Abstract: Drying kinetics of organically produced tomato slice was studied in a conventional hot-air dryer. The samples were dried at 50, 60 and 70°C air temperature with control and blanching as pretreatments. Drying of tomato occurred in falling rate period. Eight thin layer drying models were evaluated by fitting to the experimental moisture ratio data. Among the mathematical models investigated, the logarithmic model satisfactorily described the drying behaviour of organic tomato slices with high r2 values. The effective moisture diffusivity of tomato samples increased as the drying air temperature was increased. Also the moisture diffusivity and activation energy were higher for blanched samples.
INTRODUCTION
Tomato is one of the world’s largest vegetable crops next only to potato and is available round the year. Organically produced tomatoes are in higher demand recently due to the belief of consumers that they are highly nutritive and have better taste (Woese et al., 1997). Organic production of tomato has attracted premium price and brings a 10-30% higher price than the conventionally produced tomatoes. As a processing crop, it ranks first among the vegetables (Ilyas et al., 2003). Ripe tomato fruit is consumed fresh and utilized in the manufacture of a range of processed products such as puree, paste, powder, ketchup, sauce, soup and canned whole fruits. Tomatoes are important source of lycopene and vitamin C and are valued for their colour and flavour. Dried tomatoes are rich in flavour, minerals and fibre. Commercially dried tomatoes are used in the preparation of sauce, powder, etc.
Drying involves the removal of moisture contained in the fruits or vegetables in order to preserve. Although preservation for enhanced shelf life is the primary reason for drying, it also lowers the product mass and volume. The reduction in mass and volume improves the efficiency of packaging, storing and transportation. Traditionally fruits and vegetables are dried in open sunlight, which is weather dependable and also prone to microbial and other contamination. To get best quality dried product hot air industrial dryers should be used. Industrial dryers are rapid and provide uniform, hygienic dried product (Doymaz and Pala, 2002). Also, blanching of vegetables prevents loss of colour by inactivating enzymes, reduces drying time by relaxing tissue structure and yield a good quality dried product (Piga et al., 2004).
The drying kinetics of vegetables is a complex phenomenon and requires simple representations to predict the drying behaviour and for optimizing the drying parameters. Thin layer drying equations has been used for drying time prediction and for generalization of drying curves (Karathanos and Belessiotis, 1999). Extensive research in drying behaviour of vegetables was reported (Hawlader et al., 1991; Rapuscas and Driscoll, 1995; Methakhup et al., 2005; Tunde-Akintunde et al., 2005; Kaleemullah and Kailappan, 2006; Akanbi et al., 2006; Kumar et al., 2006). But, no detailed studies were found in literature on drying kinetics of organically produced tomato. The objectives of this study were: i) to study the drying kinetics of organically produced tomato ii) to calculate the effective moisture diffusivity and activation energy during the drying process.
Materials and Methods
Experimental Material
Tomato, grown in the organic system of cultivation (cv. Naveen), was
procured from the experimental farm of CIPHET, Abohar, Punjab, India for the
experiments. Matured and firm tomatoes were selected from the whole lot. The
initial moisture content of tomato was 1350.80% d.b. and was determined by the
AOAC method No. 934.06 (AOAC, 2000). Tomatoes were sliced uniformly (average
thickness: 4.3±0.5 mm) and were dried on the same day.
Drying Equipment
The drying experiments were conducted in a cabinet dryer (Narang Scientific
Works, New Delhi). Overall dimensions of the dryer are, height: 1.48 m, width:
1.02 m and depth: 1.12 m. The dryer consisted of trays (800x400x30 mm), temperature
controller (0-300°C, dry bulb temperature, accuracy ±1°C) and
a centrifugal fan for airflow (1.2 m sec-1).
Drying Procedure
Tomato slices were dried with pretreatments namely control (untreated sample)
and blanching (70°C for 2 min). Drying experiments were conducted at 50,
60 and 70°C (±1°C). The dryer was allowed to run for 30 min to
reach the set drying air temperature conditions. Tomato slices (1000 g) were
uniformly spread in rectangular aluminium trays and loaded in the dryer. Moisture
loss was recorded at 30 min interval by a digital balance of 0.01 g accuracy.
The drying was continued till the final moisture content reached 10±0.5%
d.b. Experiments were replicated three times to minimize error.
Evaluation of Thin Layer Drying Models
Moisture ratio of samples during drying was expressed by the following equation:
(1) |
where MR is the dimensionless moisture ratio; M is the moisture content at time t and Mo and Me, the initial and equilibrium moisture contents, respectively, on dry basis.
The moisture ratio was simplified according to Pala et al. (1996), since the MR values are relatively smaller when compared to M and Mo, to :
(2) |
Moisture ratio data was fitted with eight thin layer drying equations (Table
1) to select a suitable model for describing the drying process of tomato
slices. Non-linear regression analysis was performed using SPSS (Statistical
Package for Social Science) 11.5.1 program. Coefficient of correlation, r2
was one of the main criteria for selecting the best model. In addition to coefficient
of correlation, the goodness of fit was determined by various statistical parameters
such as reduced chi-square, χ2, mean bias error, MBE and root
mean square error, RMSE. For quality fit, r2 value should be higher
and χ2, MBE and RMSE values should be lower (Togrul
and Pehlivan, 2002; Erenturk et al., 2004).
| Table 1: | Thin layer drying models |
The above parameters can be calculated as follows:
(3) |
(4) |
(5) |
where N is the total number of observations, z, the number of drying constants, MRexp.i the experimental values and MRpre.i the predicted moisture ratio values.
