Physical properties of three onion varieties as affected by the moisture content

Three onion varieties (Sweet Vidalia, Spirit and Niz) were selected for this study based on their dry matter amounts ranging from 7% to 18%. The contents of their major components (protein, fat, ash and sugar) were analyzed. Three types of physical properties, thermal (conductivity, specific heat capacity, diffusivity), structural (density and porosity) and dielectric (relative permittivity and loss factor) properties were measured and calculated. The conductivity and specific heat capacity were measured using an annular heat source probe. Density was measured by liquid displacement method while a coaxial line probe was used to measure the dielectric properties. The thermal conductivity ranged from 0.12 to 0.54 W/m K for completely dried and fresh onion, whereas the specific heat capacity varied from 0.87 to 4.01 kJ/kg K. These properties varied linearly with varying moisture contents. A second order polynomial equation was fitted to the density variation with varying moisture contents. The densities increased from 970 for fresh onion to 1250 kg/m(3) for dried. Dielectric permittivity decreased from 66 to 1.6 as onion dries. Loss factor also decreased from 15.5 to 0.20. (C) 2002 Elsevier Science Ltd. All rights reserved.