Water desorption isotherm and drying characteristics of green soybean

Water desorption isotherm and drying characteristics of green soybean

Journal of Stored Products Research 60 (2015) 25e30 Contents lists available at ScienceDirect Journal of Stored Products Research journal homepage: ...

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Journal of Stored Products Research 60 (2015) 25e30

Contents lists available at ScienceDirect

Journal of Stored Products Research journal homepage: www.elsevier.com/locate/jspr

Water desorption isotherm and drying characteristics of green soybean Zhao Yang*, Enlong Zhu, Zongsheng Zhu School of Mechanical Engineering, Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, MOE, Tianjin University, China

a r t i c l e i n f o

a b s t r a c t

Article history: Accepted 24 October 2014 Available online 1 December 2014

There is very little published information on green soybean about moisture sorption and drying. Water desorption isotherms were determined by the static gravimetric method using saturated salt solutions at 20, 30, and 40  C. By comparing the index of goodness of fit, the isotherm for green soybean seeds could be better described by the Halsey equation with the desorption isotherm parameters A and B estimated to be 5.612 and 1.538 respectively. The net isosteric heat of water desorption calculated by Clausius eClapeyron equation was from 208.8 to 3627.9 kJ/kg. Thin layer drying of green soybean seeds in a range of 25e45  C and relative humidity from 0.2 to 0.4 dec were carried out with a heat pump dryer. The Page model was the most suitable model for describing the thin layer drying process of green soybean seeds compared with the Lewis, Henderson and Thompson models. The drying characteristics of green soybean seeds were tested and analyzed under various temperature and humidity conditions, and the results will be useful for the drying and storage of green soybean seeds. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Green soybean Desorption isotherm Net isosteric heat Heat pump dryer Drying characteristics

1. Introduction Green soybean (Glycine max (L.) Merr.) is one of the most important economical crops grown worldwide, especially in China. The seeds are rich in healthy and anti-cancer ingredients (RedondoCuenca et al., 2006; Trock et al., 2006). Usually green soybean seeds are consumed as a vegetable (Mateos-Aparicio et al., 2008), snack food and soy milk. The nutrient composition of green soybean is of special interest mainly because it is characterised by high protein and edible sugar content (Redondo-Cuenca et al., 2006). High sugar content could influence the bean's moisture isotherms, glass transition temperature and drying characteristics. The drying characteristics of green soybean seeds can reflect some integrated drying information of agricultural materials. Therefore, knowledge of physicochemical properties of green soybean seeds will provide useful information on drying of beans and can lead to improvement of the drying process (Miranda et al., 2012; Argyropoulos et al., 2012). Moisture content of harvested green soybean seeds can reach 20% or more, with higher moisture content enhancing physiological activities, increasing enzyme activity and lipid oxidation, leading to higher nutrient consumption and eventually resulting in quality

* Corresponding author. 92 Weijinnan Road, Nankai District, Tianjin, 300072, China. Tel./fax: þ86 022 27890627. E-mail address: [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.jspr.2014.10.006 0022-474X/© 2014 Elsevier Ltd. All rights reserved.

degradation in storage and transportation. Using effective artificial drying technologies to dehydrate seeds to a safe moisture content (10e12%) could assure good storage quality (Abalone et al., 2006; Parde et al., 2002). Quality stability of green soybean seeds requires a good knowledge of the relationship between water activity and equilibrium moisture content at a particular temperature, which is known as water sorption isotherm (Martins et al., 2008; €ǧüs, 1997; Timmermann Roopesh et al., 2009; Maskan and Go et al., 2001). Knowledge of the drying kinetics of green soybean is essential in designing and optimizing the quality control during drying process. Considerable research has been carried out on the drying kinetics of seeds, such as tomato, amaranth, pumpkin, safflower coriander seeds (Sogi et al., 2006; Sacilik et al., 2007; Sacilik, 2007), but research on green soybean seeds has been very limited. Heat pump drying is an effective drying method, because it has more technology advantages such as energy conservation, environmental protection and easy to realize automatic control (Hawlader et al., 2006; Prasertsan and Saen-saby, 1998, Chin and Law, 2010; Minea, 2012). To the authors' knowledge, no comprehensive study on the water sorption and drying characteristic of green soybean with a heat pump dryer has been reported, so experiments were carried out with the aim of providing this information. The objectives of this work were to (1) measure desorption isotherms and thin layer drying curves, (2) find the best model to describe desorption isotherms, drying characteristics of green

