Sake fermentation

In chilly a room where the temperature is maintained at 5°C temperature, when the lid of a fermenting tank is removed, a fruity aroma fills the space. Although there are many varieties of fruity aromas, this particular scent suggests a fresh green apple.

Insite the tank, bubbles occasionally rise to the surface of the sake mash that's on its way to becoming sake. Beneath the surface, fermentation is occurring in a complicated and somewhat mysterious process.

So what's going on in there, exactly? Insite the tank are koji rice, steamed rice, yeast and water. The enzyme produced by koji rice is breaking down the rice starch into glucose, which is being provided to the yeast. The yeast converts glucose into alcohol. So, the conversion of rice starch into glucose by koji  rice enzyme is occurring.

Next, the fermentation or conversion of glucose into alcohol by yeast takes place. The sake mash is gradually turning into sake. Many say that sake making and wine making are similar in their production processes. However, since wine making requires conversion of glucose into alcohol alone, it is fair to say that fermentation in sake making is somewhat more complicated. Vigilant control of the fermentation process is the key to crafting great sake.

At Asahi Shuzo, we use a long, low-temperature fermentation method. Because yeast is viable, even a slightly elevated temperature can activate it. Meanwhile, careful fermentation at a temperature so low that it is only marginally life-sustaining for the yeast to make the resulting sake smoother, and with richer aromas. The smooth, fresh aromas that please so many who taste Dassai for the first time is the result of low-temperature over many days. A maker producing sake in large volume completes the fermentation process in as little as 10 days. However, we need to spend more than 35 days on this process for Dassai to deliver its full, clean flavor profile.

Not only do we invest time but also manpower in this fermenting process. Our staff continuously monitor the temperature of sake mash with a digital thermometer, and stir the mash with a wooden oar to blend the room's cool air into the tank as needed. This is how workers adjust the temperature caused by fermentation in increments of 0.1°C and keep it at an ideal fermenting temperature.

We once tried to manage the mash temperature using sensors and a computer. However, it did not work out well because yeast is a living organism and rice varies slightly in its properties, depending on the field where it was cultivated. These are examples of the combination of factors that subtly affects the sake fermenting temperature, and it is something even the latest sensors and computers cannot deal with.

Traditionally, sake making was entrusted to a master in sake crafting called Toji. We used to depend on such a master for our sake crafting too, but there came a time when we decided to change our crafting methods from one that relies one man's hunches to one that is based on decisions taken after analyzing the data collected from ingredients, fermenting sake and tasting the sake.

To many sake producers it is unconceivable but, eventually ending this toji-system greatly improved the flavor profile of our sake. It has enabled us to pursue what we believe to be the "ideal sake" without worrying stepping on the master's toes. Also, year-round sake crafting became possible: a toji is typically a farmer or a fisherman, and would normally come to make sake only in winter - during his off-season.

These two factors, namely conducting low-temperature fermentation of sake mash by hand and crafting it year-round without a toji, are ideas that challenged common thinking in the sake industry. They were, however, done with a single purpose in mind : to make great sake.