determine the cell wall porosity of coffee samples (Schenker 2000). To exclude the effect of the
evacuated cells, the roasted coffee samples were grinded by laboratory ball mill (PM 200; Retsch
Inc., Newtown, PA, USA) to disrupt the cells. In order to confirm the efficiency of the grinding, the
particle size after grinding was determined by light microscopy after dispersing the coffee particles
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in the vegetable oil. About 0.4 g ground sample was used and the measurement was conducted at
temperature of 20 °C. Applied intrusion pressure ranged from 0 to 60000 psia, which corresponded to
pore diameter of 1068.83 to 0.0036 µm. The measured intrusion pressure was converted to
equivalent pore diameter by using the Washburn equation:
Δp=
where Δp is the intrusion pressure (psia); γ is surface tension of the mercury (480 mN/m); θ is
contact angle between solids and mercury (140°); dpore is the diameter of the pores.
2.8 Statistical analysis
The non-linear regression method (IBM SPSS Statistics 21, New York, United States) was
utilized to estimate the parameters of Weibull distribution model. The goodness of fit was evaluated
on the basis of coefficient of determination (R2) and of the root-mean-square deviation (RMSE).
Statistical comparison of values in this study was conducted based on Tukey’s multiple
comparisons using Minitab 15 software (Minitab Inc, State College, United States).
3. Results and discussion
3.1 Effect of roasting conditions on the residual CO2 content in roasted coffee
Roasted coffee beans
The residual CO2 contents in the roasted coffee beans increased from 6.29 ± 0.47-6.70 ± 1.01 mg/g
to 11.04 ± 0.99-11.51 ± 0.50 mg/g as the coffee beans were roasted from light to medium roast
(Table 2). These values are comparable with the value reported by Geiger et al., who observed 9.9
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mg/g of residual CO2 in medium roast coffee (Geiger et al. 2005). Further processing the coffee to
dark roast resulted in more than double the residual CO2 (15.62 ± 0.72-15.36 ± 0.52 mg/g) as
compared to the light roast counterparts. However, processing the beans to very dark roast degree did
not cause further increase in residual CO2 (15.11 ± 0.92-15.97 ± 2.46 mg/g). This observation may be
due to the depletion of the CO2 precursors and the rapid release of trapped CO2 when the beans
underwent the “second crack” that occurred as roasted to dark degree of roast.
Table 2 also shows that residual CO2 content for coffee beans processed to the same degree of
roast, using either HTST or LTLT processes, were not significantly different (P>0.05). In addition, it
is noteworthy that the overall rate of CO2 formation was highly temperature dependent. For instance,
to attain the comparable residual CO2, a roast time of 5.5 min was needed when the coffee beans
were roasted at 250°C, as compared to 18 min when 230°C roast temperature was used.
Ground coffee
The residual CO2 can be trapped as gaseous phase within the evacuated cells, or be solubilized in
moisture, lipid and solid matrices. To gain better understanding of the CO2 that were trapped in the
cellular voids, roasted coffee beans were grinded into coarse, medium and fine grinds. The particle
size distributions of these three grinds, determined by passing the ground samples through a series of
sieves, are shown in Figure 1, and the corresponding CO2 contents are summarized in Table 3. As
shown, substantial amounts of CO2 were lost during grinding, and the finer the grinds, the greater the
amount of CO2 loss. Compared with the whole beans, approximately 26-30, 33-38 and 45-59% of
residual CO2 were lost when the samples were grinded to coarse, medium, and fine grinds,
respectively. These observations indicated that a substantial amount of CO2 was trapped in the
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evacuated cells of coffee beans, the breakage of which allowed the gas to escape during grinding.
Conceivably, the frictional heat generated during grinding, which was greater in finely grinded
samples, would have also contributed to the increased CO2 desorption from the lipid and solid
matrices.
3.2 Effect of roasting conditions on CO2 degassing behavior
CO2 degassing in whole beans is a slow process. As shown in Figure 3, beans subjected to HTST-
D treatment took more than 800 h (~33 days) to degas ~14 mg/g of CO2, which accounted for about
90% of the residual CO2 (15.36 mg/g; Table 2). In comparison, the degassing process was relatively
slower for LTLT-D sample; about 42% of CO2 was still trapped with the same degassing duration.
Similar trends were observed with the medium roasted beans that were processed using HTST and
LTLT processes, except that the amount of CO2 evolved were lower than the dark roast beans. These
results indicated that CO2 degassing rates for coffee beans roasted at higher temperature were faster
than those roasted at lower temperature.
