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Water sorption properties of coffee fruits, pulped and green coffee
LWT - Food Science and Technology 50 (2013) 386e391
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Water sorption properties of coffee fruits, pulped and green coffee A.L.D. Goneli a, P.C. Corrêa b, G.H.H. Oliveira c, *, P.C. Afonso Júnior d a
Agricultural Sciences Dept., Universidade Federal da Grande Dourados, Dourados, MS, Brazil Agricultural Engineering Dept., Universidade Federal de Viçosa, Campus UFV, P.O. Box 270, 36570-000 Viçosa, MG, Brazil c Instituto Federal de Educação, Ciência e Tecnologia de Brasília, Campus Gama, Lote 01, DF 480 Setor de Múltiplas Atividades, 72405-980 Brasília, DF, Brazil d EMBRAPA e Café, Parque Estação Biológica e PqEB e s/n , 70770-901 Brasília, DF, Brazil b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 June 2012 Received in revised form 10 September 2012 Accepted 11 September 2012
Thermodynamic properties of coffee in three different forms (green, pulped and coffee fruit) were obtained during the desorption process. Desorption was accomplished by static method using saturated salt solutions, providing relative humidity range of 11e95%. Five different temperatures were utilized (10, 20, 30, 40 and 50 C). Equilibrium moisture content data were correlated by the GuggenheimeAndersonede Boer model, which presented good fit to the data, according to statistical procedures. Equilibrium moisture content ranged from 4.34 to 10.75 g/100 g dry solids. Enthalpy values for each model coefficient were estimated, ranging from 8.09 to 82.64 kJ kg1. Differential enthalpy and differential entropy increased with decreased equilibrium moisture content, which was also found for Gibbs free energy. Coffee fruits presented higher values of these thermodynamic parameters. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: GAB model Coffea arabica L. Wet method Enthalpy Entropy Gibbs free energy
1. Introduction Brazil is the main green coffee producer (47.9 million tons), followed by Vietnam, Colombia and Indonesia (FAO, 2010). According to OIC (2010), Brazil is the leader of consumption (per capita consumption 5.64 kg) among countries that exports coffee. Due to this importance, it is required the study of the different factors that affect cup quality of coffee. One factor is the different processing procedures. According to Corrêa, Goneli, Afonso Júnior, Oliveira, and Valente (2010), coffee can be processed in two different ways: ‘dry method’ or natural form and ‘wet method’ or hulling. In the dry process, the entire cherry after harvest, unwashed in natural form, is first cleaned and then placed in the sun to dry on tables or in thin layers on patios. In the Wet Process, the skin and some pulp covering the seeds/beans is removed before they are dried. This is done either by the classic ferment-and-wash method or a newer procedure called machineassisted wet processing, aquapulping or mechanical demucilaging. The processing procedure also affects post-harvest techniques, in which to correctly conduct drying and storage operations, it is
* Corresponding author. Tel.: þ55 61 21032259. E-mail addresses: [email protected] (A.L.D. Goneli), [email protected] (P.C. Corrêa), [email protected] (G.H.H. Oliveira), paulo.junior@ embrapa.br (P.C. Afonso Júnior). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.lwt.2012.09.006
necessary to know the relationship between air temperature (T) and relative humidity (RH), and desirable conditions for preserving the product. To obtain this information, sorption isotherms are indispensable. Several mathematical models are used to describe the sorption isotherms. However, the GuggenheimeAndersonede Boer (GAB) model was recommended by the European Project Group COST 90 (Wolf, Spiess, & Jung, 1985) as a fundamental equation to characterize water sorption in foodstuff. The advantages of this model, when compared to the others, are its theoretical background and the physical meaning of its parameters (Oliveira, Corrêa, Santos, Treto, & Diniz, 2011). Sorption isotherms are also efficient tools to determine thermodynamic interactions between water and food components. Thermodynamic properties provide useful information for the development and improvement of dryers and for studies on water properties on food surface (Corrêa, Goneli, Jaren, Ribeiro, & Resende, 2007). Thermodynamic data for coffee in different processing procedures are scarce in the literature and is required to develop and design post-harvest equipment. In this context, the aim of this study was to obtain the desorption isotherms of coffee in two different processing levels (dry and wet method), at different temperatures. Thermodynamic functions of enthalpy and entropy differentials, and Gibbs free energy were determined. The theory of enthalpyeentropy compensation was applied.
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2. Material and methods 2.1. Raw material Coffee fruits (Coffea arabica L), variety “Catuai Vermelho” were manually harvested during cherry stage. After harvested, fruits were washed and selected for characterization, and three lots were obtained. The first lot was kept as coffee fruits (group 1); the second lot of coffee fruits was hulled (pulped coffee, group 2), while the rest was processed (green coffee, group 3). During harvest, unripened, deteriorated and injured coffee beans were eliminated. Due to the high initial moisture content, coffee fruits and pulped coffee were partially dried outdoors. In order to reduce the risk of microorganisms development during experimental hygroscopic test, coffee fruits and pulped coffee were dried until moisture content reached 45 g/100 g dry solids. For green coffee tests, coffee of approximately 11% g/100 g dry solids was used. They were obtained by low temperature drying from a pulped coffee portion using an air conditioning unit manufactured by Aminco, model Aminco-Aire 150/300 (Silver Spring, Maryland e USA). At the end of drying process, the material had its parchment and silver pellicle removed by hand, and then it was submitted to analyses. Two samples were stored in polythene bags kept in a refrigerator (5 1 C) to attain moisture uniformity during 24 h. When experiments were to be made, samples were left in room conditions (25 3 C) for 6 h to achieve moisture equilibrium. According to Seeds Analysis Standard of Brazil (Brazil, 1992), moisture content of coffee fruits, pulped coffee and green coffee was determined by applying the oven method at 105 1 C for 24 h, in duplicate.
Xm CKaw ð1 Kaw Þð1 Kaw þ CKaw Þ
P ¼
n Y Y 100 X n i¼1 Y
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 i ¼ 1 ðY YÞ SE ¼ DF
(1)
In which: Xeq ¼ equilibrium moisture content, g/100 g dry solids; Xm ¼ monolayer moisture content, g/100 g dry solids; C, K ¼ model constants that are related to monolayer and multilayer properties, respectively (van den Berg, 1984), dimensionless and, aw ¼ water activity, decimal.
(2)
(3)
In which: P is the mean relative percent deviation, %; n is the number of observed data; SE is the standard error, g/100 g dry b is the estimated value through the solids; Y is the observed value; Y model; and DF is the degrees of freedom of the model. 2.3. Thermodynamic properties After achieving the model coefficients, the enthalpy values for each GAB parameter were calculated using a simple regression procedure. It requires the representation of ln Xm, ln C and ln K vs. the inverse of absolute temperature, as stated by Simal, Fermenía, Llull, and Rosselló (2000).
DH1
C ¼ C0 exp
(4)
RT
K ¼ K0 exp
Equilibrium moisture content (EMC) of green, pulped and coffee fruit were determined using the static method, which is based on using saturated salt solutions to maintain constant water activity of samples once equilibrium between room conditions and food sample is reached. Salt solutions used to obtain constant relative humidity were NH4Cl, KBr, KNO2, KNO3, K2SO4, CaCl2, Ca(NO3)2, Na2Cr2O7, MgCl2$6H2O, LiCl, LiCl$H2O and NaCl. This group of salts allows obtaining a wide range of relative humidity, from 11% to 95% RH in equilibrium. In order to determine sorption isotherms of green coffee, pulped coffee and coffee fruit the analysis was conducted at five different temperatures: 10, 20, 30, 40 and 50 C. Water activity was also measured at these five temperatures. To acquire data for drawing sorption curves, three samples containing 100 g of coffee at different processing levels and combinations of temperature and water activity were left in room conditions until reaching static equilibrium. For this purpose, samples were hanged in small baskets inside a 2.0 L glass container with 250 mL of salt solutions, hermetically closed, and kept in five climacteric boxes of regulated temperature. Baskets were weighted daily until mass variation was lower than 1 mg. Then, their moisture contents were determined, leading to the obtainment of equilibrium moisture content for each treatment condition. Experimental data of equilibrium moisture content of green, pulped and coffee fruit were fitted to GAB model (Equation (1)).
