Drying of agricultural products is the great importance for preservation of food human beings. The drying operation is mainly affected by variation of the air temperature, air velocity,, air relative humidity and other factors. The aim of this research was to investigate influence of some factors such as drying air temperature (40, 50, 60, 70, and 80 °C) and onion slices thickness (4, 6, and 8 mm) on the thin layer convection drying behavior of white onions slices at 1.5 m/s air velocity. Drying experiments were conducted using a convection tray dryer(UOP 8 Tray Dryer, Armfield, UK, S.N : 016679-001). The dryer is equipped with controllers for controlling the temperature and airflow velocity. Temperature changes of dried samples, as well as relative humidity and temperature of drying air were measured during the drying process. Rehydratation ratio, color, texture and sensory properties were used as a parameter for the dried sample quality. The drying curves were fitted with different moisture ratio equations given by several researchers.
Pag's equation (exponential model) fitted very well the experimental data and it is possible to accurately simulate the thin layer drying behavior of onion slices at different air temperature. Two well-defined falling rate periods and a very short constant rate period at lower air temperature were observed. Drying constants ( k and n) were greatly affected by drying air temperature and slice thickness. Drying time decreases considerably with increase in hot air temperature. During rehydration of dried onions,3.08 to 3.71 of rehydration ratio by the drying process was obtained. Slice thickness had a significant effect on the rehydration ratio. Drying of onion slices at range of (40 – 60°C) revealed optimum color values, meanwhile drying at 70- 80 °C gave least acceptable color values. So if the economics of the process is considered, drying at 50 to 60 °C is generally recommended