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Drying characteristics and sorption isotherm of tomato slices
Journal of Food Engineering 73 (2006) 157–163 www.elsevier.com/locate/jfoodeng
Drying characteristics and sorption isotherm of tomato slices Charles Taiwo Akanbi *, Remi Sikiru Adeyemi, Ademola Ojo Department of Food Science and Technology, Obafemi Awolowo University, Ile-Ife, Nigeria Received 9 August 2004; accepted 13 January 2005 Available online 11 March 2005
Abstract The drying behaviour of tomato slices was investigated at 45, 60 and 75 C. Three falling rate periods were observed with diffusion coefficients in the range 3.72–12.27 · 109 m2 s1. The water vapour sorption isotherm of dehydrated tomato slices in the water activity (aw) range of 0.08–0.85 was also determined at three temperature levels, i.e., 25, 30 and 40 C. Five sorption models were fitted with the adsorption data generated from the gravimetric method. GAB and Oswin models describe the adsorption characteristics of dehydrated tomato at 25 C better than other models with GAB model being the best applicable model. The isosteric heat of adsorption decreases with increasing moisture. 2005 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Isosteric heat; Water activity; Dehydrated tomato
1. Introduction Tomato (Lycopersicon esculentum Mill) is the worldÕs most commercially produced vegetable (Ensminger, Ensminger, Kolande, & Robson, 1994). The global tomato production reached 108 million metric tons in Calendar Year 2002 (FAS, 2003).Tomato is used to great extent in the fresh state, and in some processes as juice, puree, sauces and canned varieties. Tomato production in Nigeria has more than doubled in the last 10 years; and the production in 2001 alone was 879,000 metric tonnes (FAO, 2002), which is largely consumed in the fresh state. However, tomato is highly perishable in the fresh state leading to wastage and losses during the peak harvesting period. The prevention of these losses and wastage is of major interest especially when there is subsequent imbalance in supply and demand at the harvesting off-season. Dehydration processes offer an alternative way of providing tomato to commerce. The dehydration of to*
Corresponding author. E-mail address: [email protected] (C.T. Akanbi).
0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.01.015
mato has been practiced for many years as a means of preserving tomato. The most popular method of drying tomato in the tropics is hot air drying. This may result in physical and structural changes such as migration of soluble solids, shrinkage and case hardening, loss of volatiles and aroma and slower water absorption during rehydration. To minimize these changes, some drying methods are practiced such as freeze drying, air drying at low temperature and vacuum drying (Akanbi & Oludemi, 2003). It is well understood in hot air drying method that drying curves generated from the drying data may exhibit constant and one or more falling rate periods (Brennan, Butters, Cowell, & Lilly, 1981). However, some foods exhibit only falling rate periods (Jackson & Lamb, 1981). The transport mechanism of moisture movement within the foods in these drying regions have been described by Perry, Green, and Maloney (1999) with only one mechanism dominating at any given time. Thus, FickÕs equation is widely used for explaining drying mechanism of solid food material involving diffusion of vapour (Johnson, Brennan, & Addo-Yobo, 1998; Maskan & Go¨g˘u¨s, 1998; Sankat, Castaigne, & Maharaj, 1996).
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An important factor in the loss of quality of dried foods during storage is the water activity (aw) which influences the biochemical reactions and stability of dried products. Some of these reactions are lipid oxidation, caking, agglomeration and degradation of vitamins and lycopene (Saltmarch & Labuza, 1980; Akanbi & Oludemi, 2003). Several theoretical and empirical models have been proposed and used by investigators to fit equilibrium moisture data of foods and agricultural products (Chirife & Iglesias, 1978; Iglesias & Chirife, 1995). It is usual for each investigator to report which model best fits the experimental data. It is therefore necessary to understand the moisture sorption characteristics of dried tomato. However, little information exists on dehydrated tomato produced for use in the tropics. More importantly, knowledge of the sorption characteristics of dehydrated tomato is required in regard to its keeping quality and acceptability. The aim of this work was to establish the drying and sorption characteristics of tomato over the range of water activities commonly experienced in the tropical markets.
2. Materials and methods
Fresh Tomato
Washing and Sorting
Sulphiting
Slicing
Blanching
(For 180s in sodium metabisulphite solution (1%, weight/volume))
(15mm ± 1mm thick)
(For 60s in steam blancher at steam pressure of 57.8 kN/m2)
( 25s to 30s in 1%(w/v) sodium metabisulphite solution) Sulphiting
2.1. Sample preparation Fresh, mature and ripe tomatoes (Lycopersicum esculentum var. Roma) were purchased from a local market in Ile-Ife, Nigeria. The fresh tomatoes were sorted, graded and washed in water (Fig. 1). Sulphiting was done according to Baloch, Khen, and Baloch (1997) by dipping whole tomatoes in 1% sodium metabisulphite solution for 180 s and then slicing to 15 mm thickness using a stainless steel knife. The sliced tomatoes were blanched for 60 s at 57.8 kN/m2 pressure, prior to sulphiting for 30 s. 2.2. Drying of tomato slices The blanched and sulphited slices were put in drying pan and dried in cabinet air oven operated at air velocity of 0.13 m s1 at temperature 45 C, 60 C or 75 C with drying from the top surface of the drying slices. The ambient air humidity ranged between 0.008 and 0.010 kg (kg dry air)1. Dry bulb and wet bulb temperatures were measured by resistors attached to a temperature recorder. Changes in weight of 10 slices were monitored at 5 min intervals for the first hour, 15 min for another hour and 30 min for subsequent drying times to equilibrium. Drying runs were done in triplicate. Drying curves were constructed from the drying data collected. From these curves were drawn the plots of drying rate by method of gradient at points on the curves to obtain the critical moisture content and the
Drying
(Air oven, at 45 ºC for 48 hrs.)
