with the collaboration of Iranian Society of Mechanical Engineers (ISME)

Document Type : Research Article

Authors

1 Department of Mechanical Engineering of Biosystems, Tabriz University, Tabriz, Iran

2 Department of Food Science and Technology, Tabriz University, Tabriz, Iran

Abstract

Introduction
Drying foods, fruits and vegetables is a suitable method to reduce post-harvest losses of the crops. Drying is considered as a simultaneous heat and mass transfer process. Various physical, chemical and nutritional changes occur during drying of foods and are affected by a number of internal and external heat and mass transfer parameters. External parameters may include temperature, velocity and relative humidity of the drying medium (air), while internal parameters may include density, permeability, porosity, sorption–desorption characteristics and thermo physical properties of the material being dried. In this regard, understanding the heat and mass transfer in the product will help to improve drying process parameters and hence the quality. The mathematical model that reflects the drying process physics is a complex model. Particularly because of the process of convection drying of materials with high initial water content, boundary conditions should be assumed in the model describing heat and mass transfer. Ruiz-López and García-Alvarado (2007) proposed a model that provides a simple mathematical description for food drying kinetics and considered both shrinkage and a moisture dependent diffusivity. Food temperature was considered constant. The objectives of this work are: (a) to develop a mathematical model for simulating simultaneous moisture transport and heat transfer of pretreated carrot sample; (b) to study numerically the effect of the air drying conditions and pretreated on the drying of carrot and (c) to calculate the density and effective diffusion coefficients of carrot under various conditions.

Materials and Methods
In order to compare experimental and numerical analysis results, a laboratory scale convection dryer was used for experimental work. Cylindrical samples before entering the dryer were pretreated with ultrasound at frequency of 28 kHz for 10 min and microwave at 1 W g-1 power for 15 min. Experimental results of moisture evolution and volume changes during drying were used to estimate moisture diffusivity and product density. Transient three-dimensional simulation of heat and mass transfer was performed with a set of initial and boundary conditions using the finite element method. The effect of the aforementioned pretreatments was applied in terms of the modified effective moisture diffusion coefficient in the heat and mass transfer equations.

Results and Discussion
The effect of the ultrasonic pretreatment on drying was mainly observed during the air-drying stage where a significant increase in water effective diffusivity was found. Ultrasonic waves can cause a rapid series of alternative compressions and expansions, in a similar way to a sponge when it is squeezed and released repeatedly (sponge effect). Microwave pretreatment reduced the initial moisture content and slightly increased the coefficient. The values of moisture diffusivity found in this study was in the order of - m2 s-1 which is typical value for drying of agricultural product (Zielinska and Markowski, 2010). Comparison of the experimental and predicted moisture and temperature profiles showed that the model could predict the heat and mass transfer phenomena with good accuracy. In this section, some simulation results are presented. The simulated moisture contents in the center and on the surface during drying showed that moisture content on the surface decreases rapidly for a short time due to the evaporation during precooling. Then it starts to increase because of the moisture diffusion from the layers under the surface towards. The temperature inside the object increases with an increase in the drying time since the temperature of the drying air is higher than that of the object. As a result of these transient and non-uniform temperature distributions, the moisture diffusivity which depends on the moisture will vary and in turn the rate of the moisture diffusion inside the object. As seen in the figure, the distributions appear not to be symmetrical. Higher temperature and moisture gradients are obtained at the side wall due to the upstream of the drying air.

Conclusion
A theoretical analysis of pretreated and non-pretreated carrot drying process was presented. The main innovation introduced by this study was represented by the model formulation. This, in fact, simulated the simultaneous three dimensional heat and moisture transfer accounting for the variation of both air and food physical properties as functions of local values of temperature and moisture content. Moisture diffusivities of pretreated and non-pretreated carrot have been determined experimentally and moisture diffusivities of pretreated and non-pretreated carrot were found to increase with using of ultrasound pretreated. The effect of the aforementioned pretreatments was applied in terms of the modified effective moisture diffusion coefficient in the heat and mass transfer equations. Comparison of the experimental and predicted moisture and temperature profiles showed that the model could predict the heat and mass transfer phenomena with good accuracy. The model can be used as a proper tool in the design optimization and the optimal determination of the dryer performance parameters.

