Keen for Koji

Would you believe that a great deal of saké is brewed with just four ingredients? We’re breaking down each ingredient to show just how minute differences can impact the taste, aroma, and quality of any given saké. Today we’re giving the spotlight to Koji-kin, the wonderful little mold that could.

Koji-kin (aspergillus oryzae) mold is the foundation for many Asian food products such as miso, soy sauce, mirin and of course, saké. 

Rice becomes 'Koji' when it has been steamed and inoculated with Koji-kin. Its purpose is to deliver enzymes that convert the rice starch into fermentable sugar. However, the influence of Koji goes beyond being a fermentation buddy to the yeast. Depending on the strain and type of Koji-kin, varied levels of vitamins, amino acids, citric acid and other factors can affect both aroma and flavor.

Nearly all saké use Yellow strain Koji. This particular strain limits acidity and promotes a smooth mouthfeel and light sweetness due to its inability to convert all starch into sugar. 

By now you likely understand that there are two general and diametrically opposite styles of saké; umami style, which is bigger in body, acidity and may feature more earthy flavors, and Ginjo style, which is light bodied, fruity and/or floral. It should come as no surprise that the two types of Yellow strain Koji residing at each end of the spectrum help deliver either umami or Ginjo characteristics. 


Brewers who select the 'so-haze' (pronounced so-hah-zay) type of Koji-kin often times are looking to produce a table-grade Futsu-shu saké or a full bodied, high acidity and umami-rich premium style. It is the high vitamin, protein and organic acid content that creates these qualities.


The lighter mold growth pattern of 'tsuki-haze' (pronounced skee-hah-zay) means a slow and very controlled fermentation, ideal for brewing the delicate, crisp and fruity/floral Ginjo and Daiginjo style saké.

When it comes to saké, this spore grows on the steamed (but cooled) rice, digesting it with enzymes that convert the starch into sugars. Koji spores are applied to the steamed rice which is then carefully managed with temperature and humidity controls. Slight shifts in temperature, length of time, or humidity can mean the difference between perfectly fragrant and rich yeast food and old smelly sneakers. Koji also adds a good deal of flavor to saké.

Cultivating the Koji mold occurs over the course of about 48 hours. After the rice is steamed, Koji in either powder or granulated form is spread evenly across a large shallow bed in our Koji room. The next step is to mound and bundle the inoculated rice beneath layers of blankets to retain warmth and moisture so the mold has an ideal growing environment. Brewers monitor and manage temperature closely during this time as mold growth naturally creates heat. Koji grows on and into the grain, digesting the starch and converting it to sugar. Different strains of Koji create different aromas and flavor elements, so our brewing team selects for the desired effect. After about twenty-four hours we have perfect food for the yeast.


Once the steamed rice is brought down to the target temperature, brewers will uncover and break apart the rice mound to get rid of heat spots and re-distribute the Koji rice evenly across the bed. The warm temperature and humidity of the Koji room promote additional mold growth for the next 8-12 hours. It is the brewer's arduous task to execute an appropriate method of managing the thoroughness and distribution of the mold growth to achieve the perfect mold pattern for their target style. 

The Koji rice temperature will fluctuate from roughly 95F to 92F. Naturally, the temperature will rise again to peak around 100F and the grains are gradually spread thinner to cover more surface area to both manage the mold growth and gently dry the grains of rice. When performed properly, the moisture levels of the grains will draw the mold growth inward towards the center and not just the outside. 

When the brewer is satisfied with the mold coverage and penetration, they stop the Koji growth by cooling the batch to the ambient temperature of the brewery. Roughly two days later, steamed rice has become brew-ready Koji rice and yeast can finally be introduced to the process.

Fun Fact: Cedar rooms are often used during Koji cultivation for their aromatic, humidity controlling and antimicrobial properties.

Stay tuned in two weeks to learn about the final ingredient added to pure saké, yeast.

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