The mythical world of Koji-kin, Aspergillus oryzae

Looking at the storage tanks at Saijo Tsuru

What's Koji-kin? What's Aspergillus Oryzae? 


This microbe is indispensable for sake making. 


There are a lot of amazing and intriguing facts about sake brewing. Comparing how wine, beer, and sake are produced will make your love and appreciation to these beverages deeper. 


So here's a little tidbit. This time mainly about Koji-kin.


The grape juice already has sugar in there and it will be fermented into alcohol. And the wine is ready, though wine making is much more than that of course.


The barley has to be malted so that the starch in barley should be turned into sugar, which will be fermented by the yeast to alcohol named beer. These are sequential processes, one step at a time.


The sake rice ready to be used for sake making is already without husks. Thus no malting process is involved in sake making. In addition, the rice is first milled. Sometimes down to 35 % of the original weight if it's for Dai-ginjo premium sake. The milling ratios of the rice range from 70 % to 35 %. The rice powder is used for other purposes such as the ingredient for snacks, animal feed, etc. so don't worry.


The milled rice is still rice and does not have the sugar to be eaten by yeast cells and cause the fermentation process. Here comes the Koji-kin or Koji spores. Their scientific name is Aspergillus Oryzae.


The Koji spores are sprinkled over the steamed rice and they begin to propagate, making Koji-molded rice. This molded rice containing glucose is the food for yeast cells.


The Koji-molded rice, steamed rice, and pure underflow water are mixed in a vat and the yeast is added to the mixture. The yeast cells are happy with the food and the fermentation process begins while the starch in rice continues to be broken into the sugar by the enzyme the Koji spores produced. 


First the starter mash has to be prepared, followed by the serious fermentation process for 20 to 30 days. 


These processes, saccharification and fermentation, take place at the same time in the same vat. Not sequential processes like in the beer making.


The sake breweries have the special room, Koji Muro, where the Koji-molded rice is prepared. It's a delicate process which has to be controlled and monitored carefully.


In this photo, two of my guests are taking a look at the tanks where the fermentation stage is happening at the factory of Saijo Tsuru, Saijo, Hiroshima. Toward the end of November, breweries have their products of this year, their new sake. 


My husband is a great fun of Shinzui produced by Saijo Tsuru, one of the breweries in the Saijo Sake Brewery Street. This superb product is naturally pricey. I promised him to buy him a bottle for the new year's celebration if he tries to be a good boy. He has a month and a half to go.  We'll see.