Xeq ¼
The GAB parameters were estimated using a nonlinear regression by GausseNewton approximation method, which minimizes the sum of square errors in a series of interactive stages. The adequacy of the model was analyzed based on the values of mean relative percent deviation (P), the standard error (SE), determination coefficient (R2) and residual plots:
2.2. Equilibrium moisture content
387
DH2
(5)
RT
Xm ¼ X0 exp
DH3
(6)
RT
In which: C0 and K0 are parameters from equations (4) and (5), dimensionless; X0 is the parameter from equation (6), decimal dry solids; T is the temperature, K; DH1, DH2 and DH3 are the enthalpies from the model coefficients, kJ kg1; and R is the water vapor constant, 0.462 kJ kg1 K1. Remaining thermodynamic parameters, such as differential entropy of desorption (DS), differential enthalpy (DH), Gibbs free energy (DG) and enthalpyeentropy relationship, were obtained by means of a known methodology, stated by Corrêa, Oliveira, and Santos (2012), which an approximate (1 a)100% confidence interval for isokinetic temperature is used. These parameters are expressed respectively by Equations (7)e(11).
DHst DS ln aw ¼ R RT
(7)
DH ¼ DHst DHvap
(8)
DG ¼ RT ln aw
(9)
bB t TB ¼ T m2;a=2 Thm ¼
nt 1 i¼1 T i
Pnt
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi VarðTB Þ
(10) (11)
In which: DH: isosteric heat of sorption, kJ kg1; DHvap: latent heat of vaporization of pure water, kJ kg1; DHst: net isosteric heat
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of sorption, kJ kg1; DS: differential entropy of sorption, kJ kg1 K1; and DG: Gibbs free energy, kJ kg1 mol1; TB: isokinetic temperature, K; m: number of data pairs of enthalpy and entropy; t is the t value at (m 2) degrees of freedom; Thm: harmonic mean temperature, K; and nt: number of temperatures utilized. 3. Results and discussion 3.1. Mathematical modeling of sorption Statistical parameters in order to analyze model adequacy are presented in Table 1. GAB model presented random residual plots of EMC experimental data of green, pulped and coffee fruit at all temperatures investigated at the present work. Also, low values of P and SE were encountered along with high values of R2. These results indicate that GAB model is adequate to estimate equilibrium moisture content of coffee in its different forms of processing. Several researchers applied the GAB model in order to determinate sorption isotherms of different agricultural products, and its different types of processing procedures (García-Pérez, Cárcel, Clemente, & Mulet, 2008; Kaya & Kahyaoglu, 2006; Oliveira et al., 2011; Samapundo et al., 2007; Tarigan, Prateepchaikul, Yamsaengsung, Sirichote, & Tekasakul, 2006). Corrêa et al. (2010), also working with coffee beans provided by different processing procedures, concluded that the GAB model, Modified Henderson and ANN model were adequate to predict EMC values. According to Oliveira et al. (2011), the number of water molecules that are strongly adsorbed in specific sites at the food surface are indicated by the monolayer moisture content. This parameter tended to decrease as temperature increased (Table 1), varying between 10.75% and 4.34 g/100 g dry solids According to Perdomo et al. (2009), this can be explained by the number of active sites that are reduced with increased temperature, resulting into modifications of the physical and chemical characteristics of the product. The monolayer moisture content is the safest moisture content for storage purposes, providing increased time period with minimum quality loss at a certain temperature during preservation. Values below monolayer moisture content slow down deteriorative
Table 1 Parameter estimated values, mean relative error (P), standard error (SE) of estimate, determination coefficient (R2) and residual plots for GAB model to coffee sorption in different processing levels. Temperature ( C)
Parameters
R2 (%)
SE (g/100 g dry solids)
P (%)
Residual plots
Xm
C
K
Coffee fruits 10 20 30 40 50
10.75 10.14 9.63 9.31 7.64
22.30 32.08 14.24 12.67 13.23
0.76 0.75 0.76 0.75 0.82
99.75 99.72 99.95 99.92 99.66
0.40 0.33 0.15 0.18 0.34
2.05 1.30 0.98 1.24 2.78
Random Random Random Random Random
Pulped coffee 10 20 30 40 50
9.99 8.36 7.39 7.13 4.34
6.16 6.55 5.71 5.76 12.29
0.67 0.73 0.79 0.74 0.92
99.69 99.76 99.89 99.65 99.07
0.35 0.27 0.19 0.28 0.43
2.86 1.04 1.74 3.41 5.85
Random Random Random Random Random
Green coffee 10 20 30 40 50
7.92 7.60 7.54 6.37 5.67
12.59 9.99 8.29 11.70 9.69
0.74 0.75 0.75 0.77 0.81
99.89 99.54 99.95 99.77 98.82
0.19 0.35 0.12 0.21 0.47
1.44 1.30 1.05 2.31 5.96
Random Random Random Random Random
reactions, pest attacks and respiration rate of the product. Water activity values corresponding to the monolayer moisture content of coffee were obtained from the data presented in Table 1 and is tabulated in Table 2. It can be noticed that coffee fruits had lower values of aw than pulped coffee and green coffee, meaning that its storage is more difficult to accomplish at these temperatures, requiring more energy and care. This fact is expected since that the presence of hulls on coffee fruits ends up on increased energy to dry this product, increased time to this product reach equilibrium with the surrounding environment. In addition, coffee fruits also have mucilage, a component of high sugar content, leading to a high probability of microorganism and insect attack. These facts also may explain the non-patterned values of water activity for increased temperatures. Water activity of green coffee may be assumed to be constant at all temperatures investigated (Table 2), due to absence of hull and mucilage. Pulped coffee presented a decrease on water activity values with temperature increase, probably due to the presence of mucilage. According to Timmermann, Chirife, and Iglesias (2001), the C constant is associated with the chemical potential differences between the monolayer and superior layers. No defined behavior could be concluded for this parameter. Probably this fact is due to the heterogeneous characteristic of coffee product, which components variance among layers may lead to the trend noticed, observed both for the same product but between temperatures and between different products. Duggan, Noronha, O’Riordan, and O’Sullivan (2008) reported that the K constant is correlated to the chemical potential difference between the monolayer and free water state. This parameter presented a slightly increase at temperature of 50 C to the same product, being considered constant for the remaining temperatures. Higher levels of temperature (50 C) lead to a rapid water molecules transition between multilayer and free water. C0, K0 and X0 and the enthalpies associated with each GAB model constant (DH1, DH2 and DH3) are shown in Table 3. According to Yadav, Thiel, Kasting, and Pinto (2009), the preexponential factors C0 and K0 are expressions of entropic effects in the sorption process. Values of C0 were higher for pulped coffee, followed by green coffee and coffee fruit (Table 3). Furthermore, pulped coffee presented a value of C0 bigger than one and the remaining products lower than one. A value of C0 less than one is expected if the mobility of molecules within the first sorbed layer is restricted (Pradas, Sanchez, Ferrer, & Ribelles, 2004). Thus, we can conclude that the mobility of molecules within the first layer of pulped coffee occurs freely. This trend may be due to the absence of the hull but also the presence of the mucilage, in which hull are present on coffee fruits and mucilage is absent on green coffee. Values of K0 were similar for coffee fruit and green coffee, being significantly higher for pulped coffee, due to the same reason stated previously. X0 is related to the number of water molecules sorbed in the first layer. This parameter presented higher values for coffee fruits and
Table 2 Corresponding water activity (aw) values for safe storage of green, pulped and coffee fruits at different temperatures. Product
Temperature ( C) 10
20
30
40
50
