Beginning of Koji Starters
Koji has been existed since old times. It is said that koji was produced because wild fungi were seeded in nature at that time. It is presumed that repetition of such seedings selected mold that grows well in steamed rice. The mold is thought to become aspergillus oryzae.
It is said that, from around 10th century, the production method changed from natural seeding to Tomodane (Tomokoji) method where only well-made koji produced by natural seeding is used. Tomodane is a seed that cultured aspergillus oryzae, which is treated in the same way as koji starters.
Tomodane at early times was unstable. In the Muromachi period, it became possible to stabilize the aspergillus oryzae spores by adding wood ash to steamed rice. This is the base of current koji starters.
【Quoted from Rohen Kanwa Vol. 1 issued by Nihon Jyozo Kogyo (February 15, 1976 issue)】
According to Dr. Kinichiro Sakaguchi (Journal of the Brewing Society of Japan Vol.58 No.5, 1963), origin of koji starters was considered as follows:
“A book called ‘Ryounoshuge’ published at the beginning of the Heian period (9th century) describes Ritsuryo (codes) and institutions at the Nara period or earlier. It says that the proportion of making ‘Mishou’ should be 8 g? (approx. 1.44 liters) of ‘Kikushimai’ to 1 to (approx. 18 liters) of total ingredients. This ‘Kikushimai’ is suspected to be koji starters. In China, the character ‘shi’ means plant seeds. Therefore, I don't think it is completely unreasonable to consider ‘Kikushi’ as koji starters.”
Brewing method of "Hishio (sauce)" is said to have come down in the Emperor Shomu era (8th century), and a "sake brewing method" is said to have come down in 5th century. It is said that documents related to sake brewing which were written when the "sake brewing method" came down to Japan did not mention anything about koji starters. Assuming that there was no koji starters at the time of China, it is reasonable to consider that a new technique that uses koji starters started in either Japan or Baekje (Kudara) during 300 years after the sake brewing method came down to Japan.