Dehydration dynamics of potatoes in superheated steam and hot air

Superheated-steam at atmospheric pressure is an alternative drying medium for dehydrating materials insensitive to temperature equal to or above 100 degrees C. This research compared the dehydration characteristics, temperature histories, drying rates, and overall moisture diffusivities of cylindrical potato samples exposed to superheated steam and hot air at 125, 145, and 165 degrees C. A small amount of moisture (0.18 to 0.47 kg/kg db, dry basis) dependent on the steam temperature was gained from steam condensation on the sample surface during the warm-up period from the superheated-steam. The temperature of the drying medium had a greater effect on the drying rate, overall moisture diffusivity, and consequently dehydration time for the superheated-steam dehydration than for the hot-air dehydration. Increasing the temperature from 125 to 165 degrees C decreased the dehydration time by 60 and 24% for the superheated-steam and hot-air dehydration, respectively. A constant-rate drying period was only observed with superheated steam at 125 and 145 degrees C. There existed an inversion temperature point between 145 to 165 degrees C for the first dehydration stage above 2.6 kg/kg db and between 125 and 145 degrees C for the last dehydration stage below 2.6 kg/kg db.