Coffee fruits Pulped coffee Green coffee
0.229 0.429 0.297
0.201 0.383 0.319
0.276 0.375 0.344
0.291 0.399 0.294
0.264 0.242 0.301
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Parameters C0
K0
X0
D H1
DH2
DH3
Coffee fruit Pulped coffee Green coffee
0.12 2.23 0.95
1.37 5.14 1.15
0.56 0.03 0.05
16.24 13.61 31.91
8.09 5.60 5.60
35.00 75.95 82.64
were similar among the remaining products investigated (Table 3). This trend is probably due to the fact that coffee fruits are not processed and therefore its hull is not ruptured, allowing water molecules to transfer between the first layer and the subsequent layers without alteration in its normal functionality. For green coffee and pulped coffee, due to the processing procedure, this behavior is not present. Analyzing Table 3, it can be noticed that enthalpy values were positive to the X0 and C0 constants, which indicates endothermic reactions, or in other words, reactions that require the absorption of energy from the environment in which products are inserted. This fact is expected, because during drying, the product absorbs energy from the environment in order to reach equilibrium with it, through heating (sensible heat) and later through the evaporation of free water (latent heat of vaporization). At the same table, enthalpy values for K0 are negative. This signal indicates an energy release from the several layers that form the multilayer. This tendency may be explained by energy release as heat flow, because of the increasing level of molecular vibration of water molecules during drying. X0 presented the highest enthalpy values (Table 3). This trend is expected, because the monolayer moisture content indicates the amount of water molecules that are strongly adsorbed by specific sites from the food surface; thus, significantly greater energy is required to remove these water molecules from the monolayer. According to McMinn and Magee (2003), knowledge of differential enthalpy at a given moisture content indicates the state of water that is sorbed within the product, serving as a measurement of physical, chemical and biological stability of the product during storage. 3.2. Differential enthalpy Fig. 1 presents the values of differential enthalpy according to the equilibrium moisture content. According to Fig. 1, it is observed a reduction of coffee EMC in its different types of form. The required energy to remove water from the product is higher in lower levels of EMC. Elevated values of differential enthalpy at low values of EMC can be explained by the differences of bonding forces between moisture and adsorbent surface of the product. At initial stages of sorption (low values of moisture content), there are polar sorption sites highly active, of elevated interaction energy, which are covered by water molecules, forming the monomolecular layer (Al-Muhtaseb, McMinn, & Magee, 2004). Throughout time, which water molecules are being chemically connected to the sorption sites highly active, sorption starts to occur at sites less active (higher moisture content), with lower interaction energy and, consequently, lower differential enthalpy (Wang & Brennan, 1991). Values of differential enthalpy of coffee fruits are higher than those found for green and pulped coffee, at all EMC range. These results are similar to those found by Martinez and Chiralt (1996), working with hazelnuts, peanuts and almond. According to these authors, it was because of enhancement of lipidelipid interaction
Differential enthalpy (kJ kg-1)
Enthalpy (kJ kg1)
Processing level
1800 1500 1200 900 600 300 0 0
5
10
15
20
25
30
Equilibrium moisture content (g/100g d.b.) Fig. 1. Observed and estimated values (d) of differential enthalpy in function of equilibrium moisture content of green (C), pulped (,) and coffee fruits (6).
after dehulling of these products, which would increase the hydrophobicity of cellular components. Furthermore, because of the low hygroscopicity of the layer that involves pulped coffee, values of differential enthalpy of green and pulped coffees were similar.
3.3. Differential entropy Fig. 2 presents differential entropy of sorption values (kJ kg1 K1), in function of equilibrium moisture content (g/100 g dry solids) for green, pulped and coffee fruit. According to Fig. 2, there is a notable relationship between differential entropy and equilibrium moisture content, similar behavior observed in differential enthalpy values. Coffee fruits also presented higher values of differential entropy than pulped and green coffee. The number of available desorption sites corresponding to a specific energy level is proportional to the differential entropy, which describes the degree of disorder and motion randomness of water molecules. Therefore, DS values of coffee fruits are expected since this product possesses the hull, an increase of area where water molecules may move, in which leads to higher number of
5 Differential entropy (kJ kg-1 K-1)
Table 3 Calculated values of GAB model constants (C0, K0 and M0) and its respective enthalpy values.
389
4 3 2 1 0 0
5
10
15
20
25
30
Equilibrium moisture content (g/100g d.b.) Fig. 2. Observed and estimated values (d) of differential entropy of green (C), pulped (,) and coffee fruits (6).
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desorption sites at a certain EMC value, regarding green and pulped coffee. Comparable results were reported by Al-Muhtaseb et al. (2004), working with starch powder and by Kaya and Kahyaoglu (2006), analyzing sesame seeds. Increasing values of equilibrium moisture content leads to a decrease of differential entropy values, tending to become constant at high values of EMC (Fig. 2). Based on the second law of thermodynamics, a process is reversible when the sum of all entropy changes for all subsystems in a process is constant. Thus, coffee sorption in its different types of processing is irreversible (hysteresis effect), because entropy is produced during the process (Madamba, Driscoll, & Buckle, 1996). However, at high values of EMC in which differential entropy tends to become constant, sorption process may be reversible.
3.4. Enthalpyeentropy compensation theory Fig. 3 presents the relationship between differential enthalpy and differential entropy values of coffee sorption. It can be noticed linear relationships for sorption of coffee, presenting determination coefficients superior to 99%. This high level of linearity between these parameters allows us to indicate that the enthalpyeentropy compensation theory for green, pulped and coffee fruits is probably valid. In order to confirm the enthalpyeentropy compensation theory, isokinetic temperature was compared to the mean harmonic temperature (Thm). According to Krug, Hunter, and Grieger (1976a, 1976b) linear chemical compensation only exists if isokinetic temperature is different from the mean harmonic temperature. Values of isokinetic temperature and Gibbs free energy at this temperature were determined by means of linear regression, and are presented at Table 4. Thm was 302.50 K, value significantly different from all values of TB reported (Table 4). Therefore, enthalpyeentropy compensation theory is valid. According to Liu and Guo (2001), isokinetic temperature is the one in which all reactions in series happens at the same time, in other words, when the product is at an equilibrium state. If TB > Thm, the process is enthalpy controlled; if TB < Thm, the process is entropy controlled (Leffler, 1955). The first condition is in accordance with the results of the present research, thus the sorption mechanism in green, pulped and coffee fruits are enthalpy controlled. This result is in agreement with several researchers that applied the isokinetic theory over sorption of different products (Goneli, Corrêa, Oliveira, Gomes, & Botelho, 2010; Moreira, Chenlo,
-1
Differential enthalpy (kJ kg )
1800 1500 1200 900 600 300 0 0
1
2
3
4 -1
5
-1
Differential entropy (kJ kg K ) Fig. 3. Enthalpyeentropy relationship of green (C), pulped (,) and coffee fruits (6).