Keywords

1. Aversa, M., S. Curcio, V. Calabrò, and G. Iorio. 2007. An analysis of the transport phenomena occurring during food drying process. Journal of Food Engineering 78: 922-932.
2. Azarpazhooh, E., and H. Ramaswamy. 2011. Optimization of Microwave-Osmotic Pretreatment of Apples with Subsequent Air-Drying for Preparing High-Quality Dried Product. International Journal of Microwave Science and Technology.
3. Azoubel, P. M., M. D. A. M. Baima, M. D. R. Amorim, and S. S. B. Oliveira. 2010. Effect of ultrasound on banana cvPacovan drying kinetics. Journal of Food Engineering 97: 194-198.
4. Azzouz, S., A. Guizani, W. Jomaa, and A. Belghith. 2002. Moisture diffusivity and drying kinetic equation of convective drying of grapes. Journal of Food Engineering 55: 323-330.
5. Białobrzewski, I. 2006. Simultaneous Heat and Mass Transfer in Shrinkable Apple Slab during Drying. Drying Technology 24: 551-559.
6. Białobrzewski, I., M. Zielińska, A. S. Mujumdar, and M. Markowski. 2008. Heat and mass transfer during drying of a bed of shrinking particles – Simulation for carrot cubes dried in a spout-fluidized-bed drier. International Journal of Heat and Mass Transfer 51: 4704-4716.
7. Chandra Mohan, V. P. and P. Talukdar. 2010. Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying. International Journal of Heat and Mass Transfer 53: 4638-4650.
8. Fernandes, F. A. N. and S. Rodrigues. 2007. Ultrasound as pre-treatment for drying of fruits: Dehydration of banana. Journal of Food Engineering 82: 261-267.
9. Garcia-Perez, J. V., J. A. Carcel, E. Riera, and A. Mulet. 2009. Drying Technology 27: 281-287.
10. Gowen, A., N. Abu-Ghannam, J. Frias and J. Oliveira. 2006. Optimisation of dehydration and rehydration properties of cooked chickpeas (Cicer arietinum L.) undergoing microwave–hot air combination drying. Trends in Food Science and Technology 17: 177-183.
11. Kumar, C., A. Karim, S. C. Saha, M. U. H. Joardder, R. J. Brown, and D. Biswas. 2012. Multiphysics Modelling of convective drying of food materials. Proceedings of the Global Engineering, Science and Technology Conference: Global Institute of Science and Technology.
12. Mason, T. J., E. Riera, A. Vercet, and P. Lopez-Buesa. 2005. 13 - Application of Ultrasound. Pages 323-351 in Da-Wen S, ed. Emerging Technologies for Food Processing. London: Academic Press.
13. Mihoubi, D., S. Timoumi, and F. Zagrouba. 2009. Modelling of convective drying of carrot slices with IR heat source. Chemical Engineering and Processing: Process Intensification 48: 808-815.
14. Motevali, A., and S. Minaei. 2012. Effects of microwave pretreatment on the energy and exergy utilization in thin-layer drying of sour pomegranate arils. Chemical Industry & Chemical Engineering Quarterly 18: 63-72.
15. Mulet, A. 1994. Drying modeling and water diffusivity in carrots and potatoes. Journal of Food Engineering 22: 329-348.
16. Nilnont, W., S. Thepa, S. Janjai, N. Kasayapanand, C. Thamrongmas, and B. K. Bala. 2012. Finite element simulation for coffee (Coffea arabica) drying. Food and Bioproducts Processing 90: 341-350.
17. Ozbek, B. and G. Dadali. 2007. Thin-layer drying characteristics and modelling of mint leaves undergoing microwave treatment. Journal of Food Engineering 83: 541-549.
18. Ruiz-Lopez, I. I., A. V. Cordova, G. C. Rodrı́guez-Jimenes, and M. A. Garcı́a-Alvarado. 2004. Moisture and temperature evolution during food drying: effect of variable properties. Journal of Food Engineering 63: 117-124.
19. Seiiedlou Heris, S. 2009. Experimental Study and Mathematical Simulation of Drying Process in Convectional Air-Dried Apples.
20. Seiiedlou Heris, S., H. R. Ghasemzadeh, N. Hamdami, F. Talati, and M. Moghaddam. 2010. Convective Drying of Apple: Mathematical Modeling and Determination of some Quality Parameters. Iternational journal of Agriculutre and Biology 12: 171-178.
21. Srikiatden, J., and J. S. Roberts. 2008. Predicting moisture profiles in potato and carrot during convective hot air drying using isothermally measured effective diffusivity. Journal of Food Engineering 84: 516-525.
22. Zielinska, M., and M. Markowski. 2010. Air drying characteristics and moisture diffusivity of carrots. Chemical Engineering and Processing: Process Intensification 49: 212-218.
CAPTCHA Image