Propagating Koji

Propagating Koji
Or: How to turn your packet of spores into an infinite supply.

Mature kome-koji.

I’ve saved the best part for last. Are you ready for some information you can’t get anywhere else? This is the coup de grâce for the secret that every kura worldwide has so jealously guarded over the centuries that information regarding it is not available anywhere on the English-speaking or Japanese-speaking internet (I asked John Gautner to look on the Japanese side for me and even ask his toji friends). The one thing that the people who make money from koji and related products (especially Vision Brewing) does not want you to know.

This is the answer to the question that I often receive, of which this is but one example:

Comment from Forrest O. [Visitor]
Here’s a question for you, can you make koji from some of the last batch of koji, instead of koji-kin?

An infinite supply of koji to fill your sake homebrewing requirements, for nothing more than the cost of your first pack of spores and the rice you put into making it? Hmmmmm….

Yes, it can be done! But not in the way that our friend Forrest here is assuming. It needs to be understood that koji, while being a fungus, is not yeast. It’s not possible to culture more of it by simply mixing some of a previous batch of koji with some steamed rice. It won’t spread kernel-to-kernel like that, it’s just not how it works.

But, in my scouring of the internet in search of any information on the subject of harvesting koji-kin spores, I found one very brief, very obscure (so obscure that I can’t re-find the document to link it here, I’m relying on my memory) reference to the old traditional Japanese method of inoculating freshly steamed and cooled rice with aspergillus oryzae spores. With that little scrap of information, some educated guessing, and a little luck, I figured out how to replicate that technique.

And now I’m going to tell you how I did it.


An ordinary large salt shaker.I know, I know, there’s already a huge list of equipment back at the beginning of this guide. But I’m mentioning this here because there’s one particular item that you may not have around the house, and you’ll want to make sure you pick one up. I’m talking about a good size shaker with small holes on the top. I grabbed this beauty at Wal*Mart for about $3.50 specifically for this task. You need to make sure that whatever shaker you use has small enough holes on it to prevent the rice grains from falling through, which means a pizzeria style cheese shaker is not appropriate unless you modify it with some kind of screen under the lid. You just want to be able to shake koji-kin spores out of it without the rice grains that is their source falling out.

You’ll also need a small bowl or some other vessel to hold a small portion of your koji rice. Hopefully you made more than you needed for your upcoming batch of sake.


This is actually very simple. As near as I can tell, two major factors trigger aspergillus oryzae into its spore-producing state: full maturity (time) and dehydration. Once the 40-50 hours of main incubation of your batch of koji is finished, take a small amount of the kome-koji (I used 1/4 cup, but this is a very small amount for experimental purposes), separate it from the koji you’re about to refrigerate or freeze, and put it in a clean plastic bowl. This bowl you just move back to the incubator, which hopefully is still warm. Take out the cup of water or whatever other means you used to keep the incubator humid, leave the bowl of koji rice uncovered, put your temperature probe in the bowl, and seal the incubator back up.

The residual humidity inside the incubator is enough to keep the rice from drying out just long enough for the aspergillus oryzae to figure out that things are getting arid and prepare to start reproducing. In 24 hours, you’ll have something that looks like this:

Maturing koji forming spores after 24 hours.

See the green splotches forming all over the place (more evident at the edges due to lower light intensity)? Spores! Carefully stir the grains up a little bit to separate them, but don’t be too rough or you’ll end up with a cloud of spores instead of nice green-covered grains. Place the bowl back in the warm incubator and check on it in another ten hours. Chances are, you’ll see something like this:

Spore coverage is spreading quickly.

Don’t stir or otherwise disturb those grains or you’ll get a face full of spores. Just leave the bowl where it is and come back to it in 10 to 12 hours. When next you see it, there may or may not be a big visible difference:

Mature tane-koji in only 48 hours.

Grains of rice covered in yellow-green spores. This yellow-green color is the only exception to the “white-only” rule of making koji.

Provided that you did, indeed, remove the humidifier from your incubator, this tane-koji should now be dry enough to move to its final resting place. It might be prudent to consider using a piece of cellophane to cover the openings of both containers while you pour the tane-koji into the receiving vessel, just to prevent any spores that get knocked loose from flying all over the place (they’re very fine, smoke-like when they become airborne):

Carefully transfer the tane-koji to its final home. Just don't tell my wife about that yellow-green stain in her Tupperware bowl....

Here you can see some spores I've shaken out as an example.The rice and its accompanying mold will dry out further over time in this shaker, and will keep pretty much indefinitely at room temperature. This shaker came with a plastic lid that fits over the top, which is pretty nice to keep contaminants out. If you don’t have one, you could cover it with some plastic wrap and secure with a rubber band when not in use. To use this, just remove whatever cover there is from the holes, invert, and shake over the rice you wish to inoculate. Mix and repeat until you are satisfied that you’ve gotten enough spores into your freshly steamed rice – it’s not really possible to overdo it with the spores, but not adding enough might be a concern. You want the inoculated rice to have a noticeable green tinge to it when you’re done, which usually takes three applications of spores when I do it.

…And In Conclusion…

As I have herein shown, the process of growing your own koji from readily available mail-order (okay, internet-order) spores should be pretty clear and easy to follow successfully with only a few basic pieces of equipment necessary to maintain the proper environment. It’s somewhat labor-intensive with all the stirring, but no more so than sake itself is. And hey, making your own sake utterly from scratch is worth it, right?

Additionally, with the knowledge that koji can be encouraged to spore and be used in the old-school Japanese style to inoculate further batches, your first pack of spores should also be the last one you ever need to buy.

Thanks for reading!
-]Bob Taylor, Taylor-MadeAK