Drying characteristics and modeling of yam slices under different relative humidity conditions
ABSTRACT The drying characteristics of yam slices under different constant relative humidity (RH) and step down RH levels were studied. A mass transfer model was developed based on Bi Di correlations containing a drying coefficient and a lag factor to describe the drying process. It was validated using experimental data. Results showed that the drying air with constant RH levels of 20, 30, and 40% temperature of 60degC, and air velocity of 1.5 m/s had an insignificant effect on drying time. This phenomenon was likely attributed to the fact that higher RH led to a rapid increase in sample's temperature. The higher sample temperature could provide an additional driving force to water diffusion and thereby promote the moisture movement, which could minimize the negative effect of lower the drying rate in the initial drying stage. Applying air with 40% RH for 15 min in the initial stage achieved the desired color and reduced the drying time by 25% compared to the drying time under continuous dehumidification from an initial RH of 40% Using the developed Bi Di correlation, the estimated Biot number, effective moisture diffusivity, and mass transfer coefficient ranged from 0.1024 to 0.1182, 1.1133 x 10 10 to 8.8144 x 10 9 m2/s, and 1.8992 x 10 9 to 1.7364 x 10 7 m/s, respectively. A rather high correlation coefficient of determination (R2 between 0.9871 and 0.9971) was determined between the experimental and predicted moisture contents. The present findings contribute to a better understanding of the effect of relative humidity on drying characteristics. The developed Bi Di correlation provided a new method to determine the effective diffusivity of moisture in drying.