Sake is made of only 4 ingredients, water, rice, koji, and yeast. Nothing else needs to be added. The water here in Obuse, while great for growing chestnuts, is not ideal for brewing sake (Obuse water is hard, acidic, and contains iron), so all of the water must be charcoal filtered before being used. The charcoal filtering makes very clean, soft and pure tasting water, most suitable for brewing.
The rice used in sake brewing is also very important and it can be bought with a wide variety of characteristics. The outer shell of the rice grains contain lipids and other nutrients which are not ideal for brewing sake (these fats and nutrients can cause the formation of a large lactic acid bacteria colony in the starter mash, which will make the sake very sour) so this part of the rice needs to be milled off. In the center of the rice grains is pure starch, which is what the koji require in order to make sugar, which means that the more you mill a rice grain down, the more unwanted material will be discarded, and the better it will be for the sake. The problem with this is that the more you mill the rice, the smaller the grain, which makes buying ultra milled rice very expensive (as much as 70% of the rice can be lost during the milling process).
Since the water is so refined and the rice is just pure starch almost all of the flavors in sake stem from koji and yeast, which makes koji and yeast arguably the most important ingredients in the process. As mentioned before the sake rice grains are pure starch, thus the yeast has nothing to eat and cannot produce alcohol. Koji is the secret ingredient that makes this whole thing work. Koji is a type of fungus, and when sprinkled onto cooked rice and given the proper conditions (high temperature and moderate humidity) it will blossom and start converting all of the available starches into sugars. When koji inoculated rice is tossed into a fermentation tank containing yeast, the sugars that the koji produce will get eaten by the yeast, who will then in turn produce alcohol. This is called a multiple parallel fermentation since two chemical processes are going on at the same time in one tank.
Incidentally since the koji produces sugar steadily at a slow rate, the yeast can only produce alcohol at the same rate. Since the alcohol levels of the brew go up slowly the yeast has a chance to adapt to the increasingly alcoholic environment which is why sake can reach alcohol levels of 20% where as beer caps out at 5% and wine is around 14% (beer only takes a few days to brew, wine two weeks, where as sake can take up to a month). So despite what many people may think, sake is not a distilled liquor, the alcohol levels acheived are purely natural.
The first stage is to make a starter mash. The sake rice needs to be washed to remove all of the powder left on the grains from milling, then soaked for a length of time that varies depending on the type of rice, the temperature and the brewery (every brewery has their own way of washing and soaking the rice). The rice soaked and washed, it is left overnight and then steamed first thing in the morning, usually around 6am. After steaming, a portion of the rice is sent to the koji room for inoculation while the rest is cooled and thrown into the fermentation tank.
Rice that gets sent into the koji room is inoculated with the koji spores. The koji room can best be thought of as a coolish sauna. The entire room and the koji boxes are built of wood, for purposes of humidity control, and is kept at an ideal temperature for the koji, around 30 oC (86 oF), making it a very hot place to do hard work in. After a while, the koji on the rice will begin to grow and convert starch to sugar, turning the rice into what looks and tastes like sugar puffs. In addition to producing sugar though, the koji themselves will start to generate heat. The amount of heat given off is actually not negligible and if left to themselves the koji can actually kill themselves from the excessive temperatures. To prevent this, boxes containing koji have to be shifted around the room periodically (about every 5 hours or so) to keep things from heating up too much. It takes about three days to get a good koji colony going, and the brewers have to get up at midnight and at 5am every night to move the boxes around.
Koji made, it is ready to be thrown into the starter mash. The entire point of the starter mash is to create a home for as many yeast cells as possible so that later on you can get a huge vat of sake brewing as quickly as possible. If the fermentation isn't strong or fast enough then there is a chance of lactic bacteria infection, turning the sake sour. It takes about 2 weeks to get a starter mash packed with yeast cells.
After the starter mash is finished what is known as "yon-dan" in Japanese ( 4 additions ) begins. In the 1st stage the starter mash is moved to a larger tank and more koji, more rice and more water is made and added to the tank, just about doubling the size of the mash. The next day is called "odori" which means "dancing ferment". It is very obvious to see why it is called a dancing ferment as the tank basically explodes into a bubbling cauldren and the yeast have a feeding frenzy. Nothing is added to the tank this day, in order for the yeast to get fully adjusted to its new home.
For two days after the Odori the 2nd and 3rd stages take place in which the contents of the tank are doubled, then doubled again. The reason for all of this doubling, rather than just tossing in everything at once, is so that the yeast can have a chance to reproduce as quickly as possible, and also to ensure the maximum alcohol content of the sake. After the 3rd step the koji and yeast are working in harmony and the brewing is in full swing. However, the brewers still must be careful and check on the fermentation's temperature periodically. Because all of the flavors of the sake come from the koji and yeast, it is important to not let the fermentation to progress too quickly lest the yeast will become more exited about eating than producing flavor.
Yeast ferments alcohol much slower at low temperatures, which is why sake brewing takes place in the winter. The downside of this is if the temperature gets too low the fermentation will "stick" (i.e. fermentation will stop) and once again you risk the possibility of a bacterial contamination in the brew. The ideal temperature for the fermentation is around 10-15 oC ( 50-60 oF ). To keep the temperature at about this level (alcohol fermentation produces a lot of heat) the temperature of the brewery needs to be around 3 oC ( 37 oF ). As a measure of comfort to the freezing brewers, the cold tempterature of the brewery does mean that there is less bacteria in the air, which makes their jobs a tiny bit easier.
During the fermentation process, which lasts from mid-December until mid-Feburary, the Masuichi brewers busily build up a total of 16 fermentation tanks, ranging from 2,000 to 6,000 liters ( 528-1585 gallons ).
Once the last bit of rice is steamed, and the last batch of koji produced, the brewers can take a break. Brewers can take a day off every once in a while; work hours change from 5am-7:30pm to 8am-5pm; but they still have to work 7 days per week.