Modelling effective moisture difusivity of rough rice (Lido cultivar) at low drying temperatures A. Iguaz * , M.B. San Mart  ın, J.I. Mat  e, T. Fern andez, P. V  ırseda Tecnolog  ıa de Alimentos, Escuela T ecnica Superior de Ingenieros Agr anomos, Universidad P ublica de Navarra, Campus Arrosad  ıa, 31006 Pamplona, Spain Received 29 July 2002; accepted 19 November 2002 Abstract The drying rate of Lido medium rough rice was determined at 35, 25, 12 and 5 °C at relative humidity varying from 30% to 70% and at two air velocities, 2.5 and 0.75 m/s. The experimental data were fitted to four models: single exponential model, biparametric exponential model, Page model and two term exponential model. The biparametric exponential model and the Page model provided the best fit. However the biparametric exponential model was preferred due to its simplicity of application. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Thin layer drying; Grain; Simulation; Modelling; Diffusivity 1. Introduction The deep-bed drying of grain using forced ventilation has been investigated in the past (Jindal & Siebenmor- gen, 1994a; Morey, 1984). Mathematical models were developed to predict the temperature and moisture content of the air and grain with respect to time and a position within a silo. During this ventilation, the out- coming air from given grain layer is considered the in- coming air into the following layer. Thus, a deep-bed drying effect can be considered as composed of series of thin layers of grain. Using mathematical simulation, it is possible to predict the temperature and moisture profile in a bed of grain. As a consequence, the validity of a deep-bed model depends on the goodness of fit of the thin layer drying model. Thus, the thin layer drying equations, that help to comprehend the mass and energy transfer mechanisms contribute to simulate and opti- mize the design of drying and storage equipment. Grain drying is a simultaneous heat and mass transfer process. As a consequence of the heat transfer, the grain temperature tends to be the air wet bulb temperature and theoretically an internal temperature gradient can exist. However, this gradient is usually neglected and the drying process is considered as a pure mass transfer process based on the following assumptions: • The internal temperature of grain can be considered constant due to the low Biot number (Bi ¼ hr=k) usu- ally found for food drying. Parti (1990) proposed that when the Biot number is lower than 0.1, the tempe- rature difference between air and grain can be neg- lected. Therefore, the model is based on a single mass equation that describes the humidity loss of grain ex- posed to the air drying conditions. • Effective moisture diffusivity (D eff ) of rice during dry- ing can be used when the rice kernel is considered as a homogeneous material. Effective diffusivity combi- nes various mass transfer mechanisms as capillarity, gaseous and liquid diffusivity, pressure gradients, etc. Rice is a cereal with a low moisture content, around 22% (dried basis, d.b.), which determines the falling drying rate and D eff is the main process that controls drying during this period. Crank (1975) developed the mathematical models to determine for D eff based on the second FickÕs law and depending on the geometry of the product. The model for a semi-infinite slab is as follows: Journal of Food Engineering 59 (2003) 253–258 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +34-948-169-091; fax: +34-948-169- 893. E-mail address: asun.iguaz@unavarra.es (A. Iguaz). 0260-8774/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0260-8774(02)00465-X