Table 4 Isokinetic temperature (TB) and Gibbs free energy at isokinetic temperature (DGB) of coffee in different types of form. Processing level Coffee fruit Pulped coffee Green coffee
DGB (kJ kg1)
TB (K) 391.50 0.001 414.09 0.002 432.39 0.002
4
5.89 10 3.42 103 8.05 104
R2 (%) 99.99 99.99 99.99
Torres, & Vallejo, 2008; Nkolo Meze’e, Noah Ngamveng, & Bardet, 2008; Oliveira et al., 2011; Tunç & Duman, 2007). 3.5. Gibbs free energy Gibbs free energy is related to the work needed regarding the product layers to become available to sorption (Nkolo Meze’e et al., 2008); therefore, in higher values of equilibrium moisture content, there will be fewer sites available to sorption. Being that stated DG values are expected, because in higher values of moisture content, there is lower necessity of work to make the sites available to sorption, because they are already available. This trend is observed by lower values of DG. It is observed that temperature influence over Gibbs free energy is higher at lower values of EMC, in which lower temperatures provides higher values of DG. This trend can be correlated to the vibration of water molecules within the product, where at high temperatures this vibration (movement) is higher; this leads to lower work required to the product layers to become available to sorption. At high levels of EMC, temperature influence becomes negligible due to the sorption sites are already available. Positive values of DG are also expected, as it characterizes endothermic reaction, or else, reactions that require energy from the environment to occur. Green and pulped coffee presented similar values of Gibbs free energy, while coffee fruit presented higher values of this parameter. This trend may be explained by the presence of the hull in coffee fruits, as stated before at the differential entropy discussion. DG values become similar between different types of coffee in high levels of EMC (above 20%). Table 5 contains the regression equations for Gibbs free energy, for each temperature and coffee type studied in the present work and their respective values of determination coefficient. Table 5 Regression equations for Gibbs free energy (DG) in different temperatures and coffee type analyzed, related to equilibrium moisture content (Xeq). Temperature ( C)
Equation
R2 (%)
Coffee fruits 10 20 30 40 50
DG ¼ 839.24 exp(0.13Xeq) DG ¼ 720.24 exp(0.13Xeq) DG ¼ 653.44 exp(0.13Xeq) DG ¼ 612.83 exp(0.13Xeq) DG ¼ 587.76 exp(0.14Xeq)
99.84 99.96 99.89 99.42 99.18
Pulped coffee 10 20 30 40 50
DG ¼ 572.24 exp(0.16Xeq) DG ¼ 517.51 exp(0.16Xeq) DG ¼ 488.08 exp(0.17Xeq) DG ¼ 461.72 exp(0.17Xeq) DG ¼ 455.24 exp(0.18Xeq)
99.67 99.98 99.54 99.27 97.44
Green coffee 10 20 30 40 50
DG ¼ 658.22 exp(0.17 Xeq) DG ¼ 598.69 exp(0.17 Xeq) DG ¼ 556.99 exp(0.17Xeq) DG ¼ 543.63 exp(0.18Xeq) DG ¼ 518.54exp(0.18Xeq)
99.92 99.96 99.95 99.21 99.17
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4. Conclusions Modeling and the thermodynamic parameters were estimated for desorption process of green, pulped and coffee fruits, processed by two different procedures. Evaluation of these parameters is of fundamental importance to the correct preservation of the product, as well providing parameters to design and develop the several equipments utilized on the main post-harvest operations. According to the results, it can be concluded that desorption process was well represented by the GAB model. Also, monolayer moisture content decreased with increased temperature of drying. Water activity values at five different storage temperatures were acquired. Enthalpy values related to two GAB coefficients were positive, which indicates endothermic reactions; on the other hand, one coefficient from GAB model was negative, indicating an energy release from the several layers that form the multilayer. Differential enthalpy, differential entropy and Gibbs free energy presented high correlation with the equilibrium moisture content of green, pulped and coffee fruits, and the enthalpyeentropy compensation theory exists. Coffee fruits presented higher values of differential enthalpy, differential entropy and Gibbs free energy due to the presence of the hull. References Al-Muhtaseb, A. H., McMinn, W. A. M., & Magee, T. R. A. (2004). Water sorption isotherms of starch powders. Part 2: thermodynamic characteristics. Journal of Food Engineering, 62, 135e142. van den Berg, C. (1984). Description of water activity of foods for engineering purposes by means of the GAB model of sorption. In B. M. McKenna (Ed.), Engineering and food (pp. 311e321). New York: Elsevier Applied Science. Corrêa, P. C., Goneli, A. L. D., Afonso Júnior, P. C., Oliveira, G. H. H., & Valente, D. S. M. (2010). Moisture sorption isotherms and isosteric heat of sorption of coffee in different processing levels. International Journal of Food Science and Technology, 45, 2016e2022. Corrêa, P. C., Goneli, A. L. D., Jaren, C., Ribeiro, D. M., & Resende, O. (2007). Sorption isotherms and isosteric heat of peanut pods, kernels and hulls. Food Science and Technology International, 13, 231e238. Corrêa, P. C., Oliveira, G. H. H., & Santos, E. S. (2012). Thermodynamic properties of agricultural products processes. In I. Arana (Ed.), Physical properties of foods: Novel measurement techniques and applications (1st ed.) (pp 131e141). Boca Raton: CRC Press. Duggan, E., Noronha, N., O’Riordan, E. D., & O’Sullivan, M. (2008). Effect of resistant starch on the water binding properties of imitation cheese. Journal of Food Engineering, 84, 108e115. FAO e Food and Agriculture Organization. (2010). Top production e Coffee, green. García-Pérez, J. V., Cárcel, J. A., Clemente, G., & Mulet, A. (2008). Water sorption isotherms for lemon peel at different temperatures and isosteric heats. LWT, 41, 18e25. Goneli, A. L. D., Corrêa, P. C., Oliveira, G. H. H., Gomes, C. F., & Botelho, F. M. (2010). Water sorption isotherms and thermodynamic properties of pearl millet grain. International Journal of Food Science and Technology, 45, 828e838. Kaya, S., & Kahyaoglu, T. (2006). Influence of dehulling and roasting process on the thermodynamics of moisture adsorption in sesame seed. Journal of Food Engineering, 76, 139e147.
