Influence of shapes of selected vegetable materials on drying kinetics during fluidized bed drying Wijitha Senadeera a , Bhesh R. Bhandari a, * , Gordon Young a , Bandu Wijesinghe b a Gatton College Food Science and Technology, School of Land and Food Sciences, University of Queensland, St. Lucia, Gatton, Qld 4072, Australia b Department of Primary Industries, Centre for Food Technology, Brisbane, Qld, Australia Received 10 February 2002; accepted 21 September 2002 Abstract Three different particular geometrical shapes of parallelepiped, cylinder and sphere were taken from cut green beans (length:diameter ¼ 1:1, 2:1 and 3:1) and potatoes (aspect ratio ¼ 1:1, 2:1 and 3:1) and peas, respectively. Their drying behaviour in a fluidised bed was studied at three different drying temperatures of 30, 40 and 50 °C (RH ¼ 15%). Drying curves were constructed using non-dimensional moisture ratio (MR) and time and their behaviour was modelled using exponential (MR ¼ expðktÞ) and Page (MR ¼ expðkt n Þ) models. The effective diffusion coefficient of moisture transfer was determined by Fickian method using uni- and three-dimensional moisture movements. The diffusion coefficient was least affected by the size when the moisture movement was considered three- dimensional, whereas the drying temperature had a significative effect on diffusivity as expected. The drying constant and diffusivity coefficients were on the descending order for potato, beans and peas. The Arrhenius activation energy for the peas was also highest, indicating a strong barrier to moisture movement in peas as compared to beans and skinless cut potato pieces. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Peas; Potato; Green beans; Aspect ratio; Diffusivity; Length–diameter ratio 1. Introduction Fluidised bed drying is commonly used in drying particulate materials. In a fluidised bed, conditions are favourable for rapid heat and mass transfer. This is a result of the minimum or very thin boundary layer surrounding the food particles due to very rapid mixing. Due to the rapid drying it has been considered as an economical drying method compared with other drying techniques. Fluidised bed drying has been recognised as a gentle, uniform drying, down to a very low residual moisture content, with a high degree of efficiency (Borgolte & Simon, 1981). This is a very convenient method for heat sensitive food materials as it prevents them from overheating (Gibert, Baxerres, & Kim, 1980; Giner & Calvelo, 1987). Knowledge of drying kinetics is important in the design, simulation and optimisation of drying processes. Drying curves are usually modelled by defining drying rate constants based on first order kinetics. The basic model is known as the simple (exponential) model (Eq. (1)). MR ¼ expðk s tÞ ð1Þ where, MR ¼ moisture ratio, k s ¼ drying constant and t ¼ time. This model tends to over predict the early stages of drying and under-predict the later stages of drying. To overcome the shortcomings of a simple exponential model, the Page model (Eq. (2)) is applied with an em- pirical modification to the time term by introducing an exponent ÔnÕ (Madamba, Driscoll, & Buckle, 1996; Wongwises & Thongprasert, 2000). MR ¼ expðk p t n Þ ð2Þ This model was also used to fit the experimental data of beans, peas, grapes and potato (Afzal & Abe, 1999; Sawhney, Sarasavadia, Pangavhane, & Singh, 1999). Journal of Food Engineering 58 (2003) 277–283 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +61-7-5460-1196; fax: +61-7-5460- 1171. E-mail address: b.bhandari@mailbox.uq.edu.au (B.R. Bhandari). 0260-8774/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0260-8774(02)00386-2