Water Sorption Isotherm and Drying Characteristics of Tomato Seeds

Water Sorption Isotherm and Drying Characteristics of Tomato Seeds

Available online at www.sciencedirect.com Biosystems Engineering (2003) 84 (3), 297–301 doi:10.1016/S1537-5110(02)00275-1 PH}Postharvest Technology ...

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Available online at www.sciencedirect.com

Biosystems Engineering (2003) 84 (3), 297–301 doi:10.1016/S1537-5110(02)00275-1 PH}Postharvest Technology

Water Sorption Isotherm and Drying Characteristics of Tomato Seeds D.S. Sogi1; U.S. Shivhare2; S.K. Garg3; A.S. Bawa4 1

Department of Food Science and Technology, Guru Nanak Dev University, Amritsar 143 005, India; e-mail of corresponding author: [email protected] 2 Department of Chemical Engineering and Technology, Panjab University, Chandigarh, India; e-mail: [email protected] 3 Department of Molecular Biology and Biochemistry, Guru Nanak Dev University, Amritsar 143 005, India; e-mail: [email protected] 4 Defence Food Research Laboratory, Mysore, India; e-mail: [email protected] (Received 4 February 2002; accepted in revised form 26 November 2002)

Seeds were separated from tomato waste by sedimentation and dried in cabinet/fluidised bed dryers at 50, 70 and 90oC using tray load of 4, 8 and 12 kg m2. Sorption isotherms were obtained at 30, 40, 50, 60 and 708C by the static method using saturated salt solutions. The Henderson’s model described the sorption isotherm, adequately, over the entire temperature range. Drying rates indicated that the drying of tomato seeds took place under the falling rate period and the drying behaviour was well described by Page’s model. Dependence of the rate constant on air temperature was described by Arrhenius type relationship. # 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Science Ltd

1. Introduction Tomato, a native plant of South America, is one of the most important crops grown worldwide. Most of the tomatoes are consumed fresh as salad and in food preparations. The remaining produce is utilised in the manufacture of various products, such as puree, paste, ketchup, sauce and soups, etc. Industrial processing of tomatoes generates waste originating from water flumes, washing, sorting table, pulper-refiner (PR) and cleaning. The PR waste (pomace) consisting of seeds, skin and pulp, has substantial quantity of nutrients, which can be processed for food as well as feed (Sogi & Bawa, 1998). Tomato seeds can be separated from pulp and skin by a sedimentation system (Sogi et al., 1999). Considerable work has been carried out on the chemical composition of tomato and its waste (Kramer & Kwee, 1977a, b; Al-Wandawi et al., 1985), preparation of protein concentrate (Brodowski & Giesman, 1980), incorporation of tomato seeds into bread (Carlson et al., 1981) and methane production from waste (Niementowski & Nelson, 1976; Sarada & Joseph, 1993a, b, 1994). These findings have demonstrated the potential for utilisation of tomato waste. However, the high moisture content of seeds makes them susceptible to spoilage at ambient temperature. 1537-5110/03/$30.00

Drying is a widely used technique in food preservation. It is a simultaneous heat and mass transfer operation in which moisture is removed from food material and carried away by hot air. So far, it has not been verified whether the convective drying of the tomato seeds takes place in a constant/falling rate period or combination of both. The drying models being used to describe the convective drying of grains, have not been applied to tomato seeds. Drying of biological material is a diffusion-controlled process and may be represented by Fick’s law. The simplified solution to the diffusion equation has been widely used to describe the drying behaviour of biological materials (Byler et al., 1987; Salek, 1986). The simplified form, also called logarithmic model, is as follows: MR 5ðm  me Þ=ðmo  me Þ5exp ðktÞ


where: MR is the moisture ratio (dimensionless); m is the moisture content in % dry basis at any time t; mo is the initial moisture content in % dry basis; me is the equilibrium moisture content in % dry basis; k is the drying rate constant in h1 ; and t is the time in h. The logarithmic model in Eqn (1) has been used for modelling the drying behaviour of biological materials (Salek, 1986); however, it has been observed that this model did not describe the drying behaviour of many products. It underestimated the drying curve in the 297

# 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Science Ltd



beginning and overestimated in the later stages (Shivhare et al., 1991). Page’s model has also been widely used to describe the drying behaviour of a variety of biological materials with better results than the logarithmic model (Byler et al., 1987; Shivhare et al., 2000; Tan et al., 2001). MR 5expðk tn Þ


where n is a dimensionless exponent. The Arrhenius law can be used to relate the drying rate constant with air temperature. The relationship is described as k5A exp ðB=TÞ