Dehydrated Tomato
Fig. 1. Flow diagram for the production of dehydrated tomato.
corresponding drying time to reach the critical moisture content values. Analysis of drying data also utilized the dimensionless ratio, X , thus: X ¼ ½X X e =½X 0 X e
ð1Þ
where X = average moisture content (kg moisture/kg dry solid) of the slices at time t; X0 = initial moisture content (kg moisture/kg dry solid) of the slices at t = 0; Xe = equilibrium moisture content (kg moisture/kg dry solid) at the drying temperature (45, 60, 75 C). Several drying mechanisms have been proposed depending on the shape of the drying curve (Brennan et al., 1981; Geankoplis, 1983). However, when a material dries mainly in the falling rate period, then it could be assumed that internal diffusion prevails. Therefore, moisture diffusivity can be calculated from the experimental drying data using FickÕs second law for a slab shaped material drying from one surface according to Geankoplis (1983):
C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163
½X X e =½X 0 X e ¼ 8=p2 exp p2 Deff t=L2
ð2Þ
2 1
Deff = effective diffusivity (m s ) at the drying temperature; L = thickness (m) of the slices; t = drying time (s). The plot of Eq. (2) was drawn as ln [(X Xe)/ (X0 Xe)] versus t for determining the diffusion coefficient from the slope of the plot. The thickness L was assumed constant throughout the drying period. The linear plot of the equation was established using the linear regression analysis of the Excel package of the Microsoft. 2.3. Determination of sorption isotherms The dried slices were immediately transferred into a glass desiccator containing phosphorous pentoxide (P2O5) for 3 days. The equilibrium isotherms of tomato slices dried at 45 C were determined by the gravimetric method at 25, 30 and 40 C. Samples (about 2 g) were put in a dish on a mesh tray above saturated salt solutions of lithium chloride, potassium acetate, magnesium chloride, potassium carbonate, magnesium nitrate, sodium nitrite, sodium chloride and potassium chloride (Greenspan, 1977) with aw in the range (0.08–0.85) in separate tightly closed glass jars, for 15 to 25 days until constant weight was reached. Then the dry matter content was determined by drying the equilibrated samples in a vacuum oven (Model 1410-90, SHEL LAB, Sheldon Manufacturing Inc., Cornelius, USA) at 70 C to evaluate the equilibrium moisture content.
Table 1 Sorption model equations used for dehydrated tomato Model Oswin (1946)a Kuhn (1964) Halsey (1948)
Model equation k aw X ¼C 1 aw C þk X ¼ ln aw 1=k C X ¼ RT ln aw
Brunauer et al. (1938)
X ¼ X m Caw =½ð1 aw Þ
GAB (van den Berg, 1984)
aw ¼ aa2w þ baw þ c X
þ ðC 1Þð1 aw Þaw
where; a ¼ b¼
ðC 2Þ X mC
c¼
1 X m Ck
ð1 CÞ k X mC
ð3Þ
159
2.4. Empirical models for sorption isotherms and isosteric heat of sorption Parameters for the selected mathematical models for sorption of Table 1 describing moisture content as a function of aw were estimated according to Labuza (1984) using linear and non-linear least square regression analyses. To evaluate the ability of each model to fit the experimental data, the mean relative percent deviation modulus (E) and coefficient of determination (r2) were determined using Eqs. (11) and (12), respectively. " 2 , # n X xi xpi E ¼ 100 n ð11Þ xi i¼1 r2 ¼
n X
xp i xi
i¼1
2
,
n X
ðx i x i Þ
2
ð12Þ
i¼1
where xi and xpi are experimental and predicted equilibrium moisture contents, respectively. In order to determine the influence of temperature on the dynamic equilibrium between water activity, aw, and the moisture content, the Clausius-Clapeyron equation was used in the form: dLnðaw Þ=dLnð1=T Þ ¼ DH c =R
ð13Þ
Qst ¼ H c þ H v
ð14Þ
where Qst is the isosteric heat of sorption (kJ mol1), Hc is the heat of interaction of water vapour with solid substance and Hv is the heat of condensation of pure water at 31.6 C (the average temperature of 25, 30, and 40 C), R is the gas constant (kJ K1 mol1) and T is the absolute temperature (K) at the corresponding aw level.
ð4Þ ð5Þ
ð6Þ ð7Þ ð8Þ ð9Þ ð10Þ
aw = water activity; C = empirical constant; k = constant; R = universal gas constant; T = temperature (K); X = moisture content (kg/kg dry solids); Xm = monomolecular moisture content; a, b, c = constants. a See reference listing.
3. Results and discussion The drying rate curves for the slices at the three temperatures 45, 60 and 75 C are shown in Fig. 2 with the equilibrium moisture contents of 0.18, 0.11 and 0.10 kg moisture/kg dry solids, respectively. It was observed that while the samples dried at 45 C had a smooth texture with no noticeable shrinkage, those dried at 60 and 75 C were hardened especially toward the end of the drying period. In all the samples, however, dipping the slices in sodium metabisulphite solution had positive influence on colour retention. Analysis of the drying data could not establish a constant rate period. The drying rate curves consisted of three falling rate periods. The critical moisture content values and the corresponding drying times to reach these values are presented in
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C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163 0.6
Drying rate (g/g dry solids min)
Drying Time (min)
0
Data at 45, 60, and 75oC
0
20
0.5
ln[(X-X0)/(X-Xe)]
0.4
40
50
60
70
0.3 0.2
-0.5
-1
-1.5
y = -0.0197x - 0.0277 2 R = 0.9984
45°C
y = -0.0306x - 0.0479 2 R = 0.9738
60°C
-2
0 0
(a)
1
2
3
4
5
6
7
8
75°C
y = -0.0323x - 0.0883 2 R = 0.948
9
Moisture content (g/g dry solids)
(a)
-2.5
0.45
Data at 45oC and 60oC
0.4
Drying Time (min)
0 0
20
40
60
0.35
80
100
120
140
160
2nd Falling Rate
-0.5
ln[(X-X0)/(X-Xe)]
0.3 0.25 0.2 0.15 0.1
-1 -1.5 -2
45°C
0.05
60°C
-2.5
0 0
(b)
1
2
3
4
5
6
7
8
y = -0.0098x - 0.6316 R 2 = 0.9924 y = -0.0154x - 0.4886 R 2 = 0.9905
75°C
9
y = -0.0156x - 0.5647 R 2 = 0.9925
-3
Moisture content (g/g dry solids)
(b)
0.6
-3.5
Data at 60oC and 75oC Drying Time (min)
0.5
0 0
0.4
100
200
0.3 0.2 0.1
300
400
500
600
3rd Falling Rate
-1
ln[(X-X0)/(X-Xe)]
Drying rate (g/g dry solids min)
30
1st Falling Rate
0.1
Drying rate (g/g dry solids min)
10
-2 -3 -4 y = -0.0109x - 0.4634
-5
45°C
-6
60°C
2
R = 0.9992
0
(c)
0
1
2
3
4
5
6
7
8
9
Moisture content (g/g dry solids) -7
Fig. 2. Drying rate curves of sliced tomato ((n) 45 C; (r) 60 C; (h) 75 C).
