Thin layer drying characteristics of fresh tea leaves

Thin layer drying characteristics of fresh tea leaves were investigated to quantify the rate of moisture transfer of fresh tea leaves during the withering process within the temperature range of 20 – 35 o C, and a relative humidity range of 40 – 90 % at 1.2 ± 0.3 m/s airflow rate. Five different mathematical models available in the literature were used with the drying data for fresh tea leaves using a constant climatic chamber. Results indicated that the Two-term model gives better predictions for moisture transfer than others. The desorption process of tea leaves occurred in falling rate period. The effective diffusivity of water in tea leaves varied from 3.3409 – 5.4669 × 10 m/s over the temperature range investigated with an activation energy of 1477.75 kJ/kg. The temperature dependence of diffusivity coefficient was described satisfactorily by a simple Arrhenius type relationship.


INTRODUCTION
Tea withering is an important unit operation in black tea processing.Fresh tea leaves received at the factory may contain 70 -83 % (w.b.) moisture depending on the climatic condition and the type of tea cultivar (Samaraweera, 1986).The moisture is reduced to about 55 -60 % (w.b.) during withering operation by careful handling of air temperature.A thick layer of leaf is loaded to a trough and an axial flow fan delivers 0.6 -0.65 m 3 /min/kg green leaves (GL) volume of air through the bed of leaves against the pressure of the 12 -15 mm water gauge (Samaraweera, 1986).During the initial stage of withering, temperature of air is maintained around 32 o C if surface water is present in the leaves.At the later stage of withering, air temperature should be maintained well below 32 o C to preserve the quality of the leaves (Ranatunga et al., 1986).
Moisture transfer during withering influences the temperature changes of air due to the magnitude of latent heat of vaporization.To study the deep bed simulation of withering process, data on moisture transfer properties of fresh tea leaves at ambient conditions are needed.This study investigates moisture transfer characteristics of a thin layer of fresh tea leaves.Very little work has been done on thin layer drying characteristics of fresh tea leaves.Jayarathnam and Abdul Gaffer (1979) studied the desorption process of fresh tea leaves and reported that the constant rate period was observed for six different tea cultivars.However, the original source of data was not available.
Many studies have been conducted on thin layer drying characteristics of different food materials (Madamba et al., 1996;Panchariya et al., 2002) and different mathematical models have been used to describe thin layer drying data for different temperature ranges.These mathematical models can be categorized as theoretical, empirical and semi-empirical.Computational procedures of these models varied from simple to complex.
The objective of this study was to quantify the rate of moisture transfer of fresh tea leaves in the temperature range of withering operation carried out based on thin layer drying data.The experimental data were fitted into five different mathematical models which were available in the literature.Using the best-fit mathematical model, a relationship was developed between the drying coefficient of fresh tea leaves, air

METHODS AND MATERIALS
Moisture desorption of leaves of tea cultivar TRI 2025 free from surface water was measured by a dynamic ventilator method.Air temperatures of 20,25,30 1).The instrument was allowed to run empty for about 3 h to become stable, after setting dry and wet bulb temperatures.
As shown in Figure 1, 50 g leaf sample thinly spread on the tray was hung at the bottom of the electronic balance (Denver S-203) with an accuracy of ± 0.001 g.A balance was connected to the computer via RS-232 serial cable to log the weights of leaf sample at constant time intervals.Readings were taken until the weight of sample reached a constant value.The dry weight of the resulting sample was determined by drying in an air oven at 103 o C for 6 h.The constant airflow rate of 1.2 ± 0.3 m/s was maintained in the chamber throughout the sorption period.The experiment was carried out in duplicate for the temperature and RH ranges studied.
Determination of drying coefficient in the desorption process: Moisture ratio (MR) versus drying time was fitted to five mathematical equations (Table 2).MR represents (M -M e )/(M o -M e ); where M o and M are denoted as moisture contents at the beginning and at a given time during withering, respectively.A non-linear regression programme (NLREG, Phillip H. Sherrod 6430 Annandale Cove, Brentwood, USA) was employed to fit experimental data into the five mathematical models.
The equilibrium moisture content (M e ) of fresh tea leaves at a given temperature and RH were calculated using the modified Oswin equation (Botheju et al., 2008).(1) Lewis where T is temperature (K) and a w is water activity (decimal).
Analysis of data: Four statistical parameters viz.standard error of estimate (SEE) in equation ( 3), mean relative deviation percentage (P) in equation ( 2), coefficient of determination (R 2 ) and the residual plots were used in the selection of the best-fit equation.
where Y is the experimental data; Y' is the value predicted by the model; N is the number of data points; if residual plot indicates a clear pattern, the model is not accepted (Weisberg, 1986).
An Arrhenius type relationship was developed between drying coefficient (k) of tea leaves and temperature and RH of air studied.

Calculation of effective diffusivity and activation energy:
The drying characteristics of biological products in falling rate period can be described by using Fick's diffusion equation.The solution to this equation developed by Crank (1975) can be used for various regularly shaped bodies.The equation ( 4) given below can be applicable for food materials with slab geometry by assuming uniform initial moisture distribution.
where D eff is the effective diffusivity (m 2 /s) and L 0 is the half thickness of slab (m).For long drying period, equation ( 4) can be further simplified to only the first term of series (Tutuncu & Labuza, 1996) and the logarithmic form can be given as follows (equation 5), which has similar form proposed by Henderson & Pabis (1961).The temperature dependency of effective diffusivity is described by an Arrhenius type relationship (Madamba, et al., 1996;Akgun & Doymaz, 2005) as follows; where D 0 is a constant equivalent to indefinitely high temperature and E a is the activation energy.