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Krug, R. R., Hunter, W. G., & Grieger, R. A. (1976a). Enthalpyeentropy compensation. 1. Some fundamental statistical problems associated with the analysis of van’t Hoff and Arrhenius data. The Journal of Physical Chemistry, 80(21), 2335e2341. Krug, R. R., Hunter, W. G., & Grieger, R. A. (1976b). Enthalpyeentropy compensation. 2. Separation of the chemical from the statistical effect. The Journal of Physical Chemistry, 80(21), 2341e2351. Leffler, J. E. (1955). The enthalpyeentropy relationship and its implications for organic chemistry. The Journal of Organic Chemistry, 20, 1202e1231. Liu, L., & Guo, Q.-X. (2001). Isokinetic relationship, isoequilibrium relationship, and enthalpyeentropy compensation. Chemical Reviews, 101, 673e695. Madamba, P. S., Driscoll, R. H., & Buckle, K. A. (1996). Enthalpyeentropy compensation models for sorption and browning of garlic. Journal of Food Engineering, 28, 109e119. Martinez, N. N., & Chiralt, A. (1996). Influence of roasting on the water sorption isotherms of nuts. Food Science and Technology International, 2, 399e404. McMinn, W. A. M., & Magee, T. R. A. (2003). Thermodynamic properties of moisture sorption of potato. Journal of Food Engineering, 60, 155e157. Moreira, R., Chenlo, F., Torres, M. D., & Vallejo, N. (2008). Thermodynamic analysis of experimental sorption isotherms of loquat and quince fruits. Journal of Food Engineering, 88, 514e521. Nkolo Meze’e, Y. N., Noah Ngamveng, J., & Bardet, S. (2008). Effect of enthalpye entropy compensation during sorption of water vapour in tropical woods: the case of Bubinga (Guibourtia Tessmanii J. Léonard; G. Pellegriniana J.L. Thermochimica Acta, 468, 1e5. OIC. (2010). Relatório mensal sobre o mercado cafeeiro. Organização Internacional do Café (7 pp.). Oliveira, G. H. H., Corrêa, P. C., Santos, E. S., Treto, P. C., & Diniz, M. D. M. S. (2011). Evaluation of thermodynamic properties using GAB model to describe the desorption process of cocoa beans. International Journal of Food Science and Technology, 46, 2077e2084. Perdomo, J., Cova, A., Sandoval, A. J., García, L., Laredo, E., & Müller, A. J. (2009). Glass transition temperatures and water sorption isotherms of cassava starch. Carbohydrate Polymers, 76, 305e313. Pradas, M. M., Sanchez, M. S., Ferrer, G. G., & Ribelles, J. L. G. (2004). Thermodynamics and statistical mechanics of multilayer adsorption. Journal of Chemical Physics, 121, 8524e8531. Samapundo, S., Devlieghere, F., De Meulenaer, B., Atukwase, A., Lamboni, Y., & Debevere, J. M. (2007). Sorption isotherms and isosteric heats of sorption of whole yellow dent corn. Journal of Food Engineering, 79, 168e175. Simal, S., Fermenía, A., Llull, P., & Rosselló, C. (2000). Dehydration of aloe vera: simulation of drying curves and evaluation of functional properties. Journal of Food Engineering, 43, 109e114. Tarigan, E., Prateepchaikul, G., Yamsaengsung, R., Sirichote, A., & Tekasakul, P. (2006). Sorption isotherms of shelled and unshelled kernels of candle nuts. Journal of Food Engineering, 75, 447e452. Timmermann, E. O., Chirife, J., & Iglesias, H. A. (2001). Water sorption isotherms of foods and foodstuffs: BET or GAB parameters? Journal of Food Engineering, 48, 19e31. Tunç, S., & Duman, O. (2007). Thermodynamic properties and moisture adsorption isotherms of cottonseed protein isolate and different forms of cottonseeds samples. Journal of Food Engineering, 81, 133e143. Wang, N., & Brennan, J. G. (1991). Moisture sorption isotherm characteristics of potato at four temperatures. Journal of Food Engineering, 14, 269e287. Wolf, W., Spiess, W. E. L., & Jung, G. (1985). Standardization of isotherm measurements (COST-project 90 and 90 bis). In D. Simatose, & J. L. Multon (Eds.), Properties of water in foods in relation to quality and stability (pp. 661e679). Dordrecht: Martinus Nijhoff. Yadav, S., Thiel, S. W., Kasting, G. B., & Pinto, N. G. (2009). Thermodynamics of water interactions with human stratum corneum. II. Interpretation via the GuggenheimeAndersonedeBoer isotherm. Chemical Engineering Science, 64, 1480e1487.
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LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Water sorption properties of coffee fruits, pulped and green coffee A.L.D. Goneli a, P.C. Corrêa b, G.H.H. Oliveira c, *, P.C. Afonso Júnior d a
Agricultural Sciences Dept., Universidade Federal da Grande Dourados, Dourados, MS, Brazil Agricultural Engineering Dept., Universidade Federal de Viçosa, Campus UFV, P.O. Box 270, 36570-000 Viçosa, MG, Brazil c Instituto Federal de Educação, Ciência e Tecnologia de Brasília, Campus Gama, Lote 01, DF 480 Setor de Múltiplas Atividades, 72405-980 Brasília, DF, Brazil d EMBRAPA e Café, Parque Estação Biológica e PqEB e s/n , 70770-901 Brasília, DF, Brazil b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 June 2012 Received in revised form 10 September 2012 Accepted 11 September 2012
Thermodynamic properties of coffee in three different forms (green, pulped and coffee fruit) were obtained during the desorption process. Desorption was accomplished by static method using saturated salt solutions, providing relative humidity range of 11e95%. Five different temperatures were utilized (10, 20, 30, 40 and 50 C). Equilibrium moisture content data were correlated by the GuggenheimeAndersonede Boer model, which presented good fit to the data, according to statistical procedures. Equilibrium moisture content ranged from 4.34 to 10.75 g/100 g dry solids. Enthalpy values for each model coefficient were estimated, ranging from 8.09 to 82.64 kJ kg1. Differential enthalpy and differential entropy increased with decreased equilibrium moisture content, which was also found for Gibbs free energy. Coffee fruits presented higher values of these thermodynamic parameters. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: GAB model Coffea arabica L. Wet method Enthalpy Entropy Gibbs free energy
1. Introduction Brazil is the main green coffee producer (47.9 million tons), followed by Vietnam, Colombia and Indonesia (FAO, 2010). According to OIC (2010), Brazil is the leader of consumption (per capita consumption 5.64 kg) among countries that exports coffee. Due to this importance, it is required the study of the different factors that affect cup quality of coffee. One factor is the different processing procedures. According to Corrêa, Goneli, Afonso Júnior, Oliveira, and Valente (2010), coffee can be processed in two different ways: ‘dry method’ or natural form and ‘wet method’ or hulling. In the dry process, the entire cherry after harvest, unwashed in natural form, is first cleaned and then placed in the sun to dry on tables or in thin layers on patios. In the Wet Process, the skin and some pulp covering the seeds/beans is removed before they are dried. This is done either by the classic ferment-and-wash method or a newer procedure called machineassisted wet processing, aquapulping or mechanical demucilaging. The processing procedure also affects post-harvest techniques, in which to correctly conduct drying and storage operations, it is
* Corresponding author. Tel.: þ55 61 21032259. E-mail addresses: [email protected] (A.L.D. Goneli), [email protected] (P.C. Corrêa), [email protected] (G.H.H. Oliveira), paulo.junior@ embrapa.br (P.C. Afonso Júnior). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.lwt.2012.09.006
necessary to know the relationship between air temperature (T) and relative humidity (RH), and desirable conditions for preserving the product. To obtain this information, sorption isotherms are indispensable. Several mathematical models are used to describe the sorption isotherms. However, the GuggenheimeAndersonede Boer (GAB) model was recommended by the European Project Group COST 90 (Wolf, Spiess, & Jung, 1985) as a fundamental equation to characterize water sorption in foodstuff. The advantages of this model, when compared to the others, are its theoretical background and the physical meaning of its parameters (Oliveira, Corrêa, Santos, Treto, & Diniz, 2011). Sorption isotherms are also efficient tools to determine thermodynamic interactions between water and food components. Thermodynamic properties provide useful information for the development and improvement of dryers and for studies on water properties on food surface (Corrêa, Goneli, Jaren, Ribeiro, & Resende, 2007). Thermodynamic data for coffee in different processing procedures are scarce in the literature and is required to develop and design post-harvest equipment. In this context, the aim of this study was to obtain the desorption isotherms of coffee in two different processing levels (dry and wet method), at different temperatures. Thermodynamic functions of enthalpy and entropy differentials, and Gibbs free energy were determined. The theory of enthalpyeentropy compensation was applied.
A.L.D. Goneli et al. / LWT - Food Science and Technology 50 (2013) 386e391
2. Material and methods 2.1. Raw material Coffee fruits (Coffea arabica L), variety “Catuai Vermelho” were manually harvested during cherry stage. After harvested, fruits were washed and selected for characterization, and three lots were obtained. The first lot was kept as coffee fruits (group 1); the second lot of coffee fruits was hulled (pulped coffee, group 2), while the rest was processed (green coffee, group 3). During harvest, unripened, deteriorated and injured coffee beans were eliminated. Due to the high initial moisture content, coffee fruits and pulped coffee were partially dried outdoors. In order to reduce the risk of microorganisms development during experimental hygroscopic test, coffee fruits and pulped coffee were dried until moisture content reached 45 g/100 g dry solids. For green coffee tests, coffee of approximately 11% g/100 g dry solids was used. They were obtained by low temperature drying from a pulped coffee portion using an air conditioning unit manufactured by Aminco, model Aminco-Aire 150/300 (Silver Spring, Maryland e USA). At the end of drying process, the material had its parchment and silver pellicle removed by hand, and then it was submitted to analyses. Two samples were stored in polythene bags kept in a refrigerator (5 1 C) to attain moisture uniformity during 24 h. When experiments were to be made, samples were left in room conditions (25 3 C) for 6 h to achieve moisture equilibrium. According to Seeds Analysis Standard of Brazil (Brazil, 1992), moisture content of coffee fruits, pulped coffee and green coffee was determined by applying the oven method at 105 1 C for 24 h, in duplicate.
