The speed at which the moisture content of a material is reduced to a lower moisture content. The drying rate is dependent on the humidity and temperature of the drying vapour as well as the properties of the material such as its porosity. There are many theoretical and empirical methods used to estimate the drying rate. The constant rate drying period is where the rate of evaporation is constant due to continual surface replacement of moisture from within the materials, while the falling-rate period is determined by the gas phase mass transfer or rate of heat transfer within the material.