Moisture Diffusivity and Product Temperature Analysis of Carrot Slices in Microwave Vacuum Rotary Chamber Dryer

Abstract

The 3.5 mm thick carrot slices were dried in a laboratory scale microwave rotary vacuum chamber dryer at five microwave power density levels of 2, 4.66, 7.33,10 and 12.66 W/g and three vacuum chamber pressure levels of 6.66, 19.98 and 33.3 k Pa up to 4-6 % d.b. moisture content level. Inside the microwave cavity the vacuum chamber having product was rotated with the speed of 4 rpm for uniform microwaves application. The product temperature was measured at predetermined time intervals using fiber optic sensors inserted at the center of moving carrot slices during microwave vacuum drying. Drying rates were calculated at all process conditions and the effect of microwave power density levels on drying rates was observed. It was found that the drying rate was increased with increase in microwave power density at all pressure levels. The Fick's law of diffusion was used to calculate the effective moisture diffusivity values. The method of slopes was used to calculate the diffusivity values. The effective moisture diffusivity showed the third order polynomial relationship with the moisture content of the product at constant power density and pressure. The average diffusivity values calculated were in the range 0.621 10-9 to 5.529 10-9 m2/s. The average moisture diffusivity was found to be function of microwave power density by power type equation with R2 and Root Mean Square Error (RMSE) values in the range of 0.961 to 0.994 and 6.87 10-10 to 1.92 10-10 at constant pressure. The average moisture diffusivity was also dependent on the power density and pressure by a logarithmic relationship with a coefficient of multiple determination of 0.963 and RMSE of 3.8310-10. The statistical analysis showed that the effect of power density on average diffusivity was significant, whereas, the chamber pressure had non significant effect. The product temperature was found to increase with increased microwave power density. The product temperature profiles with respect to moisture content showed three distinct stages during the drying process.