(c)
75°C
y = -0.0112x - 0.9499 R 2 = 0.9917 y = -0.0116x - 1.0809 R 2 = 0.996
-8
Fig. 3. Plot of ln [(X X0)/(X Xe)] versus drying time of sliced tomato ((a) 1st falling rate-, (b) 2nd falling rate-, (c) 3rd falling rate period).
Table 2. Jackson and Lamb (1981) describe foods exhibiting this type of behaviour as porous hygroscopic materials. Hawlader, Uddin, Ho, and Teng (1991) obtained two falling rate periods for tomato slices with 5 mm thickness and dried at 60 and 80 C and constant air flow rate of 0.4 m s1. Moreover, the plot of ln
(moisture ratio) versus time shown in Fig. 3 suggested a drying curve characteristic of diffusion control. However, the determination of the diffusivity of mois-
Table 2 Critical moisture content and diffusivity values for drying tomato slices at three falling rate periods Period
Mc (g (g dry solids)1)
1st period
3.6 3.8 4.2
2nd period
1.0 1.25 1.33
3rd period
Drying temperature (C)
Diffusivity (m2 s1) · 109
r2
48 30 20
45 60 75
7.48 11.63 12.27
0.9984 0.9738 0.9480
180 105 105
45 60 75
3.72 5.85 5.92
0.9924 0.9905 0.9925
45 60 75
4.14 4.26 4.41
0.9992 0.9917 0.9960
Time (min) to reach Mc
C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163
ture from the slices using FickÕs equation for a slab resulted in high r2 values from the linear regression analysis of the three diffusion regions (Table 2). The data analysis, according to Geankoplis (1983), established the effective diffusivity in the range (3.72– 12.27 · 109 m2 s1) for the three regions (as presented in Table 2). These values fall within the range of diffusivity reported by Giovanelli, Zanoni, Lavelli, and Nani (2002) for tomato products in the range 2.3 · 109– 9.1 · 109 m2 s1. However, the values obtained were higher than the values reported by Hawlader et al. (1991) for tomato slices dried at both surfaces and various drying conditions. 3.1. The sorption model equations The estimated parameters for all the sorption models at the three temperatures are shown in Table 3. The GAB and Oswin models provided the best fit to the experimental data on the entire aw range (see Fig. 4a– c). More than one sorption model have been reported to describe sorption characteristics of foods (Iglesias & Bueno, 1999; Kaymak-Ertekin & Sultanog˘lu, 2001; Lomauro, Bakshi, & Labuza, 1985). That the BET model holds true for water activities between 0.1 and 0.4 according to Leniger and Beverloo (1975) is underscored by the value of r2 of 0.98 at 30 C and 40 C for aw range of 0.08–0.43 (see Table 4). However, GAB model is more applicable at higher aw levels than BET model. Giovanelli et al. (2002) measured and modelled adsorption and desorption isotherms at
161
20 C of tomato products by the Guggenheim– Anderson–de Boer (GAB) equation. Moisture movement within the dried tomato matrix and the transfer of molecular moisture within its constituent components would result in deviation from the idealized model situation. The chemisorption of water leads to the transformation of macromolecules (Iglesias, Chirife, & Lombardi, 1975; McLaren & Rowen, 1951; Wang & Brennan, 1991). The monomolecular moisture content of dehydrated tomato was in the range 0.116– 0.151 kg/kg dry solids and 0.119–0.205 kg/kg dry solids calculated from BET and GAB models, respectively. These values are comparable with the monolayer values reported for tomato by Kiranoudis, Maroulis, Tsami, and Marinos-Kouris (1993). 3.2. Heat of adsorption The isosteric heats of sorption at various water contents or (aw) were calculated from the plot of Eq. (13) using the slopes of the curves at the three temperatures. The total heat of adsorption of water on dehydrated tomato (obtained from Eq. (14)) reveals differences in the adsorption propensity of the product at different moisture levels. It could be observed that the heat of sorption decreases with increasing moisture (Fig. 5). A plausible reason for the shape of the curve could be the differences in hydrogen bonding as moisture is absorbed probably due to differences in attractive forces between water molecules and sorption sites; and between water molecules themselves. This observation indicates greater
Table 3 Parameter values of the models fitted to the dehydrated tomato sorption isotherms in the water activity range 0.08–0.85 Model
Temperature (C)
C
k
Xm (kg/kg dry solids)
E (%)
r2
OSWIN 25 30 40
0.313 0.288 0.244
0.408 0.387 0.355
– – –
2.72 9.24 8.10
0.94 0.96 0.93
25 30 40
0.089 0.084 0.042
0.080 0.079 0.086
– – –
7.34 13.84 21.05
0.76 0.85 0.89
25 30 40
326.7 381.5 482.5
1.615 1.694 1.847
– – –
11.95 59.56 17.58
0.93 0.89 0.89
25 30 40
– 15.75 29.72
– – –
– 0.151 0.116
– 27.36 15.41
0.81 0.75 0.66
25 30 40
16.95 8.39 23.03
0.805 0.817 0.859
0.205 0.191 0.119
1.98 7.21 7.80
0.96 0.96 0.96
KUHN
HALSEY
BETa
GAB
a
Water activity range for BET was 0.08–0.55.