where: T is the absolute temperature in kelvin; and A and B are coefficients. A sorption isotherm represents the equilibrium moisture content (EMC) of a material with the relative humidity (RH) of the surrounding environment at a particular temperature. The nature of the sorption isotherm is unique for each food material. The EMC not only dictates the physical, chemical and microbial stability of a food material but is also used as one of the input parameters in the drying models. Several models i.e. Oswin, Henderson, Hasley, Chung}Pfost, Smith, etc. have been developed to describe the relationship between ERH and EMC of the test material (Iglesias & Chirife, 1982; Rahman,1995; Lopes-filho et al., 2002; Sandoval & Barrerio, 2002). Jayas and Mazza (1993) compared modified Guggenheim–Anderson-de Boer (GAB) equation with commonly used empirical equations (modified Henderson, Chung–Pfost, Hasley) for equilibrium moisture characteristics of oat and found that Chung–Pfost equation was superior to the modified GAB equation. Diosady et al. (1996) confirmed the limitations of the GAB equation to predict EMC of canola meal. The present study was, therefore, undertaken to produce shelf-stable tomato seeds by reducing the moisture content or water activity using a cabinet/ fluidised bed dryer. An attempt has also been made to describe the drying behaviour and sorption isotherm with the help of appropriate models.

2. Materials and methods Seeds were separated from the pomace collected from a tomato paste manufacturing unit by a continuous sedimentation system (Sogi et al., 1999) and dried at 50, 70 and 908C in a cabinet as well as fluidised bed dryer with tray loads of 4–12 kg m–2. The cabinet dryer (Narang Scientific Works, New Delhi) consisted of a

043 m by 084 m by 095 m stainless steel inner chamber, a 5 kW heater and a fan to circulate air inside the chamber. The air velocity in the cabinet dryer was not regulated. The cabinet dryer was adjusted to the selected temperature for about half an hour before the start of experiment to achieve the steady state. The requisite amount of the sample was spread in a single layer on a tray in the dryer. The moisture loss was recorded at 30 min intervals. The trays containing the sample were taken out, weighed and placed back at each interval in about 30 s to avoid any significant temperature variation in the dryer. The fluidised bed dryer (FBD 2000, Endecotts, England) consisted of a 2 kW heater and a blower. The temperature, airflow rate and fluidisation velocity varied from ambient to 2008C, 04–24 m3 min1 and 09–5.0 m s1 respectively. The samples along with the glass tub were weighed after every 15 min. The RH of the drying air (50–908C) ranged from 15 to 30%. 2.1. Sorption procedure Dried seeds were used to determine the sorption isotherms at 30, 40, 50, 60 and 708C using dessicators maintained at RH ranging from 10 to 85% by the static method employing standard saturated salt solutions (Rahman, 1995). Moisture content of the seeds was determined at 1038C for 24 h using the hot-air oven (Ranganna, 1986) and results were expressed in percent dry basis.

3. Results and discussion 3.1. Water sorption isotherm Equilibrium moisture content increased with RH and decreased as temperature was increased (Fig. 1). Selected two parameter models (Iglesias & Chirife, 1982; Sandoval & Barrerio, 2002) were examined to ascertain the effect of RH and temperature on the EMC of the tomato seeds (Table 1). The standard error and coefficient of determinations indicated that Henderson’s model described best the relationship under the given conditions (Fig. 1). The Henderson’s model is represented as me 5½lnð1  aÞ=ðA1 Þ1=B1


where: a is the relative humidity in the fraction; and A1 and B1 are the coefficients. The values of Henderson’s model are given in Table 2. The experimental EMC values were related to the computed values from Henderson’s model. The



Equilibrium moisture content, % d.b.


Table 2 Coefficients A1 and B1 of the Henderson model [Eqn (4)] for tomato seeds (number of experiments 5 3)


Temperature,8C 30 40 50 60 70




47041 22099 23850 3381 2395

2291 2148 2345 1807 2140




0.4 0.6 Relative humidity, fraction


Fig. 1. Moisture sorption isotherms of tomato seeds at selected temperatures: }, predicted value 308C; ^, actual data 308C; – – -, predicted value 408C; m, actual data 408C; ......, predicted value 508C; *, actual data 508C; ––, predicted value 608C; &, actual data 608C; ––, predicted value 70oC; *, actual data 70oC Table 1 Performance of selected models for sorption isotherms to tomato seeds, (number of experiments 5 3) Model

Henderson me 5 (ln ð1  aÞ=ðA1 ÞÞ1=B1

Oswin me 5 A2 ða=ð1  aÞÞB2

Halsey me 5ðA3 =ln aÞ1=B3

Chung–Pfost Me 5 (ln (ln a/A4))/(B4)

Smith Me 5 A5+B5 ln (1a)

Temperature, Coefficient Standard error 8C of determination 30 40 50 60 70

099 094 096 098 097

0058 0147 0120 0100 0129

30 40 50 60 70

083 089 085 071 067

0257 0199 0229 0394 0439

30 40 50 60 70

097 090 092 098 098

0114 0195 0164 0097 0101

30 40 50 60 70

099 095 096 099 098

0277 0886 0626 0384 0417

30 40 50 60 70

099 092 095 094 097

0412 1051 0756 1426 0875

me , equilibrium moisture content, % d.b. a, equilibrium relative humidity, fraction. A1 , A2 , A3 , A4 , A5 , B1 , B2 , B4 and B5 , coefficients.

