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Modeling of desorption of Alfalfa (Medicago sativa) stems and leaves
Industrial Crops and Products 34 (2011) 1550–1555
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Modeling of desorption of Alfalfa (Medicago sativa) stems and leaves A. Arabhosseini a,∗ , W. Huisman b , J. Müller c a
College of Abouraihan, University of Tehran, P.O. Box 11365-4117, Tehran, Iran Farm Technology Group, Wageningen University, The Netherlands c Institute of Agricultural Engineering, University of Hohenheim, Stuttgart, Germany b
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 16 May 2011 Accepted 17 May 2011 Available online 12 June 2011 Keywords: Alfalfa Desorption Drying Medicago sativa Postharvesting
a b s t r a c t The equilibrium moisture content of agricultural products is necessary to optimize drying process and helps to keep the quality of the product during the period of storage. The main aim of this research was to find the best model which could define well, the exchange of moisture between alfalfa (Medicago sativa) and the surrounding air. The desorption isotherms of alfalfa (stem and leaf) were determined separately by using the saturated salt solutions method at three temperatures (25, 50 and 70 ◦ C) within a range of 5–90% relative humidity. Experimental curves of desorption isotherms were fitted to modified equations of Henderson, Halsey, Oswin and Chung-Pfost as well as the GAB model and then evaluated visually by using residual plots and also by some statistical error parameters. The modified Halsey model was found to be the most suitable for describing the relationship between equilibrium moisture content, relative humidity and temperature. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Alfalfa (Medicago sativa L.) is an excellent forage crop due to its digestibility, protein content and other nutrients (Deshpande et al., 2002). Alfalfa leaves are low in fiber and high in protein and carotenoids, making it suitable for feeding to monogastrics such as poultry and swine, as well as a key ingredient in high protein human health supplements. Alfalfa stems are high in fiber and can be used for ruminant feed, paper and cardboard and energy production (biofuel/ethanol) (Adapa et al., 2007). Alfalfa is cut, chopped, dried and densified to reduce handling, transportation and storage costs, especially for export. Baling alfalfa hay at high moisture content can cause negative quality (Coblentz et al., 1998). Fractional drying of alfalfa leaves and stems is required because alfalfa leaves and stems dry at significantly different rates resulting in over-dried leaves and under-dried stems (Arinze et al., 1999). There are definite market incentives to produce separate leaf and stem components of alfalfa. Alfalfa leaves are also used as herbal medicine to treat various diseases, e.g. diabetes, thyroid conditions, arthritis, high blood cholesterol levels (Medical Economic Company Inc., 2002). Fractionation is the term expressing the separation of forage into two or more component parts (usually leaves and stems). This process enables a later, controlled recombination of these components to achieve specific market nutritional require-
∗ Corresponding author. Tel.: +98 292 30 40 614; fax: +98 292 30 40 730. E-mail address: [email protected] (A. Arabhosseini). 0926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2011.05.018
ments for animal feeds or the potential use of the more valuable leaf material in the rapidly expanding eutraceutical and herbal markets (Adapa et al., 2004). Drying is an essential component of a forage/alfalfa processing industry. It is performed to reduce high moisture level of alfalfa to a safe limit for storage (usually 10% wet basis). Artificial high-temperature drying is required due to climate and seasonal constraints in the most of the Prairie Provinces in North America (Dalai et al., 2006). Moisture sorption isotherm defines the relation between equilibrium relative humidity (ERH) and equilibrium moisture content (EMC) (Soysal and Oztekin, 1999). This knowledge is required to stop the drying process at the aimed moisture content (Hamer et al., 2000) to save energy. As the drying rate for stems and leaves are different so the required time for drying of stems and leaves are different too. When drying whole plants, the leaves will be over-dried and efficiency of the dryer will be less and the energy consumption will be higher (Arabhosseini et al., 2008). Drying of the leaves and stems separately will help to optimize the process. The EMC of stems and leaves are needed to stop drying process in proper time. Not much information is available about desorption isotherms of alfalfa stems and leaves separately. As desorption isotherm is one of the basic parameters of the drying process, this work will give an important contribution to the optimization of the drying of alfalfa. The objective of this study is to find the desorption isotherm curve of alfalfa (stem and leaf) at temperatures between 25 and 70 ◦ C for ERH values in the range of 5–90% and to find appropriate mathematical models to represent it.
A. Arabhosseini et al. / Industrial Crops and Products 34 (2011) 1550–1555
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Fig. 1. The equipment for the desorption isotherm measurement: (1) cryostat; (2) water bath; (3) jars with cap including netballs with samples, thymol and saturated salt solution; and (4) water pump. Table 1 Saturated salts used for experiments and ERH at different temperatures (Greenspan, 1976; Labuza et al., 1985). Salt formula
1 2 3 4 5 6 7 8 9 10 11 12
KCl KBr NaCl NaNO3 KI NaBr K2 CO3 MgCl2 LiI LiCl ZnBr2 LiBr
Leaves
Equilibrium relative humidity (ERH) (%) T = 25 ◦ C
T = 50 ◦ C
T = 70 ◦ C
84.34 80.89 75.29 74.25 68.86 57.57 43.16 32.78 17.56 11.30 7.75 6.37
81.20 79.02 74.43 69.04 64.49 50.93 40.91 30.54 12.38 11.10 7.70 5.53
79.49 79.07 75.06 66.04 61.93 49.70 37.37 27.77 7.23 10.7 8.72 5.23
EMC, d.b. (decimal)
No.
