Steaming is the final process for rice preparation. Steaming rice has two purposes. One is to sterilize the rice and one is to change the structure of rice starch which is broken into sugar by Koji enzymes during fermentation. The steaming the rice about 700kg by traditional batch steamer takes 40 to 60 minutes. This steaming steps is done at the first work of day during brewing season. When I visited one brewer in Toyama at early morning in the winter, warm seaming and rice aroma is coming out from the brewing place.
The rice is loaded in the batch steamer in layers and separated with a cloth. Top layer of the rice is used for Koji making, because they are not too moist. On the bottom of the layer, "fake rice" is loaded in order to separate between the rice layers and steam inlets to avoid that the rice become too much moist. In order to control the moisture level of this top layer of the rice, a thick cloth is used to prevent the water condensation dripping back on to the rice. The rice under the top layer is used for main fermentation and fermentation starter.
Ideal steamed rice status is 外硬内軟 (Gaikou Nainan), outside is hard and inside is soft.
(Traditional batch steamer. The photo from Kikuhime)
Some brewers uses continuous steaming machine for a large scale steaming. The most common type of steaming machine is belt conveyor type. This machine has two parts. One is steamer and second part is cooling rice. By using this machine, it takes about 30 to 40 minutes from steaming to cooling with many tonnes of rice par a day.
Brewers make a small round cake, and judge weather the rice has been steamed perfectly or not. Some brewers check the hardness of the rice ball, and some brewers check the rice by chewing with their front teeth.
After steaming, the rice is spread out on the bamboo mats on the floor of the brewery and cooled. When the rice has reached the target temperature, it is used for making Koji or main fermentation process.