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C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163 Table 4 Water activity range and coefficient of determination for the isotherm models of water vapour on dehydrated tomato
X (kg/kg dry solids)
0.7 0.6
Model
0.5 Oswin Kuhn Halsey GAB Experimental
0.4 0.3
Xm (kg/kg dry solids)
0
0.2
0.4
0.6
0.8
0.96 0.95 0.94 0.94
0.93 0.96 0.94 0.93
Halsey
– – – –
0.08–0.85 0.08–0.76 0.08–0.65 0.08–0.55
0.93 0.91 0.92 0.94
0.89 0.89 0.83 0.74
0.89 0.91 0.84 0.86
Kuhn
– – – –
0.08–0.85 0.08–0.76 0.08–0.65 0.08–0.55
0.76 0.90 0.96 0.97
0.85 0.93 0.91 0.80
0.89 0.95 0.88 0.82
BET
– – – 0.151–0.157 0.116–0.124
0.08–0.85 0.08–0.76 0.08–0.65 0.08–0.55 0.08–0.43
0.81 0.77 0.81 0.79 0.86
0.75 0.72 0.72 0.75 0.98
0.66 0.69 0.72 0.89 0.98
GAB
0.139–0.142 0.141–0.145 0.133–0.139 0.137–0.141 0.146–0.191
0.22–0.85 0.22–0.76 0.22–0.65 0.22–0.55 0.22–0.43
0.96 0.96 0.93 0.98 0.99
0.96 0.96 0.96 0.97 0.99
0.96 0.93 0.95 0.98 0.99
X (kg/kg dry solids)
1
0.8
0.6
0.4
0.2
0 0
0.1
0.2
0.3
0.4
(b)
0.5
0.6
0.7
0.8
40 C
0.94 0.93 0.96 0.95
1.2
Oswin Kuhn Halsey BET GAB Experimental
30 C
0.08–0.85 0.08–0.76 0.08–0.65 0.08–0.55
1
aw
25 C – – – –
0
(a)
r2
Oswin
0.2 0.1
aw range
0.9
aw 1.2 16 14
Oswin Kuhn Halsey BET GAB Experimental
0.8
12
Qst (kJ/kg)
X (kg/kg dry solids)
1
0.6
10 8 6 4
0.4
2 0.2
0 0
0
(c)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.05
Fig. 4. (a) Predicted and experimental sorption at: (a) 25 C, (b) 30 C and (c) 40 C.
propensity for water adsorption probably due to cooperative and non-cooperative ability of macromolecules resulting in higher binding energy at lower moisture levels. Similar effects of moisture on the binding energy of sorption were also reported for potato (Wang & Brennan, 1991), red peppers (Kim, Song, & Yam, 1991), wheat gluten (De Jong, van den Berg, & Kokelaar, 1996), mulberry (Maskan & Go¨g˘u¨s, 1998) and agar– agar (Iglesias & Bueno, 1999).
0.15
0.2
0.25
0.3
0.35
0.4
Moisture, X (kg/kg dry solids)
0.9
aw
0.1
Fig. 5. Isosteric heat of adsorption.
4. Conclusions This study has shown that tomato slices dried in the falling rate period and the moisture loss could be described by the diffusion mechanism. Slices dried at 45 C had a smooth texture with no noticeable shrinkage, those dried at 60 and 75 C were hardened. GAB polynomial and Oswin models describe the water vapour adsorption isotherms of the dehydrated tomato at 25 C (for the entire aw range) better than other models. However, GAB model best describes its keeping quality at the three temperature levels. The results sug-
C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163
gest that the drying and sorption of dehydrated tomato described in this investigation may be useful in determining the behaviour of the product when it is presented in environments of different temperatures and water activities, especially as it is common practice in the tropics.