For ground coffee, the degassing rates were greater than the whole beans (Figure 4). The
degassing plots for the fine grind reached the plateau within 50 h, even though the coarse and
medium grinds continued to degas slowly at the end of the test period. The greater degassing rate for
the ground coffee can be attributed to the partial disruption of pore structures and increased surface-
to-volume ratio of the coffee particles due to size reduction.
To further evaluate the CO2 degassing kinetics, the Weibull distribution model was fitted to the
degassing data. The coefficient of determinations (R2) for all best fit lines are greater than 0.99 with
root mean square deviation (RMSD) less than 2 mg/L, indicating the predicted values agreed with
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the experimental data (Table 4). The α parameter ranges from 190 to 335 h for coffee beans, and 2 to
25 h for ground coffee, confirming that the CO2 degassing were faster in the latter. Comparing
ground coffees with different grinding size, the trend is clear that the smaller the size, the smaller the
α value. On the other hand, roast degree had less effect on the magnitude of the α parameter than
roasting temperature. For instance, values for LTLT-M-coarse and LTLT-D-coarse samples are 21 h
and 25 h respectively, while the values for HTST-M-coarse and HTST-D-coarse are 12 h and 13 h ,
respectively. These results imply that smaller grinding size and higher roasting temperature will
favor the rapid degassing of CO2, while roast degree had a relatively less effect.
The Dcalc values were calculated from the α parameter to normalize the effect of particle size on
CO2 degassing rate. As shown in Table 4, the Dcalc values for beans (14.7×10-12 to 22.9×10-12 m2/s)
were greater than those for the ground samples (4.9×10-12 to 15.4×10-12 m2/s), despite the lower CO2
degassing rates in the whole beans. The larger Dcalc values for the whole beans than the ground
coffees can be attributed to the higher CO2 concentrations in the former. Fitting the Ficks law
diffusion model for spherical geometry to their coffee CO2 degassing data, Anderson et al. (2003)
determined that the effective diffusion coefficients for short and long times were in the ranges of
3.7×10-12 to 9.7 × 10-12 m2/s and 0.5 × 10-13 to 9.7 × 10-13 m2/s, respectively. These values are
comparable with the Dcalc values observed in the present study.
The β values are in the range of 0.698-0.734 for the beans and 0.457-0.526 for the ground coffee
(Table 4). The lower values for the ground coffee samples are indicative of higher degassing rates in
the initial stage of the degassing process. The shape parameter β in the Weibull distribution model
has been utilized by researchers for elucidating diffusion mechanisms in a number of food systems
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(Marabi et al. 2003; Marabi and Saguy 2004; Corzo et al. 2008). For instance, during the rehydration
of food particulates, β parameter in the range of 0.67-0.81 indicates that mass transfer diffusion
predominates, while higher β value in the range of 0.97-1 is indicative of the presence of external
resistance (Marabi et al. 2003). Due to coffee’s complex microstructures and multiple components
that interact with the CO2, the actual diffusion mechanism for CO2 degassing in coffee cannot be
explicitly explained with β parameter alone. However, the different β values observed between whole
beans and ground coffee samples may suggest that the mechanisms involved were different between
them. Previous study speculated that the CO2 transfer through the outer cell barrier (epidermis) might
be a limiting factor for CO2 degassing in roasted coffee beans (Baggenstoss et al. 2007). Therefore,
destroy of outer cell barrier caused by grinding might explain the faster CO2 degassing rate at
beginning (lower β value) in ground coffee than in whole beans.
3.3 Effect of roasting conditions on chemical compositions of roasted coffee
Studies in Sections 3.1 and 3.2 show that at equal degree of roast, CO2 degassing rates in HTST
processed coffees were greater than those in LTLT processed ones, although their residual CO2
contents were comparable. To better understand this phenomenon, the chemical compositional and
physical structural differences between HTST and LTLT processed coffees were investigated. As
shown in Table 5, at any given degree of roast, HTST processed coffees had significantly higher
(p<0.05) water and oil contents than those roasted using LTLT process. The majority of moisture
detected in the roasted beans was derived from the reactions that occurred during roasting, rather
than from the initial moisture present in the green beans. Due to the longer roasting time with the
LTLT process, the lower moisture contents observed in LTLT beans could be attributed to the
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depletion of reaction substrates, as suggested by Geiger et al. (2005). This explanation agrees with
the greater roast loss for beans processed using the LTLT processes, as shown in Table 5. The higher
moisture content of HTST beans might partly explain their higher CO2 degassing rates, since
degassing was shown being faster for coffee with higher moisture content (Baggenstoss et al. 2008).
