Written By: Mannat Malhan
Edited By: Prakhar Sood
Designed By: Andria Sadar
Published By: Khushal Patel
What do pickles, yogurt, and cheese all have in common? Fermentation! It’s been used by people for thousands of years to preserve food, enhance flavours, and craft new recipes from familiar ingredients. One of the most interesting examples is koji, a cultivated fungus that’s used to produce foods such as miso, soy sauce, sake, and amazake. Although koji is only one part of these foods, the process behind it links microorganisms with agriculture, food processing, and the way food moves from farms to consumers.
So, what is koji? It’s made by cultivating a fungus called Aspergillus oryzae on a food like steamed rice, barley, or soybeans. The fungus then grows through the food and produces enzymes as it develops. These enzymes are what make koji useful. Many agricultural products contain nutrients in forms that can’t be easily used by the microorganisms involved in later fermentation. Starches, for example, are made up of long chains of molecules and proteins are large molecules made from smaller building blocks. Koji produces enzymes that break these larger molecules down. Amylases help break starches into smaller sugars, while proteases break proteins into smaller compounds like peptides and amino acids. These changes affect the flavour and characteristics of the food. Sugars can contribute to sweetness (obviously), while amino acids can contribute to savoury flavours. Instead of adding ingredients to a food, koji changes the ingredients that are already there.
Furthermore, koji cultivation is more controlled than it may seem. Producers have to pay attention to factors like 
temperature, moisture, oxygen, and time because these conditions will affect how the fungus grows. For example, high heat can kill the spores, low moisture stops growth, and lack of air can cause poor enzyme production. The grain also needs to be prepared properly, depending on what it’s turning into. For rice koji, the rice is generally cooked so that it has enough moisture for the fungus to grow while maintaining its structure. The fungus is then introduced and allowed to develop under controlled conditions. As it grows, the koji becomes covered with fungi and has to be monitored because the goal is to limit unwanted microbial growth. This knowledge has been developed through both traditional practices and modern food science. Generations of food producers developed ways of controlling these processes through observation. After all, people were cultivating koji and using fermentation long before microorganisms and enzymes could be identified.
Koji also shows how much can happen to an agricultural crop after it leaves a farm. Instead of being eaten in its original form, it can be processed and fermented into products with different flavours, textures, and uses. Take soybeans as an example. They can be used to produce foods such as tofu, but they can also become part of the process used to make miso and soy sauce. In koji-based products, microorganisms and their enzymes help transform the original ingredients over time. This is what creates additional steps between agriculture and the food that eventually reaches consumers. The crop supply chain spans from growth and harvesting through processing, fermentation, and final distribution. Each stage needs different people, equipment, businesses, and resources. Therefore, koji is a part of a food system, and not just a fermentation ingredient. Fermentation can also change the value of food. A crop that’s relatively simple on its own can become an ingredient with a completely different use after processing. Koji helps make this possible. This is one reason fermentation has remained important across several cultures. It creates new tastes while also helping produce foods that can be stored and used in various ways.
So, the next time you eat miso soup or use soy sauce, there’s a much longer story behind that food than what appears on the package. It can begin with a crop grown on a farm, continue through cultivation, and eventually involve processing, transportation, and preparation. Koji may be a small part of that process, but it shows how closely agriculture, biology, and food production are connected. In conclusion, koji is an exceptional example of how much can happen between a crop being harvested and a food ending up on our plate.
References
Johnson, L. A. (2008). Soybeans. Academic Press and AOCS Press.
Piggott, J. (2011). Alcoholic Beverages. Woodhead Publishing.
Preedy, V. R. (2009). Beer in health and disease prevention. Burlington, Mass. Elsevier/Academic Press.
Batt, C. A., & Tortorello, M. L. (2014). Encyclopedia of food microbiology (2nd ed.). Elsevier/Academic Press.
Yamashita, H. (2021). Koji Starter and Koji World in Japan. Journal of Fungi, 7(7), 569.
Marui, J., Shiraishi, Y., Takeura, M., Shompoosang, S., Varichanan, P., & Boulom, S. (2025). Combined use of rice-koji cultured with Aspergillus oryzae and Aspergillus luchuensis for enhanced dephosphorylation of phytate to myo-inositol in brown rice-koji-amazake, a sweet brown rice beverage. Food Science and Technology Research, 31(2), 147–153.
The Wasabi Company. (2024, November). What is Koji and Why is it So Special?