What is koji?
I generally describe it as a sort of starter for different foods, similar to a sourdough starter or a kombucha SCOBY. The underlying biological processes are very different.
Koji is a type of edible mold called aspergillus oryzae. There are other members of the aspergillus family that are also used in similar ways all over Asia. Koji is specifically Japanese and has been used there for well over 1500 years.
Molds have been used in fermentation to provide complexity and depth in all sorts foods, from brie to salami. At the heart of it, like any fermentation, the microbes and enzymes break down some of the molecules in the starting ingredients into their constituent parts.
Historically koji and similar solid substrate mold cultures have been used to make all kinds of things like soy sauce, miso, tamari, saké and mirin. That rice wine vinegar at the back of your cupboard? Made from koji. The tasty little jar of chilli and black bean oil next to it? Yep, koji.
When you dig into it, it is amazing how many foods have been completely embraced without the knowledge of the underlying ingredient and process that created them.
Koji has the particularly fantastic ability to fragment proteins and starches into amino acids (one of which is the fabled glutamate, which provides that fabulous umami taste) and sugars respectively.
This is why using a little drop of shio koji can add that ‘little bit extra’ to any dish. You have essentially made all of the ingredients ‘taste more like themselves’ to shamelessly butcher a phrase from René Redzepi.
For me, the beauty of koji lies in that it plays well with almost any ingredient once you know a few of the basic techniques.
So, what is koji?
Tasty, tasty mold.
If you want to read more than this very brief description there are some amazing and far more articulate sources out there. I would immediately recommend:
Koji Alchemy by Schih & Umansky
Koji For Life by Nakaji
The Art of Fermentation by Katz
The Noma Guide to Fermentation by Redzepi & Zilber
The Book of Miso by Shurtleff & Aoyagi