Koji
The topic of koji making actually constitutes a large enough body of information to not only have it's own page, but it's own WIKI and many of us contribute to that as well. However, as sake is a very specific category, we will do our best to describe the difference and go over the basics so you can absorb the world of Koji a little easier.
The purpose of making koji is to sake as malting grain is to beer brewing: to produce enzymes that will breakdown starch and produce simple sugars that yeast will consume and make ethanol (alcohol). It is however, a VERY different process for achieving this goal.
Topical articles and pages:
Terminology and stages in sei-kiku 製麹 – The koji making process for sake
Troubleshooting guide for koji making
Ueda koji making method adapted for the homebrew scale
Resources included:
Background
In beer brewing, the grain is soaked in water in a controlled process that allows the seed to germinate. This activates natural enzymes which stimulate the creation of new enzymes. During the germination phase the grain is modified as rootlets emerge and the enzymes breakdown carbohydrates and proteins, which opens up the seeds' starch reserves. The grain is then dried in a kiln to stop the germination process, thereby ensuring the seed has plenty of starch left over. When the grain is later boiled in water to create beer wort, the high temperature of the water allows the enzymes to undergo hydrolysis, breaking down the starch reserves and creating simple sugars.
In sake brewing, the husk of the grain was shown to produce many unwanted flavors and so it is removed through the milling process. Therefore, it cannot simply be germinated like barley is in beer. To achieve the required enzymes, a special type of mold spore (usually of the Aspergillus Oryzae, Aspergillus Niger, or similar varieties) is inoculated onto steamed rice at about 29-33°C. The spore grows into the grain, consuming the starch, and producing alpha-amylase, beta-amylase, gamma-amylase, and glucoamylase enzymes. Different temperature ranges favor the production of one of each of these types. While the starch in these grains is broken down as well, the enzymes from the koji are released into the sake mash and continue to consume the starch from all the additional rice grains added in each of the brewing steps particular to sake.
Keep these in Mind:
We will go into much more detail on the process of koji making, but just so you have some bearings when you start down this path, Koji making is an art AND a science. There are aspects that are HIGHLY subjective and other that are very much objective. For instance… many charts (if you can find them… and we have found them for you) illustrate ideal temperature graphs. It will show a serious of time based temperature increases with beautiful arcs and no fuzzy fluctuations. There will be one mix around 10-12 hours, then another around 22, then 6-8 hours later and finally 6-8 hours after that. Each of these have a specific name and when 48 hours is elapsed, that moment is when you cool the koji down to stop its growth. This presents a problem for beginners for several reasons:
When you attempt to make koji, some things will take a while to understand, but controlling the temperature is very difficult to master. When you see those pretty charts, you might think, “OH NO! I'm doing something totally wrong!”. And you may very well be, but that's usually not the case if you follow the basic process.
If you don't know which koji spores they use, type of sake they are making, flavor profile they prefer, method they are using to make koji (there are many), or many other variables… than these charts mean nothing more than “an average". This is simply because everything you tweak will have a large impact, all the way down to what kind of materials you use to make the koji in. You could have good insulation, or bad insulation. You could do table koji or tray koji. There are some breweries that keep their koji chamber at a higher temperature than others. There are koji spores that can tolerate different temperature ranges and even generate more or less heat than others.
Other factors start to come into play like humidity, type of cloth, heating source, room/chamber size, elevation/dryness, etc. In fact, Colorado's dry air makes it fairly necessary to use a plastic wrap of sorts to keep the rice from drying out too much during the incubation phase, while other areas in the country, under normal air conditions, might never need a tweak like this, but might struggle to dry out the room/chamber on the second day when we want to alter the koji's growth.
These three items really do alter the production process immensely and there are more I'm sure aren't even included here. The point to take away is that when it comes to koji, growing it is more about which lever to pull to cause various things to change. I started making a list of these to explain what those are.
Process
Overview: one person's cheat-sheet of step-by-step instruction reminders
The most important things to remember about the process of koji making are:
Heat, Humidity, and Oxygen are the three largest levers you have to adjusting how you koji grows.
Temperature ranges are about creating a flavor profile in your final sake. If you follow what you see on a chart very closely it will not product the “best koji” it will produce something similar to who ever made that chart. That might be more or less umami, more or less sweet… you don't really know the full picture, so don't treat it like a “bible” to go by, just think of charts as “guidelines”.
What works for large scale production will not work for small scale, homebrew. If you are making 1-3kg of koji, the chamber you will need will probably be much smaller and everything from inoculation to temperature control will be very different that how it is done in large rooms with 100+kg of rice.
If you let the rice get too hot or too cold, it will either die from too much heat or it will go dormant from too little. (The spores are pretty hardy though, so unless you let them get over 50°C, they should survive getting too cold or too hot, you'll just have to adjust the environment to get the koji back on track)
Below, is an image prepared for a larger presentation that gives one brewer’s perspective on the different factors that go into changing the koji. These factors are not a simple decision, but many are considered several times throughout the 44-55 hour growth period.