0.2
0.16
Stems
0.12
0.08
0.04
0 0
0.2
0.4
0.6
0.8
1
ERH, (decimal) Fig. 2. Desorption isotherm data of alfalfa stems and leaves at 70 ◦ C and the fitted the modified Oswin equation.
2. Materials and methods 2.1. Samples Alfalfa stems and leaves were examined in this research. The crops were harvested in summer. Stems and leaves were separated immediately after harvesting. The stems were chopped to 20–30 mm long pieces. The samples of 8–10 g fresh leaves and 15–17 g fresh stem were used for desorption tests. 2.2. Experimental procedure The apparatus for providing various ERHs consists of an insulated water bath (56 cm × 36 cm × 26 cm) with 12 glass jars and
a cryostat, which includes a cooling and heating system with an accuracy of 0.1 ◦ C (Fig. 1). The water circulates between cryostat and water bath to prepare a constant temperature of water around the glass-jars. The glass jars are closed with a cap. Every glass jar of 800 ml contains 150 ml of saturated salt solution. A plate with base prevents contact of sample and salt solution. Stainless steel netballs were used to put the stems and leaves separately in the jars. In tests with relative air humidity above 60% crystalline thymol was placed in the bottle to prevent microbial spoilage (Menkov, 2000). The cryostat was separately adjusted to 25, 50 and 70 ◦ C. Every three days, the samples were weighed with an accuracy
Table 2 Estimated values and statistical parameters of four equations fitted to desorption isotherm data of alfalfa stems and leaves at three temperatures 25, 50 and 70 ◦ C. Parameters
Estimated values and the variance of the models and statistical parameters Sorption models
Stems C1 C2 C3 RSS SEE MRD R2 Residual Leaves C1 C2 C3 RSS SEE MRD R2 Residual
Henderson
Halsey
Oswin
Chung-Pfost
12.03 ± 2.93 × 10−2 1.133 ± 0.091 13.277 ± 6.95 0.0315 0.0305 0.2673 0.9324 Systematic
−2.181 ± 0.0578 −2.24 ± 0.15 × 10−2 1.351 ± 0.047 0.0092 0.0165 0.100 0.9814 Random
24.34 ± 0.85 × 10−2 −23.36 ± 1.34 × 10−4 1.817 ± 0.072 0.0096 0.0168 0.137 0.9816 Random
10.19 ± 0.85 −2.57 ± 7.17 143.4 ± 27.6 0.0765 0.0474 0.461 0.8303 Systematic
38.25 ± 9.14 × 10−3 0.715 ± 0.070 46.896 ± 15.10 0.065 0.0437 0.416 0.9269 Systematic
−1.879 ± 0.054 −1.65 ± 0.15 × 10−2 1.042 ± 0.047 0.0207 0.0247 0.193 0.9741 Random
21.06 ± 1.57 × 10−2 −19.67 ± 2.16 × 10−4 1.304 ± 0.081 0.0328 0.0311 0.292 0.9615 Systematic
8.559 ± 0.91 3.68 ± 12.64 147.8 ± 41.3 0.1787 0.0725 0.524 0.7193 Systematic
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A. Arabhosseini et al. / Industrial Crops and Products 34 (2011) 1550–1555
Modified Henderson
Modified Halsey
0 .5
0 .5 25 °C
50 °C
EMC, d.b. (decimal)
EMC, d.b. (decimal)
25 °C
0 .4
70 °C
0 .3 0 .2 0 .1 0
0
0 .5
70 °C
0 .3 0 .2 0 .1 0
1
50 °C
0 .4
0
ERH, (decimal) Modified Oswin 0 .5 25 °C
25 °C
50 °C
0 .4
EMC, d.b. (decimal)
EMC, d.b. (decimal)
1
Modified Chung-Pfost
0 .5
70 °C
0 .3 0 .2 0 .1 0
0 .5
ERH, (decimal)
0
0 .5
0 .4
50 °C 70 °C
0 .3 0 .2 0 .1 0 .0 0 .0
1
ERH, (decimal)
0 .5
1 .0
ERH, (decimal) GAB equation
0 .5
EMC, d.b. (decimal)
25 °C 50 °C
0 .4
70 °C
0 .3 0 .2 0 .1 0
0
0 .5
1
ERH, (decimal) Fig. 3. Desorption isotherms of alfalfa leaves at 25 (), 50 (♦) and 70 ◦ C () and the fitted curves of the modified Henderson, modified Halsey, modified Oswin, modified Chung-Pfost and GAB equations.
±0.1 mg. Equilibrium was acknowledged when three consecutive weight measurements showed a difference less than 1 mg. Three replications were taken for each test. When equilibrium was reached (approximately after 3–5 weeks depending on the temperature and relative humidity), the samples were dried using oven method (at 105 ◦ C for 24 h) in order to obtain dry matter contents (Park et al., 2002). The EMCs were determined by triplicate measurements of each sample.