On the other hand, the higher oil contents measured for the HTST beans than the LTLT counterparts
could be caused by the higher extraction efficiency of oil in the formers since their structure were
more porous as reflected by their lower density values measured (Table 5).
To further evaluate the differences in chemical composition, ATR-FTIR spectroscopy and PCA
methodologies were applied. FTIR technique allows rapid and nondestructive determination of
infrared fingerprints of test samples, which has been successfully applied in coffee authentication
studies and investigations of chemical compositional changes in coffee during roasting (Lyman et al.
2003; Nalawade et al. 2006; Wang et al. 2009; Wang, Fu, and Lim 2011; Wang and Lim 2012).
FTIR spectra of medium and dark roast coffees, produced using LTLT and HTST processes, are
shown in Figure 5a. As shown, medium roast coffees (HTST-M, LTLT-M) have stronger absorbance
at 2918, 2850 and 1743 cm-1, which correspond to the stretching of C-H and C=O. In addition, some
differences were observed in 1000-1600 cm-1 wavenumber region, which includes C-H, C-O, C-N,
and P-O vibrations (Wang et al. 2009). This region is unique for every organic molecule and is
believed to represent the overall “fingerprints” of many different compounds in coffee. To evaluate if
the infrared spectra can be used to discriminate the different roasted coffee samples, the PCA is
conducted. The resulting score plot is presented in Figure 5b, showing that the four samples (LTLT-
M, HTST-M, LTLT-D, HTST-D) are well separated. The clusters for the medium and dark roasted
coffees from the LTLT process (LTLT-M and LTLT-D) were farther apart than those from the HTST
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process (HTST-M and HTST-D), implying that the low temperature roasting resulted in greater
chemical compositional changes than high temperature roasting, when the beans were processed to
different roast degrees. Further inspection of the loading plot (Figure 5c) revealed that the main
spectral regions that contributed to the differences between samples were 1714-1739 cm-1 and 600-
1500 cm-1 wavenumber ranges, which corresponds to the C=O stretching and the fingerprint regions,
respectively. Strong loading was also observed at 2850 - 2920 cm-1 wavenumber region, which can
be attributed to the stretching of C-H. The C=O absorption region can be attributed to a number of
characteristic coffee components, including aromatic acids, aliphatic acids, ketones, aldehydes,
aliphatic/vinyl esters, and lactones (Lyman et al. 2003; Wang et al. 2009; Wang, Fu, and Lim 2011).
Carbonyl group is known to possess strong affinity for CO2 due to Lewis acid-base interactions
(Nalawade et al. 2006; Perko and Marko 2011). Therefore, the different contents of C=O containing
compounds might contribute to the different degassing behaviors observed in roasted coffee samples.
34 Effect of roasting conditions on the cell wall porosity of roasted coffee
Figure 6 shows the averaged particle size distribution of four ground coffee samples subjected to
mercury porosimetry test. As shown, the particle diameter was in the range of 3-26 µm, indicating
the effective disruption of evacuated cells of coffee by grinding. Figure 7 shows the development of
cumulated pore volume as function of intrusion pressure and pore diameter for samples of HTST-M,
HTST-D, LTLT-M and LTLT-D. As shown, the cumulated pore volume increased with the increasing
of intrusion pressure up to equivalent pore diameter of ~2.5-3.5 µm, which indicates the filling
process of inter-particle pores since the minimum particle diameter of the tested samples was around
3 µm (Figure 6). Further increase in the intrusion pressure did not result in the increasing of
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cumulated pore volume. However, at intrusion pressure of around 4,000-10,000 psi that
corresponded to ~15-35 nm pore size, the intruded volume increased again, indicating that filling of
the mesopores in the cell walls occurred. This observation is in agreement with Schenkers finding,
who reported 20-50 nm pores existed in the cell walls of roasted coffee. The increasing trend of
intruded volume after 3.6 nm indicated the presence of even smaller pores, which is out of the test
range of mercury porosimetry. The properties of these smaller pores (<3.6 nm) need to be
investigated in the future study in order to further elucidate the coffee’s microstructure. Overall, the
mercury porosimetry data from the present study did not reveal any detectable differences in cell
wall porosity between the four coffee samples tested, indicating that cell wall porosity may not be the
main contributor to the different CO2 degassing behaviors observed. However, further studies
including wider test range of pore size need to be conducted to confirm the above conclusion.