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soybean, and (3) estimate the numerical solutions of equilibrium moisture content and of net isosteric heat. 2. Materials and methods 2.1. Experimental Samples were obtained from Heilongjiang Province in northeast China. Initial moisture content of the samples used for determination of desorption isotherms and thin layer drying was tested before experiments. Sample moisture content was determined in an air oven at a fixed temperature of 103  C for 8 h (State Bureau of Technical Supervision, 1995). The required moisture content was adjusted by adding water to the samples, then sealing in double plastic bags and stored in refrigerator at 3e4  C for 5 days. Before the experiments began, samples had been taken out of the refrigerator and kept in double-layer polyethylene bags at room temperature for about 24 h. The desorption equilibrium moisture content was determined at 20, 30, and 40  C using the static gravimetric method (Corzo and Fuentes, 2004; Tarigan et al., 2006; Cruz et al., 2010), which is based on the use of saturated salt solutions to maintain a fixed relative humidity at a particular temperature while the equilibrium is reached. Eleven saturated salt solutions were prepared corresponding to a wide range of water activities from 0.112 to 0.946 at different temperatures as shown in Table 1 (Lahsasni et al., 2002; Zhang, 2005; Mark et al., 2011; Cassini et al., 2006). Samples (10 g) were placed into a plastic basket hanging above the saturated salt solution in each bottle, and the bottles were placed in a temperature controlled chamber (±0.5  C). Measurements were made at 20, 30, and 40  C for desorption. Early stage measurements were made every 2 days, later stage was every day. Once three consecutive weight measurements showed a difference within 0.001 g, the sample mass was considered to be stationary and the experiment was ceased. At least three replicates of each experiment were made. A heat pump dryer was used with an auxiliary condenser in this study, which is illustrated schematically in Fig. 1. The dryer can be run at two stages: warming or cooling. The function of the auxiliary condenser is to discharge the redundant heat from the dryer when the temperature is higher than the set temperature limit. The heat pump dryer allows drying air to remain at a constant temperature and relative humidity. When the heat pump dryer is in heating mode, the refrigerant in the heat pump system follows a sequential path: 1/2/3/4/5/6/7/8/9/1. If the process temperature is higher than set point, heat pump dryer will run in a cooling mode and the refrigerant will follow a different path: 1/10/11/4/5/6/7/8/9/1. The transition between the

Table 1 Saturated salt solutions and water activities used in measuring the sorption isotherms of green soybean at 20, 30 and 40  C. Salt

LiCl CH3COOK MgCl2 K2CO3 Mg(NO3)2 NaBr CuCl2 NaCl (NH4)2SO4 KCl KNO3

Fig. 1. Schematic diagram of the laboratory heat pump dryer: 1 e compressor, 2 e solenoid stop valve, 3 e condenser, 4 e liquid storage tank, 5 e filter dryer, 6 e liquid viewing glass, 7 e expansion valve, 8 e evaporator, 9 e main solenoid stop valve, 10 e solenoid stop valve, 11 e auxiliary condenser and fan, 12 e main fan, 13 e auxiliary fan, 14 e air baffle, 15 e material screen.

heating and cooling modes can keep the drying air at a constant temperature. Drying air is circulated by the main and the auxiliary fans. The drying air flows through 3/12/13/14/15/8/3. The drying air in the chamber is circulated horizontally at a constant speed over the samples. A continuous weight recording system was designed to weight the samples so as to monitor moisture content change in the drying process without removing them from the chamber. A tray of metallic mesh was used to hold the green soybean sample.

2.2. Mathematical modelling Many models have been used in describing adsorption and desorption in foodstuffs and agricultural materials in the range of water activities from 0.05 to 0.95. Seven commonly used models

Table 2 Seven mathematical models used to describe the desorption isotherm of green soybean. Models

Equations

Henderson

h i aw ¼ 1  exp A$T$MeB

(1)

h i   aw ¼ 1  exp  A$ T þ C $MeB

(2)

aw ¼ exp½  A=ðT þ CÞ$expðB$Me Þ

(3)

h i  aw ¼ exp A=R$T $MeB

(4)

h i   aw ¼ exp  exp A þ C$T $MeB

(5)

Me ¼ ðA þ C$TÞðaw =1  aw Þ1=B

(6)

