Filtering & Racking

The state of the sake after pressing will often be a yellowish-green hazy liquid with a residual turbidity. Depending on how coarse the press fabric was, this might even contain a substantial amount of small rice particles, especially if the intention is to package the product as nigorizake (cloudy sake).

If we want to produce clear, refined sake, there are a few more actions to take and these can have drastic effects on the final flavor and time is a big factor.

Settling

The Purpose of Settling is to settle out the polyphenols and other small rice particles (ori = おり), yeast cells (酵母), and enzymes (酵素) that remain suspended in the newly pressed sake. This can take some time to accomplish depending on the type of yeast you use, the density, temperature, pH, nutrient conditions, and dissolved oxygen (see: flocculation).

Settling is accomplished by cold-crashing the newly pressed sake in a tank to around 5°C or colder and waiting for the particles to fall or “settle” into the bottom of the tank. Usually the tank has a conical bottom to collect the sediments but, this can take several days (usually 1-3) before the particles settle out of solution.

The longer these elements remain suspended in the sake the more they will continue to change the flavor and aroma. Enzymes will continue to cleave proteins, lipids, and sugars. Yeast will continue to ferment sugar into alcohol. Any rice particles will tend to leave either a sweet or bitter flavor in the sake as well. For this reason, Koseki-San, who is a Senior Advisor of the Yamagata Sake and Shochu Makers Association, said that you should try to complete this step as soon as possible after pressing and pasteurize to lock in the desired flavor.

Racking

Purpose of Racking pulling clear sake off the top of the tank, and leaving a concentrated liquid of the flocculated elements in the previous tank. This ensures that the final sake is as clear as possible.

The “Settling Tank” described above usually has a separate valve which is a few inches above the bottom, which can control how liquid is pulled from the tank. This crude process allows a curved pipe inside the tank to be manipulated from the outside so that it only transfers clear sake and leaves the sediments at the bottom of the settling tank.

There are products such as bentonite, biofine (silicic acid), and others which are known as fining agents. These can help remove the elements faster by binding to them and pulling them to the bottom of the tank so the clear sake can be transferred to another tank.

Filtering

The Purpose of Filtering is to produce an even clearer sake, removing discoloration, microscopic rice particles, yeast, proteins, and even flavor compounds. Charcoal filters and fine paper filtration systems can remove particles of many various sizes. Even 0.5 microns is not surprising, but the more fine the filtration, the more flavor is removed, so be discerning in your choices.

It should be noted that during storage, even after filtering, it is possible that you may find the proteins still dissolved in the sake can become insoluble, creating a slight haze. This is avoidable using the silicic acid or other approved products, but be warned that it can remove desired flavors as well.