4. Conclusion
This study showed that the amount of CO2 retained in the coffee beans after roasting is strongly
dependent on the degree of roast but not on the roasting temperature, implying that roast degree
could be used as an indicator of residual CO2 content. Moreover, at equal degree of roast, CO2
degassing rates for coffee roasted at higher temperature were significantly faster than those roasted at
lower temperature. CO2 degassing in ground coffee was mostly dependent on the grinding size; the
finer the grinds, the higher the degassing rate. These observations indicate that in order to achieve the
optimal coffee quality, the tempering process and the design of one-way vent valve must take the
temperature-time roasting profile, degree of roast, and grind sizes into account. In addition, some
physicochemical differences were observed between HTST and LTLT roasted coffee samples, which
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helped to explain their different degassing behaviors.
5. Acknowledgement
This project was funded by Natural Science and Engineering Research Council of Canada
(NSERC). We also gratefully acknowledge material and funding supports from Mother Parkers Tea
& Coffee Inc.
6. Reference
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Table 1- Roasting temperature-time conditions used to obtain roasted coffee samples
Sample abbreviations Roasting temperature (°C) Time (min)
LTLT-L
230
4
LTLT-M 8
LTLT-D 18
LTLT-VD 26
HTST-L
250
2.5
HTST-M 3.5
HTST-D 5.5
HTST-VD 7.5
Table 2-Residual CO2 content of roasted coffee beans at various temperature-time roasting
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conditions
Samples Roast degree L* valueaResidual CO2 content (mg/g) a
LTLT-L Light 32.64 ± 0.38a6.29 ± 0.47a
HTST-L 33.15 ± 0.33a6.70 ± 1.01a
LTLT-M Medium 24.07 ± 0.34b11.51 ± 0.50b
HTST-M 24.44 ± 0.90b11.04 ± 0.99b
LTLT-D Dark 20.75 ± 0.34c15.62 ± 0.72c
HTST-D 20.27 ± 0.81c15.36 ± 0.52c
LTLT-VD Very dark 18.53 ± 0.22d15.11 ± 0.92c
HTST-VD 18.14 ± 0.29d15.97 ± 2.46c
a In the same column, the values with same letter means there is no significant difference (p>0.05) by
Tukey’s multiple comparison test.
Table 3-Effect of grinding on residual CO2 content in ground coffee samples
Ground coffee
samples
Residual CO2 content
(mg/g)
Percentage of lost due to
grinding (%)a
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LTLT-M-coarse 8.54 ± 0.08 26
LTLT-M-medium 7.66 ± 0.18 33
LTLT-M-fine 5.49 ± 0.23 52
HTST-M-coarse 7.75 ± 0.10 30
HTST-M-medium 6.80 ± 0.21 38
HTST-M-fine 4.46 ± 0.15 59
LTLT-D-coarse 11.63 ± 0.07 26
LTLT-D-medium 10.26 ± 0.09 34
LTLT-D-fine 8.54 ± 0.09 45
HTST-D-coarse 11.82 ± 0.34 29
HTST-D-medium 9.56 ± 0.16 38
HTST-D-fine 6.62 ± 0.20 57
a the value was calculated by the following equation: (1- residual CO 2 content in ground coffee/
residual CO2 content in roasted coffee beans) ×100%
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Table 4-Derived Weibull distribution model parameters (α, β, C), Dcacl (calculated diffusion coefficients), coefficient of determination (R2), and
RMSE (root mean square error).