Xm CKaw ð1 Kaw Þð1 Kaw þ CKaw Þ
P ¼
n Y Y 100 X n i¼1 Y
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 i ¼ 1 ðY YÞ SE ¼ DF
(1)
In which: Xeq ¼ equilibrium moisture content, g/100 g dry solids; Xm ¼ monolayer moisture content, g/100 g dry solids; C, K ¼ model constants that are related to monolayer and multilayer properties, respectively (van den Berg, 1984), dimensionless and, aw ¼ water activity, decimal.
(2)
(3)
In which: P is the mean relative percent deviation, %; n is the number of observed data; SE is the standard error, g/100 g dry b is the estimated value through the solids; Y is the observed value; Y model; and DF is the degrees of freedom of the model. 2.3. Thermodynamic properties After achieving the model coefficients, the enthalpy values for each GAB parameter were calculated using a simple regression procedure. It requires the representation of ln Xm, ln C and ln K vs. the inverse of absolute temperature, as stated by Simal, Fermenía, Llull, and Rosselló (2000).
DH1
C ¼ C0 exp
(4)
RT
K ¼ K0 exp
Equilibrium moisture content (EMC) of green, pulped and coffee fruit were determined using the static method, which is based on using saturated salt solutions to maintain constant water activity of samples once equilibrium between room conditions and food sample is reached. Salt solutions used to obtain constant relative humidity were NH4Cl, KBr, KNO2, KNO3, K2SO4, CaCl2, Ca(NO3)2, Na2Cr2O7, MgCl2$6H2O, LiCl, LiCl$H2O and NaCl. This group of salts allows obtaining a wide range of relative humidity, from 11% to 95% RH in equilibrium. In order to determine sorption isotherms of green coffee, pulped coffee and coffee fruit the analysis was conducted at five different temperatures: 10, 20, 30, 40 and 50 C. Water activity was also measured at these five temperatures. To acquire data for drawing sorption curves, three samples containing 100 g of coffee at different processing levels and combinations of temperature and water activity were left in room conditions until reaching static equilibrium. For this purpose, samples were hanged in small baskets inside a 2.0 L glass container with 250 mL of salt solutions, hermetically closed, and kept in five climacteric boxes of regulated temperature. Baskets were weighted daily until mass variation was lower than 1 mg. Then, their moisture contents were determined, leading to the obtainment of equilibrium moisture content for each treatment condition. Experimental data of equilibrium moisture content of green, pulped and coffee fruit were fitted to GAB model (Equation (1)).
Xeq ¼
The GAB parameters were estimated using a nonlinear regression by GausseNewton approximation method, which minimizes the sum of square errors in a series of interactive stages. The adequacy of the model was analyzed based on the values of mean relative percent deviation (P), the standard error (SE), determination coefficient (R2) and residual plots:
2.2. Equilibrium moisture content
387
DH2
(5)
RT
Xm ¼ X0 exp
DH3
(6)
RT
In which: C0 and K0 are parameters from equations (4) and (5), dimensionless; X0 is the parameter from equation (6), decimal dry solids; T is the temperature, K; DH1, DH2 and DH3 are the enthalpies from the model coefficients, kJ kg1; and R is the water vapor constant, 0.462 kJ kg1 K1. Remaining thermodynamic parameters, such as differential entropy of desorption (DS), differential enthalpy (DH), Gibbs free energy (DG) and enthalpyeentropy relationship, were obtained by means of a known methodology, stated by Corrêa, Oliveira, and Santos (2012), which an approximate (1 a)100% confidence interval for isokinetic temperature is used. These parameters are expressed respectively by Equations (7)e(11).
DHst DS ln aw ¼ R RT
(7)
DH ¼ DHst DHvap
(8)
DG ¼ RT ln aw
(9)
bB t TB ¼ T m2;a=2 Thm ¼
nt 1 i¼1 T i
Pnt
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi VarðTB Þ
(10) (11)
In which: DH: isosteric heat of sorption, kJ kg1; DHvap: latent heat of vaporization of pure water, kJ kg1; DHst: net isosteric heat
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of sorption, kJ kg1; DS: differential entropy of sorption, kJ kg1 K1; and DG: Gibbs free energy, kJ kg1 mol1; TB: isokinetic temperature, K; m: number of data pairs of enthalpy and entropy; t is the t value at (m 2) degrees of freedom; Thm: harmonic mean temperature, K; and nt: number of temperatures utilized. 3. Results and discussion 3.1. Mathematical modeling of sorption Statistical parameters in order to analyze model adequacy are presented in Table 1. GAB model presented random residual plots of EMC experimental data of green, pulped and coffee fruit at all temperatures investigated at the present work. Also, low values of P and SE were encountered along with high values of R2. These results indicate that GAB model is adequate to estimate equilibrium moisture content of coffee in its different forms of processing. Several researchers applied the GAB model in order to determinate sorption isotherms of different agricultural products, and its different types of processing procedures (García-Pérez, Cárcel, Clemente, & Mulet, 2008; Kaya & Kahyaoglu, 2006; Oliveira et al., 2011; Samapundo et al., 2007; Tarigan, Prateepchaikul, Yamsaengsung, Sirichote, & Tekasakul, 2006). Corrêa et al. (2010), also working with coffee beans provided by different processing procedures, concluded that the GAB model, Modified Henderson and ANN model were adequate to predict EMC values. According to Oliveira et al. (2011), the number of water molecules that are strongly adsorbed in specific sites at the food surface are indicated by the monolayer moisture content. This parameter tended to decrease as temperature increased (Table 1), varying between 10.75% and 4.34 g/100 g dry solids According to Perdomo et al. (2009), this can be explained by the number of active sites that are reduced with increased temperature, resulting into modifications of the physical and chemical characteristics of the product. The monolayer moisture content is the safest moisture content for storage purposes, providing increased time period with minimum quality loss at a certain temperature during preservation. Values below monolayer moisture content slow down deteriorative
Table 1 Parameter estimated values, mean relative error (P), standard error (SE) of estimate, determination coefficient (R2) and residual plots for GAB model to coffee sorption in different processing levels. Temperature ( C)
Parameters
R2 (%)
SE (g/100 g dry solids)
P (%)
Residual plots
Xm
C
K
Coffee fruits 10 20 30 40 50
10.75 10.14 9.63 9.31 7.64
22.30 32.08 14.24 12.67 13.23
0.76 0.75 0.76 0.75 0.82
99.75 99.72 99.95 99.92 99.66
0.40 0.33 0.15 0.18 0.34
2.05 1.30 0.98 1.24 2.78
Random Random Random Random Random
Pulped coffee 10 20 30 40 50
9.99 8.36 7.39 7.13 4.34
6.16 6.55 5.71 5.76 12.29
0.67 0.73 0.79 0.74 0.92
99.69 99.76 99.89 99.65 99.07
0.35 0.27 0.19 0.28 0.43
2.86 1.04 1.74 3.41 5.85
Random Random Random Random Random
Green coffee 10 20 30 40 50
7.92 7.60 7.54 6.37 5.67
12.59 9.99 8.29 11.70 9.69
0.74 0.75 0.75 0.77 0.81
99.89 99.54 99.95 99.77 98.82
0.19 0.35 0.12 0.21 0.47
1.44 1.30 1.05 2.31 5.96
Random Random Random Random Random
