The process of passing a cloudy liquid through paper, cloth or other filtration medium, removing fine solids in suspension to give a clear liquid. Fine solids include insoluble particles, yeast, bacteria or colloidal material. Filtration media include filtration paper or cloth and various types of filter.
In seishu production, the process of separating liquid sake and sake kasu from moromi is the first example of filtration to occur. The filtration of moromi cannot be completed without compression (assaku). For this reason, this first filtration is distinguished from the later roka filtration of sake itself by the term assaku , and is also known as joso or shibori (搾り).
The sake resulting from this preliminary filtration (pressing) is called the tare-kuchi (垂口) , and still contains a certain quantity of ori sediment. A second filtration is carried out to give as clear a sake as possible. At this stage, filter aids such as fibrous materials like pulp, or mineral materials such as Celite or diatomaceous earth, may be used to increase filtration efficiency.
Sediment settles during storage, so a third filtration takes place after storage or prior to bottling.
It is important to remember that the word roka is used to refer to the second and third filtrations, not to the initial pressing of the moromi, though this is in fact also a form of filtration.