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Iglesias, H. A., Chirife, J., & Lombardi, J. L. (1975). Water sorption isotherm in sugar beet root. Journal of Food Technology, 10, 299–308. Iglesias, O., & Bueno, J. L. (1999). Water agar–agar equilibrium: Determination and correlation of sorption isotherms. International Journal of Food Science and Technology, 34, 209–216. Jackson, A. T., & Lamb, J. (1981). Calculations in food and chemical engineering: Theory, worked examples and problems (p. 88). London and Basingstoke: Macmillan Press Ltd. Johnson, P.-N. T., Brennan, J. G., & Addo-Yobo, F. Y. (1998). Airdrying characteristics of plantain (Musa AAB). Journal of Food Engineering, 37, 233–242. Kaymak-Ertekin, F., & Sultanog˘lu, M. (2001). Moisture sorption isotherm characteristics of peppers. Journal of Food Engineering, 47, 225–231. Kim, H. K., Song, Y., & Yam, K. L. (1991). Water sorption characteristics of dried red peppers (Capsicum annum L.). International Journal of Food Science and Technology, 29, 339–345. Kiranoudis, C. T., Maroulis, Z. B., Tsami, E., & Marinos-Kouris, D. (1993). Equilibrium moisture content and heat of desorption of some vegetables. Journal of Food Engineering, 20, 55–74. Kuhn, I. (1964). A new theoretical analysis of adsorption phenomena: Introductory part: The characteristic expression of the main regular types of adsorption isotherms by a simple equation. Journal of Colloid Science, 19, 685–698. Labuza, T. P. (1984). Moisture sorption: Practical aspects of isotherm measurement and use (pp. 69–73). St. Paul, MN: The American Association of Cereal Chemists. Leniger, H. A., & Beverloo, W. A. (1975). Food process engineering Dordrecht—Holland (p. 408). Boston—USA: D. Reidel Publishing Company. Lomauro, C. J., Bakshi, A. S., & Labuza, T. P. (1985). Evaluation of food moisture sorption isotherm equations: Part I. Fruit, vegetable and meat product. Lebensmittel-Wissenschaft und Technologie, 18, 111–117. Maskan, M., & Go¨g˘u¨s, F. (1998). Sorption and drying characteristics of mulberry (Morus alba). Journal of Food Engineering, 37, 437–449. McLaren, A. D., & Rowen, J. W. (1951). Sorption of water vapour by proteins and polymers: a review. Journal of Polymer Science, 57, 289–324. Oswin, C. R. (1946). The kinetics of package life. III. The isotherm. Journal of Chemistry and Industry, 65, 419–423. Perry, R. H., Green, D. W., & Maloney, J. O. (1999). Chemical engineers’ handbook (7th ed.). New York: McGraw-Hill Company Inc. Saltmarch, M., & Labuza, T. P. (1980). Influence of relative humidity on the physico-chemical state of lactose in spray-dried sweet whey powders. Journal of Food Science, 45, 1231–1236, 1242. Sankat, C. K., Castaigne, F., & Maharaj, R. (1996). The air drying behaviour of fresh and osmotically dehydrated banana slices. International Journal of Food Science and Technology, 31, 123– 135. 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. 311–321). London: Elsevier Applied Science Publishers. Wang, N., & Brennan, J. G. (1991). Moisture sorption isotherm characteristics of potatoes at four temperatures. Journal of Food Engineering, 14, 269–287.
Drying characteristics and sorption isotherm of tomato slices Charles Taiwo Akanbi *, Remi Sikiru Adeyemi, Ademola Ojo Department of Food Science and Technology, Obafemi Awolowo University, Ile-Ife, Nigeria Received 9 August 2004; accepted 13 January 2005 Available online 11 March 2005
Abstract The drying behaviour of tomato slices was investigated at 45, 60 and 75 C. Three falling rate periods were observed with diffusion coefficients in the range 3.72–12.27 · 109 m2 s1. The water vapour sorption isotherm of dehydrated tomato slices in the water activity (aw) range of 0.08–0.85 was also determined at three temperature levels, i.e., 25, 30 and 40 C. Five sorption models were fitted with the adsorption data generated from the gravimetric method. GAB and Oswin models describe the adsorption characteristics of dehydrated tomato at 25 C better than other models with GAB model being the best applicable model. The isosteric heat of adsorption decreases with increasing moisture. 2005 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Isosteric heat; Water activity; Dehydrated tomato
1. Introduction Tomato (Lycopersicon esculentum Mill) is the worldÕs most commercially produced vegetable (Ensminger, Ensminger, Kolande, & Robson, 1994). The global tomato production reached 108 million metric tons in Calendar Year 2002 (FAS, 2003).Tomato is used to great extent in the fresh state, and in some processes as juice, puree, sauces and canned varieties. Tomato production in Nigeria has more than doubled in the last 10 years; and the production in 2001 alone was 879,000 metric tonnes (FAO, 2002), which is largely consumed in the fresh state. However, tomato is highly perishable in the fresh state leading to wastage and losses during the peak harvesting period. The prevention of these losses and wastage is of major interest especially when there is subsequent imbalance in supply and demand at the harvesting off-season. Dehydration processes offer an alternative way of providing tomato to commerce. The dehydration of to*
Corresponding author. E-mail address: [email protected] (C.T. Akanbi).
0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.01.015
mato has been practiced for many years as a means of preserving tomato. The most popular method of drying tomato in the tropics is hot air drying. This may result in physical and structural changes such as migration of soluble solids, shrinkage and case hardening, loss of volatiles and aroma and slower water absorption during rehydration. To minimize these changes, some drying methods are practiced such as freeze drying, air drying at low temperature and vacuum drying (Akanbi & Oludemi, 2003). It is well understood in hot air drying method that drying curves generated from the drying data may exhibit constant and one or more falling rate periods (Brennan, Butters, Cowell, & Lilly, 1981). However, some foods exhibit only falling rate periods (Jackson & Lamb, 1981). The transport mechanism of moisture movement within the foods in these drying regions have been described by Perry, Green, and Maloney (1999) with only one mechanism dominating at any given time. Thus, FickÕs equation is widely used for explaining drying mechanism of solid food material involving diffusion of vapour (Johnson, Brennan, & Addo-Yobo, 1998; Maskan & Go¨g˘u¨s, 1998; Sankat, Castaigne, & Maharaj, 1996).
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An important factor in the loss of quality of dried foods during storage is the water activity (aw) which influences the biochemical reactions and stability of dried products. Some of these reactions are lipid oxidation, caking, agglomeration and degradation of vitamins and lycopene (Saltmarch & Labuza, 1980; Akanbi & Oludemi, 2003). Several theoretical and empirical models have been proposed and used by investigators to fit equilibrium moisture data of foods and agricultural products (Chirife & Iglesias, 1978; Iglesias & Chirife, 1995). It is usual for each investigator to report which model best fits the experimental data. It is therefore necessary to understand the moisture sorption characteristics of dried tomato. However, little information exists on dehydrated tomato produced for use in the tropics. More importantly, knowledge of the sorption characteristics of dehydrated tomato is required in regard to its keeping quality and acceptability. The aim of this work was to establish the drying and sorption characteristics of tomato over the range of water activities commonly experienced in the tropical markets.
