Filtering stabilises the flavour of beer, holding it at a point acceptable to the brewer, and preventing further development from the yeast, which under poor conditions can release negative components and flavours Filtering also removes haze, clearing the beer, and so giving it a “polished shine and brilliance”. Beer with a clear appearance has been commercially desirable for brewers since the development of glass vessels for storing and drinking beer, along with the commercial success of pale lager, which – due to the lagering process in which haze and particles settle to the bottom of the tank and so the beer “drops bright” (clears) – has a natural bright appearance and shine
There are several forms of filters; they may be in the form of sheets or “candles”, or they may be a fine powder such as diatomaceous earth (also called kieselguhr) which is added to the beer to form a filtration bed which allows liquid to pass but holds onto suspended particles such as yeast. Filters range from rough filters that remove much of the yeast and any solids (e.g., hops, grain particles) left in the beer to filters tight enough to strain colour and body from the beer. Filtration ratings are divided into rough, fine, and sterile. Rough filtration leaves some cloudiness in the beer, but it is noticeably clearer than unfiltered beer. Fine filtration removes almost all cloudiness. Sterile filtration removes almost all microorganisms.
Sheet (Pad) Filters
These filters use sheets that allow only particles smaller than a given size to pass through. The sheets are placed into a filtering frame, sanitized (with boiling water, for example) and then used to filter the beer. The sheets can be flushed if the filter becomes blocked. The sheets are usually disposable and are replaced between filtration sessions. Often the sheets contain powdered filtration media to aid in filtration.
Pre-made filters have two sides. One with loose holes, and the other with tight holes. Flow goes from the side with loose holes to the side with the tight holes, with the intent that large particles get stuck in the large holes while leaving enough room around the particles and filter medium for smaller particles to go through and get stuck in tighter holes.
Sheets are sold in nominal ratings, and typically 90% of particles larger than the nominal rating are caught by the sheet.
Filters that use a powder medium are considerably more complicated to operate but can filter much more beer before regeneration. Common media include diatomaceous earth and perlite.