2.3. Salt solution The saturated salt solutions method is based on the fact that ERHs of specific salt solutions are a well known physical property (Greenspan, 1976). Table 1 gives the equivalent of relative humidity of the selected salt solutions at three temperatures. Salt solutions were provided by mixing and dissolving of the salt crystals in distilled water at the temperatures of 45, 70 and 90 ◦ C which these
temperatures were 20 ◦ C higher than the temperature of each test in the water bath, to guarantee a saturated solution. The salt solutions were agitated from time to time to avoid the formation of a condensation gradient in the liquid phase (Kouhila et al., 2001). 2.4. Desorption models A number of models have been suggested in literature to describe the relationship between EMC and ERH. The modified Henderson, Oswin, Halsey, Chung-Pfost and GAB equations (Chen, 2002) have been adopted by the American Society of Agricultural Engineers as standard equations for describing sorption isotherms of agricultural products (ASAE, 2003). The equations have been transformed to get EMC as dependent parameter and ERH as independent variable. Modified Henderson EMC =
−
1/C3
1 ln(1 − ERH) C1 (T + C2 )
(1)
A. Arabhosseini et al. / Industrial Crops and Products 34 (2011) 1550–1555
Modified Henderson
Modified Halsey
0.5
0.5 25 °C
EMC, d.b. (decimal)
EMC, d.b. (decimal)
25 °C 50 °C
0.4
70 °C
0.3 0.2 0.1 0
0
0.5
70 °C
0.3 0.2 0.1 0
1
50 °C
0.4
0
0.5
ERH, (decimal) Modified Oswin
Modified Chung-Pfost 0.5 25 °C
EMC, d.b. (decimal)
25 °C
EMC, d.b. (decimal)
1
ERH, (decimal)
0.5 50 °C
0.4
70 °C
0.3 0.2 0.1 0
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0
0.5
70 °C
0.3 0.2 0.1 0
1
50 °C
0.4
0
0.5
ERH, (decimal)
1
ERH, (decimal) GAB equation
0.5
EMC, d.b. (decimal)
25 °C 50 °C
0.4
70 °C
0.3 0.2 0.1 0
0
0.5
1
ERH, (decimal) Fig. 4. Desorption isotherms of alfalfa stems at 25 (), 50 (♦) and 70 ◦ C () and the fitted curves of the modified Henderson, modified Halsey, modified Oswin, modified Chung-Pfost and GAB equations.
Modified Halsey EMC =
−exp(C + C T ) 1/C3 1 2 ln(ERH)
ERH 1/C3
Modified Oswin EMC = (C1 + C2 T )
1 − ERH
1 (C2 − T ) Modified Chung-Pfost EMC = ln ln(ERH) C1 C3
GAB equation EMC =
(2)
The parameters C2 and C3 in the GAB model are correlated with temperature using the following equations (Lahsasni et al., 2004):
(3)
C2 = C4 exp
(4)
C3 = C5 exp
C1 C2 C3 (ERH) [1 − C2 (ERH)][1 − C2 (ERH) + C2 C3 (ERH)] (5)
In which EMC is equilibrium moisture content (dry basis) in decimal, ERH is equilibrium relative humidity in decimal. C1 , C2 and C3 are the model coefficients and T is temperature in ◦ C.
C 6 RTa
C 7 RTa
(6)
(7)
C4 , C5 , C6 and C7 are coefficients and Ta is the absolute temperature (K) and R is the universal gas constant (R = 8.314 kJ/kmolK). The modified equations of Henderson, Halsey, Oswin and Chung-Pfost as well as the GAB model (Eqs. (1)–(5)) were fitted to the experimental desorption data of the alfalfa (leaf and stem) through a nonlinear regression analysis of the transformed nonlinear equations using Matlab (Mathwork). The quality of the fitted curves was evaluated by using the statistical parameters, residual sum of square (RSS), standard error estimation (SEE) and mean relative deviation (MRD) (Sun, 1999). The coefficient of determination
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Table 3 Estimated values and statistical parameters of the GAB equation fitted to desorption isotherm data of alfalfa stems and leaves at three temperatures 25, 50 and 70 ◦ C. Stems (5.59 ± 0.46) × 10 0.285 ± 0.057 (3.85 ± 1.42) × 10−4 2849 ± 491 25000 0.0196 0.0192 0.213 0.9337 Random
C1 C4 C5 C6 C7 RSS SEE MRD R2 Residual
0.04
Leaves −2
−2
(6.42 ± 0.53) × 10 0.125 ± 0.044 (6.88 ± 2.81) × 10−4 4841 ± 864 25000 0.0159 0.0216 0.1923 0.8165 Random
EMC, d.b. (decimal)
Parameters
0.06
0.02
0
-0.02 T=2 5
-0.04
T=5 0
(R2 ) was also used to give information about the ability of prediction and represent the total variation. The RSS, which is a parameter for nonlinear regression, is defined as: RSS =
n
(EMC − EMC)
2
(8)
T=7 0
-0.06
0
0.2
0.4
0.6
0.8
1
ERH, (decimal) Fig. 5. Residual plots for the modified Halsey equation for desorption data of alfalfa leaves at three temperatures with random distribution.
i=1
The SEE, which is the conditional standard deviation of EMC, represents the fitting ability of the models for the given data points.
n i=1
SEE =
(EMC − EMC)
2
df
(9)
The value of MRD shows the fitting of the curves.