reactions, pest attacks and respiration rate of the product. Water activity values corresponding to the monolayer moisture content of coffee were obtained from the data presented in Table 1 and is tabulated in Table 2. It can be noticed that coffee fruits had lower values of aw than pulped coffee and green coffee, meaning that its storage is more difficult to accomplish at these temperatures, requiring more energy and care. This fact is expected since that the presence of hulls on coffee fruits ends up on increased energy to dry this product, increased time to this product reach equilibrium with the surrounding environment. In addition, coffee fruits also have mucilage, a component of high sugar content, leading to a high probability of microorganism and insect attack. These facts also may explain the non-patterned values of water activity for increased temperatures. Water activity of green coffee may be assumed to be constant at all temperatures investigated (Table 2), due to absence of hull and mucilage. Pulped coffee presented a decrease on water activity values with temperature increase, probably due to the presence of mucilage. According to Timmermann, Chirife, and Iglesias (2001), the C constant is associated with the chemical potential differences between the monolayer and superior layers. No defined behavior could be concluded for this parameter. Probably this fact is due to the heterogeneous characteristic of coffee product, which components variance among layers may lead to the trend noticed, observed both for the same product but between temperatures and between different products. Duggan, Noronha, O’Riordan, and O’Sullivan (2008) reported that the K constant is correlated to the chemical potential difference between the monolayer and free water state. This parameter presented a slightly increase at temperature of 50 C to the same product, being considered constant for the remaining temperatures. Higher levels of temperature (50 C) lead to a rapid water molecules transition between multilayer and free water. C0, K0 and X0 and the enthalpies associated with each GAB model constant (DH1, DH2 and DH3) are shown in Table 3. According to Yadav, Thiel, Kasting, and Pinto (2009), the preexponential factors C0 and K0 are expressions of entropic effects in the sorption process. Values of C0 were higher for pulped coffee, followed by green coffee and coffee fruit (Table 3). Furthermore, pulped coffee presented a value of C0 bigger than one and the remaining products lower than one. A value of C0 less than one is expected if the mobility of molecules within the first sorbed layer is restricted (Pradas, Sanchez, Ferrer, & Ribelles, 2004). Thus, we can conclude that the mobility of molecules within the first layer of pulped coffee occurs freely. This trend may be due to the absence of the hull but also the presence of the mucilage, in which hull are present on coffee fruits and mucilage is absent on green coffee. Values of K0 were similar for coffee fruit and green coffee, being significantly higher for pulped coffee, due to the same reason stated previously. X0 is related to the number of water molecules sorbed in the first layer. This parameter presented higher values for coffee fruits and
Table 2 Corresponding water activity (aw) values for safe storage of green, pulped and coffee fruits at different temperatures. Product
Temperature ( C) 10
20
30
40
50
Coffee fruits Pulped coffee Green coffee
0.229 0.429 0.297
0.201 0.383 0.319
0.276 0.375 0.344
0.291 0.399 0.294
0.264 0.242 0.301
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Parameters C0
K0
X0
D H1
DH2
DH3
Coffee fruit Pulped coffee Green coffee
0.12 2.23 0.95
1.37 5.14 1.15
0.56 0.03 0.05
16.24 13.61 31.91
8.09 5.60 5.60
35.00 75.95 82.64
were similar among the remaining products investigated (Table 3). This trend is probably due to the fact that coffee fruits are not processed and therefore its hull is not ruptured, allowing water molecules to transfer between the first layer and the subsequent layers without alteration in its normal functionality. For green coffee and pulped coffee, due to the processing procedure, this behavior is not present. Analyzing Table 3, it can be noticed that enthalpy values were positive to the X0 and C0 constants, which indicates endothermic reactions, or in other words, reactions that require the absorption of energy from the environment in which products are inserted. This fact is expected, because during drying, the product absorbs energy from the environment in order to reach equilibrium with it, through heating (sensible heat) and later through the evaporation of free water (latent heat of vaporization). At the same table, enthalpy values for K0 are negative. This signal indicates an energy release from the several layers that form the multilayer. This tendency may be explained by energy release as heat flow, because of the increasing level of molecular vibration of water molecules during drying. X0 presented the highest enthalpy values (Table 3). This trend is expected, because the monolayer moisture content indicates the amount of water molecules that are strongly adsorbed by specific sites from the food surface; thus, significantly greater energy is required to remove these water molecules from the monolayer. According to McMinn and Magee (2003), knowledge of differential enthalpy at a given moisture content indicates the state of water that is sorbed within the product, serving as a measurement of physical, chemical and biological stability of the product during storage. 3.2. Differential enthalpy Fig. 1 presents the values of differential enthalpy according to the equilibrium moisture content. According to Fig. 1, it is observed a reduction of coffee EMC in its different types of form. The required energy to remove water from the product is higher in lower levels of EMC. Elevated values of differential enthalpy at low values of EMC can be explained by the differences of bonding forces between moisture and adsorbent surface of the product. At initial stages of sorption (low values of moisture content), there are polar sorption sites highly active, of elevated interaction energy, which are covered by water molecules, forming the monomolecular layer (Al-Muhtaseb, McMinn, & Magee, 2004). Throughout time, which water molecules are being chemically connected to the sorption sites highly active, sorption starts to occur at sites less active (higher moisture content), with lower interaction energy and, consequently, lower differential enthalpy (Wang & Brennan, 1991). Values of differential enthalpy of coffee fruits are higher than those found for green and pulped coffee, at all EMC range. These results are similar to those found by Martinez and Chiralt (1996), working with hazelnuts, peanuts and almond. According to these authors, it was because of enhancement of lipidelipid interaction
Differential enthalpy (kJ kg-1)
Enthalpy (kJ kg1)
Processing level
1800 1500 1200 900 600 300 0 0
5
10
15
20
25
30
Equilibrium moisture content (g/100g d.b.) Fig. 1. Observed and estimated values (d) of differential enthalpy in function of equilibrium moisture content of green (C), pulped (,) and coffee fruits (6).
after dehulling of these products, which would increase the hydrophobicity of cellular components. Furthermore, because of the low hygroscopicity of the layer that involves pulped coffee, values of differential enthalpy of green and pulped coffees were similar.
3.3. Differential entropy Fig. 2 presents differential entropy of sorption values (kJ kg1 K1), in function of equilibrium moisture content (g/100 g dry solids) for green, pulped and coffee fruit. According to Fig. 2, there is a notable relationship between differential entropy and equilibrium moisture content, similar behavior observed in differential enthalpy values. Coffee fruits also presented higher values of differential entropy than pulped and green coffee. The number of available desorption sites corresponding to a specific energy level is proportional to the differential entropy, which describes the degree of disorder and motion randomness of water molecules. Therefore, DS values of coffee fruits are expected since this product possesses the hull, an increase of area where water molecules may move, in which leads to higher number of
5 Differential entropy (kJ kg-1 K-1)
Table 3 Calculated values of GAB model constants (C0, K0 and M0) and its respective enthalpy values.
389
4 3 2 1 0 0
5
10
15
20
25
30
Equilibrium moisture content (g/100g d.b.) Fig. 2. Observed and estimated values (d) of differential entropy of green (C), pulped (,) and coffee fruits (6).
390
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desorption sites at a certain EMC value, regarding green and pulped coffee. Comparable results were reported by Al-Muhtaseb et al. (2004), working with starch powder and by Kaya and Kahyaoglu (2006), analyzing sesame seeds. Increasing values of equilibrium moisture content leads to a decrease of differential entropy values, tending to become constant at high values of EMC (Fig. 2). Based on the second law of thermodynamics, a process is reversible when the sum of all entropy changes for all subsystems in a process is constant. Thus, coffee sorption in its different types of processing is irreversible (hysteresis effect), because entropy is produced during the process (Madamba, Driscoll, & Buckle, 1996). However, at high values of EMC in which differential entropy tends to become constant, sorption process may be reversible.
