At the end of 2018 I picked up a copy of the NOMA Fermentation Guide and was absolutely blown away by the techniques used it. It’s part cookbook, part textbook, filled to the brim with insanely awesome technical information about how the various fermentation processes work. I quickly put together a koji chamber, and started fermenting away. After making lots of koji, much of which turned into misos and shoyus, some I bartered for discounts on new knives or a free bottle of wine, I moved from Oakland to NYC and sadly sold off my chamber to some person at Lagunitas. It took some time before I finally settled in, and re-sourced a new chamber. After DM’ing multiple people on reddit with the part list, I figured I should make a website with the list and various nuances. Disclaimer: For most parts I’m using an Amazon affiliate link. If you’re trying to reduce cost, there are more than likely cheaper options out there but keep in mind the criteria for why something was picked.
First thing first, you need a container! I personally use a tray rack. It feels really versatile, especially since my girlfriend uses it when she bakes a lot of cookies. I can scale up the amount of koji being made at a time by just adding more trays. You’ll need to snag a vinyl cover and a tray for the bottom of the rack to act as the floor. Here's the link for the rack, vinyl cover, and full sized tray.
A cooler is a great starting point as they’re relatively cheap. The main caveat is you need to keep the lid slightly open most of the time to let fresh air in. Something like this would probably work really well, but if you can go to a local walmart I imagine there’s even cheaper coolers to be had. Link to Coleman cooler.
The perforation allows the Aspergillus Oryzae to breath. The hotel pans are really nice because it gives you some depth to work with, but a perforated sheet tray totally works. If you want to get fancy, you can build a traditional tray out of untreated cedar. I like half sized personally since they’re easier to handle and store. Links for the perforated sheet pan and 2 1/2” deep hotel pan.
N.B.: Sheet pans and hotel pans use fundamentally different sizing! A half sized hotel pan will not be able to use the rails in a sheet pan rack, as it's not wide enough. A full sized hotel pan will be too big. For some reason hotel pan racks are considerably more expensive than a sheet pan rack, so what I did was simply run some wood across the rails so the hotel pans can sit on them.
Most koji recipes call for lining your tray and covering your koji with a damp towel. Having an oversized tea towel or a smooth cotton towel will make life better should your rice/grain be a bit sticky, as it won't stick nearly as bad as it would to a terry cloth.
Because Aspergillus Oryzae needs to be incubated at approximately 30 degrees Celsius and at 80% humidity, you’re going to need some temperature controllers. I think the InkBird controllers work super well, especially for the price point. One thing to keep in mind though is that they are not true PID controllers, meaning there will be some temperature error. It’ll let the temperature drop to, say, 29.5 degree before turning the heating element on, and then wait until the temperature is at 30 degrees before turning it off. However, the temperature will keep rising since the heating element gives off residual heat for a short period of time, so the peak temperature is something like 31.5 degrees. Real, tuned PID controllers can keep the temperature at a consistent 30 degrees by having a model of the heating element's characteristics and constantly making micro adjustments. If you're tech inclined, you can really nerd out building your own controller using an Arduino with the relevant sensors. Link to Inkbird Humidity and Temperature controller.
Per above, you’re going to need a heating element. The size of the chamber should dictate the wattage of heater you buy. I personally use a half sized tray rack, and find 500w keeps up pretty easily at room temp. Make sure you pick one with a physical on/off switch! If it’s not physical, your heater won’t turn on when your temperature controller starts providing power to it again. Link to 500 watt space heater. For a cooler setup, you could use a heating mat to save space.
Same idea goes for a humidifier. Physical switch is a must, and chamber size dictates the amount of water you need. This will keep my chamber at 80% humidity for the whole grow time without needing to be filled, which is really nice. Link to humidifier w/ physical switch.
If you’re going to use a cooler instead of a rack style set up, you can probably use one of these small USB powered stick humidifiers in glass of water to get some space back. I haven’t tried it though so buyer beware.
N.B.: Humidity and moisture will tend to collect on the side of your chamber. This will lead to incorrect humidity readings. To get more accurate readings, I highly recommend taping the cable to the top of chamber so that the hygrometer dangles in midair, close to where your koji will be.
Once you have all your pieces, set it up! It should be pretty intuitive, make sure you keep your cord managed. If you're using a cooler, you can use something like a few mason jar lids or cooling tray to keep the koji tray off the floor.
Since I've never made a cooler based fermentation chamber, I'd strongly recommend reading /u/nesevis's post on making a cooler chamber.
Koji Alchemy is an incredible book filled with awesome recipe and knowledge, I highly recommend checking it out.
I also have a blog post on Medium about actually making koji.