2. Materials and methods
Fresh Tomato
Washing and Sorting
Sulphiting
Slicing
Blanching
(For 180s in sodium metabisulphite solution (1%, weight/volume))
(15mm ± 1mm thick)
(For 60s in steam blancher at steam pressure of 57.8 kN/m2)
( 25s to 30s in 1%(w/v) sodium metabisulphite solution) Sulphiting
2.1. Sample preparation Fresh, mature and ripe tomatoes (Lycopersicum esculentum var. Roma) were purchased from a local market in Ile-Ife, Nigeria. The fresh tomatoes were sorted, graded and washed in water (Fig. 1). Sulphiting was done according to Baloch, Khen, and Baloch (1997) by dipping whole tomatoes in 1% sodium metabisulphite solution for 180 s and then slicing to 15 mm thickness using a stainless steel knife. The sliced tomatoes were blanched for 60 s at 57.8 kN/m2 pressure, prior to sulphiting for 30 s. 2.2. Drying of tomato slices The blanched and sulphited slices were put in drying pan and dried in cabinet air oven operated at air velocity of 0.13 m s1 at temperature 45 C, 60 C or 75 C with drying from the top surface of the drying slices. The ambient air humidity ranged between 0.008 and 0.010 kg (kg dry air)1. Dry bulb and wet bulb temperatures were measured by resistors attached to a temperature recorder. Changes in weight of 10 slices were monitored at 5 min intervals for the first hour, 15 min for another hour and 30 min for subsequent drying times to equilibrium. Drying runs were done in triplicate. Drying curves were constructed from the drying data collected. From these curves were drawn the plots of drying rate by method of gradient at points on the curves to obtain the critical moisture content and the
Drying
(Air oven, at 45 ºC for 48 hrs.)
Dehydrated Tomato
Fig. 1. Flow diagram for the production of dehydrated tomato.
corresponding drying time to reach the critical moisture content values. Analysis of drying data also utilized the dimensionless ratio, X , thus: X ¼ ½X X e =½X 0 X e
ð1Þ
where X = average moisture content (kg moisture/kg dry solid) of the slices at time t; X0 = initial moisture content (kg moisture/kg dry solid) of the slices at t = 0; Xe = equilibrium moisture content (kg moisture/kg dry solid) at the drying temperature (45, 60, 75 C). Several drying mechanisms have been proposed depending on the shape of the drying curve (Brennan et al., 1981; Geankoplis, 1983). However, when a material dries mainly in the falling rate period, then it could be assumed that internal diffusion prevails. Therefore, moisture diffusivity can be calculated from the experimental drying data using FickÕs second law for a slab shaped material drying from one surface according to Geankoplis (1983):
C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163
½X X e =½X 0 X e ¼ 8=p2 exp p2 Deff t=L2
ð2Þ
2 1
Deff = effective diffusivity (m s ) at the drying temperature; L = thickness (m) of the slices; t = drying time (s). The plot of Eq. (2) was drawn as ln [(X Xe)/ (X0 Xe)] versus t for determining the diffusion coefficient from the slope of the plot. The thickness L was assumed constant throughout the drying period. The linear plot of the equation was established using the linear regression analysis of the Excel package of the Microsoft. 2.3. Determination of sorption isotherms The dried slices were immediately transferred into a glass desiccator containing phosphorous pentoxide (P2O5) for 3 days. The equilibrium isotherms of tomato slices dried at 45 C were determined by the gravimetric method at 25, 30 and 40 C. Samples (about 2 g) were put in a dish on a mesh tray above saturated salt solutions of lithium chloride, potassium acetate, magnesium chloride, potassium carbonate, magnesium nitrate, sodium nitrite, sodium chloride and potassium chloride (Greenspan, 1977) with aw in the range (0.08–0.85) in separate tightly closed glass jars, for 15 to 25 days until constant weight was reached. Then the dry matter content was determined by drying the equilibrated samples in a vacuum oven (Model 1410-90, SHEL LAB, Sheldon Manufacturing Inc., Cornelius, USA) at 70 C to evaluate the equilibrium moisture content.
Table 1 Sorption model equations used for dehydrated tomato Model Oswin (1946)a Kuhn (1964) Halsey (1948)
Model equation k aw X ¼C 1 aw C þk X ¼ ln aw 1=k C X ¼ RT ln aw
Brunauer et al. (1938)
X ¼ X m Caw =½ð1 aw Þ
GAB (van den Berg, 1984)
aw ¼ aa2w þ baw þ c X
þ ðC 1Þð1 aw Þaw
where; a ¼ b¼
ðC 2Þ X mC
c¼
1 X m Ck
ð1 CÞ k X mC
ð3Þ
159
2.4. Empirical models for sorption isotherms and isosteric heat of sorption Parameters for the selected mathematical models for sorption of Table 1 describing moisture content as a function of aw were estimated according to Labuza (1984) using linear and non-linear least square regression analyses. To evaluate the ability of each model to fit the experimental data, the mean relative percent deviation modulus (E) and coefficient of determination (r2) were determined using Eqs. (11) and (12), respectively. " 2 , # n X xi xpi E ¼ 100 n ð11Þ xi i¼1 r2 ¼
n X
xp i xi
i¼1
2
,
n X
ðx i x i Þ
2
ð12Þ
i¼1
where xi and xpi are experimental and predicted equilibrium moisture contents, respectively. In order to determine the influence of temperature on the dynamic equilibrium between water activity, aw, and the moisture content, the Clausius-Clapeyron equation was used in the form: dLnðaw Þ=dLnð1=T Þ ¼ DH c =R
ð13Þ
Qst ¼ H c þ H v
ð14Þ
where Qst is the isosteric heat of sorption (kJ mol1), Hc is the heat of interaction of water vapour with solid substance and Hv is the heat of condensation of pure water at 31.6 C (the average temperature of 25, 30, and 40 C), R is the gas constant (kJ K1 mol1) and T is the absolute temperature (K) at the corresponding aw level.
ð4Þ ð5Þ
ð6Þ ð7Þ ð8Þ ð9Þ ð10Þ
aw = water activity; C = empirical constant; k = constant; R = universal gas constant; T = temperature (K); X = moisture content (kg/kg dry solids); Xm = monomolecular moisture content; a, b, c = constants. a See reference listing.