Calculation of Moisture Diffusivity and Activation Energy
Fick’s diffusion equation for particles with slab geometry was used for
calculation of effective moisture diffusivity by method of slopes. Since the
tomato was dried after slicing, the samples were considered of slab geometry.
The equation is expressed as (Maskan et al., 2002):
(6) |
where MR is the dimensionless moisture ratio, Deff the effective moisture diffusivity in m2/s, t-time of drying in seconds and L slab thickness in meters.
The activation energy for diffusion was estimated using simple Arrhenius equation as given below (Kaleemullah et al., 2006):
(7) |
where D0 is the constant equivalent to the diffusivity at infinitely high temperature (m2 sec-1), Ea the activation energy (kJ/mol), R the universal gas constant (8.314x10-3 kJ/mol K) and T is the absolute temperature (K). Ea was determined by plotting ln (Deff) versus 1/T.
Results and Discussion
Drying Characteristics of Organic Tomato in a Convective Dryer
It is evident that the drying air temperature has an important effect on drying.
When the temperature was increased, due to the quick removal of moisture, the
drying time reduced (Table 2). The results are similar with
the earlier observations on drying of garlic slices (Madamba et al.,
1996) and onion slices (Sarsavadia et al., 1999).
Curves of moisture ratio versus drying time for the samples dried at different
temperature and treatment are shown in Fig. 1-3.
The moisture ratio decreased continuously with drying time and drying rate increased
with the increase in temperature. Drying of tomato slices occurred in falling
rate period and due to quick removal of moisture, no constant rate period was
observed. Similar drying behaviour has been reported for red chillies (Chandy
et al., 1992) and onion slices (Rapusas et al., 1995). The drying
in falling rate period shows that, internal mass transfer has occurred by diffusion.
| Fig. 1: | Moisture ratio of tomato slices dried at 50°C |
| Fig. 2: | Moisture ratio of tomato slices dried at 60°C |
| Fig. 3: | Moisture ratio of tomato slices dried at 70°C |
Selection of Thin-layer Drying Model
The coefficient of correlation of the thin-layer drying models (Table
1) fitted with moisture ratio data and results of statistical analyses are
listed in Table 3. In all cases, the r2-values
for the mathematical models were greater than 0.90, indicating a good fit. However,
values of r2 for the Page, Wang and Singh and logarithmic model were
above 0.99. But, the χ2, MBE and RMSE values were
lower when the values were fitted in the logarithmic model. Thus the logarithmic
model may be assumed to represent the thin layer drying behaviour of organically
produced tomato slices. Similar findings were reported for hot air drying of
apricots (Togrul et al., 2002) and rosehip (Erenturk et al., 2004)
and plum slices (Goyal et al., 2006). Accuracy of the selected model
was compared by plotting the experimental moisture ratio and the predicted values
from the logarithmic model (Fig. 4). The banding of predicted
values around the straight line indicates the suitability of logarithmic model
for describing the drying character of organically produced tomato.
| Fig. 4: | Comparison of experimental moisture ratio and predicted values
by the logarithmic model |
| Table 2: | Drying time of tomato slices |
| Table 3: | Values of statistical parameters |
| Table 5: | Moisture diffusivity values of tomato slices |
Moisture Diffusivity and Activation Energy
Values of Deff with coefficient of correlation, r2 are
given in Table 4. Effective moisture diffusivity of tomato
ranged from 1.68 to 2.84 μm sec-2. These values are within the
general range 0.1 to 10 μm sec-2 for drying of food materials
(Maskan et al., 2002). The moisture diffusivity increased as drying
air temperature was increased. Due to the influence of blanching on internal
mass transfer of tomato during drying, blanched samples had higher moisture
diffusivity values. Similar results of the influence of pretreatments on the
moisture diffusivity during air drying have been found in apricots (Pala et
al., 1996).
Activation energy of tomato slices was found to be 21.1 and 22.41 kJ-1 mol for untreated and blanched samples, respectively. The values were within the range (15-40 kJ-1 mol) of activation energy values reported by Rizvi (1986) for different foods. Activation energy of organically produced tomato slices was higher than soybean (Giner et al., 1994) and lower than red chillies (Kaleemullah et al., 2006) and green beans (Doymaz, 2005).
Conclusions
The effect of temperature and blanching on thin layer drying of organically grown tomato slices in a hot-air dryer was investigated. Increase in drying air temperature from 50 to 70°C decreased the drying time from 450 to 330 min. The entire drying process occurred in falling rate period. The logarithmic thin layer drying model showed better fit, than the other seven models evaluated, with high correlation coefficient and low χ2, MBE and RMSE values. The moisture diffusivity of the tomato slices ranged from 1.68 to 2.84 μm sec-2 and activation energy of blanched and untreated samples were 22.42 and 21.1 kJ-1 mol, respectively.