What is koji? (And why make it at home?)

Koji, (Aspergillus oryzae) is a type of mould that is used in making many Japanese foods. It is used in making soy sauce, miso paste, sake. It changes starch into sugar and uses protein to create savoury "umami" tastes.

The word "mould" conjures up images of something disgusting for some people--something that looks horrid and could even be toxic.

But, of course, there are moulds that transform food in positive ways. This is true in Western cuisine as well as Japanese. Who would want to get rid of the mould from a mature Stilton?

In a recent Guardian article about Japanese food, Charlie Brooker described another mould prepared food, Katsuobushi, as follows.

In summary: it's a dry, mouldy fish that has been sitting around for months. [...] desiccated mould-encrusted tuna. That's the source. Sorry to break that to you.

You might as well describe Stilton as mouldy curdled milk sitting around for months.

Anyway, back to koji. Because we are dealing with something that is alive, it can be tricky to make. You need to get the temperature and humidity absolutely right and there are craftspeople who devote their whole  lives to getting the process right.

This blog is about making it at home. It is born of necessity. I am a Japanese woman living in the Yorkshire dales and I don't have access to the ingredients I would be able to buy in Japan. I expect plenty of failure, but there is a little voice encouraging me as I set out to make my own koji: "Didn't women make miso at home for ages? Did they ALWAYS buy koji from shops? And hey? Didn't you let plenty of food go mouldy in your student days, anyway? In a sort of blundering sort of way, you must be an expert."

As G.K. Chesterton once put it, if a thing is worth doing, it's worth doing badly.