Experimental and Mathematical Investigations of Apple Slices Convective Drying

Abstract: The aim of this work was to determine sorption isotherms and thin layer convective drying behavior of apple slices and to compare the experimental and calculated results by using models available in the literature. The sorption isotherm was determined at four temperature levels 40， 50， 60 and 70 °C and at water activity ranging from 0.058 to 0.89， using the static gravimetric method. A non-linear regression procedure was used to fit experimental sorption isotherms and drying curves with most used empirical mathematical models available in literature. It was found that the Peleg model suitably represent the sorption experimental data in the mentioned investigated ranges of temperature and water activities. The Midilli et al. model satisfactorily described the thin layer drying behavior of apple slices. The drying characteristic curve and the thin layer drying rate expression of apple slices have been established from experimental convective drying kinetics. The values of the diffusivity coefficients at each condition were obtained using Fick’s second law of diffusion. They varied from 8.51 × 10-8 to 3.28 × 10-7 m2/s in the temperature range of 40-70 °C and in the relative humidity range of 20%-40%. The activation energies for moisture diffusion were calculated to vary from 26.72 to 35.83 kJ/mol.

Key words: Convective drying， sorption isotherm， isosteric heat， apple， empirical modeling， effective diffusivity， activation energy.

In order to improve the quality of the dried product， industrial dryers should be used in place of traditional sun drying. Industrial dryers provide uniform dried product， hygiene and also the process is more rapid [1]. 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 [2， 3]. Knowledge of sorption isotherm and drying behavior are of great importance in food dehydration， especially in the quantitative approach to the prediction of the shelf life of dried foods [4]. Equations for modelling water sorption isotherms are of special interest for many aspects of food preservation by dehydration， including evaluation of the thermodynamic functions of the water sorbet in foods. Knowledge of the thermodynamic properties associated with sorption behavior of water in foods is important to dehydration in several respects， especially in the design and optimization of unit operation， and further help the understanding and interpretation of sorption mechanisms and food-water interactions.

Several thin layer equations available in the literature for explaining drying behavior and sorption isotherms of agricultural products have been used by Diamente and Munro for sweet potato slices[5]， Toujani et al. for silverside fish [6]， Madamba et al. for garlic slices [7]， Midilli for pistachio [8]， Yald?z et al. for grape [9]， Basunia and Abe for rough rice[10]， Panchariya et al. for black tea [11]， Dandamrongrak et al. for banana [12]， Lahsasni et al. for prickly pear peel [13]， Kalpana et al. for betel leaf

70 °C of the drying air and the relative humidity are a small effect of drying rate. The results were generally in agreement with some literature studies on drying of various food products [4， 35， 51]. 3.4 Characteristic Drying Curve

Marquardt-Levenberg nonlinear optimization method， using the computer program Origin 6.0， was used to find the best equation for apple slices characteristic drying curve. According to our results， the characteristic fitting of the drying curves drying curve of thin layer of apple slices is depicted in Fig. 5. The best fitting of this curve is carried out by a polynomial equation of degree 3:

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