1 EMC − EMC MRD = n EMC n
(10)
i=1
where in Eqs. (8)–(10), EMC is the experimental equilibrium moisture content at observation i, EMC is the predicted equilibrium moisture content at this observation, n is the number of observations and df is the degree of freedom. The smaller the values of these statistical parameters better fits the model. The residuals were also used as an index of the difference between observed data from the experiments and predicted values by the models. The residuals of the EMC, obtained for each model, were plotted against the measured values and assessed visually as random or patterned. If the residual plots indicate a clear pattern, the model should not be accepted (Chen and Morey, 1989).
and leaves of the GAB model. The curves of the model were plotted at each temperature by replacing the coefficients of the models in Tables 2 and 3 in the equations to estimate the equilibrium moisture content at different temperatures. The residuals of EMC of the selected models at the three temperatures were plotted against the experimental data. The pattern was visually judged to be either randomly or systematically distributed. In most of the cases the GAB and the modified Halsey equations had a random distribution in the residual plots (Figs. 5 and 6 as an example). 4. Discussion The comparison of the desorption curves at three temperatures show that at constant relative humidity, moisture content increases by decreasing temperature. This aspect is in line with the results of other researcher on agricultural and food products (Kouhila et al., 2001; Park et al., 2002). The coefficients of the models were found by fitting the results data to the models by programming in Matlab. Since the variance of the C7 in the GAB equation was too wide in the first calculations, 0.05
3. Results
0.04 0.03
EMC, d.b. (decimal)
The desorption of alfalfa leaves and stems were examined in the same conditions to find out the differences of these two particles. Fig. 2 presents the experimental data of the desorption of alfalfa stems and leaves at temperature 70 ◦ C. The desorption level of the leaves was a little higher than the stems. The experimental results as well as the fitted model are shown in Fig. 3 for desorption isotherms of alfalfa leaves and the fitted curves of the selected equations at 25, 50 and 70 ◦ C. The typical S-shape curves were found for all three temperatures. The desorption data for alfalfa stems, for the three temperatures, are shown in Fig. 4. The same typical S-shape curves were found also for stems, but the levels are slightly lower compared to leaves. The selected models (Eqs. (1)–(5)) were fitted to the experimental data of the alfalfa (leaf and stem). The resulting coefficients of the equations are shown in Tables 2 and 3 along with RSS, SEE, MRD and the visual judgment of the residual plots. The desorption results of the first four models are shown in Table 2 for leaves and stems. Table 3 presents the results of the desorption data for the stems
0.02 0.01 0 -0.01 -0.02 -0.03
T=2 5
-0.04
T=5 0 T=7 0
-0.05
0
0.2
0.4
0.6
0.8
1
ERH, (decimal) Fig. 6. Residual plots for the modified Oswin equation for desorption data of alfalfa stems at three temperatures with systematic distribution.
A. Arabhosseini et al. / Industrial Crops and Products 34 (2011) 1550–1555
this parameter was fixed at a value of C7 = 25,000, taken from the literature (Menkov, 2000). The modified Halsey equation can be used to describe the desorption of alfalfa leaves, based on the levels of RSS, SEE, MRD and R2 . The modified Oswin also showed a good fitting only for the alfalfa stems. Similar finding is reported for red clover stems (Stencl and Homola, 2001). The results are comparable with the results of literature, in which was found that the GAB equation is appropriate for the isotherms of most agricultural and food materials and the modified Halsey equation tends better to approximate the isotherms of oil-rich materials (Yang and Siebenmorgen, 2003). The results of this research are applicable to use in the drying process of alfalfa stems and leaves and the products which are similar to alfalfa. The results can also be used for designing of the dryers which are going to use for drying of alfalfa and similar products. 5. Conclusion The experimental data of moisture isotherms of alfalfa stems and leaves showed higher EMC for the leaves compared to the stems. At constant RH, the EMC increased with decreasing of temperature. As a whole the modified Halsey equation was found to be relevant to the desorption estimation of alfalfa. Acknowledgment Financial support from College of Abouraihan, University of Tehran, is gratefully acknowledged. References Adapa, P.K., Schoenau, G.J., Arinze, E.A., 2004. Fractionation of Alfalfa into leaves and stems using a three pass rotary drum dryer. Biosyst. Eng. 91 (4), 455–463. Adapa, P., Schoenau, G., Tabil, L., Sokhansanj, S., Singh, A., 2007. Compression of fractionated sun-cured and dehydrated alfalfa chops into cubes—specific energy models. Bioresour. Technol. 98, 38–45.
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Arabhosseini, A., Huisman, W., van Boxtel, A., Müller, J., 2008. Modeling of thin layer drying of tarragon (Artemisia dracunculus L.). Ind. Crops Prod. 28 (2), 53–59. Arinze, E.A., Schoenau, G.J., Sokhansanj, S., 1999. Design and experimental evaluation op a solar dryer for commercial high quality hay production. Renew. Energy 16, 639–642. ASAE. 2003. Moisture relations of plant-based agricultural products. ASAE Standards; Standards Engineering Practics Data (50th Edition) D245.5:538. Chen, C., 2002. Sorption isotherms of sweet potato slices. Biosyst. Eng. 83 (1), 85–95. Chen, C., Morey, R.V., 1989. Comparison of 4 EMC/ERH equations. Trans. ASAE 32 (3), 983–990. Coblentz, W.K., Fritz, J.O., Bolsen, K.K., King, C.W., Cochran, R.C., 1998. The effects of moisture concentration and type on quality characteristics of alfalfa hay baled under two density regimes in a model system. Anim. Feed Sci. Technol. 72, 53–69. Dalai, A., Schoenau, G., Das, D., Adapa, P., 2006. Volatile organic compounds emitted during high-temperature alfalfa drying. Biosyst. Eng. 94 (1), 57–66. Deshpande, S.D., Sokhansanj, S., Irudayaraj, J., 2002. Effect of moisture content and storage temperature on rate of respiration of alfalfa. Biosyst. Eng. 82 (1), 79–86. Greenspan, L., 1976. Humidity fixed points of binary saturated aqueous solutions. J. Res. Natl. Bur. Stand. A: Phys. Chem. 81A (1), 89–96. Hamer, P.J.C., Knight, A.C., McGechan, M.B., Cooper, G., 2000. Model for predicting the field drying characteristics of grass conditioned by maceration (severe treatment). J. Agric. Eng. Res. 75 (3), 275–289. Kouhila, M., Belghit, A., Daguenet, M., Boutaleb, B.C., 2001. Experimental determination of the sorption isotherms of mint (Mentha viridis), sage (Salvia officinalis) and verbena (Lippia citriodora). J. Food Eng. 47 (4), 281–287. Labuza, T.P., Kaanane, A., Chen, J.Y., 1985. Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods. J. Food Sci. 50, 385–391. Lahsasni, S., Kouhila, M., Mahrouz, M., 2004. Adsorption–desorption isotherms and heat of sorption of prickly pear fruit (Opuntia ficus indica). Energy Convers. Manage. 45 (2), 249–261. Menkov, N.D., 2000. Moisture sorption isotherms of chickpea seeds at several temperatures. J. Food Eng. 45 (4), 189–194. Park, K.J., Vohnikova, Z., Brod, F.P.R., 2002. Evaluation of drying parameters and desorption isotherms of garden mint leaves (Mentha crispa L.). J. Food Eng. 51 (3), 193–199. Soysal, Y., Oztekin, S., 1999. Equilibrium moisture content equations for some medicinal and aromatic plants. J. Agric. Eng. Res. 74 (3), 317–324. Stencl, J., Homola, P., 2001. Water sorption isotherms of leaves and stems of Trifolium pratense L. Grass Forage Sci. 55 (2), 159–165. Sun, D.W., 1999. Comparison and selection of EMC ERH isotherm equations for rice. J. Stored Prod. Res. 35 (3), 249–264. Yang, W., Siebenmorgen, T.J., 2003. Drying theory. In: Encyclopedia of Agricultural, Food, and Biological Engineering, 10-1081: 231–241.
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Modeling of desorption of Alfalfa (Medicago sativa) stems and leaves A. Arabhosseini a,∗ , W. Huisman b , J. Müller c a
College of Abouraihan, University of Tehran, P.O. Box 11365-4117, Tehran, Iran Farm Technology Group, Wageningen University, The Netherlands c Institute of Agricultural Engineering, University of Hohenheim, Stuttgart, Germany b
a r t i c l e
i n f o
Article history: Received 18 April 2011 Received in revised form 16 May 2011 Accepted 17 May 2011 Available online 12 June 2011 Keywords: Alfalfa Desorption Drying Medicago sativa Postharvesting
a b s t r a c t The equilibrium moisture content of agricultural products is necessary to optimize drying process and helps to keep the quality of the product during the period of storage. The main aim of this research was to find the best model which could define well, the exchange of moisture between alfalfa (Medicago sativa) and the surrounding air. The desorption isotherms of alfalfa (stem and leaf) were determined separately by using the saturated salt solutions method at three temperatures (25, 50 and 70 ◦ C) within a range of 5–90% relative humidity. Experimental curves of desorption isotherms were fitted to modified equations of Henderson, Halsey, Oswin and Chung-Pfost as well as the GAB model and then evaluated visually by using residual plots and also by some statistical error parameters. The modified Halsey model was found to be the most suitable for describing the relationship between equilibrium moisture content, relative humidity and temperature. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Alfalfa (Medicago sativa L.) is an excellent forage crop due to its digestibility, protein content and other nutrients (Deshpande et al., 2002). Alfalfa leaves are low in fiber and high in protein and carotenoids, making it suitable for feeding to monogastrics such as poultry and swine, as well as a key ingredient in high protein human health supplements. Alfalfa stems are high in fiber and can be used for ruminant feed, paper and cardboard and energy production (biofuel/ethanol) (Adapa et al., 2007). Alfalfa is cut, chopped, dried and densified to reduce handling, transportation and storage costs, especially for export. Baling alfalfa hay at high moisture content can cause negative quality (Coblentz et al., 1998). Fractional drying of alfalfa leaves and stems is required because alfalfa leaves and stems dry at significantly different rates resulting in over-dried leaves and under-dried stems (Arinze et al., 1999). There are definite market incentives to produce separate leaf and stem components of alfalfa. Alfalfa leaves are also used as herbal medicine to treat various diseases, e.g. diabetes, thyroid conditions, arthritis, high blood cholesterol levels (Medical Economic Company Inc., 2002). Fractionation is the term expressing the separation of forage into two or more component parts (usually leaves and stems). This process enables a later, controlled recombination of these components to achieve specific market nutritional require-
∗ Corresponding author. Tel.: +98 292 30 40 614; fax: +98 292 30 40 730. E-mail address: [email protected] (A. Arabhosseini). 0926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2011.05.018
ments for animal feeds or the potential use of the more valuable leaf material in the rapidly expanding eutraceutical and herbal markets (Adapa et al., 2004). Drying is an essential component of a forage/alfalfa processing industry. It is performed to reduce high moisture level of alfalfa to a safe limit for storage (usually 10% wet basis). Artificial high-temperature drying is required due to climate and seasonal constraints in the most of the Prairie Provinces in North America (Dalai et al., 2006). Moisture sorption isotherm defines the relation between equilibrium relative humidity (ERH) and equilibrium moisture content (EMC) (Soysal and Oztekin, 1999). This knowledge is required to stop the drying process at the aimed moisture content (Hamer et al., 2000) to save energy. As the drying rate for stems and leaves are different so the required time for drying of stems and leaves are different too. When drying whole plants, the leaves will be over-dried and efficiency of the dryer will be less and the energy consumption will be higher (Arabhosseini et al., 2008). Drying of the leaves and stems separately will help to optimize the process. The EMC of stems and leaves are needed to stop drying process in proper time. Not much information is available about desorption isotherms of alfalfa stems and leaves separately. As desorption isotherm is one of the basic parameters of the drying process, this work will give an important contribution to the optimization of the drying of alfalfa. The objective of this study is to find the desorption isotherm curve of alfalfa (stem and leaf) at temperatures between 25 and 70 ◦ C for ERH values in the range of 5–90% and to find appropriate mathematical models to represent it.
