Two-stage drying is also referred to as combination drying.
Considering the theoretical drying curve of paddy (Figure 4) and the requirements for quick drying immediately after harvest to a MC that is safe for temporary storage the two-stage drying system or combination drying system was developed.
A typical first stage dryer takes advantage of the different drying rates during the three drying periods and that surface moisture can be removed rapidly from very wet paddy without causing damage to the grains by using very high temperatures for a short period of time. Drying air temperatures in first stage dryers can reach over 100ºC in fluidized bed dryers where the grain is exposed to the drying air only for a few minutes. After this rapid pre-drying to a MC of 18%, the grain is considered safe for up to two weeks of storage. The grain is then transferred to a storage bin with aeration facilities where it is slowly dried to the desired moisture content of 14% or lower with only slightly pre-heated air or even ambient air if the climatic conditions are feasible.
Although two-stage drying has many advantages since it uses two different drying principles well suited to the different drying phases of paddy grains at different MC ranges the introduction of two stage drying in Southeast Asia has so far failed. This strategy is used only in Thailand by the commercial sector.