Coffee sample α (h) β C (mg/L) Dcacl (m2/s, ×10-12) R2RMSE (mg/L)
Coffee beans
LTLT-M 335.2 ± 54.6 0.72 ± 0.00 72.5 ± 5.4 72.5 ± 5.4 0.999 0.52
HTST-M 222.9 ± 1.3 0.73 ± 0.01 82.4 ± 5.3 82.4 ± 5.3 0.998 1.02
LTLT-D 305.3 ± 69.5 0.73 ± 0.03 101.0 ± 2.3 101.0 ± 2.3 0.999 0.73
HTST-D 190.9 ± 4.2 0.70 ± 0.01 149.1 ± 5.9 149.1 ± 5.9 0.999 1.58
Ground coffee
LTLT-M-coarse 20.7 ± 1.7 0.52 ± 0.01 73.4 ± 4.1 73.4 ± 4.1 0.998 0.86
LTLT-M-medium 9.5 ± 0.8 0.48 ± 0.00 68.9 ± 6.4 68.9 ± 6.4 0.999 0.81
LTLT-M-fine 4.8 ± 0.1 0.53 ± 0.00 45.5 ± 0.3 45.5 ± 0.3 0.998 0.63
HTST-M-coarse 11.5 ± 0.5 0.49 ± 0.00 80.4 ± 0.6 80.4 ± 0.6 0.999 0.76
HTST-M-medium 6.6 ± 0.6 0.47 ± 0.00 69.1 ± 2.7 69.1 ± 2.7 0.998 0.70
HTST-M-fine 2.8 ± 0.0 0.48 ± 0.00 42.9 ± 0.5 42.9 ± 0.5 0.997 0.63
LTLT-D-coarse 24.9 ± 1.2 0.51 ± 0.01 110.3 ± 1.4 110.3 ± 1.4 0.999 0.85
LTLT-D-medium 11.7 ± 0.6 0.48 ± 0.01 92.2 ± 0.3 92.2 ± 0.3 0.998 1.10
LTLT-D-fine 4.7 ± 0.1 0.52 ± 0.01 55.7 ± 0.8 55.7 ± 0.8 0.995 1.14
HTST-D-coarse 12.8 ± 0.8 0.47 ± 0.01 128.6 ± 2.0 128.6 ± 2.0 0.998 1.50
HTST-D-medium 5.8 ± 0.4 0.46 ± 0.01 104.1 ± 0.5 104.1 ± 0.5 0.997 1.44
HTST-D-Fine 2.2 ± 0.1 0.49 ± 0.01 55.2 ± 0.5 55.2 ± 0.5 0.996 0.94
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510
Table 5-Some physiochemical properties of roasted coffee
Coffee samples Total roast loss (%)
Oil content
(dry basis, %)
Moisture content (%)
Density
(g/ml)
LTLT-M 15.7 ± 0.2a13.7 ± 0.1a1.3 ± 0.0a0.626 ± 0.000a
HTST-M 14.9 ± 0.2a14.5 ± 0.3b1.5 ± 0.1b0.585 ± 0.006b
LTLT-D 18.1 ± 0.3b14.1 ± 0.0b0.9 ± 0.1c0.561 ± 0.002c
HTST-D 17.6 ± 0.4b15.0 ± 0.3c1.3 ± 0.1a0.522 ± 0.010d
In the same column, the values with same letter means there is no significant difference (p>0.05) by
Tukey’s multiple comparison test.
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Figure 1-Averaged particle size (in radius) distribution of coarse, medium and fine ground coffee.
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Figure 2- Schematic diagram of CO2 degassing test apparatus (a) and typical FTIR spectrum of
headspace air, showing the strong absorbance at 2341 cm-1 due to CO2 (b)
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Figure 3-Representive CO2 degassing kinetics data (symbols) at 25 °C for coffee beans roasted
using different temperature-time conditions. Solid line represents the best fit of Weibull
distribution model, showing its goodness of fit to the experimental data. HTST: high-
temperature-short-time; LTLT: low-temperature-long-time; D: dark roast degree; M: medium
roast degree.
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Figure 4-Representive CO2 degassing kinetics data (symbols) at 25 °C for coffee samples grinded to
different sizes (coarse, medium and fine). Solid lines represent the best fit curves of Weibull
distribution model to the experimental data. HTST: high-temperature-short-time; LTLT: low-
temperature-long-time; D: dark roast degree; M: medium roast degree.
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Figure 5- (a) Representative FTIR spectra of coffee samples LTLT-M, LTLT-D, HTST-M, and HTST-
D; (b) two factor score plots of PCA analysis of FTIR data; (c) loading plot of first principle
component.
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Figure 6-Averaged particle size (in diameter) distribution of coffee samples subjected to the mercury
porosimetry test, showing particle size less than 30 µm.
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Figure 7- Porosimetric curves of finely ground coffee samples of LTLT-M, LTLT-D, HTST-M, and
HTHT-D, showing mesopores (2-50 nm) present in the cell walls (data are the average of two
duplicates).