A. Arabhosseini et al. / Industrial Crops and Products 34 (2011) 1550–1555
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Fig. 1. The equipment for the desorption isotherm measurement: (1) cryostat; (2) water bath; (3) jars with cap including netballs with samples, thymol and saturated salt solution; and (4) water pump. Table 1 Saturated salts used for experiments and ERH at different temperatures (Greenspan, 1976; Labuza et al., 1985). Salt formula
1 2 3 4 5 6 7 8 9 10 11 12
KCl KBr NaCl NaNO3 KI NaBr K2 CO3 MgCl2 LiI LiCl ZnBr2 LiBr
Leaves
Equilibrium relative humidity (ERH) (%) T = 25 ◦ C
T = 50 ◦ C
T = 70 ◦ C
84.34 80.89 75.29 74.25 68.86 57.57 43.16 32.78 17.56 11.30 7.75 6.37
81.20 79.02 74.43 69.04 64.49 50.93 40.91 30.54 12.38 11.10 7.70 5.53
79.49 79.07 75.06 66.04 61.93 49.70 37.37 27.77 7.23 10.7 8.72 5.23
EMC, d.b. (decimal)
No.
0.2
0.16
Stems
0.12
0.08
0.04
0 0
0.2
0.4
0.6
0.8
1
ERH, (decimal) Fig. 2. Desorption isotherm data of alfalfa stems and leaves at 70 ◦ C and the fitted the modified Oswin equation.
2. Materials and methods 2.1. Samples Alfalfa stems and leaves were examined in this research. The crops were harvested in summer. Stems and leaves were separated immediately after harvesting. The stems were chopped to 20–30 mm long pieces. The samples of 8–10 g fresh leaves and 15–17 g fresh stem were used for desorption tests. 2.2. Experimental procedure The apparatus for providing various ERHs consists of an insulated water bath (56 cm × 36 cm × 26 cm) with 12 glass jars and
a cryostat, which includes a cooling and heating system with an accuracy of 0.1 ◦ C (Fig. 1). The water circulates between cryostat and water bath to prepare a constant temperature of water around the glass-jars. The glass jars are closed with a cap. Every glass jar of 800 ml contains 150 ml of saturated salt solution. A plate with base prevents contact of sample and salt solution. Stainless steel netballs were used to put the stems and leaves separately in the jars. In tests with relative air humidity above 60% crystalline thymol was placed in the bottle to prevent microbial spoilage (Menkov, 2000). The cryostat was separately adjusted to 25, 50 and 70 ◦ C. Every three days, the samples were weighed with an accuracy
Table 2 Estimated values and statistical parameters of four equations fitted to desorption isotherm data of alfalfa stems and leaves at three temperatures 25, 50 and 70 ◦ C. Parameters
Estimated values and the variance of the models and statistical parameters Sorption models
Stems C1 C2 C3 RSS SEE MRD R2 Residual Leaves C1 C2 C3 RSS SEE MRD R2 Residual
Henderson
Halsey
Oswin
Chung-Pfost
12.03 ± 2.93 × 10−2 1.133 ± 0.091 13.277 ± 6.95 0.0315 0.0305 0.2673 0.9324 Systematic
−2.181 ± 0.0578 −2.24 ± 0.15 × 10−2 1.351 ± 0.047 0.0092 0.0165 0.100 0.9814 Random
24.34 ± 0.85 × 10−2 −23.36 ± 1.34 × 10−4 1.817 ± 0.072 0.0096 0.0168 0.137 0.9816 Random
10.19 ± 0.85 −2.57 ± 7.17 143.4 ± 27.6 0.0765 0.0474 0.461 0.8303 Systematic
38.25 ± 9.14 × 10−3 0.715 ± 0.070 46.896 ± 15.10 0.065 0.0437 0.416 0.9269 Systematic
−1.879 ± 0.054 −1.65 ± 0.15 × 10−2 1.042 ± 0.047 0.0207 0.0247 0.193 0.9741 Random
21.06 ± 1.57 × 10−2 −19.67 ± 2.16 × 10−4 1.304 ± 0.081 0.0328 0.0311 0.292 0.9615 Systematic
8.559 ± 0.91 3.68 ± 12.64 147.8 ± 41.3 0.1787 0.0725 0.524 0.7193 Systematic
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Modified Henderson
Modified Halsey
0 .5
0 .5 25 °C
50 °C
EMC, d.b. (decimal)
EMC, d.b. (decimal)
25 °C
0 .4
70 °C
0 .3 0 .2 0 .1 0
0
0 .5
70 °C
0 .3 0 .2 0 .1 0
1
50 °C
0 .4
0
ERH, (decimal) Modified Oswin 0 .5 25 °C
25 °C
50 °C
0 .4
EMC, d.b. (decimal)
EMC, d.b. (decimal)
1
Modified Chung-Pfost
0 .5
70 °C
0 .3 0 .2 0 .1 0
0 .5
ERH, (decimal)
0
0 .5
0 .4
50 °C 70 °C
0 .3 0 .2 0 .1 0 .0 0 .0
1
ERH, (decimal)
0 .5
1 .0
ERH, (decimal) GAB equation
0 .5
EMC, d.b. (decimal)
25 °C 50 °C
0 .4
70 °C
0 .3 0 .2 0 .1 0
0
0 .5
1
ERH, (decimal) Fig. 3. Desorption isotherms of alfalfa leaves at 25 (), 50 (♦) and 70 ◦ C () and the fitted curves of the modified Henderson, modified Halsey, modified Oswin, modified Chung-Pfost and GAB equations.
