Koji is a filamentous fungus, it creates a network of filaments which colonise. When it takes up residency on the surface of a substrate it breaks down the compound into its molecular components. This creates flavourful and aromatic by-products, breaking proteins into amino acids etc.
The Koji process is referred to as ‘autolysis’, a process of self-digestion. This is when enzymes break down from within a cell. Ageing meat or baking bread are processes of autolysis.
Autolysis takes place when water and flour is mixed and then rested. Naturally occurring enzymes then break down the flour’s proteins splitting them into gluten.
Technically speaking koji as a fungus is performing this process from outside the cell, extracellular digestion, as well as in it, so it is ultimately a process involving extracellular digestion, autolysis, as well as lactic and acetic acid fermentation.
The metabolism of the substrate is the koji’s way of feeding its life cycle. The fungus releases enzymes that will break down fats, proteins, and starch molecules. This process means it can be grown on pretty anything.
Koji is domesticated for alcohol production because it produces more alpha-amylase enzymes than other compounds.
These are extremely useful when breaking starch into sugar. Koji is essentially a tool to break down other substrates in order to let other fermentation microbes flourish. In sake brewing, it is breaking down the starches to allow for yeast fermentation. In Europe we use germinated barley, to break down starches for the yeast.
Strains are cultivated for the different abilities to work on the substrate. For example, soybean koji, has a particularly high level of protease. This means it is well adapt to breaking down the high levels of protein in soybeans.
When to use it?
40-50 hours is the sweet spot, it should be cottony and blurry. Refrigerate and use within ten days.