3. Results and discussion The drying rate curves for the slices at the three temperatures 45, 60 and 75 C are shown in Fig. 2 with the equilibrium moisture contents of 0.18, 0.11 and 0.10 kg moisture/kg dry solids, respectively. It was observed that while the samples dried at 45 C had a smooth texture with no noticeable shrinkage, those dried at 60 and 75 C were hardened especially toward the end of the drying period. In all the samples, however, dipping the slices in sodium metabisulphite solution had positive influence on colour retention. Analysis of the drying data could not establish a constant rate period. The drying rate curves consisted of three falling rate periods. The critical moisture content values and the corresponding drying times to reach these values are presented in
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C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163 0.6
Drying rate (g/g dry solids min)
Drying Time (min)
0
Data at 45, 60, and 75oC
0
20
0.5
ln[(X-X0)/(X-Xe)]
0.4
40
50
60
70
0.3 0.2
-0.5
-1
-1.5
y = -0.0197x - 0.0277 2 R = 0.9984
45°C
y = -0.0306x - 0.0479 2 R = 0.9738
60°C
-2
0 0
(a)
1
2
3
4
5
6
7
8
75°C
y = -0.0323x - 0.0883 2 R = 0.948
9
Moisture content (g/g dry solids)
(a)
-2.5
0.45
Data at 45oC and 60oC
0.4
Drying Time (min)
0 0
20
40
60
0.35
80
100
120
140
160
2nd Falling Rate
-0.5
ln[(X-X0)/(X-Xe)]
0.3 0.25 0.2 0.15 0.1
-1 -1.5 -2
45°C
0.05
60°C
-2.5
0 0
(b)
1
2
3
4
5
6
7
8
y = -0.0098x - 0.6316 R 2 = 0.9924 y = -0.0154x - 0.4886 R 2 = 0.9905
75°C
9
y = -0.0156x - 0.5647 R 2 = 0.9925
-3
Moisture content (g/g dry solids)
(b)
0.6
-3.5
Data at 60oC and 75oC Drying Time (min)
0.5
0 0
0.4
100
200
0.3 0.2 0.1
300
400
500
600
3rd Falling Rate
-1
ln[(X-X0)/(X-Xe)]
Drying rate (g/g dry solids min)
30
1st Falling Rate
0.1
Drying rate (g/g dry solids min)
10
-2 -3 -4 y = -0.0109x - 0.4634
-5
45°C
-6
60°C
2
R = 0.9992
0
(c)
0
1
2
3
4
5
6
7
8
9
Moisture content (g/g dry solids) -7
Fig. 2. Drying rate curves of sliced tomato ((n) 45 C; (r) 60 C; (h) 75 C).
(c)
75°C
y = -0.0112x - 0.9499 R 2 = 0.9917 y = -0.0116x - 1.0809 R 2 = 0.996
-8
Fig. 3. Plot of ln [(X X0)/(X Xe)] versus drying time of sliced tomato ((a) 1st falling rate-, (b) 2nd falling rate-, (c) 3rd falling rate period).
Table 2. Jackson and Lamb (1981) describe foods exhibiting this type of behaviour as porous hygroscopic materials. Hawlader, Uddin, Ho, and Teng (1991) obtained two falling rate periods for tomato slices with 5 mm thickness and dried at 60 and 80 C and constant air flow rate of 0.4 m s1. Moreover, the plot of ln
(moisture ratio) versus time shown in Fig. 3 suggested a drying curve characteristic of diffusion control. However, the determination of the diffusivity of mois-
Table 2 Critical moisture content and diffusivity values for drying tomato slices at three falling rate periods Period
Mc (g (g dry solids)1)
1st period
3.6 3.8 4.2
2nd period
1.0 1.25 1.33
3rd period
Drying temperature (C)
Diffusivity (m2 s1) · 109
r2
48 30 20
45 60 75
7.48 11.63 12.27
0.9984 0.9738 0.9480
180 105 105
45 60 75
3.72 5.85 5.92
0.9924 0.9905 0.9925
45 60 75
4.14 4.26 4.41
0.9992 0.9917 0.9960
Time (min) to reach Mc
C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163
ture from the slices using FickÕs equation for a slab resulted in high r2 values from the linear regression analysis of the three diffusion regions (Table 2). The data analysis, according to Geankoplis (1983), established the effective diffusivity in the range (3.72– 12.27 · 109 m2 s1) for the three regions (as presented in Table 2). These values fall within the range of diffusivity reported by Giovanelli, Zanoni, Lavelli, and Nani (2002) for tomato products in the range 2.3 · 109– 9.1 · 109 m2 s1. However, the values obtained were higher than the values reported by Hawlader et al. (1991) for tomato slices dried at both surfaces and various drying conditions. 3.1. The sorption model equations The estimated parameters for all the sorption models at the three temperatures are shown in Table 3. The GAB and Oswin models provided the best fit to the experimental data on the entire aw range (see Fig. 4a– c). More than one sorption model have been reported to describe sorption characteristics of foods (Iglesias & Bueno, 1999; Kaymak-Ertekin & Sultanog˘lu, 2001; Lomauro, Bakshi, & Labuza, 1985). That the BET model holds true for water activities between 0.1 and 0.4 according to Leniger and Beverloo (1975) is underscored by the value of r2 of 0.98 at 30 C and 40 C for aw range of 0.08–0.43 (see Table 4). However, GAB model is more applicable at higher aw levels than BET model. Giovanelli et al. (2002) measured and modelled adsorption and desorption isotherms at
161
