Drying kinetics of dill leaves in a convective dryer

Abstract

A b s t r a c t. Thin layer drying characteristics of dill leaves under fixed, semi-fluidized, and fluidized bed conditions were studied at air temperatures of 30, 40, 50, and 60°C. In order to find a suitable drying curve, 12 thin layer-drying models were fitted to the experimental data of the moisture ratio. Among the applied mathematical models, the Midilli et al. model was the best for drying behavior prediction in thin layer drying of dill leaves. To obtain the optimum network for drying of dill leaves, various numbers of multilayer feed-forward neural networks were made and tested with different numbers of hidden layers and neurons. The best neural network feed-forward back-propagation topology for the prediction of drying of dill leaves (moisture ratio and drying rate) was the 3-45-2 structure with the training algorithm trainlm and threshold functions logsig and purelin. The coefficient of determination for this topology for training, validation, and testing patterns was 0.9998, 0.9981, and 0.9990, respectively. Effective moisture diffusivity of dill leaves during the drying process in different bed types was found to be in the range from 7.10 10 -12 to 1.62 10 -10 m 2 s -1. Also, the values of activation energy were determined to be between 75.435 and 80.118 kJ mol Drying is rated as an important post-harvest process for foods and fruits with respect to high consumption of energy and quality concerns. The final and major purpose is to minimize water activity and reduce microbial and chemical reactions. Sun drying is still used for drying fruits and vegetables in spite of considerable drawbacks such as long drying time, pollutions, product deterioration, and unwanted damage. These drawbacks are sufficient enough to substitute industrial and technological trends for drying in order to guarantee demands and preserve the quality (Doymaz, 2007).In the fluidized bed drying method, the drying time is reduced rather than in the fixed bed method, due to the high heat and mass transfer between warm air and products (Giner and Calvelo, 1987). The advantages of fluidized bed drying are high heat and mass transfer rates, uniform temperature and bulk moisture content of particles, and excellent temperature control and operation up to the high ratio of mass of air to mass of product (Izadifar and Mowla, 2003). Fluidization is defined as suspending the vegetable bed (dill leaves, Anethum sowa L.) in air flow. With gradually increasing airflow through a bed of vegetable, fixed bed, minimum fluidized bed (semifluidized bed), bubbling fluidized bed and transportation will be created, respectively (Chayjan et al., 2011).Several authors had attempted to determine mathematical models for estimation of the moisture ratio and drying rate of food and biological materials during the drying process in eg potato (Bakal et al., 2011), Cuminum cyminum (Zomorodian and Moradi, 2010), carrot (Doymaz, 2004, sweet cherry (Doymaz and Ismail, 2011), and berberies (Gorjian et al., 2011). Some researchers have studied the effective moisture...

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