PURPOSE: To improve the conversion of sweet potato to alcohol, to increase the thermal efficiency and save the fuel consumption, and to shorten the time for steaming and improve the operation efficiency, by heating sweet potato indirectly with steam, and then heating directly.

CONSTITUTION: Sweet potatoes used as the raw material are charged to the main body of the steamer by opening the lid 4, and distributed almost uniformly to each steaming chamber 6 by rotating the distribution blades 8. The valves 30 and 36 are opened to introduce steam to each jacket 7, and the sweet potatoes are heated for a definite time interval. After the completion of the indirect heating, the introduction of steam to the jacket 7 is stopped, and the valve 29 is opened to introduce steam directly to the main body of the steamer 1. Steam is passed from the top of the steamer through the spaces between the sweet potatoes to the bottom of the steamer, and discharged from the discharge pipe 41. When the steaming of the sweet potatoes is finished, the introduction of steam is stopped, cold water is poured into the jacket 7 and cold air is sent to the steaming chamber 6 to cool the potatoes. The steamed sweet potatoes are dropped to the bottom plate 26 by inclining the bottom plate of the steaming chamber 6, and taken out through the outlet 24 port by rotating the rotary blades 27.