Hot-air Drying Kinetics of Yam Slices under Step Change in Relative Humidity

The drying kinetics and mathematical modeling of hot-air drying of yam slices were investigated under two-stage relative humidity (RH) control strategy with 60 degrees C and 1.5 m/s as its constant drying temperature and air velocity, respectively. Results indicate high RH in the initial stage results in high sample's temperature that enhances water diffusion in the falling rate drying period. Within the scope of current work, change in RH in the later drying period has insignificant influence on sample's temperature rise while low RH can accelerate the drying rate. Compared to drying at constant 20% RH at the same drying air temperature, the drying strategy of using 40% RH over the first 15 min and then lowing to 20% RH for the remainder time yields a shorter drying time. Weibull model adequately described the moisture content variation with time for all experiments with the scale parameter ranging from 105.02 to 122.38 min and the values of shape parameters from 0.988 to 1.183. The effective moisture diffusivity determined from the Weibull model varied from 2.032 to 2.610 x 10(-8) m(2)/s. The rehydration ratio increased as the overall drying time was reduced. Microstructure examination shows that higher RH in the initial drying stage can lead to a more porous microstructure which enhances drying, while fast drying rate in the initial drying period generates a crust layer which hinders drying.