±0.1 mg. Equilibrium was acknowledged when three consecutive weight measurements showed a difference less than 1 mg. Three replications were taken for each test. When equilibrium was reached (approximately after 3–5 weeks depending on the temperature and relative humidity), the samples were dried using oven method (at 105 ◦ C for 24 h) in order to obtain dry matter contents (Park et al., 2002). The EMCs were determined by triplicate measurements of each sample.
2.3. Salt solution The saturated salt solutions method is based on the fact that ERHs of specific salt solutions are a well known physical property (Greenspan, 1976). Table 1 gives the equivalent of relative humidity of the selected salt solutions at three temperatures. Salt solutions were provided by mixing and dissolving of the salt crystals in distilled water at the temperatures of 45, 70 and 90 ◦ C which these
temperatures were 20 ◦ C higher than the temperature of each test in the water bath, to guarantee a saturated solution. The salt solutions were agitated from time to time to avoid the formation of a condensation gradient in the liquid phase (Kouhila et al., 2001). 2.4. Desorption models A number of models have been suggested in literature to describe the relationship between EMC and ERH. The modified Henderson, Oswin, Halsey, Chung-Pfost and GAB equations (Chen, 2002) have been adopted by the American Society of Agricultural Engineers as standard equations for describing sorption isotherms of agricultural products (ASAE, 2003). The equations have been transformed to get EMC as dependent parameter and ERH as independent variable. Modified Henderson EMC =
−
1/C3
1 ln(1 − ERH) C1 (T + C2 )
(1)
A. Arabhosseini et al. / Industrial Crops and Products 34 (2011) 1550–1555
Modified Henderson
Modified Halsey
0.5
0.5 25 °C
EMC, d.b. (decimal)
EMC, d.b. (decimal)
25 °C 50 °C
0.4
70 °C
0.3 0.2 0.1 0
0
0.5
70 °C
0.3 0.2 0.1 0
1
50 °C
0.4
0
0.5
ERH, (decimal) Modified Oswin
Modified Chung-Pfost 0.5 25 °C
EMC, d.b. (decimal)
25 °C
EMC, d.b. (decimal)
1
ERH, (decimal)
0.5 50 °C
0.4
70 °C
0.3 0.2 0.1 0
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0
0.5
70 °C
0.3 0.2 0.1 0
1
50 °C
0.4
0
0.5
ERH, (decimal)
1
ERH, (decimal) GAB equation
0.5
EMC, d.b. (decimal)
25 °C 50 °C
0.4
70 °C
0.3 0.2 0.1 0
0
0.5
1
ERH, (decimal) Fig. 4. Desorption isotherms of alfalfa stems at 25 (), 50 (♦) and 70 ◦ C () and the fitted curves of the modified Henderson, modified Halsey, modified Oswin, modified Chung-Pfost and GAB equations.
Modified Halsey EMC =
−exp(C + C T ) 1/C3 1 2 ln(ERH)
ERH 1/C3
Modified Oswin EMC = (C1 + C2 T )
1 − ERH
1 (C2 − T ) Modified Chung-Pfost EMC = ln ln(ERH) C1 C3
GAB equation EMC =
(2)
The parameters C2 and C3 in the GAB model are correlated with temperature using the following equations (Lahsasni et al., 2004):
(3)
C2 = C4 exp
(4)
C3 = C5 exp
C1 C2 C3 (ERH) [1 − C2 (ERH)][1 − C2 (ERH) + C2 C3 (ERH)] (5)
In which EMC is equilibrium moisture content (dry basis) in decimal, ERH is equilibrium relative humidity in decimal. C1 , C2 and C3 are the model coefficients and T is temperature in ◦ C.
C 6 RTa
C 7 RTa
(6)
(7)
C4 , C5 , C6 and C7 are coefficients and Ta is the absolute temperature (K) and R is the universal gas constant (R = 8.314 kJ/kmolK). The modified equations of Henderson, Halsey, Oswin and Chung-Pfost as well as the GAB model (Eqs. (1)–(5)) were fitted to the experimental desorption data of the alfalfa (leaf and stem) through a nonlinear regression analysis of the transformed nonlinear equations using Matlab (Mathwork). The quality of the fitted curves was evaluated by using the statistical parameters, residual sum of square (RSS), standard error estimation (SEE) and mean relative deviation (MRD) (Sun, 1999). The coefficient of determination
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A. Arabhosseini et al. / Industrial Crops and Products 34 (2011) 1550–1555
Table 3 Estimated values and statistical parameters of the GAB equation fitted to desorption isotherm data of alfalfa stems and leaves at three temperatures 25, 50 and 70 ◦ C. Stems (5.59 ± 0.46) × 10 0.285 ± 0.057 (3.85 ± 1.42) × 10−4 2849 ± 491 25000 0.0196 0.0192 0.213 0.9337 Random
C1 C4 C5 C6 C7 RSS SEE MRD R2 Residual
0.04
Leaves −2
−2
(6.42 ± 0.53) × 10 0.125 ± 0.044 (6.88 ± 2.81) × 10−4 4841 ± 864 25000 0.0159 0.0216 0.1923 0.8165 Random
EMC, d.b. (decimal)
Parameters
0.06
0.02
0
-0.02 T=2 5
-0.04
T=5 0
(R2 ) was also used to give information about the ability of prediction and represent the total variation. The RSS, which is a parameter for nonlinear regression, is defined as: RSS =
n
(EMC − EMC)
2
(8)
T=7 0
-0.06
0
0.2
0.4
0.6
0.8
1
ERH, (decimal) Fig. 5. Residual plots for the modified Halsey equation for desorption data of alfalfa leaves at three temperatures with random distribution.