RESULTS AND DISCUSSION
TRI 2025, a common cultivar in tea estates in Sri Lanka, was selected to study thin layer drying properties of fresh tea leaves.Eleven combinations of temperature and relative humidity (Table 1) were studied using a constant climatic chamber.Initial moisture content of leaf samples varied between 75 -80 % (w.b.).Equilibrium moisture contents (M e ) at selected temperature and RH were calculated using equation (1).
Moisture ratios (MR) verses drying time for tea leaves for eight combinations of temperature and RH are shown in Figure 2. It indicates that increasing air temperature or decreasing RH speeds up the drying/ withering process and thus shortens the withering time.Similar results were obtained for dehydration of apple (Vergara et al., 1997), drying of kiwifruit and apple pomace (Fenton & Kennedy, 1998) etc.
The plots of drying rate verses drying time and the drying rate verses moisture content are shown in Figures 3 and 4 (six combinations were considered to avoid congestion) respectively.Figure 3 indicates that no constant rate period is observed in the withering process of tea leaves.Withering process takes place in a falling rate period and the rate of moisture removal is faster at the beginning than that at the end (Figure 4).This observation is in agreement with results on withering experiments carried out by Ghodake et al. (2006).Two falling rate periods could be observed in the tea withering process (Figure 4).The first falling rate period occurred when the moisture content of tea leaves was above 72 % (w.b.) or 260 % (d.b.), while the other was observed below 260 %.(d.b.).
The colloidal and hyperbolic nature of food materials causes the water molecules to be tightly bound to the material (Mazza & Le Maguer, 1980).Hence, the drying of almost all biological products takes place in the falling rate period.A constant rate period was observed before the falling rate in the study done by Jayaratnam and Abdul Gaffar (1979) for drying tea leaves of six different cultivars.Until the free surface water was removed from tea leaves, a constant rate period was observed in their drying curve.Temple et al. (2000) reported that a single falling rate period was enough to describe the whole process of tea.However, using some industrial equipment a constant rate period could be observed due to limited evaporative capacity of the air.In such cases, constant rate period was a property of air supply rather than a drying property of the material.

Modelling thin layer drying of tea leaves
The curve of MR versus drying time (Figure 2) at different temperature and RH values was fitted to five mathematical models.Non-linear regression programme (NLREG) was performed to analyze the data.R 2 , SEE, P and residual plots were employed to evaluate the bestfit model and the calculated coefficients of each model are presented in Table 3.
R 2 obtained for all five models were greater than 0.99.However, Page and Two-term models gave consistently higher R 2 than the other three models.SEE and P calculated for Two-term model gave comparatively very low values than others.Moreover, when the residual plots for all model equations were compared, the Twoterm model characterized a random distribution for all treatments except the graph drawn for 35 o C and 60 % RH.When comparing all statistical parameters Two-term model fitted well with the experimental data than the other four models.Therefore, Two-term model was selected as the best-fit mathematical equation to explain the experimental desorption data of fresh tea leaves.
Regression analysis was used to find the relationship between withering temperature, RH as against drying coefficients k o and k 1 (min -1 ) of Two-term model.Thus the drying coefficients k o , k 1 against temperature and RH can be expressed as an Arrhenius type relationship.A similar type of relationship has been developed for deep bed rice drying (Murata et al., 1996).where h and T are relative humidity (decimal) and temperature (Kelvin) respectively.
The constants 'a' and 'b' did not show any significant variation for the range of temperature and RH (Table 3) studied.Therefore, the average values of 'a' and 'b' were considered.The final Two-term model can be expressed as follows.

Determination of effective diffusivities and activation energy
The results have shown that internal mass transfer resistance controls the drying time due to presence of the falling rate period.The values of effective diffusivity (D eff ) at different temperatures could be obtained by using equations ( 5) and (6).The average values of effective diffusivities of tea leaves in the desorption process at 20 -35 o C varied in the range of 3.3409 -5.4669 × 10 -10 m 2 /s (Table 4).The effective diffusivity increases exponentially with the increase of air temperature.These results are in agreement with the previous investigations within the general range of 10 -9 to 10 -11 m 2 /s for food materials (Botheju et al., 2008).
A linear relationship was derived from the equation ( 7) and natural logarithmic of D eff was plotted as against 1/T (Figure 5).The energy of activation (E a ) for water diffusion of tea leaves calculated from the slope of the straight line was found to be 1477.75kJ / kg, which was in the range of drying onion 1200 kJ/kg (Mazza & Le Maguer, 1980), rice 1183 kJ/kg (Pinaga et al., 1984) and paprika 2036 kJ/kg (Carbonell et al., 1986).The activation energy barrier must be overcome to activate moisture diffusion.To increase drying rates by increasing moisture diffusion, use of high temperatures would be beneficial but it is advisable to use optimum temperatures (Ranatunga et al., 1986;Keegal, 1965) to maintain the quality of tea leaves during withering.

Figure 1 :
Figure 1: Schematic diagram of the constant climatic chamber used for dynamic desorption of tea leaves are determined by plotting experimental drying data in terms of ln MR versus drying time (t) in equation (5).The slope of the straight line gives;

Figure 4 :
Figure 4: Drying rate versus moisture content of tea leaves at different temperatures and relative humidity levels

Figure 5 :Figure 2 :
Figure 5: The relationship between ln D eff and 1/T of drying

Figure 3 :
Figure 3: Drying rate versus drying time of tea leaves at different temperatures and relative humidity levels

Table 1 :
Relative humidity of air corresponding to different dry and wet bulb temperatures

Table 2 :
Mathematical models given by various authors for thin layer drying of materials Where k, k o and k 1 are drying coefficients; a, b, c and n are constants and t is time

Table 3 :
Drying curve parameters for five models and the curve fitting criteria for each model for withering of tea leaves

Table 4 :
Effective diffusivities of fresh tea leaves at different temperatures