Brewing Sake-a student experiment at RWTH Aachen University
The Chair of Biochemical Engineering at RWTH Aachen University introduces students to the art of sake brewing during the practical course in biotechnology and biochemical engineering.
Traditional sake brewing in Japan (picture kindly provided by Gekkeikan Sake Co., Ltd.)
Students of RWTH Aachen University have been brewing sake in their practical course since 2002 . Students of biotechnology, biology and mechanical engineering (majoring in biochemical engineering) attend this course which is offered at least once per semester. There are also students from mathematics and information technology major taking this practical course as an elective. Different fermentation methods like batch, fed-batch and continuous fermentations are taught by the Chairs of Biotechnology and Biochemical Engineering.
In the field of food biotechnology the students produce fruit wine at the Chair of Biotechnology and sake at the Chair of Biochemical Engineering. Thus, different methods of fermentation are presented.
Plastic vessel with fermenting sake
In comparison to wine, sake is produced from starch-containing rice, instead of sugar-containing juices. To cleave the starch into sugar the fungus Aspergillus oryzea is added to steamed rice. The fungus produces the amylase the enzyme, which cleaves starch to sugar. The result is Koji, an amylase-rich, sweet mixture of rice and fungus. Koji is mixed with new, steamed rice and water. The brewing yeast Sacharomyces cerevisie is added to ferment the sugar into ethanol resulting in the production of sake the beverage.
The fungus Aspergillus oryzea, the starting of the sake fermentation
Sake brewing in Aachen
Brewing sake is an art with an old and rich tradition. The complex process of sake brewing is not easily achieved in a practical course for students. Fortunately, our Japanese Guest Professor, Prof. Yoshihito Kato had designed a protocol for easy sake brewing at home, which can be applied in our course. The necessary cultures of Sacharomyces cerevisie and Aspergillus oryzae were kindly provided by Gekkeikan Sake Co., Ltd..
The quality of sake depends on the quality of the used rice. To prevent undesired by-products the rice is polished. This removes fatty acids and proteins in the outer layer of the rice grain.These substances could negatively influence the taste . The pure, starch-rich inner part of the rice grain remains and is used in sake brewing.
Highly polished (left) and unpolished (right) rice in comparison (picture kindly provided by Gekkeikan Sake Co., Ltd.)
The polished rice is washed to remove rice flour. Thereafter the rice is watered. Watering is very important for sake quality as it greatly influences the quality of the resulting Koji and also the whole fermentation.
Traditional rice watering in Japan (picture kindly provided by Gekkeikan Sake Co., Ltd.)
The polished and watered rice is then steamed. It is important that the rice will not stick together. Therefore the rice is steamed instead of boiled.
Rice is steamed in Aachen in a pressure cooker
Rice steamed in the traditional way (picture kindly provided by Gekkeikan Sake Co., Ltd.)
Rice steamed in a modern, industrialized way (picture kindly provided by Gekkeikan Sake Co., Ltd.)
The rice is cooled and than inoculated with Aspergillus oryzae spores. The resulting Koji is incubated at a warm place with a defined humidity. After two days the rice is densely settled by Aspergillus oryzae.
Koji in the practical course
Traditional way of Koji production in Japan (picture kindly provided by Gekkeikan Sake Co., Ltd.)
The Koji is mixed with new rice, water and the yeast starter culture (Shubo) to start the fermentation. Three more times (every two days) Koji, rice and water are added until the final fermentation can begin. The sake fermentation is a kind of fed-batch fermentation.
A student adding Koji to the main culture (Moromi)
The main culture (Moromi) is mixed regularly and the carbon dioxide production is monitored. This enables the students to observe the progress of the fermentation.
Carbon dioxide production during sake fermentation
The final sake batch is weighed
Big tanks for industrial sake production (picture kindly provided by Gekkeikan Sake Co., Ltd.)
At the end of fermentation the sake is filtered. The rice and the microorganisms are separated from the resulting sake. The sake is then pasteurized. This is not done in our practical course. Instead the sake is consumed directly.
Filtering sake at Gekkeikan (picture kindly provided by Gekkeikan Sake Co., Ltd.)
After the written exam of the course the students and supervisors gather for a collective breakfast. During this occasion the new sake is tasted.
Students from RWTH Aachen University trying their own sake
High appreciation and gratitude are given to Prof. Yoshihito Kato, Gekkeikan Sake Co., Ltd. and Ms. Sachiko Wakamura for their great supports.