The key to understanding saké is to understand that it comes from rice. Wines are made by taking simple sugars found in grapes and fermenting, a simpler one-step process. Beer goes through a brewing process involving breaking down starches into sugars and then yeast takes the sugars and produces alcohol, much closer to the process for making saké. In saké production however, those processes take place in the same vat; commonly called parallel multiple fermentation.
Different varieties of rice allow for different levels of milling or polishing. This is the highest level distinction of different types of saké - how much of the outer surface of the rice has been removed. More on this later.
The rice varieties used in saké production tend to have highly concentrated starch cores surrounded by fats and proteins in the outer layers. Those are milled away because they produce unwanted flavors. Also proper polishing prepares the rice for efficient steaming and effective Kōji growth. The polished rice is allowed to rest before soaking in water in order to avoid cracking. The more that was milled away the less time the rice will be soaked in water, then steamed. Over steaming will ruin the flavor profile of the finished product, and not steaming it long enough will not allow the rice to entirely be fermented.
Kōji is the speciic kind of mold (aspergillus oryzae) used in the saké fermentation process. Kōji growth is critical to good brewing because the Koji mold efficiently converts the rice starches into sugar making the rice grain ideal food for the yeast. The mold is often allowed to grow for a couple of days before the next step. Careful management of rice and Kōji has a huge impact on brewing.
When the rice and mold mixture is ready, it's time for yeast to be added to produce the starter mash, or Moto. The yeast can survive much longer than in the case of beer brewing where it typically lasts only until the content reaches around 9% alcohol, or in the case of wine say 13%. In saké brewing the resulting drink naturally can get up to 20% alcohol. Usually this is reduced to around 15% by the addition of water.
This starter mash is where the Kōji, rice, yeast, steamed rice and water all come together for the first intense brew. The mash is then moved to a larger tank where more rice and water is added to keep the brew going. After this the mixture is allowed to ferment for two to six weeks. This process can be regulated by controlling the temperature; premium sakés often lower the temperatures and slow the process down.
When the fermentation is complete the mixture is pressed to separate the solids from the liquid. Pay attention, here's where it gets a bit tricky. A lot of the flavors the brewer was after are trapped in the solids, but most sakés are carbon filtered. There was a time when only filtered product could be considered saké. Now there are commercial products where carbon filtering has been skipped, and the rough resulting beverage tends to be earthy. Many cloudy sakés have been filtered then had some of the sediment added back in!
In many cases the liberation of flavors trapped in the solids is achieved by adding small amounts of distilled alcohol to the saké before pressing. This can result in a more aromatic end product, Honjozo, with a pronounced enhancement of the subtle flavors. Some purists prefer the Junmai sakés, which have no alcohol added.
To be clear, adding brewers alcohol is a deliberate step that can improve the quality of the end product. It is sometimes not done in lower and midrange sakés in order to avoid higher import fees. And this is not to be confused with large manufacturers adding distilled alcohol to cheaper grades of saké in order to fortify them. Got all that?