20 C of tomato products by the Guggenheim– Anderson–de Boer (GAB) equation. Moisture movement within the dried tomato matrix and the transfer of molecular moisture within its constituent components would result in deviation from the idealized model situation. The chemisorption of water leads to the transformation of macromolecules (Iglesias, Chirife, & Lombardi, 1975; McLaren & Rowen, 1951; Wang & Brennan, 1991). The monomolecular moisture content of dehydrated tomato was in the range 0.116– 0.151 kg/kg dry solids and 0.119–0.205 kg/kg dry solids calculated from BET and GAB models, respectively. These values are comparable with the monolayer values reported for tomato by Kiranoudis, Maroulis, Tsami, and Marinos-Kouris (1993). 3.2. Heat of adsorption The isosteric heats of sorption at various water contents or (aw) were calculated from the plot of Eq. (13) using the slopes of the curves at the three temperatures. The total heat of adsorption of water on dehydrated tomato (obtained from Eq. (14)) reveals differences in the adsorption propensity of the product at different moisture levels. It could be observed that the heat of sorption decreases with increasing moisture (Fig. 5). A plausible reason for the shape of the curve could be the differences in hydrogen bonding as moisture is absorbed probably due to differences in attractive forces between water molecules and sorption sites; and between water molecules themselves. This observation indicates greater
Table 3 Parameter values of the models fitted to the dehydrated tomato sorption isotherms in the water activity range 0.08–0.85 Model
Temperature (C)
C
k
Xm (kg/kg dry solids)
E (%)
r2
OSWIN 25 30 40
0.313 0.288 0.244
0.408 0.387 0.355
– – –
2.72 9.24 8.10
0.94 0.96 0.93
25 30 40
0.089 0.084 0.042
0.080 0.079 0.086
– – –
7.34 13.84 21.05
0.76 0.85 0.89
25 30 40
326.7 381.5 482.5
1.615 1.694 1.847
– – –
11.95 59.56 17.58
0.93 0.89 0.89
25 30 40
– 15.75 29.72
– – –
– 0.151 0.116
– 27.36 15.41
0.81 0.75 0.66
25 30 40
16.95 8.39 23.03
0.805 0.817 0.859
0.205 0.191 0.119
1.98 7.21 7.80
0.96 0.96 0.96
KUHN
HALSEY
BETa
GAB
a
Water activity range for BET was 0.08–0.55.
162
C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163 Table 4 Water activity range and coefficient of determination for the isotherm models of water vapour on dehydrated tomato
X (kg/kg dry solids)
0.7 0.6
Model
0.5 Oswin Kuhn Halsey GAB Experimental
0.4 0.3
Xm (kg/kg dry solids)
0
0.2
0.4
0.6
0.8
0.96 0.95 0.94 0.94
0.93 0.96 0.94 0.93
Halsey
– – – –
0.08–0.85 0.08–0.76 0.08–0.65 0.08–0.55
0.93 0.91 0.92 0.94
0.89 0.89 0.83 0.74
0.89 0.91 0.84 0.86
Kuhn
– – – –
0.08–0.85 0.08–0.76 0.08–0.65 0.08–0.55
0.76 0.90 0.96 0.97
0.85 0.93 0.91 0.80
0.89 0.95 0.88 0.82
BET
– – – 0.151–0.157 0.116–0.124
0.08–0.85 0.08–0.76 0.08–0.65 0.08–0.55 0.08–0.43
0.81 0.77 0.81 0.79 0.86
0.75 0.72 0.72 0.75 0.98
0.66 0.69 0.72 0.89 0.98
GAB
0.139–0.142 0.141–0.145 0.133–0.139 0.137–0.141 0.146–0.191
0.22–0.85 0.22–0.76 0.22–0.65 0.22–0.55 0.22–0.43
0.96 0.96 0.93 0.98 0.99
0.96 0.96 0.96 0.97 0.99
0.96 0.93 0.95 0.98 0.99
X (kg/kg dry solids)
1
0.8
0.6
0.4
0.2
0 0
0.1
0.2
0.3
0.4
(b)
0.5
0.6
0.7
0.8
40 C
0.94 0.93 0.96 0.95
1.2
Oswin Kuhn Halsey BET GAB Experimental
30 C
0.08–0.85 0.08–0.76 0.08–0.65 0.08–0.55
1
aw
25 C – – – –
0
(a)
r2
Oswin
0.2 0.1
aw range
0.9
aw 1.2 16 14
Oswin Kuhn Halsey BET GAB Experimental
0.8
12
Qst (kJ/kg)
X (kg/kg dry solids)
1
0.6
10 8 6 4
0.4
2 0.2
0 0
0
(c)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.05
Fig. 4. (a) Predicted and experimental sorption at: (a) 25 C, (b) 30 C and (c) 40 C.
propensity for water adsorption probably due to cooperative and non-cooperative ability of macromolecules resulting in higher binding energy at lower moisture levels. Similar effects of moisture on the binding energy of sorption were also reported for potato (Wang & Brennan, 1991), red peppers (Kim, Song, & Yam, 1991), wheat gluten (De Jong, van den Berg, & Kokelaar, 1996), mulberry (Maskan & Go¨g˘u¨s, 1998) and agar– agar (Iglesias & Bueno, 1999).
0.15
0.2
0.25
0.3
0.35
0.4
Moisture, X (kg/kg dry solids)
0.9
aw
0.1
Fig. 5. Isosteric heat of adsorption.
4. Conclusions This study has shown that tomato slices dried in the falling rate period and the moisture loss could be described by the diffusion mechanism. Slices dried at 45 C had a smooth texture with no noticeable shrinkage, those dried at 60 and 75 C were hardened. GAB polynomial and Oswin models describe the water vapour adsorption isotherms of the dehydrated tomato at 25 C (for the entire aw range) better than other models. However, GAB model best describes its keeping quality at the three temperature levels. The results sug-
C.T. Akanbi et al. / Journal of Food Engineering 73 (2006) 157–163
gest that the drying and sorption of dehydrated tomato described in this investigation may be useful in determining the behaviour of the product when it is presented in environments of different temperatures and water activities, especially as it is common practice in the tropics.
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