Dehydration Characteristics and Mathematical Modeling of Thyme Leaves using the Microwave Process
drying kinetics, microwave, modeling, thyme leaves, activation energy., activation energy, Modeling
Abstract
Thyme leaves (Thymus vulgaris) with 50 g weight and 83.7% humidity on wet basis were dried in a laboratory scale microwave dryer using six different microwave power levels ranging between 200 to 700 W. The effect of drying power microwave on the coefficients of the best moisture ratio model was determined by non-linear regression method. Results from the mathematical modeling showed that the Midilli et al. model gave the best fit to describe the drying curve of thyme. In addition, the effective diffusivities of thyme under microwave drying were obtained from 2.94×10-7 to 7.38×10-7 m2/s. Also, the activation energy for the moisture diffusion was found to be 16.471 W/g.
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Roberts (1978) (Roberts et al., 2008) (Motevali et al., 2010) MR=a.exp (-kt) Henderson and Pabis (Chhinnan, 1984) MR=1+a.t+bt2 Wang and Singh (Wang and Singh, 1978) MR=a.exp (-kt)+c Logarithmic (Dandamrongrak et al., 2002) MR=a.exp(-ktn)+b.t Midili et al.
MR − MR exp i , N − z 1 N MR − MR 2 2 RMSE = i = 1 pre i.
Midilli (2002) N (Midilli et al., 2002) Where, MRexp is the experimental dimensionless moisture ratio, MRpre is the predicted dimensionless moisture ratio, N is the number of experimental data points, and z is the number of parameters in model.
Aghbashlo (2008) DR = ∆ t where, Mt+Δt is moisture content at time t+Δt (kg water/kg dry mater), t is the time (min) and DR is the drying rate (kg water/kg dry mater.min). d) Moisture Diffusivity The mechanism of moisture movement within a food and agricultural product can be represented with effective moisture diffusion.
Crank (1975) = D ∂ t ∂ x The solution to the Fick's equation with the assumptions of moisture migration by diffusion only, negligible volume shrinkage, diffusion coefficients and constant temperature is solved Eq. (7) (Crank, 1975): 8 1 D t MR = π exp − (2 n + 1) π 2 = (2 n + 1) 2 4 L 2 n 0 Where, Deff is the effective diffusivity (m2/s), and L is the half-thickness of samples (m), n is a 1 0.9 200 W 0.8 300 W 0.7 400 W Moisture Ratio 500 W 0.6 ) positive integer.
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2013-11-28
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