Modified Henderson

Modified Chung-Pfost

Halsey

Values of activities 20  C

30  C

40  C

0.113 0.231 0.331 0.432 0.559 0.591 0.680 0.755 0.813 0.851 0.946

0.113 0.216 0.324 0.432 0.514 0.560 0.670 0.751 0.806 0.836 0.923

0.112 0.230 0.316 0.400 0.484 0.570 0.670 0.745 0.790 0.823 0.891

Modified Halsey

Oswin

Modified GAB Me ¼

A$B$C=T$aw ð1  B$aw Þð1  B$aw þ C=T$B$aw Þ

(7)

Z. Yang et al. / Journal of Stored Products Research 60 (2015) 25e30

were fitted to desorption isotherms (Table 2) which describe the relationship of equilibrium moisture content, water activity and temperature (Al-Muhtaseb et al., 2002; Corzo and Fuentes, 2004; Chen, 2006). Sorption is always accompanied by heat absorption or release. Desorption heat can be calculated from the famous ClausiuseClapeyron thermodynamic model which can be decomposed into two parts (Corzo and Fuentes, 2004; Chen, 2006):

Q ¼ qsn þ Lr

(8)

qsn

is defined as the net isosteric heat of water sorption and described as following:

qsn ¼ R

  Ta1 Ta2 a ln w2 Ta2  Ta1 aw1

(9)

(10)

Drying of biological material is a diffusion-controlled process, which can be represented by Fick's second law. Numerous mathematical models have been proposed by researchers to describe the thin layer drying of agriculture products. Four well known mathematical models (Abalone et al., 2006; Sogi et al., 2006) were used in this work, and those are: Lewis model:

MR ¼

M  Me ¼ expðkL tÞ M0  Me

(11)

Page model:

MR ¼

  M  Me ¼ exp kp t n M0  Me

(12)

Henderson model:

M  Me MR ¼ ¼ A1 expðA2 tÞ M0  Me

(13)

Thompson model:

t ¼ A3 ln MR þ A4 ðln MR Þ2

respectively. The parameters of the four factors are shown in Table 3. Air speed was held at 1 m/s. 2.4. Statistical analysis Several criteria were used for determining the adequacy of isotherm models and the thin layer drying models. Non-linear regression was used to fit data to the eleven equations by using ‘cftool’ function of MATLAB software (Xie, 2010; pp. 275e283). The goodness of fit was evaluated with the coefficient of determination (R2), adjusted coefficient of determination (adjusted R2), error sum of squares (SSE) and root mean square error (RMSE) (Xie, 2010). If the goodness of fit indexes met the following conditions: high values of R-square, adjusted R-square and low values of SSE, RMSE, the goodness of fit was judged to be high. 3. Results

The Lr is value of pure water can be calculated by Eq. (10):

Lr ¼ Rð6887  5:31  Ta Þ

27

(14)

The resulting sorption isotherms for green soybean are presented in Fig. 2. They show that the equilibrium moisture content increased with the increasing aw and decreased with increasing temperature. Table 4 shows the coefficients of the models obtained by the least squares parameter estimation method for the seven models listed in Table 2. The results include the estimated parameter values, as well as the respective values of the error sum of squares (SSE), coefficient of determination (R2), adjusted coefficient of determination (adjusted R2), root mean square error (RMSE). It can be seen that the Halsey model had the highest R2, adjusted R2, and the lowest SSE, RMSE. The indices of goodness of fit i.e. R2, adjusted R2, SSE, and RMSE were 0.9894, 0.9882, 0.0072, and 0.0283, respectively. Therefore, the Halsey model is shown for the first time to provide the best description of the desorption isotherms for green soybean with coefficients A and B of 5.612 and 1.538 respectively. The aw equation for desorption data from 20 to 40  C can be described as follows:

  12:002 1:538 $Me aw ¼ exp T

(15)

The relationship between qsn and different moisture contents from 5 to 32% (d.b.) is shown in Fig. 3. Applying Eq. (9), the net desorption isosteric heat was calculated and its range was from 208.8 to 3627.9 kJ/kg. The prediction of drying rates for thin layer drying is essential for an efficient moisture transfer analysis and benefit for optimal energy usage. Figure 4 shows typical results obtained in the thin

2.3. Drying method The orthogonal collocation method was available for studying the drying kinetics of soybean seeds (Barrozo et al., 2006), and also adopted in thin layer drying of green soybeans in this paper. The drying experiments were carried out at five levels of air temperature, initial moisture content, relative humidity and drying time,

Table 3 Experimental parameters encoding table of the four factors. Factors

x1 x2 x3 x4

Air temperature ( C) Initial moisture content (d.b.)% Relative humidity Drying time (h)

Levels and actual values 2

1

0

1

2

25 6 0.2 1

30 14 0.25 2

35 22 0.3 3

40 30 0.35 4

45 38 0.4 5

Fig. 2. Desorption isotherms of green soybean at 20 (*), 30 (B) and 40  C (C).