3.4. Enthalpyeentropy compensation theory Fig. 3 presents the relationship between differential enthalpy and differential entropy values of coffee sorption. It can be noticed linear relationships for sorption of coffee, presenting determination coefficients superior to 99%. This high level of linearity between these parameters allows us to indicate that the enthalpyeentropy compensation theory for green, pulped and coffee fruits is probably valid. In order to confirm the enthalpyeentropy compensation theory, isokinetic temperature was compared to the mean harmonic temperature (Thm). According to Krug, Hunter, and Grieger (1976a, 1976b) linear chemical compensation only exists if isokinetic temperature is different from the mean harmonic temperature. Values of isokinetic temperature and Gibbs free energy at this temperature were determined by means of linear regression, and are presented at Table 4. Thm was 302.50 K, value significantly different from all values of TB reported (Table 4). Therefore, enthalpyeentropy compensation theory is valid. According to Liu and Guo (2001), isokinetic temperature is the one in which all reactions in series happens at the same time, in other words, when the product is at an equilibrium state. If TB > Thm, the process is enthalpy controlled; if TB < Thm, the process is entropy controlled (Leffler, 1955). The first condition is in accordance with the results of the present research, thus the sorption mechanism in green, pulped and coffee fruits are enthalpy controlled. This result is in agreement with several researchers that applied the isokinetic theory over sorption of different products (Goneli, Corrêa, Oliveira, Gomes, & Botelho, 2010; Moreira, Chenlo,
-1
Differential enthalpy (kJ kg )
1800 1500 1200 900 600 300 0 0
1
2
3
4 -1
5
-1
Differential entropy (kJ kg K ) Fig. 3. Enthalpyeentropy relationship of green (C), pulped (,) and coffee fruits (6).
Table 4 Isokinetic temperature (TB) and Gibbs free energy at isokinetic temperature (DGB) of coffee in different types of form. Processing level Coffee fruit Pulped coffee Green coffee
DGB (kJ kg1)
TB (K) 391.50 0.001 414.09 0.002 432.39 0.002
4
5.89 10 3.42 103 8.05 104
R2 (%) 99.99 99.99 99.99
Torres, & Vallejo, 2008; Nkolo Meze’e, Noah Ngamveng, & Bardet, 2008; Oliveira et al., 2011; Tunç & Duman, 2007). 3.5. Gibbs free energy Gibbs free energy is related to the work needed regarding the product layers to become available to sorption (Nkolo Meze’e et al., 2008); therefore, in higher values of equilibrium moisture content, there will be fewer sites available to sorption. Being that stated DG values are expected, because in higher values of moisture content, there is lower necessity of work to make the sites available to sorption, because they are already available. This trend is observed by lower values of DG. It is observed that temperature influence over Gibbs free energy is higher at lower values of EMC, in which lower temperatures provides higher values of DG. This trend can be correlated to the vibration of water molecules within the product, where at high temperatures this vibration (movement) is higher; this leads to lower work required to the product layers to become available to sorption. At high levels of EMC, temperature influence becomes negligible due to the sorption sites are already available. Positive values of DG are also expected, as it characterizes endothermic reaction, or else, reactions that require energy from the environment to occur. Green and pulped coffee presented similar values of Gibbs free energy, while coffee fruit presented higher values of this parameter. This trend may be explained by the presence of the hull in coffee fruits, as stated before at the differential entropy discussion. DG values become similar between different types of coffee in high levels of EMC (above 20%). Table 5 contains the regression equations for Gibbs free energy, for each temperature and coffee type studied in the present work and their respective values of determination coefficient. Table 5 Regression equations for Gibbs free energy (DG) in different temperatures and coffee type analyzed, related to equilibrium moisture content (Xeq). Temperature ( C)
Equation
R2 (%)
Coffee fruits 10 20 30 40 50
DG ¼ 839.24 exp(0.13Xeq) DG ¼ 720.24 exp(0.13Xeq) DG ¼ 653.44 exp(0.13Xeq) DG ¼ 612.83 exp(0.13Xeq) DG ¼ 587.76 exp(0.14Xeq)
99.84 99.96 99.89 99.42 99.18
Pulped coffee 10 20 30 40 50
DG ¼ 572.24 exp(0.16Xeq) DG ¼ 517.51 exp(0.16Xeq) DG ¼ 488.08 exp(0.17Xeq) DG ¼ 461.72 exp(0.17Xeq) DG ¼ 455.24 exp(0.18Xeq)
99.67 99.98 99.54 99.27 97.44
Green coffee 10 20 30 40 50
DG ¼ 658.22 exp(0.17 Xeq) DG ¼ 598.69 exp(0.17 Xeq) DG ¼ 556.99 exp(0.17Xeq) DG ¼ 543.63 exp(0.18Xeq) DG ¼ 518.54exp(0.18Xeq)
99.92 99.96 99.95 99.21 99.17
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4. Conclusions Modeling and the thermodynamic parameters were estimated for desorption process of green, pulped and coffee fruits, processed by two different procedures. Evaluation of these parameters is of fundamental importance to the correct preservation of the product, as well providing parameters to design and develop the several equipments utilized on the main post-harvest operations. According to the results, it can be concluded that desorption process was well represented by the GAB model. Also, monolayer moisture content decreased with increased temperature of drying. Water activity values at five different storage temperatures were acquired. Enthalpy values related to two GAB coefficients were positive, which indicates endothermic reactions; on the other hand, one coefficient from GAB model was negative, indicating an energy release from the several layers that form the multilayer. Differential enthalpy, differential entropy and Gibbs free energy presented high correlation with the equilibrium moisture content of green, pulped and coffee fruits, and the enthalpyeentropy compensation theory exists. Coffee fruits presented higher values of differential enthalpy, differential entropy and Gibbs free energy due to the presence of the hull. References Al-Muhtaseb, A. H., McMinn, W. A. M., & Magee, T. R. A. (2004). Water sorption isotherms of starch powders. Part 2: thermodynamic characteristics. Journal of Food Engineering, 62, 135e142. van den Berg, C. (1984). Description of water activity of foods for engineering purposes by means of the GAB model of sorption. In B. M. McKenna (Ed.), Engineering and food (pp. 311e321). New York: Elsevier Applied Science. Corrêa, P. C., Goneli, A. L. D., Afonso Júnior, P. C., Oliveira, G. H. H., & Valente, D. S. M. (2010). Moisture sorption isotherms and isosteric heat of sorption of coffee in different processing levels. International Journal of Food Science and Technology, 45, 2016e2022. Corrêa, P. C., Goneli, A. L. D., Jaren, C., Ribeiro, D. M., & Resende, O. (2007). Sorption isotherms and isosteric heat of peanut pods, kernels and hulls. Food Science and Technology International, 13, 231e238. Corrêa, P. C., Oliveira, G. H. H., & Santos, E. S. (2012). Thermodynamic properties of agricultural products processes. In I. Arana (Ed.), Physical properties of foods: Novel measurement techniques and applications (1st ed.) (pp 131e141). Boca Raton: CRC Press. Duggan, E., Noronha, N., O’Riordan, E. D., & O’Sullivan, M. (2008). Effect of resistant starch on the water binding properties of imitation cheese. Journal of Food Engineering, 84, 108e115. FAO e Food and Agriculture Organization. (2010). Top production e Coffee, green. García-Pérez, J. V., Cárcel, J. A., Clemente, G., & Mulet, A. (2008). Water sorption isotherms for lemon peel at different temperatures and isosteric heats. LWT, 41, 18e25. Goneli, A. L. D., Corrêa, P. C., Oliveira, G. H. H., Gomes, C. F., & Botelho, F. M. (2010). Water sorption isotherms and thermodynamic properties of pearl millet grain. International Journal of Food Science and Technology, 45, 828e838. Kaya, S., & Kahyaoglu, T. (2006). Influence of dehulling and roasting process on the thermodynamics of moisture adsorption in sesame seed. Journal of Food Engineering, 76, 139e147.
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