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Application of Weibull distribution model was investigated for describing the moisture content of coroba slices during air drying. One set of experiments was performed following a full factorial design at three levels for air temperature (71, 82 and 93°C) and velocity (0.82, 1.00 and 1.18m/s). The set was designed to assess the adequacy of the Weibull model to describe water losses. The high regression coefficients (R2>0.99) and low reduced chi-square indicated the acceptability of Weibull model for predicting moisture content. Values of scale parameter ranged from 41.77 to 71.52 (min) and values of shape parameter ranged from 1.06 to 1.21. Temperature sensitivity of scale parameter increased with increasing air velocity from 0.82m/s (Ea=210.45J/mol) to 1.00m/s (Ea=214.93J/mol) and then decreased with increasing velocity to 1.18m/s (Ea=139.03J/mol). The normalized Weibull model was investigated for determining the effective diffusion coefficient (De). The De values ranged approximately from 2.51×10−12m2/s to 4.27×10−12m2/s.
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Thin layer drying kinetics of apple slices (variety-Golab) was experimentally investigated in a convective dryer and the mathematical modeling was performed by using thin layer drying models in the literature. Drying characteristics of apple slices were determined using heated ambient air at temperatures from 40 to 80 °C, velocity of 0.5 m/s and slice layers of 2, 4, 6 mm thickness. Beside, the effects of drying air temperature, effects of slice thickness on the drying characteristics and drying time were also determined. Thirteen thin-layer drying models were studied. The fitting ability of the models is compared using the root mean square error, chisquare and modeling efficiency. The results showed that, increasing the drying air temperature and decreasing slice thickness causes shorter drying times. The Midilli et al. model was found to be the best model for describing the drying curves of the apple slices. Also, the effects of drying air temperature and thickness of layers on the model constants and coefficients were predicted by multiple regression analysis. According to the results of regression method, Henderson and Pabis model could satisfactorily describe the drying curve of apples with a correlation coefficient (R 2) of 0.9762.
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Great effort has been devoted towards developing models that describe cooking processes. A difficulty towards developing such models arises from the fact that during cooking, the physical properties and quite often sample geometry are time dependent. In this work a finite element model describing cooking of rice and water uptake using a Fickian diffusion model was developed, assuming axisymmetric conditions. Effective diffusivity was considered a function of moisture content. The numerical model compared favorably with experimental results. The value of the effective water diffusivity was estimated to be in the order of 7×10−10m2/s, by minimizing the error between experimental and numerically predicted results.The effect of grain size on the cooking was also investigated using the model. Cooking time, i.e. the time to reach about 70% moisture content (wet basis), appeared to be a strong function of the initial size distribution.
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The diffusion coefficients and solubility of CO2 in methyl cellulose (Mw = 40 000 g·mol–1) and sodium carboxymethyl cellulose (CMC) of different molecular weights, Mw = (90 000, 250 000, and 700 000) g·mol–1, were determined by a gravimetric method. The MBS (magnetic suspension balance) was used for measurements at three different temperatures, (313, 333, and 353) K, and pressures up to 30 MPa. High values of solubility (up to 36.11 mol %) and diffusion coefficients [(2.55·10–9 to 8.61·10–8) cm2·s–1] were obtained. The solubility of CO2 in the polymers depends on the temperature and pressure, while diffusion coefficients are concentration-dependent. The solubility and diffusivity in the various CMCs are influenced by molecular weight and the degree of substitution.
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Rehydration of food particulates is a complex phenomenon affected by numerous factors that typically include pre-drying treatments, mode of dehydration, structure, composition and medium viscosity. Freeze drying (FD), air drying (AD) and their combinations, were utilized to produce an array of porosities, ranging from very high to very low values for FD and AD carrots, respectively. Bulk porosity correlated significantly with open, but not with closed, porosity. Bulk and open porosities decreased with AD time. Scanning electron micrographs of the FD samples verified their organized and more open structure in comparison with the AD carrots. Rehydration ratio increased with bulk and open porosity, and was not affected by the closed porosity. The effective moisture diffusivity, derived from fitting the normalized Weibull distribution, increased with bulk and open porosity and was about two orders of magnitude higher for the FD, than for the AD, carrots. The Weibull shape parameter, β, was inversely related to porosity. Its values indicated that water uptake of only the AD carrots followed a Fickian diffusion. A critical porosity value above which water mechanism changed from a Fickian diffusion to imbibition into a porous medium is suggested. Copyright © 2004 Society of Chemical Industry