Soybeans are first cooked to soften the bean, and then bacterial and fungal cultures are added to begin the fermentation process. Roasted wheat or other grains may also be included to provide a unique flavor. The soybean culture mixture is then combined with a salt brine and allowed to “brew” for a specific amount of time.

During fermentation, enzymes from the mold act on the soy and wheat proteins, gradually breaking them down into amino acids. The starches are converted to simple sugars, then fermented to lactic acid and alcohol. After the aging process is complete, the mixture is laid out onto cloth and pressed to release the liquid.

Fungal spores, usually Aspergillus coryza, A. sojae, and A. tamarii, are added in and left for 3 days. The next step is fermentation, where a brine solution is added. This may be left to ferment for 1 month up to 4 years.

After the mold colonizes the substrate, a process that takes about three days, the culture is combined with salt water and transferred to large vats where lactobacillus —a bacteria that breaks down sugars into lactic acid —is added, and the resulting mixture ferments for a period of time, anywhere from six months for some standard supermarket brands to several years for more pricey bottles.