i=1
The SEE, which is the conditional standard deviation of EMC, represents the fitting ability of the models for the given data points.
n i=1
SEE =
(EMC − EMC)
2
df
(9)
The value of MRD shows the fitting of the curves.
1 EMC − EMC MRD = n EMC n
(10)
i=1
where in Eqs. (8)–(10), EMC is the experimental equilibrium moisture content at observation i, EMC is the predicted equilibrium moisture content at this observation, n is the number of observations and df is the degree of freedom. The smaller the values of these statistical parameters better fits the model. The residuals were also used as an index of the difference between observed data from the experiments and predicted values by the models. The residuals of the EMC, obtained for each model, were plotted against the measured values and assessed visually as random or patterned. If the residual plots indicate a clear pattern, the model should not be accepted (Chen and Morey, 1989).
and leaves of the GAB model. The curves of the model were plotted at each temperature by replacing the coefficients of the models in Tables 2 and 3 in the equations to estimate the equilibrium moisture content at different temperatures. The residuals of EMC of the selected models at the three temperatures were plotted against the experimental data. The pattern was visually judged to be either randomly or systematically distributed. In most of the cases the GAB and the modified Halsey equations had a random distribution in the residual plots (Figs. 5 and 6 as an example). 4. Discussion The comparison of the desorption curves at three temperatures show that at constant relative humidity, moisture content increases by decreasing temperature. This aspect is in line with the results of other researcher on agricultural and food products (Kouhila et al., 2001; Park et al., 2002). The coefficients of the models were found by fitting the results data to the models by programming in Matlab. Since the variance of the C7 in the GAB equation was too wide in the first calculations, 0.05
3. Results
0.04 0.03
EMC, d.b. (decimal)
The desorption of alfalfa leaves and stems were examined in the same conditions to find out the differences of these two particles. Fig. 2 presents the experimental data of the desorption of alfalfa stems and leaves at temperature 70 ◦ C. The desorption level of the leaves was a little higher than the stems. The experimental results as well as the fitted model are shown in Fig. 3 for desorption isotherms of alfalfa leaves and the fitted curves of the selected equations at 25, 50 and 70 ◦ C. The typical S-shape curves were found for all three temperatures. The desorption data for alfalfa stems, for the three temperatures, are shown in Fig. 4. The same typical S-shape curves were found also for stems, but the levels are slightly lower compared to leaves. The selected models (Eqs. (1)–(5)) were fitted to the experimental data of the alfalfa (leaf and stem). The resulting coefficients of the equations are shown in Tables 2 and 3 along with RSS, SEE, MRD and the visual judgment of the residual plots. The desorption results of the first four models are shown in Table 2 for leaves and stems. Table 3 presents the results of the desorption data for the stems
0.02 0.01 0 -0.01 -0.02 -0.03
T=2 5
-0.04
T=5 0 T=7 0
-0.05
0
0.2
0.4
0.6
0.8
1
ERH, (decimal) Fig. 6. Residual plots for the modified Oswin equation for desorption data of alfalfa stems at three temperatures with systematic distribution.
A. Arabhosseini et al. / Industrial Crops and Products 34 (2011) 1550–1555
this parameter was fixed at a value of C7 = 25,000, taken from the literature (Menkov, 2000). The modified Halsey equation can be used to describe the desorption of alfalfa leaves, based on the levels of RSS, SEE, MRD and R2 . The modified Oswin also showed a good fitting only for the alfalfa stems. Similar finding is reported for red clover stems (Stencl and Homola, 2001). The results are comparable with the results of literature, in which was found that the GAB equation is appropriate for the isotherms of most agricultural and food materials and the modified Halsey equation tends better to approximate the isotherms of oil-rich materials (Yang and Siebenmorgen, 2003). The results of this research are applicable to use in the drying process of alfalfa stems and leaves and the products which are similar to alfalfa. The results can also be used for designing of the dryers which are going to use for drying of alfalfa and similar products. 5. Conclusion The experimental data of moisture isotherms of alfalfa stems and leaves showed higher EMC for the leaves compared to the stems. At constant RH, the EMC increased with decreasing of temperature. As a whole the modified Halsey equation was found to be relevant to the desorption estimation of alfalfa. Acknowledgment Financial support from College of Abouraihan, University of Tehran, is gratefully acknowledged. References Adapa, P.K., Schoenau, G.J., Arinze, E.A., 2004. Fractionation of Alfalfa into leaves and stems using a three pass rotary drum dryer. Biosyst. Eng. 91 (4), 455–463. Adapa, P., Schoenau, G., Tabil, L., Sokhansanj, S., Singh, A., 2007. Compression of fractionated sun-cured and dehydrated alfalfa chops into cubes—specific energy models. Bioresour. Technol. 98, 38–45.
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