Drying Characteristics of Japanese Yam (Dioscorea opposita Thunb.)

The drying characteristics (changes of mass (moisture content), volume, drying surface area, and surface and internal temperature of sample during drying) of Japanese yam (Dioscorea opposita Thunb.) were measured under various operating conditions (temperature, relative humidity, and absolute pressure (vacuum level)). Japanese yam was dried by two methods, hot air drying and continuous vacuum drying. The effects of temperature, relative humidity, and absolute pressure on the drying rate of the sample were investigated. The moisture content change of the sample was analyzed under two conditions, considering or not considering the drying surface area change. When surface area change is considered, two periods, the constant rate period and the falling rate period, existed in both hot air drying and vacuum drying of the sample. In each case, mathematical models to describe the changes of the moisture content were derived, and the measured data agreed well with the values calculated from the models. The effects of temperature, relative humidity, and vacuum level on the drying rate constant were investigated. The drying rate constants were related to the temperature and relative humidity for hot air drying, or the temperature and vacuum level for vacuum drying using empirical equations which are modified form an Arrhenius-type equation.