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Table 4 Estimated parameters and comparison criteria for the equilibrium moisture content models of green soybean desorption data. Coefficients

Henderson

Modified Henderson

Modified Chung-Pfost

Halsey

Modified Halsey

Modified Oswin

Modified GAB

A B C SSE R2 Adj R2 RMSE

0.8348 1.526 e 0.0119 0.9835 0.9817 0.0364

0.8219 1.526 0.316 0.0119 0.9835 0.9794 0.0386

28.770 12.950 11.760 0.0133 0.9817 0.9771 0.0407

5.612 1.538 e 0.0072 0.9894 0.9882 0.0283

50.770 1.570 2.728 1.7760 0.8946 0.8729 0.0893

50.190 1.522 1.211 0.0617 0.9090 0.8863 0.0878

66.200 4.153 0.254 0.0533 0.9266 0.9083 0.0816

layer experiment with the heat pump dryer. The initial moisture of green soybean seeds was 22% (d.b.) and the drying conditions are listed in Table 5. The models used in this paper were all semi-theoretical drying models which were generally derived by simplifying general series solutions of Fick's second law. The drying kinetics were investigated by considering four classical models according to the data. The constants and statistical parameters used in all modes were obtained for each period, seen in Table 5. Compared with R2, adjusted R2, SSE, and RMSE, in the four thin layer drying models discussed above, the Page and Thompson models give better result than the other two models. Especially the Page model's adjusted R2 is 0.9994 which is bigger than the other three models, and its SSE is 8.089  105, RMSE is 0.002596, which are smaller than the three others. This study proved that the Page equation is more suitable for describing thin layer drying process of green soybean for the first time, which can be written as:

 MR ¼ exp 0:3138t 0:4658

(16)

4. Discussion On the basis of the BET classification, the isotherm curve follows type III behaviour (Brunauer, 1943; Al-Muhtaseb et al., 2002), which is different from many other cereal products (Oyelade et al., 2007). The desorption curves of green soybean have similar trends at all temperatures. However, the isotherms curves are very close, mainly for low values of (between 0.1 and 0.25). This indicates a low influence of temperature (from 20  C to 40  C) on the desorption isotherms of green soybean. Green soybean isotherm curves crossed over at a water activity value in the range of 0.6e0.9, which is consistent with many studies on various types of sugars-rich foods (Tsami, 1991; Cassini et al., 2006). The cross phenomenon is

Fig. 3. Model fit for net isosteric heat of desorption of water in green soybean using Halsey (◊), Modified Henderson (▽), Henderson (B) and Modified Chung-Pfost (,) models.

mainly caused by the fact that green soybean has high sugar content about 38.6 g/100 g (with a moisture content of 10.81 kg/kg d.b.) (Redondo-Cuenca et al., 2006). Net isosteric heat of desorption is a valuable tool in designing the drying process, which is important in understanding the mechanism of sorption (Li et al., 2011). The isosteric heat for desorption decreased rapidly with increase of moisture content until a moisture content of about 20% was reached, but above 20% the heat decreased smoothly with increase in seed moisture content. This phenomenon for green soybean was not found before, but reported for some other agricultural products (Li et al., 2011; Bonner and Kenney, 2013; Toǧul and Arslan, 2007). Isosteric heats of desorption presented higher values at lower moisture content and then decreased with an increase in moisture content. The step increase of qsn at low moisture contents is because of the monomolecular layer formed by highly active polar sites on the surface of the food material that are covered with water molecules (Mrad et al., 2012). As the moisture content was higher than 20%, the values calculated from Eq. (9) were similar. The qsn values calculated by the Halsey model had the highest values among the four aw models at lower moisture content. It can be seen from Fig. 4 that the drying rate decreased continuously throughout the drying period, which indicated that green soybean seeds drying process took place in the falling rate period. It can be considered as a diffusion controlled process that the rate of removed moisture is limited by diffusion of moisture from inside to surface (Duc et al., 2011). Comparable results have also been reported for various other agricultural products (Fatouh et al., 2004; Shi, 2013). From Fig. 4 it also can be seen that at the same drying condition of temperature and time, a lower relative humidity of the drying air leads to a lower moisture ratio MR. It indicated that if other drying conditions were the same, an increasing temperature and decreasing relative humidity of the drying air would lead to a decrease in the MR.

