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Koji is the heart of sake
Going back the the basic concept of sake brewing, the very first step of the fermentation process is to use enzymes to break down the starch in the rice grain and convert it to sugar (a.k.a Saccharification).
Koji is the guy for this job!
In the world of sake brewing, the saying goes "Koji first, Shubo second, brewing third". Koji is considered to be the most important step in sake brewing. The development of Koji has a significant impact on the style and quality of the resultant sake.
Most of the breweries make their own koji. Koji rice is made by cultivating koji-fungi on streamed rice. Steamed rice is laid out over a table and koji fungus is sprinkled over it. Koji-fungi are most active at a temperature of around 36°C, but will cease all activity at a temperature above 45°C. With an environment of temperature and humidity carefully controlled, the process of saccharification last for around 2 days.
During the process of saccharification, sake brewers have to closely monitor the development of the koji fungus. They will regularly hand-mix the batch to ensure the even distribution of fungus, heat and humidity in the batch.
In general, there are two types of Koji rice style- Sohaze and Tsukihaze.
sohaze
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koji fungus propagates entire grain
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quicker saccharification
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used for Futsu-shu and full bodied sake
tsukihaze
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koji fungus propagates in spotted pattern
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slower saccharification
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used for Ginjo level sake
shubo, the yeast starter
Shubo, literally translated to "Mother of sake", is to be prepared before the main fermentation. In a small tank, the brewers add the koji rice to the mixture of warm water, steamed rice, yeast and lactic acid to cultivate a robust yeast culture.
Sake Yeast (a.k.a. Kobo) has 2 important roles:
1) converting sugar into alcohol
2) creating aroma compounds
Historically, sake used to be produced with yeasts that grew naturally at the breweries which led to inconsistent quality of sake. In the modern days, cultured yeasts are used instead. With the nation-wide "Brewing Association Yeast" program established in the early 1900s, different types of standardized yeast are developed by the Brewing Society of Japan (a.k.a. Kyokai-kobo 協會酵母), and adopted by the breweries according to the style of sake they want to make.
Apart from choosing the right yeast, it is also crucial for the shubo to be highly acidic- which suppresses the microbes that can spoil the sake during fermentation process. Unlike grapes, rice grain itself contains no acid. The different methods in creating this acidic Shubo are:
1) Sokujo (速釀)- adding artificial lactic acid directly to the shubo
2) Kimoto (生酛)/ Yamahai (山廃)- using lactic acid bacilli to cultivate natural lactic acid
moromi, the main fermentation
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When the Shubo is ready, the next step is to add the rest of the ingredients to the big tank for the main fermentation process. Yet, the entire amount cannot be added all in one go (although it would have saved some time), as the yeast would become too diluted for a proper fermentation. The mixture of a standard ratios of steamed rice 80, koji 20 and water 130 is added to the tank of shubo in three stages over four days. Throughout the process, the temperature of the mix is strictly regulated, lowering from 12°C in the first step, to around 8°C in the last step of addition.
In the sake moromi (main mash), the magic of "multiple parallel fermentation" happens over the course of 3- 4 weeks. The enzymes in koji dissolve the steamed rice and the yeast ferments the resulting sugars at the same time, in the same tank.
At the end of the fermentation process, th sake will yield an alcohol content of around 17% to 20%
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