Predicted EMC, % d.b.



10 8 6 4 2 0



10 4 8 6 Experimental EMC, % d.b.


Fig. 2. Correlation between the values predicted by Henderson model and the experimental values of the equilibrium moisture content (EMC) of tomato seeds

coefficients of determination and standard error for the linear regression were 1 and 0015, respectively (Fig. 2). Values for the EMC as computed by the Henderson’s model were used in the drying model. 3.2. Drying characteristics The initial moisture content of the seeds was 268% dry basis. The Langrangian differentiation technique (O’Neil, 1983), employing three data points was used to calculate the drying rate. The drying rate decreased continuously throughout the drying period indicating that drying of tomato seeds took place in the falling rate period. It can, therefore, be considered a diffusioncontrolled process in which the rate of moisture removal is limited by diffusion of moisture from inside to surface of the product. Linear regression of Page’s model [Eqn (2)] was carried out using the least-squares techniques and the coefficients were determined. Typical drying curves (Fig. 3) indicate that Page’s equation describe the drying behaviour adequately over a wide range of temperature. The values for the coefficient of determination R2 were greater than 087 while the standard errors were less than 014 (Table 3).



Total drying time was reduced substantially with the increase in temperature of air for all tray loads. The rate constant k, which is a measure of the drying rate, increased with temperature (Fig. 4). Results indicated that the Arrhenius law could be used to relate the rate constant with drying air temperature (Table 4). Variation of coefficient n with temperature and tray load was non-systematic. Ahmed et al. (2001) have reported similar behaviour. The rate constant decreased with

increase in tray load at selected drying temperature but the effect was more pronounced at 4 kg/m2 (Table 3). The drying time was reduced considerably at selected temperature and tray loads in case of fluidised bed dryers. It took about 7 h at 708C to reduce moisture content of tomato seeds from 268 to 10% (dry basis), in cabinet dryer, while it took about 1.5 h in fluidised bed system to achieve similar moisture reduction for 8 kg m2 tray load (Fig. 5).

250 Drying rate constant, h-1

Moisture content, % d.b.


200 150 100 50 0 0



6 Time, h



10 1 0.1 0.01 0.0028



Fig. 3. Drying curves of tomato seeds for cabinet dryer with 8 kg m2 tray load at selected temperatures: }, predicted value 508C; m, actual data 508C; – – -, predicted value 708C; &, actual data 708C; ......, predicted value 908C; *, actual data 908C


Absolute temperature,



Fig. 4. Dependence of rate constant on drying air temperatures in cabinet dryer: } , predicted value 4 kg m2; m, actual data 4 kg m2, – – -, predicted value 8 kg m2; ^, actual data 8 kg m2; ...., predicted value 12 kg m2; *, actual data 12 kg m2

Table 3 Coefficients of Page’s model [Eqn (2)] for tomato seeds (number of experiments 5 3) Drying system

Temperature, 8C

Feed rate, kg m2

Drying rate constant (k), h1

Dimensionless coefficient, (n)

Cabinet dryer


4 8 12

0298 0107 0077

1214 1363 1191

095 098 095

0082 0044 0053


4 8 12

0431 0190 0093

0152 0384 0347

094 099 098

0114 0036 0054


4 8 12

1334 0257 0106

1172 1331 1291

099 098 095

0071 0055 0068


4 8 12

1730 1643 1727

0557 0606 0697

1 097 096

0020 0044 0062


4 8 12

3019 2521 2285

0784 0611 0652

098 092 087

1111 0127 0144


4 8 12

3390 3107 1708

0493 0622 0676

1 091 096

0002 0141 0057

Fluidised bed dryer

Coefficient of determination

Standard error


Table 4 Coefficients A and B of the Arrhenius model [Eqn (3)] for cabinet dryer (number of experiments 5 3) Tray load, kg m2 4 8 12



Coefficient of determination

Standard error

183  105 366  102 142

4340 2620 9392

090 098 099

0361 0092 0016



Moisture Content, % d.b.







4 5 Time, h




Fig. 5. Effect of drying systems on the drying behaviour of tomato seeds at 708C and 8 kg m2 tray load: }, predicted value cabinet dryer; m, actual value cabinet dryer; ....., predicted value fluidised bed dryer; &, actual value fluidised bed dryer

4. Conclusions Sorption isotherm of tomato seeds is best described by Henderson’s model. Page’s equation adequately described the drying behaviour of the seeds over the range of temperature and tray load used. Dependence of the rate constant on drying air temperature follows Arrhenius law. The drying time decreased for lower tray loads and higher air temperatures. The drying time reduced considerably in case of fluidised bed dryer.

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