Senadeera, Wijitha W. and Bhandari, Bhesh R. and Young, Gordon and Wijesinghe, Bandue (1998) Change of physical properties of green beans during drying and its influence on fluidization. In Akitidis, C. B. and Marinos-Kouris, D. and Sarvakos,

Changes in the physical properties (size, particle density, bulk density, shrinkage and porosity) and fluidization behaviour of cut green beans with change in moisture content during drying were investigated using a heat pump and fluidized bed dryers. Three length:diameter ratios 1:1, 2:1, 3:1 of cut green beans were considered. All drying experiments were conducted at 50 + 2 0 C and 13 + 2 % RH using a heat pump dehumidifier system. Fluidization experiments were undertaken for the bed heights of 100, 80, 60 and 40 mm at 10 moisture levels in order to determine the critical moisture level of fluidization and optimum bed height for improved fluidization behaviour. INTRODUCTION Drying is a major food processing operation in the food industry which consumes larger amount of energy. It is a very complex process and involves mass and heat transfer processes simultaneously. Also several physical and structural changes are happening simultaneously (Fusco et al., 1991). The quality of food materials undergoing drying is not only depend on their initial quality, but also on the changes occurring during drying (Karel, 1991). Bulk density, particle density, shrinkage and porosity are important physical properties in food materials which influence fluidization properties of the dry foods. Food particles are coming in different shapes and sizes and therefore are of irregular in nature. During drying, shape and size of the products change appreciably, further influencing their particle properties (Ratti, 1994). Shrinkage is the change in volume of the food particulate mainly as a result of removal of water. But some amount of air removal can be contributed to it, the degree of which will depend on the type of the product (Khraisheh et al., 1997). Shrinkage is important as it causes stresses and may also result in cracks (Balaban, 1989). Better understanding of the drying processes can be achieved by understanding shrinkage as it is occurring at the same time with moisture diffusion which influences moisture removal rate (Suarez and Viollas, 1991). Shrinkage of the product during drying, also influences the particle density, bulk density and porosity of the bed. Bulk density, particle density and porosity are some of the important parameters in designing equipment pertaining to food industry. Flow resistance through beds, food storage and packaging of the products are related to bulk density and porosity (Madamba et al., 1994a). The change in density of food particulates during drying are generally modelled using empirical relations, but there are also theoretical models that are based on mass and volume (Rahman and Driscoll, 1994). Porosity of the bed changes as a result of deformations as well as shrinkage of the cells itself (Lozano et al., 1980). By selecting a suitable drying method final product porosity can thus be controlled (Krokida and Maroulis, 1997). Several authors studied the effect of drying method and conditions on various fruits and vegetables and developed models for bulk density, particle density and porosity with change of moisture (Krokida and Maroulis, 1997, McMinn and Magee, 1997a,b, Chou et al., 1997, Wang and Brennan, 1995, Madamba et al., 1994a,b, Zogas et al., 1994, Lozano et al., 1980, Suzuki et al., 1976). Simultaneous moisture and heat transfer models were also compared with or without volume changes (Balaban, 1989). Although, several publications have dealt with changes in physical properties of various food products during drying, there are no published reports on green beans. The objectives of this study were to determine shrinkage, density and porosity changes for cylindrical green beans with varying length diameter ratios and relate these changes to moisture content by suitable models, and to study the fluidization behaviour of the product with changing physical properties during successive drying. MATERIALS AND METHODS