Fig. 4. Moisture ratio curves of green soybean seeds at different drying conditions: (▽) 35  C, RH ¼ 0.3, t ¼ 5 h; (,) 25  C, RH ¼ 0.3, t ¼ 3 h; (C) 35  C, RH ¼ 0.2, t ¼ 3 h; (B) 35  C, RH ¼ 0.4, t ¼ 3 h; (*) 45  C, RH ¼ 0.3; t ¼ 3 h.

Z. Yang et al. / Journal of Stored Products Research 60 (2015) 25e30

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Table 5 Curve fitting of four models for drying green soybean. Model

Temperature  C

RH dec

Drying time h

SSE

R-square

Adjusted R-square

RMSE

Lewis

25 35 35 35 45 25 35 35 35 45 25 35 35 35 45 25 35 35 35 45

0.30 0.40 0.20 0.30 0.30 0.30 0.40 0.20 0.30 0.30 0.30 0.40 0.20 0.30 0.30 0.30 0.40 0.20 0.30 0.30

3 3 3 5 3 3 3 3 5 3 3 3 3 5 3 3 3 3 5 3

0.05175 0.01897 0.05906 0.1801 0.07155 0.00013 0.00014 8.089e5 0.00096 0.00017 0.01621 0.00556 0.01888 0.04268 0.02568 0.01083 0.03391 0.01043 0.02491 0.00837

0.6055 0.7590 0.6147 0.4711 0.7396 0.9990 0.9982 0.9995 0.9994 0.9994 0.8764 0.9294 0.8768 0.8747 0.9065 0.9989 0.9967 0.9990 0.9972 0.9992

0.6055 0.7590 0.6147 0.4711 0.7396 0.9990 0.9981 0.9994 0.9993 0.9993 0.8661 0.9235 0.8666 0.8669 0.8988 0.9988 0.9964 0.9989 0.9972 0.9991

0.06309 0.0382 0.0674 0.1029 0.07419 0.003232 0.003392 0.002596 0.007747 0.003762 0.03676 0.02152 0.03967 0.0516 0.04626 0.03005 0.05316 0.02948 0.03946 0.02641

Page

Henderson

Thompson

5. Conclusions In this work desorption isotherm and thin layer drying test were conducted to study the drying characteristic of green soybean seeds. From the results, it can be concluded that desorption isotherm belongs to type III. The Halsey model provided a better fit to experimental data, the net isosteric heat of desorption increased with increasing moisture content and decreased with increasing temperature in the range of temperature 20e40  C and water activity 0.112e0.946. There was a cross-over among green soybean seeds due to high sugar content. The value of net isosteric heat of water desorption ranges between 208.8 and 3627.9 kJ/kg, and the Page model was found to be best fitted for describing thin layer drying behaviour of green soybeans. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51276124 and 51476111), Research Fund for the Doctoral Program of Higher Education of China (No. 20130032130006), and a Science and Technology Project of Tianjin City (Grant No. 12ZCDGGX49400). Nomenclature A, B, C, Ai model coefficients aw, awi water activity(decimal) i ¼ 1, 2 d.b. dry basis df number of degrees of freedom kL drying parameter of Lewis model/h kp drying parameter of Page's model/h Lr heat of evaporation for pure water kJ/kg M seeds moisture content kg/kg M0 initial moisture content (d.b.) kg/kg MR moisture ratio dimensionless ∧

MR N n p R R2

predictive value of moisture ratio number of experimental values Page model exponent, dimensionless independent variable gas constant kJ/(mol K) coefficient of determination

QS qsn T,Tai t xi yi yi

sorption isosteric heat, kJ/kg net heat of sorption isosteric kJ/kg absolute temperature i ¼ 1, 2 K drying time h experimental factor i ¼ 1, 2, 3, 4 experimental value mean value of experiment

yi

predictive value of experiment



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